EP4399034A1

EP4399034A1 - Acoustic field visualisation

Info

Publication number: EP4399034A1
Application number: EP22773292.2A
Authority: EP
Inventors: Rachel Edwards; Oksana Trushkevych; David Hutchins; Duncan Billson; Rachel Watson
Original assignee: University of Warwick
Current assignee: University of Warwick
Priority date: 2021-09-08
Filing date: 2022-09-08
Publication date: 2024-07-17
Also published as: GB2610588A; WO2023037108A1; GB202112813D0

Abstract

Apparatus for acoustic field visualisation are disclosed. One apparatus (1; 1₄) comprises: an ultrasonic transducer (2; 2₃) having an acoustic field emitting surface (3; 3₄); and a visualisation layer (4; 4₄) disposed on the surface, such that, when the ultrasonic transducer is driven, the layer visualises an acoustic field, the layer comprising a polymer (10) and thermochromic liquid crystal (11; 11₄) dispersed in the polymer. When the ultrasonic transducer is driven, an acoustic field has a direction of propagation having a non-zero component normal to the surface. The apparatus is operable in air to visualise the acoustic field.

Description

Acoustic field visualisation

Field

The present invention relates to acoustic field visualisation.

Background

Laser vibrometry is used for a broad range of applications including in aerospace, automotive, and electronics industries, as well as for industrial and medical ultrasonics and ultrasound visualisation, condition monitoring and non-destructive testing (NDT), and in materials research and development. A typical laser vibrometry system consists of one or more lasers. It can measure out-of-plane velocity of a point on a surface under test using the Doppler effect (laser beam frequency changing slightly depending on the velocity of displacement), and the point can be scanned across the sample to build up an image of the wave-field. Laser vibrometry measurements have high lateral resolution and are sensitive to sub-nm displacements.

Commercial laser vibrometry systems can work up to 2.4 GHz, but measurements at higher frequencies are difficult and systems operating above 1 MHz are significantly more expensive.

Acoustography is an imaging process that employs a liquid crystal sensor to convert ultrasound into an image in near real-time. The technique allows centimetre-squared area imaging at one time (rather than requiring scanning) and has been applied as an alternative to C-scan ultrasonic scans with higher resolution, almost real-time imaging speed and comparable or superior defect detection capability. It has been applied to transducer field imaging and acoustic holography. Currently acoustography uses a thick aligned layer of liquid ciystal in a complex setup involving oblique incidence, polarisers, and a water bath, and is limited to operation at 3.3 MHz. Both laser vibrometry and acoustography are employed in a variety of applications where an ultrasound field needs to be visualised. This ranges from biomedical to aerospace, with a large user base in NDT. However, these techniques are limited in the sense that they are not simple, low-cost, fast, visual, and capable of imaging large areas without scanning. Summary

According to a first aspect of the invention, there is provided apparatus including an ultrasonic transducer having an acoustic field emitting surface. When the ultrasonic transducer is driven, an acoustic field has a direction of propagation has a non-zero component normal to the acoustic field emitting surface. The apparatus also includes a visualisation layer disposed on the acoustic field emitting surface, such that, when the ultrasonic transducer is driven, the layer visualises an acoustic field. The visualisation layer includes a polymer and liquid ciystal dispersed in the polymer. The liquid crystal is thermochromic liquid crystal. The apparatus is operable in air to visualise the acoustic field.

A visualisation layer in which liquid crystal is dispersed in a polymer can allow the visualisation layer to take the form of a paint-on coating layer, a sheet, a film, or a tape. It can also allow the visualisation layer to be applied to non-flat surfaces.

The visualisation layer maybe a removable visualisation layer. The visualisation layer may be reusable.

When the ultrasonic transducer is driven, it may produce a bulk wave (e.g., longitudinal or shear).

According to a second aspect of the invention, there is provided apparatus including an ultrasonic transducer. The ultrasonic transducer is disposed on a sample having an acoustic field emitting surface. The apparatus also includes a visualisation layer disposed on the acoustic field emitting surface such that, when the ultrasonic transducer is driven, the visualisation layer visualises an acoustic field. The visualisation layer includes a polymer and liquid crystal dispersed in the polymer. The apparatus is operable in air to visualise the acoustic field. According to a third aspect of the invention, there is provided apparatus including an ultrasonic transducer having an acoustic field emitting surface. The apparatus also includes a removable visualisation layer disposed on the acoustic field emitting surface such that, when the ultrasonic transducer is driven, the layer visualises an acoustic field. The visualisation layer includes a polymer and liquid crystal dispersed in the polymer. The apparatus is operable in air to visualise the acoustic field. The liquid crystal may include thermochromic liquid crystal. Alternatively, the liquid crystal maybe thermochromic liquid crystal.

According to a fourth aspect of the invention, there is provided apparatus including an ultrasonic transducer, an acoustic field emitting surface, and a visualisation layer disposed on the surface. When the ultrasonic transducer is driven, the visualisation layer visualises an acoustic field. The visualisation layer includes a polymer and a liquid dispersed in the polymer, with ferromagnetic platelets suspended in the liquid. When the ultrasonic transducer is driven, it produces a bulk wave (e.g., longitudinal or shear).

