WO2018197858A1 - Microstructured sapphire substrates - Google Patents

Abstract

The present invention relates to sapphire substrates having anti-fouling properties, to methods for their preparation and to their use as optical components in devices which transmit or receive light. The sapphire substrates have a micropatterned surface which comprises a plurality of sapphire pillars which project from the surface. The pillars have an aspect ratio less than or equal to about 1.0, and the ratio of the spacing between adjacent pillars to the pillar height is greater than or equal to about 0.5.

Description

Microstructured sapphire substrates

The present invention relates to sapphire substrates having anti-fouling properties, to methods for their preparation, and to their use as optical components in devices which transmit or receive light.

More specifically, the invention relates to microstructured sapphire substrates which are both oleophobic in water and hydrophobic in air, whilst retaining excellent optical properties. These properties reduce surface fouling and so make the materials particularly suitable for use in subsea applications, for example as optical windows in underwater surveillance cameras in the oil and gas industry.

Sapphire is the crystalline form of Al₂0₃ (alpha alumina) and the hardest material in the world after diamond. It is an important material in a range of industrial applications and may, for example, be used for optical purposes due to its transparency in the wavelength range ~ 250-5000 nm.

Due to its excellent mechanical and thermal properties, sapphire substrates can be used in harsh environments, for example as an optical window in technologies applied in the offshore oil and gas industry, such as optical sensors and cameras. Optical technologies can facilitate increased production and cost efficiency of oil and gas, reduce safety risks, and avoid production interruptions by providing online environmental surveillance for remote installations. However, when used in subsea installations, the sapphire surfaces are continuously exposed to oil (hydrocarbons) and other fouling mixtures such as precipitated salts and biological specimens which can lead to contamination of the surface and operational difficulties due to reduced light transmission of the surface. Maintenance interventions underwater are costly and can involve risks related to both human safety and the environment. Marine bio-fouling is the undesired growth of living organisms on submerged surfaces. Bio-fouling is a highly dynamic process, which spans numerous length scales depending on factors such as the submerged surface, geographical location and season. Bio-fouling of a clean surface immersed in natural seawater is typically a multiphase process that includes initial adsorption of a molecular "conditioning" film consisting of dissolved organic material. In the second phase, initial settlement of bacteria and diatoms occurs creating a biofilm matrix. This is followed by colonisation by algal spores and attachment of multicellular macro- fouling species in later phases. Build-up of foul such as oil, scales and bio-foul on optical components installed subsea is thus a major challenge faced by various technologies including oil-gas sensors, oil-water separation, small oil droplet transportation and self-cleaning of marine equipment. All of these applications require controlled oil-adhesion. During the past decade, much progress has been made in developing surfaces with various types of wettability with regard to water and organic phases in air and underwater (see Nature Comm. 3: 1025, 2012; Langmuir 25: 14165, 2009; ACS nano 10: 1386, 2015). Recently, the inventors have investigated the underwater surface properties of unstructured sapphire (see Hoist et al., J. Phys. Chem. 1 19: 15333, 2015). To the inventors' knowledge this is the only investigation which has been carried out on the underwater surface properties of sapphire.

Most surfaces that repel oil in air are typically not oil-repellant once submerged in water and vice versa (see Langmuir 25: 14165, 2009). Taking inspiration from nature, many aquatic animals provide a model subsea oil-repellent and self- cleaning surface. For instance, shark skin is covered with small individual tooth-like scales ribbed with longitudinal grooves that allow efficient flow of water over this surface. For fish scales, the hierarchical rough structures combined with the hydrophilic chemistry of scales leads to underwater superoleophobicity. This is due to the trapped water layer at the oil-solid interface that acts as an oil-repellent layer. This is similar to the superhydrophobic phenomenon where an air layer trapped between water and a low surface energy solid surface in an air environment reduces the contact between water and the solid surface. Inspired by this concept, various underwater oil-repellent surfaces have been developed. However, most of the reported methods involve the use of hydrophilic chemistry (i.e. a surface coating) combined with specifically designed patterns. Some examples involve graphene oxide (see Chem. Comm. 50: 190, 2014), polymers (see Nanoscale 16: 190, 2014; and Langmuir 25: 11 137, 2009), hydrogels (see Adv. Mat. 22: 4826, 2010; and Adv. Mat. 23: 4270, 201 1), metal oxides (see J. Mat. Chem. 22: 19652, 2012; and J. Mat. Chem. 2: 17666, 2014), colloidal crystals (see Adv. Funct. Mat. 21_: 4436, 2011) and microstructures (see Langmuir 25: 14165, 2009; Adv. Mat. 21 : 665, 2009; and J. Mat. Chem. 2: 8790, 2014). Jiang et al. (Adv. Mat. 21 : 665, 2009) and Hou et al. (J. Mat. Chem. A 3: 9379, 2015) alter substrate surface properties by modification of the substrate itself, without applying a coating.

In Jiang et al. silicon is used as the substrate and the pillars made are 30 μηι high and 150 μηι x 150 μηι in lateral dimension. However, these dimensions would strongly change the optical properties of the silicon surface and would not be feasible in respect of sapphire where it is important that optical transmission is not adversely impacted. In Hou et al. a transparent underwater superoleophobic surface on silica glass is made using femtosecond laser-induced ablation. Nanoscale rough microstructures on the surface are formed due to the ablation under laser pulses and the

resolidification of ejected particles. The resulting surface consists of numerous randomly distributed holes and grooves. This method provides a relatively simple and cost-effective route for transparent oleophobic underwater silica glass surfaces, but at the same time poses a concern to time-stable performance and robustness subsea, where silica glass cannot be used as it does not possess the required degree of hardness. This method would also not be expected to be suitable for use on a sapphire surface as the sapphire may turn amorphous (and hence white) during the ablation process thus destroying its optical transmission properties.

Microstructured surfaces having hydrophobic and/or oleophobic characteristics for use in multiphase fluid mixtures are proposed in WO 2010/028752. These surfaces work by trapping a wetting liquid phase (e.g water) within the microstructured surface using a surface priming process which relies on capillary action. The wetting phase which becomes trapped in the pores is immiscible with at least one phase of the surrounding multiphase fluid mixture (e.g. oil) and so provides the desired wetting properties of the surface. However, careful design of the microstructured surface is required to reduce the probability that the wetting liquid phase may leave the microstructure thereby destroying the desired wetting characteristics. In order to trap and lock the wetting liquid in the structured surface, relatively high aspect ratios are required to take advantage of the capillary effect, i.e. the height of the microstructures is much larger than their diameter. The structures described in WO 2010/028752 have several drawbacks for use in subsea applications. Escape of trapped wetting liquid from within the

microstructure allows diffusion of oil and foul particles into the structure. This reduces long-term antifouling properties and can even lead to permanent contamination of the surface. Due to capillary action and a lack of fluid mobility across the surface there is an increased risk that foul particles (e.g. oil) become trapped and/or aggregate at the surface. Due to their high aspect ratios, the microstructures are also prone to damage resulting from high pressures and/or mechanical vibrations underwater. Any damage to the surface structure would result in fouling at a higher rate than for a non-structured surface.