When the ultrasonic transducer is driven, the acoustic field may have a direction of propagation having a non-zero component normal to the acoustic field emitting surface.

The acoustic field emitting surface may be a surface of the ultrasonic transducer.

Alternatively, the acoustic field emitting surface maybe a surface of a sample on which the ultrasonic transducer is disposed.

The liquid dispersed in the polymer may be in the form of droplets.

The liquid may include oil. The liquid maybe oil. The ferromagnetic platelets may include nickel platelets. The ferromagnetic platelets may be nickel platelets.

A magnetic field source may be used to align the ferromagnetic platelets. Thus, order imparted to the visualisation layer by application of an acoustic field can be erased and the appearance of the visualisation layer set to an initial state. In other words, the visualisation layer may be configured to be set to an initial state in which the ferromagnetic platelets are aligned by application of a magnetic field from a magnetic field source.

The visualisation layer may be removable from the acoustic field emitting surface. The visualisation layer may be reusable. The visualisation layer may be a paint-on coating layer. Alternatively, the visualisation layer may be a sheet, film or tape.

The visualisation layer may be directly disposed on the acoustic field emitting surface. Alternatively, at least one other layer may be interposed between the acoustic field emitting surface and the visualisation layer.

A coupling layer may be interposed between the ultrasonic transducer and the visualisation layer. The coupling layer may guide the acoustic field to the visualisation layer by reducing acoustic impedance mismatch. Thus, the coupling layer may help to improve performance. The coupling layer may selectively transmit the component of the acoustic field oriented normal to the acoustic field emitting surface to the visualisation layer. Alternatively, the coupling layer may transmit both the component of the acoustic field oriented normal to the acoustic field emitting surface and the components of the acoustic field oriented parallel to the acoustic field emitting surface to the visualisation layer.

A backing layer may be interposed between the acoustic field emitting surface and the visualisation layer, in thermal contact with the latter. The backing layer may convert displacement to heat through friction or absorption. Thus, the backing layer may help to improve performance. The backing layer may provide or improve visual contrast. The backing layer may comprise more than one layer. For example, the backing layer may be a bilayer including a first layer for improving absorption of the acoustic field into the visualisation layer and a second layer for improving visual contrast.

A thermal camera, also known as an infrared camera, may be used for thermal imaging of the backing layer instead of the visualisation layer. A thermal camera can allow broad and flexible thermal operation conditions. The visualisation layer may have a first surface disposed on the acoustic field emitting surface, and a second surface, opposite to the first surface, on which a protective later is disposed. The protective later may be transparent and/ or thermally insulating. Alternatively, the second surface may be uncovered. The apparatus may be operable outside a tank containing a second liquid. The apparatus may be operable when immersed in the second liquid. The second liquid may be water or oil. A point on the visualisation layer may have a colour indicating acoustic field amplitude at that point.

According to a fifth aspect of the invention, there is provided a system for acoustic field visualisation including an apparatus according to any one of the first, second, third, or fourth aspects of the invention. The system also includes a power supply and control circuitiy for driving the ultrasonic transducer and an image capture unit having a camera for capturing a visual-spectrum image of the visualisation layer. The system also includes a processor having memory for processing an image captured by the camera.

The system may also include a temperature control unit. The temperature control unit may have active means of temperature control, such as a heater controlled by a temperature controller. The temperature control unit may have passive means of temperature control, such as thermal insulation. Thus, the system can have a tunable and stable operational temperature.

The image capture unit may also include a light source. The light source may illuminate the visualisation layer at a constant level during system operation. Thus, the light source can help to ensure stable imaging conditions.

The light source may illuminate the visualisation layer for a short time or continuously with low intensity light that does not lead to heating. The light source may include an LED or array of LEDs. The system may include a second ultrasound source. The system may also include a heat source. The heat source may be a halogen lamp, a device producing a stream of hot air or water, or resistive or inductive heating element built into the sample or applied externally. Thus, the system can be used to make thermographic measurements and/ or thermosonic measurements in addition to visualising the acoustic field emitted by the ultrasonic transducer using the same visualisation layer. The visualisation layer may map temperature by rendering visible the temperature distribution. Such a visualisation layer works differently to infrared thermal cameras that detect infrared radiation. Thermosonic measurements may use the ultrasound transducer or a second ultrasound source to generate ultrasound. These measurements may be carried out sequentially, concurrently, or simultaneously.

The system can be used for safety and quality control of acoustic energy transfer systems. The system can be used to visualise an acoustic field emitted by air-coupled transducer arrays. The system may be used for characterising transducers or ultrasonic cleaning baths.