Furthermore, at least some of the surface structure configurations which are described in WO 2010/028752 would not be considered suitable for enhancing the anti-fouling properties of a sapphire surface for use in applications which rely on its optical characteristics. For such structures with high aspect ratios, the side wall angle is a major source of light scattering. To the extent that this earlier document provides any detailed discussion relating to sapphire surfaces, it suggests the need for a photocatalytic film of titanium dioxide to maintain the hydrophilic

characteristics of the surface and aid its ability to trap water as the wetting liquid phase. As with earlier methods, a hydrophilic coating would therefore be required.

To date, the most successful strategy to combat bio-fouling has been the incorporation of biocidal additives into paints. Biocidal paints based on tributyltin have been highly effective to fight bio-fouling and used until recently. However, due to their toxicity, the use of such paints has now been banned. The legislation prohibiting the use of highly effective anti-fouling paints due to their harmful effects on the environment has driven tremendous interest in recent years in the development of non-biocidal and non-toxic anti-fouling strategies. Current methods to control bio-fouling include low-drag and low-adhesion surfaces achieved through coating and surface nano/micro texturing. Current commercial coatings are based on poly(dimethylsiloxane) elastomers. These coatings are non-polar with low surface energies and are expected to reduce the adhesion of proteins. However, use of such coatings is limited by their weak bonding to the substrate, durability and robustness. Recent attention in the context of anti-fouling has also been devoted to surface designs based on bio-inspired topographical features. Topographies inspired by shark skin scales have shown an 85% reduction in settlement of motile spores of the macroalga Ulva (Carman et al., Biofouling 22(1): 11-21 , 2006).

Contamination of underwater sapphire surfaces nevertheless remains a significant problem and there is a strong and increasing demand for alternative (e.g. improved) anti-fouling surfaces, in particular surfaces having underwater oleophobic properties.

For other industries, condensation of water in air is a problem and leads to increased wettability of light transmissive surfaces such as sapphire. This is a problem, for example, for surfaces of electronic devices such as mobile phones where contact of the surface with moisture in the air for any prolonged period of time can lead to surface damage. In such cases, a light transmitting surface which is hydrophobic in air is desirable. Other instances in which a 'hydrophobic in air' surface may be desired include optical sensors, aeroplane windows, lenses (e.g. camera lenses), etc.

The invention seeks to address these needs and to provide optical surfaces having desirable wettability for use in oil/water-related applications. The inventors have now appreciated that the surface of sapphire can be

microstructured in a way which not only makes it possible to change its properties from hydrophilic in air to hydrophobic in air and oleophobic in water without the need for any additional coating, but which can be done without compromising its optical properties.

The sapphire substrates described herein have a micropatterned surface comprising a plurality of micropillars which project from the surface of the sapphire crystal. Although it is envisaged that such micropillars will typically extend across the entire sapphire surface, this need not be the case provided these are formed on at least a portion of the sapphire surface. More specifically, the inventors have found that the optical properties of sapphire can be maintained whilst achieving the desired wettability characteristics by selecting specific structural parameters for micropatterning of its surface. The invention involves the formation (e.g. by etching) of a plurality of small pillars in the sapphire surface to achieve the properties as described herein. The inventors have appreciated that certain aspects of the pillaring of the surface are key to achieving these properties and, in particular, to ensuring that the optical transmission of the "as received" sapphire is essentially unchanged following microstructuring of its surface. The use of pillar structures with specific dimensions in order to alter the wettability of a sapphire surface whilst maintaining optical properties has not previously been investigated in this way because patterning has mainly been focussed on non-transparent materials, and hence optical properties have not been a concern.

The inventors have appreciated that the desired wettability properties for use in oil/water-related applications and optical characteristics can be achieved by modification of the sapphire surface itself, i.e. without the application of any coatings. This has the advantage that the surface structure of the sapphire remains robust and durable - this can only be removed if the sapphire surface itself is damaged which, due to its hardness, is very difficult. As will be understood, the invention relates to micropatterning of the sapphire crystal surface. Accordingly, the micropillars consist of sapphire and these form an integral part of the sapphire substrate.

The invention thus provides a modified sapphire surface structure having controlled wettability of water and oil underwater without unduly compromising its optical properties. It also provides a modified sapphire surface having controlled wettability of water in air.

The invention further provides structured sapphire surfaces which are effective in the prevention or reduction of bio-fouling. Without wishing to be bound by theory, anti-biofouling properties are believed to arise from the low aspect ratio of the pillar structures formed in the sapphire surface. The pillars are dimensioned such that they inhibit the bioadhesives in creating a connected film, whilst at the same time ensuring an efficient flow of water in between the pillars hence prohibiting the strong interfacial bond between bioadhesives and the surface of the sapphire substrate. The presence of a water layer that is continually replenished is believed to shed away the bioorganisms ensuring stable, self-cleaning properties. In certain embodiments, the rough edges of the pillars are also believed to disrupt and/or inhibit the settlement and eventual colonisation of microorganisms.

The pillar structures project from the surface of the sapphire such that the exposed sapphire surface is uneven (i.e. modulates) in structure. Preferably, adjacent pillars are non-contacting at their base and so form a micropattern on the surface of the sapphire which consists of a plurality of peaks and troughs.

Each pillar structure forms a projection on the sapphire surface which may be defined by way of its lateral dimension (d), its height (h) and the angle which it forms with the sapphire surface (sidewall angle, Θ). The periodic distance between the centres of adjacent pillars (pitch, p) and the pillar spacing (s) further defines the microstructure of the sapphire surface. These features are illustrated, by way of example only, in accompanying Fig. 1. As will be understood, not all pillars will necessarily be identical and there may be some variation between individual pillars in respect of their shape, dimensions (lateral dimension and height), spacing, pitch and sidewall angle, etc. Where reference is made herein to any dimensions of the microstructured sapphire surface, it will be understood that these refer to average values. As used herein, the term "lateral dimension" is intended to refer to the maximum width of the pillar. For example, where the pillar is circular in cross-section the lateral dimension will be the diameter of the cross-section of the pillar. Where it is rectangular, it will be the length of the longest diagonal, etc. The "height" of a pillar according to the invention is the maximum distance from the base of the pillar to the highest point on its uppermost surface (i.e. the surface which projects away from the sapphire surface).

The "aspect ratio" of a pillar according to the invention is the ratio of the height of the pillar to its lateral dimension (i.e. h/d). The "spacing" between adjacent pillars refers to the minimum distance between adjacent pillars, i.e. the minimum distance between the sidewalls of adjacent pillars. As used herein, the term "pitch" is intended to refer to the distance between the midpoints of adjacent pillars and is intended to indicate the periodicity of the microstructure.

The "sidewall angle" is the angle formed between the base of a pillar and the adjacent sapphire surface.

Certain features of the microstructures herein described are important in achieving the desired characteristics. One such feature is the aspect ratio. In contrast to prior art structures, such as those which are described in WO 2010/028752, the aspect ratio should be low. This should be less than or equal to 1.0, i.e. the height of the pillars should be less than or equal to their lateral dimension (h≤ d). Another feature which is key to achieving the desired properties is the spacing between adjacent pillars and, in particular, the spacing compared to the pillar height. The spacing should be greater than or equal to half the pillar height (s≥ h/2).

In one aspect the invention thus provides a sapphire substrate having a

micropatterned surface which comprises a plurality of sapphire pillars which project from the surface, wherein the pillars have an aspect ratio less than or equal to about 1.0, and wherein the ratio of the spacing between adjacent pillars to the pillar height is greater than or equal to about 0.5.

In one embodiment, the spacing between adjacent pillars is greater than or equal to half the lateral dimension, i.e. s≥ d/2. In another embodiment, the spacing between adjacent pillars is less than or equal to 3 times the lateral dimension, i.e. s≤ 3d. In a further embodiment the spacing, s, satisfies the following conditions: d/2≤ s≤ 3d. The spacing between adjacent pillars may, for example, range from 0.5 to 3 times, or from 1 to 2.5 times, e.g. from 1 to 2 or from 2 to 3 times, the lateral dimension. In certain embodiments the spacing between adjacent pillars may be 0.5, 1 , 1.5, 2, 2.5 or 3 times the lateral dimension. The lateral dimension of each pillar may be in the range from 0.05 to 12 μηι, preferably from 0.5 to 12 μηι, more preferably from 1 to 10 μηι, e.g. from 3 to 5 μηι. In some embodiments, the lateral dimension may be 0.8 to 12 μηι, 2 to 12 μηι, 3 to 8 μηι, 2 to 7 μηι, 4 to 6 μηι, e.g. about 5 μηι. Although the lateral dimensions of the pillars need not be identical to one another, it is preferred that these are

substantially identical throughout the patterned sapphire surface.

The height of each pillar may be in the range of from 100 to 800 nm, preferably 150 to 700 nm, e.g. from 200 to 650 nm, or from 200 to 600 nm. Heights in the range of from 400 to 800 nm, preferably 500 to 700 nm, e.g. about 600 nm are particularly preferred. Although the heights of the pillars need not be identical to one another, it is preferred that these are substantially identical throughout the patterned sapphire surface. As discussed, the height of each pillar should be less than or equal to the lateral dimension (i.e. h≤ d, preferably h < d). Typical aspect ratios (h/d) may range from 0.03 to 1.0, preferably from 0.04 to 0.5, e.g. from 0.04 to 0.2. In some

embodiments, the aspect ratio may be from 0.05 to 0.2, from 0.1 to 0.2, e.g. from 0.11 to 0.2. In a further embodiment, the aspect ratio is less than 0.6. Although the aspect ratio of each of the pillars need not be identical to one another, it is preferred that this ratio is substantially the same throughout the patterned sapphire surface.

As discussed, another feature which is key to achieving the desired properties is the spacing between adjacent pillars and, in particular, the spacing compared to the pillar height.

The spacing between adjacent pillars may be in the range from 0.025 to 40 μηι, preferably from 0.5 to 30 μηι, e.g. from 1 to 12 μηι. In some embodiments, the pillar spacing may be 1 to 25 μηι, 5 to 30 μηι, 10 to 25 μηι, 15 to 30 μηι or 15 to 25 μηι. Although the spacing between pillars need not be identical to one another, it is preferred that this is substantially the same throughout the patterned sapphire surface.

The pillar spacing should be greater than or equal to half the pillar height (i.e. s≥ h/2). The pillar spacing to height ratio (s/h) may range from 0.5 to 800, preferably from 5 to 400, more preferably from 10 to 200. In some embodiments, the pillar spacing to height ratio may be from 5 to 100, from 10 to 75, e.g. from 15 to 75, from 25 to 60, or from 25 to 50. Although the pillar spacing to height ratio need not be uniform throughout the structure, it is preferred that this ratio is substantially the same throughout the patterned sapphire surface.

The pitch may range from 0.075 to 40 μηι, preferably from 0.5 to 30 μηι, e.g. from 1 to 12 μηι. In some embodiments, the pitch may be 1 to 25 μηι, 5 to 30 μηι, 8 to 17 μηι, 10 to 25 μηι, 15 to 30 μηι or 15 to 25 μηι. Although the pitch need not be uniform throughout the structure, it is preferred that the pitch values are

substantially identical throughout the patterned sapphire surface.

Examples of suitable d/p/h ratios of pillars include: 5 μηι /15 μηι /200 nm and 5 μηι /10 μηι /600 nm.

As described herein, the invention relates to sapphire substrates comprising surface microstructures having a low aspect ratio and a relatively large separation between adjacent micropillars. Without wishing to be bound by theory, it is believed that under these conditions the capillary forces between adjacent micropillars are negligible and the surfaces are structured in such a way that these do not trap a water layer. Rather, when these structures are submerged in water, water is readily able to flow in and through the structure thus creating a dynamic water layer which provides a protective, immiscible layer against oil wetting. This leads to a higher oil contact angle. In contrast to earlier proposals, the inventors have thus realised that the water layer does not need to be trapped in the surface structure to provide the desired oleophobic characteristics. In fact, they have found that it is an advantage that it is not trapped since efficient water flow not only on the top of the

microstructures, but also between the microstructures enhances anti-fouling properties. These have enhanced long-term self-cleaning properties due to minimal trapping of particles. The low aspect ratio of the microstructures herein described also makes these particularly robust for use in underwater applications.

As will be described herein, a variety of shapes and sizes of pillars may be capable of providing the desired anti-fouling and optical properties. However, the inventors have appreciated that the upper surface of the pillars, i.e. the surface which protrudes away from the sapphire surface, should preferably be substantially planar. By "substantially planar" it is intended that any variations on the surface of each pillar is less than 10% of the average pillar height, preferably less than 5%, e.g. less than 3% or less than 1 %. As will be understood, any pillar which tapers to form a peak will not be considered to have a substantially planar surface.

In one set of embodiments the upper surface of the pillars will be substantially parallel to the adjacent sapphire surface. By "substantially parallel" it is intended that the difference in height from one side of a pillar to the other will not be more than 30% of the average pillar height, preferably not more than 20%, more preferably not more than 10%, e.g. not more than 5%.

The pillar structures may vary in size and shape and need not all be identical. The size and shape of individual pillars may be varied independently of one another, though in practice these will typically be substantially identical (at least to within the tolerance limits of the process used for their preparation).

The cross-sectional shape of the pillars is not critical and it is envisaged that a wide range of different shapes may be capable of providing the desired properties. Suitable shapes include, for example, circular, oval, triangular, square, rectangular, pentagonal, hexagonal, heptagonal and octagonal cross-sections. In a preferred embodiment, a circular, square or rectangular shaped cross-section may be used due their ease of manufacture. The shape of the pillars can be chosen

independently of one another. It is, however, preferred that all pillars are of substantially the same shape.

The cross-section of the pillars may vary with increasing structure height, but typically these will have a substantially uniform cross-section. As will be appreciated, those which taper (e.g. to form a peak) do not have a substantially uniform cross-section.

In a further embodiment, the surface of the sapphire between adjacent pillars is substantially planar, i.e. at no point between the pillars is there a projection with a height greater than 10% of that of the average pillar height, preferably no greater than 5%. The inventors have appreciated that in order to maximise the optical characteristics of the microstructured sapphire surface the pillars should project substantially perpendicular to the sapphire surface. By "substantially perpendicular" it is meant that the sidewall angle is between 70° and 110°, preferably between 80° and 100°, more preferably between 85° and 95° or 87° and 93°, e.g. between 88° and 92°. Typically, the sidewall angle will be about 90°.