The system can provide a simple, low-cost, fast, and visual approach to acoustic field visualisation. Further, the system can be capable of imaging large areas without scanning. According to a sixth aspect of the invention, there is provided a system for acoustic field visualisation including an apparatus comprising an ultrasonic transducer, an acoustic field emitting surface, and a backing layer for converting displacement to heat. The backing layer is disposed on the surface, such that, when the ultrasonic transducer is driven, the backing layer emits a heat signature indicative of an acoustic field. The system also includes a power supply and control circuitiy for driving the ultrasonic transducer, a thermal camera for capturing a thermal image of the backing layer, and a processor for processing the thermal image captured by the camera. Thus, the system according to the sixth aspect of the invention does not comprise a visualisation layer and a visible spectrum camera, but instead comprises a thermal camera for capturing a thermal image of the backing layer, and a processor for processing an image captured by the thermal camera.

According to a seventh aspect of the invention, there is provided a method for operating an apparatus according to any one of the first, second, third, or fourth aspects of the invention or the system according to the fifth aspect of the invention. The method includes driving the ultrasonic transducer, capturing a visible spectrum image of the visualisation layer, and processing the visible spectrum image.

According to an eighth aspect of the invention, there is provided a method for operating the system according to any one of the fifth or sixth aspects of the invention. The method includes applying ultrasound, capturing a thermal image of the backing layer, and processing the thermal image.

According to an ninth aspect of the invention, there is provided a method for operating the system according to the fifth aspect of the invention. The method includes applying heat, capturing a visible spectrum image of the visualisation layer, and processing the visible spectrum image. Here, it is to be appreciated that the visualisation layer allows for simultaneous application of ultrasound and heat to the sample. According to a tenth aspect of the invention, there is provided a computer program for performing a method according to the seventh, eighth, or ninth aspects of the invention.

According to a eleventh aspect of the invention, there is provided a computer program product including a computer readable medium for storing the computer program according to the tenth aspect of the invention.

Brief Description of the Drawings

Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of an ultrasonic transducer and a visualisation layer; Figure 2 is a side view of an ultrasonic transducer and a visualisation layer;

Figure 3 is a side view of an ultrasonic transducer, a sample, and a visualisation layer;

Figure 4 is a plan view of a visualisation layer including a polymer and liquid crystal droplets dispersed in the polymer;

Figure 5 is a side view of an ultrasonic transducer and a visualisation layer; Figure 6 is a side view of an ultrasonic transducer, coupling layer, backing layer, visualisation layer, and protective layer;

Figure 7 is a side view of an apparatus for acoustic field visualisation immersed in a tank containing a liquid;

Figure 8 is a side view of a visualisation layer including a liquid and ferromagnetic platelets suspended in the liquid in which the ferromagnetic platelets are aligned by an external magnetic field source;

Figure 9 is a schematic diagram of a system for visualising acoustic fields emitted by an ultrasonic transducer;

Figure 10 is a schematic diagram of an image capture unit; Figure 11 is a process flow diagram of a method of acoustic field visualisation;

Figure 12 is a process flow diagram of a method of acoustic field visualisation;

Figure 13 is a process flow diagram of a method of thermographic measurement;

Figure 14 is an image of high order vibration modes of an air-coupled transducer obtained using laser vibrometiy; Figure 15 is an image of high order vibration modes of an air-coupled transducer obtained using a visualisation layer comprising a polymer and liquid ciystal droplets dispersed in the polymer;

Figure 16 is an image of high order vibration modes of an air-coupled transducer obtained using a visualisation layer comprising a polymer and thermochromic liquid crystal droplets dispersed in the polymer;

Figure 17 is an image of standing Lamb waves on a 1.5mm thick Perspex plate obtained using a visualisation layer comprising a polymer and thermochromic liquid ciystal droplets dispersed in the polymer;

Figure 18 is a thermal image of high order vibration modes of an air-coupled transducer obtained using a visualisation layer comprising a polymer and thermochromic liquid crystal droplets dispersed in the polymer; and Figure 19 is an image of high order vibration modes of an air-coupled transducer obtained using visualisation layer comprising oil and nickel platelets suspended in the oil. Detailed description of certain embodiments

Apparatus

Referring to Figures 1 and 2, a first apparatus 1, ii including an ultrasonic transducer 2, 2i having an acoustic field emitting surface 3, 31 is shown. The first apparatus ii also includes a visualisation layer 4, 41 disposed on the acoustic field emitting surface 31, such that, when the ultrasonic transducer 2i is driven, the visualisation layer 41 visualises an acoustic field by rendering it visible.

Referring also to Figure 3, a second apparatus i₂ including an ultrasonic transducer 2₂ on which a sample 5 having an acoustic field emitting surface 3₂ is disposed is shown. A visualisation layer is disposed on the acoustic field emitting surface 3₂, such that, when the ultrasonic transducer 2₂ is driven, the visualisation layer 4₂ visualises an acoustic field by rendering it visible.

The colour of a point on the visualisation layer 4_1? 4₂ indicates the acoustic field amplitude at that point. No scanning is required, and the lateral dimensions of the visualisation layer 4_1? 4₂ can be set based on the size of the acoustic field emitting surface 31, 3₂ in any given situation. Thus, this approach to visualising acoustic fields can be suitable for visualising an acoustic field over a large area. Moreover, this approach can be simple and low-cost, particularly when compared against laser vibrometry and acoustography.

In some examples, the visualisation layer 41, 4₂ is removable from the acoustic field emitting surface 3i, 3₂. For example, such a visualisation layer 4_1? 4₂ may be removed by sliding or peeling. Thus, this approach can be suitable for visualising acoustic fields in situations where it is either not preferable or not possible to permanently attach the visualisation layer 41, 4₂ to the acoustic field emitting surface 31, 3₂. In other examples, the visualisation layer 41, 4₂ is permanently applied to the acoustic field emitting surface 31, 3₂. Such a permanently applied visualisation layer can be used for long-term safety monitoring of parts of equipment that do not change shape and are subject to ageing or wear, for example, a pipe, or part of the wing of an aircraft. Thus, this approach to visualising acoustic fields can be suitable for in-situ monitoring for short periods.

In some examples, the ultrasonic transducer 2₂ is permanently built into the sample 5, and is in acoustic contact with the sample 5. In other examples, the ultrasonic transducer is removable from the sample 5. In some examples, the sample 5 is directly disposed on the ultrasonic transducer 2₂. In some examples, other layers are interposed between the sample 5 and the ultrasonic transducer 2₂. For example, a layer of ultrasonic coupling gel can be interposed between the sample 5 and the ultrasonic transducer 2₂ to improve transmission of the acoustic field to the sample 5.

Likewise, in some examples, including some in which the visualisation layer 4.1, 4₂ is permanently disposed on the acoustic field emitting surface 31, 3₂, the visualisation layer 41, 4₂ is disposed directly onto the acoustic field emitting surface 31, 3₂. In other examples, including some in which the visualisation layer 41, 4₂ should be or is removable, other layers are interposed between the visualisation layer 41, 4₂ and the acoustic field emitting surface 31, 3₂. The performance of the apparatus ii, i₂, particularly the visualisation layer 41, 4₂, can be improved by the addition of such other layers.

When the ultrasonic transducer is driven, it produces a bulk wave (e.g., longitudinal or shear). In some examples, the ultrasonic transducer 2_X, 2₂ includes a piezoelectric active element. However, this approach to acoustic field visualisation is applicable to other arrangements in which a visualisation layer 41, 4₂ can be disposed upon an acoustic field emitting surface 31, 3₂. In other examples, the ultrasonic transducer 2i, 2₂ is an electromagnetic acoustic transducer (EMAT) or a magnetostrictive transducer (MsT).

Further details are provided in the following examples. Third example

A third apparatus i₃ has the same structure as the first apparatus ii. In the third apparatus i₃, the visualisation layer 4₃ is removable from the acoustic field emitting surface 3₃. Referring to Figure 4, a plan view of the visualisation layer 4₃ of the third apparatus 1₃ is shown. The visualisation layer 4₃ includes a polymer 10 and liquid crystal droplets 11, n₃ dispersed in the polymer 10. In the visualisation layer 4₃, the liquid crystal n₃ is nematic liquid crystal. However, in other examples the liquid crystal n₃ merely includes nematic liquid crystal. Likewise, in other examples the liquid crystal n₃ includes or is thermochromic liquid crystal, otherwise known as chiral nematic liquid crystal. In some examples, the liquid crystal n₃ has an operational temperature range that is tunable at the stage of preparing the liquid crystal material. Thus, the third apparatus i₃ can have a customisable operational temperature range.

A visualisation layer in which liquid crystal is dispersed in a polymer can allow the visualisation layer to take the form of a paint-on coating layer, a sheet, a film, or a tape.

It can also allow the visualisation layer to be applied to non-flat surfaces.

In the visualisation layer 4₃, the colour change is primarily driven by local temperature change in the thermochromic liquid crystal through the change in the helical pitch of the chiral liquid crystal n₃, and the colour is continuously dependent on the acoustic field amplitude. In other examples, the appearance only changes when a threshold acoustic field amplitude is reached or exceeded. The visualisation layer 4₃ shows a monochromatic gradient of colour when the ultrasonic transducer 2₃ is driven and emits an acoustic field. In other examples, the colour gradient can be polychromatic. As the liquid crystal n₃ experiences the acoustic field at the acoustic field emitting surface 3₃ in near-real time and the timescale on which the liquid crystal n₃ reacts to the acoustic field is short (milliseconds), this approach to acoustic field visualisation can be fast. The visualisation layer 4₃ includes a polymer 10 which include droplets of the liquid crystal n₃. However, in other examples the liquid crystal n₃ is not fully formed as droplets. In some examples, droplets of liquid crystal n₃have a diameter of less than too pm and their shape is not necessarily spherical. Indeed, droplet shape is likely to be irregular in most cases. In other examples, the droplet diameter is different. In the visualisation layer 4₃, the liquid crystal n₃ is confined to a part of the visualisation layer

4₃ having a thickness of less than 1 mm. However, in other examples the liquid crystal is confined to a part of the visualisation layer 4₃ having a thickness of less than 1 mm, and preferably less than 0.1 mm. The contrast and spatial resolution of the visible representation of the acoustic field shown by the visualisation layer 4₃ can be dependent on the droplet diameter and the thickness of the part of the visualisation layer 4₃ having liquid crystal n₃. In some examples, the contrast and spatial resolution is also be dependent on any backing layer (if included), particularly the thickness of any backing layer, and on the degree of the thermal insulation (if any).