In the case where h/d is less than 0.2, it is particularly preferred that the pillars should project substantially perpendicular to the sapphire surface in order to ensure good optical properties. In such cases, it is preferable that the degree of variation from a 90° sidewall angle is less than or equal to 5°. Preferred sidewall angles may thus range from 85° to 95° where d/h is less than 5. Preferably, the side-wall angle will be substantially constant around the base of any given pillar.

The pillars can be arranged in different geometries on the sapphire surface including both regular and irregular patterning. In one embodiment these are regularly spaced. Where these are arranged in a regular, repeating pattern this may, for example, be simple cubic, close packed, or rectangular (hexagonal) packing. Preferably the pillars may form a regular 2D lattice pattern.

The inventors have appreciated that a variation in the spacing of the pillars across the sapphire surface may be beneficial under certain circumstances, for example when the surface is to be used underwater in conditions where there is water flow across the surface. For use in conditions where the water flow is likely to be turbulent (i.e. flow is non-uniform), regular patterning of the surface structure may be preferred. However, in one embodiment, it may be beneficial to vary the packing density of pillars across the surface, for example to have closer packed pillars at the centre of the surface (i.e. a smaller pitch) with the separation increasing when moving away from the centre of the surface. The inventors have found that such an arrangement of pillars can help to reduce adhesion of fouling particles at the centre of the surface whilst still allowing an efficient flow of water towards the edges of the surface, hence assisting in the removal of the particles by the flow of water. In one set of embodiments, only a proportion of the sapphire surface may be provided with pillar structures. The precise positioning of the pillars and extent of their coverage of the surface may be varied dependent on the intended use of the sapphire substrate, for example the likely degree of fouling and/or the turbulence of water flow.

In some embodiments the inventors have found that the spacing of the pillars on the sapphire surface may be varied dependent on their positioning relative to the flow of water when in use, for example whether these are intended to be positioned in alignment with the intended flow of water or against the flow.

In one set of embodiments, the micropatterned sapphire surface is uncoated. For example, this may be free from any hydrophilic coating (for example, where this is to be used underwater), and/or free from any hydrophobic coating (for example, where this is to be used in air).

As used herein, the term "sapphire" refers to the crystalline form of Al₂0₃.

However, this need not be pure Al₂0₃ and any grade of sapphire having the desired optical characteristics may be used in the invention. Where this is impure, this may contain any type and amount of impurities provided that the desired optical properties of the sapphire are retained. Impurities which may be present include doped metal ions, for example, iron, titanium, chromium, copper or magnesium.

The nature and amount of any dopants which may be present will depend on various factors, including the effect (if any) that these may have on the optical properties of the sapphire. Suitable dopants and dopant levels may readily be determined by those skilled in the art.

The crystal structure and crystal cut of the sapphire substrate for use in the invention is not critical subject to the requirement that this should provide the desired optical characteristics for the intended use. It is envisaged that a wide range of crystal structures and cuts could be used which provide the desired optical properties. Suitable crystal structures include, for example, 0001 , 1010, 1120, 1012, 1120(0.05 Ti), and 0001 (0.1Cr). A suitable crystal structure for use in the invention is 0001. Any crystal cut of sapphire may be microstructured as herein described. The sapphire substrate can contain crystal defects including, but not limited to, any of the following: point defects (such as Schottky defects, interstitial defects and Frenkel defects), line defects (such as edge dislocations and screw dislocations), planar defects or bulk defects. Where any defects are present these should not affect the desired optical characteristics of the sapphire.

Suitable sapphire substrates for microstructuring in accordance with the invention may be obtained from companies such as Freudiger, Rayotek, Crystran and MTI Corporation.

The microstructured sapphire surface as herein described may be produced by various methods known in the art, for example by photolithography followed by wet or dry etching. Methods for the preparation of the sapphire substrates as herein described using such techniques form a further aspect of the invention.

The process of patterning of the sapphire substrate by photolithography may involve at least the following steps: (a) deposition of a photoresist layer on the surface of the substrate; (b) provision of a photomask on the photoresist layer followed by UV exposure to pattern the photoresist layer; (c) removal of the photomask and development of the photoresist whereby to produce a patterned photo-resist layer; (d) deposition of an etch mask; (e) removal (i.e. lift-off) of the remaining photoresist; (f) dry etching of the sapphire substrate using the etch mask as an etching template; and (g) removal of the etch mask whereby to form the micropatterned substrate.

Suitable dry etching methods include inductively coupled plasma etching (ICP etching), ion beam etching or a combination of both. Wet etching using standard methods in the art may also be used to etch the sapphire surface although this is generally less preferred.

Suitable photoresist layers comprise organic molecules sensitive to interaction with the UV light which penetrates through the mask. The photoresist layer may, for example, be chosen from AZNLof2000 (negative photoresist purchased from Microchemicals) or TI 35ES (image reversal photoresist purchased from

Microchemicals). Where a lift-off procedure is used for the patterning of sapphire surfaces, photoresists that are designed for lift-off procedures and also exhibit suitable undercut/retrograde wall profile in the structures may be chosen.

Suitable photomasks are known in the art and include those made from any non- transmissive materials, for example quartz photomasks with Cr metallization purchased from JD Photodata.

Etching masks which are suitable for use in the methods herein described are known in the art and include those made from Ni and Cr. The use of appropriate etching masks and etching times results in the desired patterning, i.e. the size and shape of the pillar structures on the sapphire surface. An example of a suitable micropatterning method is as follows in which an Oxford Plasmalab System 100 is used for the etching of sapphire windows. The following parameters may be used to produce the desired patterning:

BCI₃ 25sccm

Pressure 2mT (Strike at 5mTorr or higher if required)

I CP Power 1750 W

Electrode 300 W

Temperature 60°C

Backside He 0 mbar

DC Bias -400V

Selectivity >7: 1

Etch Rate: >150nm/min

-1.4: 1

Etch Rate: ~65nm/min

Thus, for example, the sapphire surface may be dry etched using BCI₃ at an operating pressure of 2mTorr whilst maintaining inductive power (ICP), electrode, DC bias, flow rate and substrate temperature at 1.75 kW, 300 W, -400V, 25sccm, and 60°C, respectively. With these processing parameters, a sapphire etch rate of -65 nm/min with an etch selectivity of ~1.4 over Cr may be achieved. Prior to micropatterning the "as received" sapphire may be cleaned, for example to remove any dirt and grease (i.e. organic contaminants). Any suitable cleaning methods may be used, including cleaning with a mixture of sulfuric acid and hydrogen peroxide, for example a 3: 1 mixture. In other embodiments, the sapphire surface may be polished prior to micropatterning. Suitable polishing methods include mechanical and chemical polishing, as well as epi-polishing. Generally, "as received" sapphire substrates which are commercially available will be optical grade and already subjected to polishing.

The micropatterned sapphire surfaces of the invention are, by their nature, hydrophobic in air and oleophobic in water. In one embodiment the surfaces of the invention will exhibit a water contact angle in air of greater than 90°, preferably greater than 100°, e.g. greater than 1 10°, and/or an oil contact angle in water of greater than 100°, preferably greater than 130°. In one particular embodiment the sapphire surface will exhibit an oil contact angle in water of greater than 150°, i.e. the surface will be superoleophobic.