Referring to Figure 5, a side view of the third apparatus 1₃ is shown. When the ultrasonic transducer 2₃ is driven, the acoustic field has a direction of propagation 15 having a component 16 normal to the acoustic field emitting surface 3₃. In some examples, the component 16 normal to the acoustic field emitting surface 3₃ is non- zero. In some examples, the components parallel to the acoustic field emitting surface

3₃ are non-zero. In some examples, the acoustic field emitted by the ultrasonic transducer 2₃ includes bulk acoustic waves, for example longitudinal or shear waves. In other examples, the acoustic field emitted by the ultrasonic transducer 2₃ includes surface acoustic waves. Alternatively, the acoustic field may have no direction of propagation and take the form of evanescent waves. The propagation direction of surface waves can be along the acoustic field emitting surface 3₃, but when the visualisation layer 4₃ is placed on the acoustic field emitting surface 3₃, the acoustic field can leak (by mode conversion) into the visualisation layer 4₃ and thereby be visualised. Evanescent waves can be visualised through a similar leaking/tunnelling process, in which the acoustic field travels along the acoustic field emitting surface 3₃ (or into the air at an edge of said surface) along the same orientation as the visualisation layer 4₃, and leaky waves propagate into the visualisation layer 4₃. Thus, it is to be appreciated that the apparatus i₃ can be suitable for imaging various acoustic fields.

Referring to Figure 6, in some examples the third apparatus i₃ includes additional layers. Namely, a coupling layer 20 and/or a backing layer 21 interposed between the acoustic field emitting surface 3₃, and/or a protective layer 23. These additional layers can help to improve the performance of the apparatus i₃.

The coupling layer 20 helps to guide the acoustic field to the visualisation layer 4₃ by reducing acoustic impedance mismatch. In some examples, the coupling layer 20 is a layer of ultrasonic coupling gel. In some examples, the coupling layer 20 selectively transmits the component of the acoustic field oriented normal to the acoustic field emitting surface to the visualisation layer 4₃. In other examples, the coupling layer 20 transmits both the component of the acoustic field oriented normal to the acoustic field emitting surface 3₃ and the components of the acoustic field oriented parallel to the acoustic field emitting surface 3₃to the visualisation layer 4₃. Thus, the third apparatus i₃ allows for various coupling strategies to be used. In some examples, the backing layer 21 is in thermal contact with the acoustic field emitting surface 3₃ and visualisation layer 4₃, and converts displacement to heat through friction or absorption of part of the acoustic field. Thus, the backing layer 21 can help to improve performance. In some examples, the backing layer 21 can improve contrast in images taken of the visualisation layer 4₃. In some examples the backing layer 21 includes more than one layer. For example, the backing layer 21 can be a bilayer including a first layer for improving absorption of the acoustic field into the visualisation layer and a second layer for improving visual contrast. Thus, a backing layer 21 can be designed to provide multiple functionalities and/or performance improvements to the apparatus i₃. In some examples, the backing layer is black.

In some examples, the visualisation layer 4₃ has a first surface disposed on the acoustic field emitting surface 3₃, and a second surface 22, opposite to the first surface, on which a protective later 23 is disposed. In some examples the protective later 23 is transparent and has a matte finish. In some examples, the protective layer 23 is made from plastic sheet or transparent varnish. In other examples, the second surface 22 is uncovered. This may be because it is unnecessary to add an additional layer for protection, or alternatively to ensure adequate homogenisation and/or thickness control of the visualisation layer 4₃. The third apparatus i₃ is operable in air to visualise the acoustic field.

Referring to Figure 7, the third apparatus 1₃ is shown immersed in a tank 25 containing a second liquid 26. In some examples, the third apparatus i₃ is operable to visualise the acoustic field when immersed in the second liquid 26. In some examples the second liquid 26 is water or oil. In contrast, acoustic field visualisation by acoustography cannot be carried out without immersing a measurement apparatus in water.

Fourth example

A fourth apparatus i₄ (not shown) is the same as the third apparatus i₃, but differs in that the visualisation layer 4₄ can be permanently applied to an acoustic field emitting surface 3₄. The fourth apparatus i₄ also differs from the third apparatus i₃ in that the component 16 normal to the acoustic field emitting surface 3₃ is non-zero. The fourth apparatus i₄ also differs from the third apparatus i₃ in that the liquid crystal n₄ in the visualisation layer 4₄ is based on thermochromic liquid crystal. In the visualisation layer 4₄, the observed colour change is primarily driven by heating associated with the absorption of the acoustic field in the visualisation layer 4₄, in particular the heating of the thermochromic liquid crystal included in the visualisation layer 4₄. At higher operational frequencies, the acoustic field is easier to visualise. This is due to heating being more efficient and absorption more effective as the wavelength of the acoustic field becomes smaller and comparable with the thickness of the visualisation layer. The colour gradient in the visualisation layer 4₄ is polychromatic. Preferably, the fourth apparatus i₄ includes a backing layer 21 for optical contrast and for increasing ultrasound absorption at lower frequencies. Fifth example

A fifth apparatus i₅ (not shown) is the same as the third apparatus i₃, but differs in that it has the same structure as the second apparatus i₂ instead of the first apparatus ii.

Thus, the fifth apparatus i₅ can be used for non-destructive testing of samples. The fifth apparatus i₅ also differs from the third apparatus i₃ in that the visualisation layer 4₅ can be permanently applied to an acoustic field emitting surface 3₅.

In some examples, the polymer 10 includes droplets of the liquid crystal n₅. The droplets of liquid crystal n₅ may have a diameter similar to a characteristic length of a part of the sample 5, for example a cell. A viscosity of the dispersion may be similar to that of a part of the sample 5, for example cytoplasm. Thus, the fifth apparatus i₅ can be used to image cell heating in tissue.

Sixth example

A sixth apparatus 16 (not shown) is the same as the fifth apparatus 1₅, but differs in that it can have the structure of the first apparatus ii or the second apparatus 1₂, and in that it is not required to be operable in air to visualise an acoustic field.

Referring to Figure 8, the sixth apparatus 16 also differs from the fifth apparatus 1₅ in that the visualisation layer includes a liquid 27 containing ferromagnetic platelets 28 suspended in the liquid 27 dispersed in the polymer 10, instead of having liquid crystal n₅ dispersed in the polymer 10. In the visualisation layer 4₆, the colour change is primarily driven by reorientation of ferromagnetic platelets 28 due to acoustic streaming. When the liquid 27 is oil and the ferromagnetic platelets 28 are nickel platelets, the colour change is from dark green (low acoustic field amplitude) to light green (high acoustic field amplitude).

The image shown by the visualisation layer for at least a week after of the transducer 16 is switched from a state in which it is emits an acoustic field to a state in which it does not emit an acoustic field. Thus, the sixth apparatus 16 can be used in applications where it is preferable to apply the acoustic field for a short time only, or where it is preferable to remove the visualisation layer before capturing an image of the acoustic field at the acoustic field emitting surface 36 which it was previously disposed upon.

In some examples, the ferromagnetic platelets 28 are confined to a part of the visualisation layer 46 having a thickness of less than 100 pm. However, in other examples the ferromagnetic platelets 28 may be confined to a part of the visualisation layer 46 having a thickness of less than 1 mm. In some examples, the ferromagnetic platelets 28 are confined in droplets of the liquid 27 having a distribution of sizes, roughly under too pm.

In some examples, the acoustic field 36 emitting surface is a surface of the ultrasonic transducer 26. In other examples, the acoustic field emitting surface 36 is a surface of a sample 5 on which the ultrasonic transducer 26 is disposed. In some examples, the visualisation layer 46 includes droplets of the liquid 27 with dispersed ferromagnetic platelets 28. Droplets of the liquid 27 may have a diameter similar to a characteristic length of a part of the sample 5, for example a cell. A viscosity of the dispersion may be similar to that of a part of the sample 5, for example cytoplasm. The visualisation layer with fluid and platelets is bistable and will retain platelet orientation upon exposure to ultrasound. Thus, the sixth apparatus 16 can be used as a phantom to image cell damage through acoustic streaming in tissue.

Likewise, in some examples the ferromagnetic platelets 28 may have a diameter similar to a characteristic length of a part of the sample 5, for example a cell. A viscosity of the dispersion may be similar to that of a part of the sample 5, for example cytoplasm. Thus, the sixth apparatus 16 can be used to image cell damage through acoustic streaming in tissue in tissue.

In some examples, a magnetic field source 29 may be used to align the ferromagnetic platelets 28. Thus, order imparted to the visualisation layer 46 by application of an acoustic field can be erased and the appearance of the visualisation layer 46 set to an initial state. In some examples the magnetic field source 29 is a permanent magnet.

Seventh example A seventh apparatus i₇ (not shown) is the same as the sixth apparatus 1₅, but differs in that it does not include a visualisation layer. In place of a visualisation layer, the seventh apparatus i₇ includes a backing layer 21 for converting displacement associated with the acoustic field to heat through ultrasound absorption and/ or friction. A thermal camera 45, otherwise referred to as an infrared camera, can be used to capture a thermal image of the heat signature emitted by the backing layer 21, which is indicative of the acoustic field at the acoustic field emitting surface 3₇.

System

Referring to Figure 9, a system 3O1 for acoustic field visualisation is shown. The system 3O1 includes an apparatus 1, for example the third apparatus i₃, the fourth apparatus i₄, the fifth apparatus 1₅, the sixth apparatus 16, or the seventh apparatus i₇. The system 3O1 also includes power supply and control circuitry 31 for driving the ultrasonic transducer 2 and an image capture unit 32 for capturing an image of the visualisation layer 4.