The patterned sapphire surfaces according to the invention display favourable optical properties. In one set of embodiments, these have an optical transmission of at least 50%, preferably at least 60%, e.g. at least 65%.

As used herein, the term "optical transmission" means the proportion of light energy which is transmitted through the sapphire substrate. The term "light energy" includes not only light in the visible region of the electromagnetic spectrum, but also light in the ultra-violet and infra-red ranges. The incident light may thus range from UV to visible to IR. In one set of embodiments, the incident light may have a wavelength in the range from 250 to 5000 nm, preferably from 250 to 4000 nm, and the "optical transmission" should be construed accordingly.

The inventors have found that the microstructured sapphire substrates described herein are particularly effective in transmitting light in the visible region. In one set of embodiments, the optical transmission of the sapphire substrate is at least 50%, preferably at least 60%, e.g. at least 65%, in the wavelength range of from 400 to 900 nm. The optical transmission of sapphire may be illustrated with reference to attached Fig. 2 (E.R. Dobrovinskaya et al. , Sapphire, Material, Manufacturing, Applications, 2009, Springer, page 85). This curve was obtained for pure, sufficiently perfect sapphire crystals grown by the Czochralski method (Linde Cz UV grade). The samples were 1 mm thick.

As will be understood, optical transmission will be dependent on the thickness of the substrate. In one set of embodiments, the optical transmission of the sapphire substrate is at least 50%, preferably at least 60%, e.g. at least 65%, for a substrate thickness in the range of from 0.2 mm to 10 mm, preferably 0.5 to 5 mm, more preferably 0.5 to 2 mm, e.g. about 1 mm.

Optical transmission may be measured using any known methodology and apparatus. Typically this will be measured using a spectrophotometer, for example spectrometer model No. UV-3100PC available from VWR International.

Optical characteristics of the sapphire substrates of the invention may also be defined with reference to the optical transmission of the unstructured or "as received" sapphire. The inventors have found that microstructuring of the sapphire surface as described herein does not adversely impact the optical transmission characteristics of the substrate. In one set of embodiments the microstructured sapphire substrate is oleophobic in oil and hydrophobic in air and has an optical transmission of at least 60%, preferably at least 70%, e.g. at least 75%, of that of the unstructured sapphire. The incident light may range from UV to visible to IR. In one set of embodiments, the incident light may have a wavelength in the range from 250 to 5000 nm, preferably from 250 to 4000 nm and the "optical transmission" should be construed accordingly. In one embodiment, the optical transmission may be determined in the visible region, for example, in the wavelength range of from 400 to 900 nm.

As used herein, the terms "unstructured sapphire" and "as received" sapphire are used interchangeably and are intended to refer to untreated, optical grade sapphire. Sapphire can be obtained in different qualities and optical grades based on the amount of surface polishing (e.g. chemical or mechanical polishing), and the number of defects it contains. Examples of unstructured sapphire substrates include sapphire with scratch/dig 40/20 or 20/10, and sapphire with epi-polished surfaces. The terms "scratch" and "dig" are terms of the art and a person skilled in the art would understand that "scratch" is the apparent width of hairline scratches according to a visual standard and that "dig" is the diameter of the largest defect, given in units of 0.01 mm. Epi-polished surfaces are atomically smooth surfaces and are mainly used as substrates for the growth of epitaxial films. Unstructured sapphire can be purchased from suppliers such as Freudiger, Rayotek, Crystran and MTI Corporation.

The sapphire substrates herein described have oleophobic and thus anti-fouling properties when used underwater whilst still retaining their desired optical transparency. A further aspect of the present invention therefore provides the use of the microstructured sapphire substrates to achieve a self-cleaning effect. Due to their self-cleaning properties underwater, the microstructured sapphire substrates are particularly suitable for use in any underwater application in which transmission of light is important. For example, these may be used as windows, lenses (e.g. for cameras and optical sensors), screens (e.g. for cameras and electronic equipment), or as other components installed subsea such as oil-gas sensors and marine equipment.

In a further aspect the invention provides the use of a micropatterned sapphire substrate as herein described as an optical surface in a device which receives or transmits light, especially in the optical range (to near IR), e.g. 400 to 900 nm, for example an optical surface in subsea equipment such as windows, optical sensors, vehicles and lights.

The sapphire substrates herein described can also have anti-biofouling properties when used underwater. A further aspect of the invention thus provides a method of reducing or preventing marine biofouling of a sapphire substrate when used underwater, said method comprising the step of microstructuring the surface of the sapphire substrate as herein described. Use of a micropatterned sapphire substrate as herein described to reduce or prevent marine biofouling also forms an aspect of the invention. Organisms which may be responsible for biofouling include tunicates, phytoplanktons, algae, bacteria, diatoms, tubeworm larvae, spores of Ulva, barnacle larvae, adult barnacles, adult tubeworms, mussels, and other small nektons.

The sapphire substrates described herein also exhibit hydrophobic properties in air. This makes these suitable for use as optical components in any device which receives or transmits light. For example, these may be used as display surfaces in any device which shows an image which has been produced electronically, for example those found in television receivers, computer monitors, projection display systems, phones, ipads, etc.

Any device (e.g. subsea equipment) comprising an optical surface comprising the microstructured sapphire as described herein forms a further aspect of the invention. Although the structures, methods and uses herein described are focused on sapphire substrates, these extend to other optical materials. These include, but are not limited to, other crystallisation materials. Non-limiting examples of other materials include quartz, diamond, glass and calcium fluoride. In a broader aspect the invention thus provides a quartz, diamond, glass or calcium fluoride substrate having a micropatterned surface, wherein said surface comprises a plurality of pillars which are formed from the substrate material and which project from the surface, wherein the pillars have an aspect ratio less than or equal to 1.0 and wherein the ratio of the spacing between adjacent pillars to the pillar height is greater than or equal to 1.0. These substrates are oleophobic in water and hydrophobic in air and the optical transmission of the substrates is preferably at least 50%, more preferably at least 60%, e.g. at least 65%. Any of the features of the sapphire substrates and methods for their micropatterning which are described herein may be applicable to these other optical materials.

Certain preferred embodiments of the invention will now be described by way of the following non-limiting examples and with reference to the accompanying drawings in which: Figure 1 is a schematic representation of a microstructured sapphire surface in an embodiment of the invention.

Figure 2 shows the optical transmission curve of sapphire.

Figure 3 is a schematic illustration of a lift-off procedure for patterning of a sapphire surface in an embodiment of the invention.

Figure 4 shows SEM images of structured sapphire surfaces in accordance with the invention.

Figure 5 shows the contact angle of (a) water in air, (b) oil (hexadecane) under water and (c) shows an image taken after 46 hours exposure to a contamination mixture. All images are shown on an as-received standard sapphire surface.

Figure 6 shows the contact angle of water in air (top panel) and oil (hexadecane) under water (bottom panel) of a structured sapphire surface. The left panel shows measurements on sapphire with surface structures having d/p/h of 5 μηι/15 μηι/200 nm and the right panel shows sapphire with structures having d/p/h of 5 μηι/10 μηι/600 nm.