In some examples, the system 3O1 includes a temperature control unit 33 for controlling the temperature, particularly at the visualisation layer 4. The temperature control unit 33 includes a heat source such as a heater 33i controlled by a temperature controller 34, and thermal insulation 332. Thus, the system 3O1 can have a tunable and stable operational temperature. In some examples, the temperature control unit 33 includes at least one of a halogen lamp, a device producing a stream of hot air or water, or a resistive or inductive heating element built into the sample 5 or applied externally as a heat source. Referring also to Figure 10, the image capture unit 32 of the system 3O1 is shown. The image capture unit 32 includes a camera 40 for capturing a visible spectrum image of the visualisation layer 4. The image capture unit 32 also includes a processor 41 having memory 42 for processing the image captured by the camera. In some examples, the processor 41 is a part of a computer 43. In some examples, the image capture unit 32 includes a light source 44. In some examples, the light source 44 illuminates the visualisation layer 3₃ at a constant level during system operation. Thus, the light source 44 can help to ensure stable imaging conditions. In some examples, the image capture unit 32 includes a thermal camera 45, also known as an infrared camera, for thermal imaging of the backing layer 21 instead of using the visualisation layer 4. In some examples, the system 3O1 includes a heating element or system. Thus, the system 3O1 can be used to make thermographic measurements and/or thermosonic measurements in addition to visualising the acoustic field emitted by the ultrasonic transducer 2 using either the thermal camera 45 and/ or the visualisation layer 4. In some examples these measurements can be carried out sequentially, but in other examples they may be carried out concurrently or simultaneously. The system 3O1 can be used for characterising transducers. In some examples, the system 3O1 can be used for safety and quality control of acoustic energy transfer systems, to visualise an acoustic field emitted by air-coupled transducer arrays, or to characterise ultrasonic cleaning baths. In some examples, the system 3O1 has an operational frequency from at least 40 kHz, with upper frequency limit set by the generation and sample transmission of ultrasound.

Operation Referring to Figure 11, a process flow diagram for a method of acoustic field visualisation using any one of the third apparatus 1₃, fourth apparatus 1₄, fifth apparatus 1₅, sixth apparatus 16, or the system 3O1 is shown. The method comprises driving the ultrasonic transducer, capturing a visible-spectrum image of the visualisation layer, and processing. Referring to Figure 12, a process flow diagram for a method of acoustic field visualisation measurement using the system 3O1 is shown. This method is applicable when the system 3O1 includes an apparatus 1 including a backing layer 21. The method includes applying ultrasound, capturing a thermal image of the backing layer 21, and processing the image.

In some examples, the ultrasound transducer 5 is used to apply ultrasound. In other examples, the second ultrasound source 35 is used to apply ultrasound. Referring to Figure 13, a process flow diagram for a method of thermographic measurement using the system 3O1 is shown. The method comprises applying heat from a heat source, capturing a visible spectrum image of the visualisation layer 4, and processing the image. Each of the methods may be performed by a computer program. The computer program may be stored on a computer readable medium of a computer program product.

Experimental data Referring to Figure 14, an image of high order vibrational modes of an air-coupled transducer driven at 740 kHz is shown. The image was obtained using laser vibrometry. Light and dark regions of the image are labelled with L and D respectively.

Referring to Figure 15, an image of high order vibration modes of an air-coupled transducer driven at 740 kHz is shown. The image was obtained using a system including an apparatus in accordance with the third apparatus i₃ described previously. The visualisation layer of the apparatus used comprises a polymer and nematic liquid crystal dispersed in the polymer. Light and dark regions of the image are labelled with L and D respectively. The visualisation layer used was a paint-on coating layer.

Referring to Figure 16, an image of high order vibrational modes of an air-coupled transducer driven at 740 kHz is shown. The image was obtained using a system including an apparatus in accordance with the fourth apparatus i₄ described previously. The visualisation layer of the apparatus used included thermochromic liquid crystal. The edges of light and dark regions of the image are labelled with arrows. The visualisation layer used was a removable sheet. Referring to Figure 17, an image of standing Lamb waves on a 1.5 mm thick Perspex plate is shown. The image was obtained using a system including an apparatus in accordance with the fifth apparatus i₅ described previously. The image was obtained using a visualisation layer comprising a polymer and thermochromic liquid crystal dispersed in the polymer. Light and dark regions of the image are labelled with L and D respectively. The visualisation layer used was a removable sheet.

Referring to Figure 18, a thermal image of high order vibration modes of an air-coupled transducer driven at 740 kHz is shown. The image was obtained using a system including an apparatus in accordance with the seventh apparatus i₇ described previously. The visualisation layer comprising a polymer and thermochromic liquid crystal dispersed in the polymer. The edges of light and dark regions of the image are labelled with arrows. The visualisation layer used was a removable sheet.

Referring to Figure 19, an image of high order vibration modes of an air-coupled transducer driven at 6.67 MHz is shown. The image was obtained using a system including an apparatus in accordance with the sixth apparatus 16 described previously. The image was obtained using obtained using visualisation layer comprising oil and nickel platelets suspended in the oil. The visualisation layer used was a removable sheet.

Modifications

It will be appreciated that various modifications may be made to the embodiments hereinbefore described. Such modifications may involve equivalent and other features which are already known in the design and use of acoustic field visualisation systems, and which maybe used instead of or in addition to features already described herein. Features of one embodiment may be replaced or supplemented by features of another embodiment.

Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel features or any novel combination of features disclosed herein either explicitly or implicitly or any generalization thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims

1. Apparatus comprising: an ultrasonic transducer having an acoustic field emitting surface, wherein, when the ultrasonic transducer is driven, an acoustic field has a direction of propagation having a non-zero component normal to the surface; and a visualisation layer disposed on the surface, such that, when the ultrasonic transducer is driven, the layer visualises an acoustic field, the layer comprising a polymer and thermochromic liquid crystal dispersed in the polymer, wherein the apparatus is operable in air to visualise the acoustic field.

2. The apparatus of claim i, wherein the visualisation layer is a removable visualisation layer.

3. The apparatus of claim i or 2, wherein the visualisation layer is reusable.

4. Apparatus comprising: an ultrasonic transducer disposed on a sample, the sample having an acoustic field emitting surface; and a visualisation layer disposed on the surface such that, when the ultrasonic transducer is driven, the layer visualises an acoustic field, the layer comprising a polymer and liquid crystal dispersed in the polymer, wherein the apparatus is operable in air to visualise the acoustic field.

5. Apparatus comprising: an ultrasonic transducer having an acoustic field emitting surface; and a removable visualisation layer disposed on the surface such that, when the ultrasonic transducer is driven, the layer visualises an acoustic field, the layer comprising a polymer and liquid crystal dispersed in the polymer, wherein the apparatus is operable in air to visualise the acoustic field.

6. The apparatus of claim 4 or 5, wherein the liquid crystal comprises or consists of thermochromic liquid crystal.

7- Apparatus comprising: an ultrasonic transducer; an acoustic field emitting surface; and a visualisation layer disposed on the surface, such that, when the ultrasonic transducer is driven, the layer visualises an acoustic field, wherein the layer comprises a polymer and a liquid dispersed in the polymer, with ferromagnetic platelets suspended in the liquid.

8. The apparatus of any one of claims 4 to 7, wherein, when the ultrasonic transducer is driven, the acoustic field has a direction of propagation having a non-zero component normal to the acoustic field emitting surface. . The apparatus of claim 7 or 8, wherein the acoustic field emitting surface is a surface of the ultrasonic transducer.

10. The apparatus of claim 7 or 8, wherein the acoustic field emitting surface is a surface of a sample on which the ultrasonic transducer is disposed.

11. The apparatus of any one of claims 7 to 10, wherein the liquid dispersed in the polymer is in the form of droplets.

12. The apparatus of any one of claims 7 to 11, wherein the liquid comprises oil and the ferromagnetic platelets comprise nickel platelets.

13. The apparatus of any one of claims 7 to 12, wherein the visualisation layer is configured to be set to an initial state in which the ferromagnetic platelets are aligned by application of a magnetic field from a magnetic field source.

14. The apparatus of any one of claims 4 to 13, wherein the visualisation layer is removable from the acoustic field emitting surface.

15. The apparatus of any one of claims 4 to 14, wherein the visualisation layer is reusable.

16. The apparatus of any one of claims 1 to 15, wherein the visualisation layer is a paint-on coating layer. 17. The apparatus of any one of claims 1 to 15, wherein the visualisation layer is a sheet, film, or tape. 18. The apparatus of any one of claims 1 to 17, wherein the apparatus further comprises a coupling layer interposed between the acoustic field emitting surface and the visualisation layer.

19. The apparatus of any one of claims 1 to 18, wherein the apparatus further comprises a backing layer disposed between the acoustic field emitting surface and the visualisation layer. 20. The apparatus of any one of claims 1 to 19, wherein the visualisation layer has a first surface disposed on the acoustic field emitting surface and a second surface opposite to the first surface, wherein the apparatus further comprises a protective layer disposed on the second surface. 21. The apparatus of any one of claims 1 to 19, wherein the visualisation layer has a first surface disposed on the acoustic field emitting surface and a second surface opposite to the first surface, and wherein the second surface is uncovered.

22. The apparatus of any one of claims 1 to 21, wherein a point on the visualisation layer has a colour indicating acoustic field amplitude at that point.

23. The apparatus of any one of claims 1 to 22, wherein the apparatus is operable outside a tank containing a second liquid. 24. The apparatus of claim 23, wherein the apparatus is operable when immersed in the second liquid.

25. A system comprising: the apparatus of any one of claims 1 to 24; a power supply and control circuitry for driving the ultrasonic transducer; a camera for capturing an image of the visualisation layer; and a processor for processing an image captured by the camera.

26. A system comprising: an apparatus comprising: an ultrasonic transducer; an acoustic field emitting surface; and a backing layer for converting displacement to heat disposed on the surface, such that, when the ultrasonic transducer is driven, the backing layer emits a heat signature indicative of an acoustic field; a power supply and control circuitry for driving the ultrasonic transducer; a thermal camera for capturing a thermal image of the backing layer; and a processor for processing the thermal image captured by the camera.

27. A method of using the apparatus of any one of claims 1 to 24 or the system of claim 25 or 26, comprising: driving an ultrasonic transducer; capturing an image of the visualisation layer; and processing the image.