Figure 7 shows images taken after 46 hours exposure to a contamination mixture (top panel), and the corresponding images taken after generating turbulence (bottom panel), (a) is "as-received" sapphire, (b) is sapphire with surface structures having d/p/h of 5 μηι/15 μηι/200 nm, (c) is sapphire with surface structures having d/p/h of 5 μιη/10 μηι/600 nm.

Figure 8 shows SEM images of a structured sapphire surface in accordance with the invention with pillar diameter/pitch/height as 5μΓη/8μΓη/600ηηι. The left and right images are taken at a 0° and 45° sample tilt, respectively (before removal of the etching mask).

Figure 9 shows: Left: static contact angles for an oil droplet in water measured on structured sapphire surfaces with varying pitch values p. The diameter (d) and height (h) of the structures is 5 μηι and 600 nm, respectively. Right: Static contact angles for an oil droplet in water measured on structured sapphire surfaces with different pillar heights h of the structures. The diameter (d) and pitch (p) of the structures is 5 μηι and 10 μηι, respectively. The predicted static contact angle values are obtained using the Wenzel and Cassie-Baxter equations.

Figure 10 shows (a) a 15 μΙ oil droplet underwater on as-received sapphire after chemical cleaning treatment; and (b) a 15μΙ oil droplet underwater on a structured sapphire surface with 5μΓη/15μΓη/200ηηι as diameter/pitch/height. Left panels show the droplets at 0° sample inclination. The right panels illustrate the movement of the underwater oil droplets on tilted samples. The oil droplet slips down the structured surface already at a 20° inclination while a similar oil droplet remains static on the cleaned as-received surface at an inclination as high as 85°.

Figure 11 is a numerical model showing the flow velocity distribution at a height of 100 nm above the bottom surface of the flow tube for unstructured surface (top left), 200 nm high pillars (top right), 600 nm high pillars (bottom left), 10 μηι high pillars (bottom right). All structured surfaces fixed at 5 μηι/15 μηι as d/p. Water flows from bottom to top in each image and the overall flow velocity in the flow tube is 1.2 m/s. The velocity in between the pillars is noticeably reduced for 10 μηι height.

Figure 1 shows, schematically, a cross-section through a section of a plurality of pillars which form part of a microstructure in accordance with an embodiment of the invention. The sapphire structure 1 is provided with a plurality of identical pillars 2 whose cross-section does not vary with increasing height. Each pillar has an upper surface 3 which is blunt and which is substantially parallel to the sapphire surface 4. Each pillar has a height "h" and a lateral dimension "d". The pitch of the structure is denoted by "p" and "Θ" denotes the side wall angle. The spacing between adjacent pillars is denoted by "s". The plurality of pillars forms a series of peaks and troughs in the surface of the sapphire.

Figure 3 shows, schematically, a lift off procedure which may be used to produce a micropatterned sapphire surface in accordance with an embodiment of the invention. After development of the photoresist, an etch mask is deposited over the entire surface including the surface that covers the photoresist and areas where the photoresist has been removed. During the lift-off procedure, the sapphire is immersed in a solvent that removes the photoresist and any etch mask deposited on it. The etch mask deposited directly on the sapphire surface remains intact. Example 1 - Patterning/microstructuring of sapphire surfaces

Sapphire patterning was achieved through patterning the etching mask via photolithography followed by inductively coupled plasma (ICP) etching of the sapphire using the etching mask.

Sapphire samples were cleaned in a 3: 1 mixture solution of H₂S0₄ and H₂0₂ at 80°C for 20 minutes, then rinsed with deionized water and dried. Samples were dehydrated on a hotplate at 110°C. The etching mask patterning was carried out as follows: 1) spin coating of the UV photoresist on the sapphire sample followed by soft baking; 2) UV exposure of the samples by contact lithography using a photomask followed by post exposure baking and then cooling down to ambient temperature; 3) samples development; 4) deposition of the etching mask onto the sapphire surface exposed through the small photoresist openings; 5) removal of the remaining photoresist.

Sapphire samples with the patterned etching mask layer were etched using Cr- based ICP with an etcher (Plasmalab System 100, Oxford Instruments). All experiments were performed using BCI₃ gas with a flow rate of 25sccm, 1750 W of ICP power, bias power of 300 W, 2 mT of operating pressure and a 60°C of substrate temperature. With this method -65 nm/min of the sapphire etch rates with the etch selectivity over Cr of 1.4 was achieved.

Figure 4 displays the scanning electron microscope (SEM) images of the resulting microstructured sapphire surface with two different pitch (p) values. This is achieved through two steps: (1) patterning the microstructure etching mask via photolithography on the sapphire windows, and (2) dry etching of the sapphire windows using the etching mask. For desired heights of the structural features, the thickness of the etching mask was chosen according to the etch rates and selectivity over the sapphire. By varying the etching time, structures with varied aspect ratio (diameter/height; d/h) were obtained.

Example 2 - Contact angle measurements

Contact angle measurements were carried out in respect of 'as received' sapphire surfaces and those produced according to the method of Example 1.

The experimental set-up used was that described in J. Phys Chem. 1 19: 15333, 2005. A video-based optical contact angle measurement system, OCA20 LHT, from Dataphysics with SCA software (version 4.3.19) was used to measure the contact angle of oils and water on sapphire windows. For measurements of the water contact angle in air, a water droplet of about 2 μΙ_ was directly placed on the sapphire surface. For oil contact angle measurements in water, an oil droplet having a volume varying between 3-10 μΙ_ was gently deposited from the bottom of the system onto the sapphire window surface submerged in water. This method is known as the captive bubble technique and is used for testing liquids that have lower density than the surrounding media. Figure 5 shows the contact angle of water in air and oil under water measured on as-received standard sapphire surface (chemically polished, c-plane with 30° random orientation crystal miscut). These windows are hydrophilic (θ=86°±2°) and oleophilic (θ=74°±2°) underwater. In order to explore the long term anti-fouling character, the sapphire window was exposed to anti-fouling mixture (water containing crude oil Oseberg, sand and CaC0₃) for about 2 days. Figure 5(c) shows the build-up of particles on the as-received sapphire surface after 46 hour exposure to the fouling mixture.

The water wetting properties in air (top panel of Figure 6) and oil wetting properties under water (bottom panel of Figure 6) for the modified sapphire surfaces were studied through contact angle measurements. The left hand panel of Figure 6 shows the contact angles for water and oil (underwater) on structured sapphire with d/p/h as 5 μηι/15 μηι/200 nm and the right hand panel of Figure 6 shows the contact angles with d/p/h as 5 μηι/10 μηι/600 nm. As is evident from the images, the wetting properties of the sapphire surface could be tuned from hydrophilic to hydrophobic and underwater oleophobic.

The contact angle of water in air and oil under water was measured on structured sapphire surfaces where the pitch values were varied. The diameter and height of the structures was kept constant at 5 μηι and 600 nm respectively. The results are summarised in Table 1 .

Table 1 : Contact angle of water in air and oil underwater measured on structured sapphire surface with varying pitch values. Diameter (d) and height (h) of structures is 5 μηι and 600 nm respectively. Error in values is + 2°.

The contact angle of water in air and oil underwater was measured on structured sapphire surfaces where the height was varied. The diameter and pitch of the structures was kept constant at 5 μηι and 10 μηι respectively. The results are summarised in Table 2.

Table 2: Contact angle of water in air and oil underwater measured on structured sapphire surface with different heights of structures. Diameter (d) and pitch (p) of structures is 5 μηι and 10 μηι respectively. Error in values is + 2°.

The results show that the wetting properties could be changed by varying the pitch values and heights of the structures.

The optical transmission was measured separately in the range 400-900 nm. The optical transmission of all structured sapphire surfaces remained above 75% for this wavelength range. Exam le 3 - Anti-fouling properties

To study the long-term anti-fouling properties of the sapphire surfaces the experimental set-up described in J. Phys Chem. 119: 15333, 2005 was used. The set-up consisted of a 20 L polypropylene cylindrical test vessel, equipped with window mounts, copper coils for cooling, a temperature control unit (Julabo F12-EH refrigerated/reating circulator and Tygon S3 B-44-4X tubing), IKA Eurostar 60 mixing element, and I 50 digital Ultra-Turrax emulsifying element. The window mounts allowed for the test surfaces to be imaged using two CCD cameras placed on the outside of the test vessel (Thorlabs high sensitivity USB 3.0 CMOS camera 1280 x 1024 global shutter NIR sensor). The cameras were fitted with Telecentric lenses (Thorlabs model MVTC23053) and were placed on translation stages for position optimisation. Images were analysed using the freeware ImageJ software developed at the National Institutes of Health, Bethesda, Maryland.

All the test parameters (e.g. type and concentration of contamination, type of flow in the test vessel and experiment time) were kept the same as for standard as- received sapphire surfaces.

Images taken after 46 hours of exposure of the structured/patterned sapphire surfaces to the fouling mixture are shown in Figure 7 (top panel). To observe the adhesion strength of foul particles on the sapphire surface, the flow of foul water mixture was first stopped and then started at once with maximum available flow speed (2000 rpm) to generate slight turbulence close to the surface. The bottom panel in Figure 7 shows the corresponding images taken after generating turbulence. Figure 7(a) shows as-received sapphire, Figure 7(b) shows sapphire with surface structures having 5μηι/15μΓΤΐ/200 nm diameter/pitch/height, and Figure 7(c) shows sapphire with surface structures having 5μηι/10μΓΤΐ/600 nm

diameter/pitch/height.

As a result of the turbulence, foul particles (dark spots in the images) moved away from the modified sapphire surfaces as shown in the bottom panel of Figure 7. However the effect of the turbulence was not observed in the as-received standard sapphire surface. This is clear evidence of the reduced adhesion of foul particles as a result of surface structuring/patterning. Without in any way wishing to be bound by theory, it is believed the water layer in the structures may act as an oil/foul repellent layer. As evident from Figure 7, this effect was more pronounced in the sapphire surface with structures having d/p/h as 5 μηι/10 μηι/600 nm. This could be attributed to the enhanced water cushioning effect.

Example 4 - Surface structuring of sapphire samples, contact angle measurements and numerical modelling Methods:

Surface structuring of sapphire samples:

Sapphire crystals were purchased from Freudiger with a diameter between 12.67- 12.73 mm (with bevel edge of 45°, 0.2 mm), a thickness between 1.55-1.60 mm and a crystal miscut specified as less than 30° relative to the Z-axis. The crystal surface was chemically polished with Scratch/Dig number of 40/20.

Standard photolithography was employed to pattern the sapphire surface, involving the following steps: (a) spin coating of the photoresist (AZNLof2000 from

Microchemicals) on the surface of the substrate; (b) UV exposure through a photomask to pattern the photoresist layer; (c) removal of the photomask and development of the photoresist to produce a patterned photo-resist layer; (d) deposition of an etch mask (Cr layer); (e) removal (lift-off) of the remaining photoresist; (f) inductively coupled plasma etching (Oxford Plasmalab System 100) of the sapphire substrate using the etch mask as an etching template; and (g) removal of the remaining etching mask layer using Cr etchant 1020 purchased from Transene Company, Inc.

Contact angle measurements:

A video-based optical contact angle measurement system, OCA20 LHT, from Dataphysics with SCA software (version 4.3.19) was used to measure contact angles of oils and water on sapphire windows. The system was equipped with an electronic tilting base unit TBU 90E that allows software controlled inclination of the instrument up to an angle of 90° with accuracy of ±0.1 °. For measurements of the water contact angle in air, a water droplet of about 2 μΙ_ was directly placed on the sapphire surface. For oil contact angle measurements in water, an oil droplet (hexadecane) having a volume varying between 3-15 μΙ- was gently deposited from the bottom of the system onto the sapphire window surface, which was submerged in water.

Chemical cleaning:

Surface cleaning treatment involved the following steps: (a) soaking of samples in a 3: 1 solution of H₂S0₄ and H₂0₂ for 20 min at 80°C; (b) soaking of samples in a 1 : 1 :5 solution of NH₃, H₂0₂, and water for 20 min at 80°C; and (c) soaking of samples in a 1 : 1 :5 solution of HCI, H₂0₂, and water for 20 min at 80°C. All three steps were followed by rinsing with ultrapure ion free water with a resistivity greater than 18 ΜΩ-cm water, and drying with nitrogen stream.

Numerical modelling:

Numerical modelling was conducted with COMSOL Multi physics 5.3. The model consists of a quadratic pattern of 5x5 pillars with diameter of 5 microns and pitch of 15 microns. The height is varied between 0, 200 nm, 600 nm and 10 microns. These pillars were placed at the bottom of a rectangular flow tube with dimensions: width = 90 microns, height = 40 microns and length = 180 microns. Both inflow and outflow velocity were set to 1.2 m/s. A non-slip boundary condition was applied between flowing water and the bottom surface and pillars. The flow regime was assumed to be laminar.

Results and conclusions:

Figure 8 shows structured sapphire surfaces prepared as described in the methods section. The lateral pillar dimension (d) was kept fixed at 5 μηι. This gave the desired self-cleaning properties with minimum impact on optical properties. The height (h) was varied between 200 and 600 nm and the pitch (p) between 8 and 17 μΐη.

A series of static and tilted underwater contact angle measurements on oil droplets were carried out. The results showed that all structured sapphire surfaces were oleophobic and in the Cassie-Baxter state with static contact angles of more than 130° (see Figure 9). The as-received sapphire surface was underwater oleophilic with a contact angle of 74°±2°. After chemical cleaning it became underwater oleophobic with a contact angle of 120°±2°. It returned to the oleophilic state after being exposed to ambient atmosphere for a few days. This is due to the accumulation of foreign materials such as carbon-based contamination on the surface due to a high chemical activity of the cleaned sapphire surface. The properties of the structured surfaces did not change after chemical cleaning, and no changes were observed after several weeks in an ambient atmosphere.

Figure 9 shows the measured static contact angle Θ for an oil droplet in water as a function of pitch and pillar height. All structured surfaces exhibited underwater oleophobicity with contact angles of 130° or more. The predicted static contact angle values in Figure 9 are obtained using the Wenzel and Cassie-Baxter equations: Wenzel:

ndh\ ,

c^os(⁰) = (^{1 +} ^J ^cos(⁰/)^<

Cassie-Baxter:

nd² ,

∞s(0) = ^j (cos(0 ) + l) - l,

Q_f is the measured value for the as-received sapphire surface; and

p, d and h are the pitch, diameter and height, respectively, of the pillars for the structured surface.

For all structured sapphire surfaces, the oil wetting behaviour underwater was indicative of the Cassie-Baxter state, which means that the space in between the pillars is filled with water. Droplets of sizes larger than p were observed to only contact the flat-top of the pillars.

Tilted-drop measurements (see Figure 10) showed that an oil droplet with a volume of 15 μΙ slides off the structured surface at 20° inclination, while it remains adhered to the as-received sapphire surface at an inclination as high as 85°. Droplets of smaller volume did not stick to the structured surfaces even after being pushed onto the surface for at least 8 hours. This is a further demonstration of the very low oil adhesion on the structured surfaces. It was no problem to get smaller droplets to stick to the chemically cleaned, as-received surface.

From the fact that the structured surfaces are in the Cassie-Baxter state and at the same time prevent oil-adhesion in the presence of flow, it can be concluded that the presence of a dynamic water layer in between the pillars makes the oil

contamination slip more readily. This was confirmed by numerical modelling.

Figure 11 shows a simulation of the water flow distribution between the pillars at a height of 100 nm above the bottom surface of the flow tube. The water flow velocity between the low aspect ratio pillars (200 and 600 nm for d/p as 5 μηι/15 μηι) was significantly higher than between the high aspect ratio pillars. The contact angle between the as-received, chemically cleaned surface and the structured surfaces only differs by about 10°. They are all underwater oleophobic, but only the structured surfaces displayed self-cleaning in the presence of flow. Trapping of contamination in the structures over time as has been suggested for high aspect ratio structures with static water layers was not observed. This is because the water can flow freely to carry away contaminants which do not get trapped into the structure.

Claims

Claims:

1. A sapphire substrate having a micropatterned surface which comprises a plurality of sapphire pillars which project from the surface, wherein the pillars have an aspect ratio less than or equal to about 1.0, and wherein the ratio of the spacing between adjacent pillars to the pillar height is greater than or equal to about 0.5.

2. A sapphire substrate as claimed in claim 1 , wherein the spacing between adjacent pillars is greater than or equal to about half the lateral dimension of the pillars, and/or wherein the spacing between adjacent pillars is less than or equal to about 3 times the lateral dimension of the pillars, preferably wherein the spacing, s, satisfies the following conditions:

d/2≤ s≤ 3d (in which d is the lateral dimension).

3. A sapphire substrate as claimed in claim 1 or claim 2, wherein the pillars have a lateral dimension in the range from 0.05 to 12 μηι, preferably from 1 to 10 μηι, e.g. from 3 to 5 μηι.

4. A sapphire substrate as claimed any one of claims 1 to 3, wherein the height of each pillar is in the range of from 100 to 800 nm, preferably 150 to 700 nm, e.g. from 200 to 650 nm.

5. A sapphire substrate as claimed in any one of the preceding claims, wherein the pillars have an aspect ratio in the range from 0.03 to 1.0, preferably from 0.04 to 0.5, e.g. from 0.04 to 0.2.

6. A sapphire substrate as claimed in any one of the preceding claims, wherein the spacing between adjacent pillars is in the range from 0.025 to 40 μηι, preferably from 0.5 to 30 μηι, e.g. from 1 to 12 μηι.

7. A sapphire substrate as claimed in any one of the preceding claims, wherein the pillar spacing to height ratio (s/h) is in the range from 0.5 to 800, preferably from 5 to 400, e.g. from 10 to 200.

8. A sapphire substrate as claimed in any one of the preceding claims, wherein the pitch of the pillars is in the range from 0.075 to 40 μηι, preferably from 0.5 to 30 μηι, e.g. from 1 to 12 μηι.

9. A sapphire substrate as claimed in any one of the preceding claims, wherein the pillars are circular, square or rectangular in cross-section.

10. A sapphire substrate as claimed in any one of the preceding claims, wherein the pillars have a substantially uniform cross-section.

11. A sapphire substrate as claimed in any one of the preceding claims, wherein the upper surface of the pillars is substantially planar.

12. A sapphire substrate as claimed in any one of the preceding claims, wherein the upper surface of the pillars is substantially parallel to the adjacent sapphire surface.

13. A sapphire substrate as claimed in any one of the preceding claims, wherein the surface of the sapphire between adjacent pillars is substantially planar.

14. A sapphire substrate as claimed in any one of the preceding claims, wherein the pillars project substantially perpendicular to the sapphire surface.

15. A sapphire substrate as claimed in any one of the preceding claims, wherein the pillars are arranged in a regular, repeating pattern, for example, simple cubic, close packed, or rectangular (hexagonal) packed.

16. A sapphire substrate as claimed in any one of the preceding claims, wherein the separation of the pillars is non-regular across the sapphire surface.

17. A sapphire substrate as claimed in any one of the preceding claims which is uncoated.

18. A sapphire substrate as claimed in any one of the preceding claims which is oleophobic in oil and hydrophobic in air and has an optical transmission of at least 50%, preferably at least 60%, e.g. at least 65%.

19. A sapphire substrate as claimed in claim 18, wherein the optical

transmission is measured in the wavelength range of from 250 to 5000 nm, preferably from 250 to 4000 nm, e.g. from 400 to 900 nm.

20. A sapphire substrate as claimed in any one of the preceding claims having a thickness in the range of from 0.2 mm to 10 mm, preferably 0.5 to 5 mm, more preferably 0.5 to 2 mm, e.g. about 1 mm.

21. A sapphire substrate as claimed in any one of the preceding claims, wherein the optical transmission of the substrate is at least 60%, preferably at least 70%, e.g. at least 75%, of that of an unstructured sapphire substrate.

22. A method of preparing a sapphire substrate having a micropatterned surface as claimed in any one of claims 1 to 21 , said method comprising subjecting the surface of a sapphire substrate to photolithography followed by wet or dry etching of said surface.

23. A method as claimed in claim 22 comprising at least the following steps: (a) deposition of a photoresist layer on the surface of a sapphire substrate; (b) provision of a photomask on the photoresist layer followed by UV exposure to pattern the photoresist layer; (c) removal of the photomask and development of the photoresist whereby to produce a patterned photo-resist layer; (d) deposition of an etch mask; (e) removal (i.e. lift-off) of the remaining photoresist; (f) dry etching of the sapphire substrate using the etch mask as an etching template; and (g) removal of the etch mask whereby to form the micropatterned substrate.

24. Use of a sapphire substrate as claimed in any one of claims 1 to 21 as an optical surface in a device which receives or transmits light, especially in the optical range, for example an optical surface in subsea equipment such as windows, optical sensors, vehicles and lights.

25. Use of a sapphire substrate as claimed in any one of claims 1 to 21 as a component of a display surface in a device which displays an electronically produced image, for example in a television receiver, computer monitor, projection display system, phone, or ipad.

26. A device (e.g. subsea equipment) having an optical surface which comprises a sapphire substrate as claimed in any one of claims 1 to 21.