GB2482908A

GB2482908A - Rheometer with optical interferometer

Info

Publication number: GB2482908A
Application number: GB1013945.9A
Authority: GB
Inventors: Thomas Andrew Waigh; Matthew Harvey
Original assignee: University of Manchester
Current assignee: University of Manchester
Priority date: 2010-08-20
Filing date: 2010-08-20
Publication date: 2012-02-22
Anticipated expiration: 2030-08-20
Also published as: GB201013945D0; GB2482908B

Abstract

A measurement apparatus 100 comprises force application means 160 for applying a known force to a sample material, and velocity determining means for determining a velocity of a portion of the sample material using a Doppler-shift optical coherence tomography technique. The force application means is a rheometer 160 and the velocity determining means is an interferometer (Michelson or mach-zender). The velocity of a portion of the sample is determined when force is applied thereto. The invention also comprises a kit to provide a rheometer with a transparent sample window and Doppler shift optical coherence tomography apparatus to determine sample velocity.

Description

Improvements in Instrumentation

Background

Rheometry is the study of the flow behaviour of materials. A rheometer is a scientific instrument for measuring the rheology of a material. The rheometer creates a well defined stress field in the material and measures the resultant stress or vice versa. The response of the material to the stress or strain fields provides an accurate measurement of its viscoelasticity i.e. the degree to which a material is viscous or elastic. All real materials are intermediate between the two ideal responses i.e. viscous or elastic.

However, measurement of the viscoelasticity of a material only provides limited information about the behaviour of the material.

It is an object of embodiments of the invention to at least mitigate one or more of the

problems of the prior art.

Brief Description of the Drawings

Embodiments of the invention will now be described by way of example only, with reference to the accompanying figures, in which: Figure 1 shows an apparatus according to a first embodiment of the invention; Figure 2 shows an apparatus according to a second embodiment of the invention; Figure 3 shows an apparatus according to a third embodiment of the invention; Figure 4 shows an apparatus according to a fourth embodiment of the invention; Figure 5 shows an apparatus according to a fifth embodiment of the invention; Figure 6 shows an apparatus according to a sixth embodiment of the invention; Figure 7 shows an apparatus according to a seventh embodiment of the invention; Figure 8 shows an apparatus according to a eighth embodiment of the invention; Figure 9 illustrates a method according to an embodiment of the invention; Figure 10 illustrates a correlation function produced by an embodiment of the invention; and Figure 11 illustrates velocity profiles for various sample materials produced by embodiments of the invention.

Detailed Description of Embodiments of the Invention Embodiments of the present invention will now be described, by way of example.

Embodiments of the present invention allow velocity measurement of a sample whilst a known stress and/or strain field is applied to the sample. Embodiments of the invention are described with reference to fluids rheometers. However, it is envisaged that other rheometer designs may be utilised such as melt polymer shear rheometers.

Embodiments of the invention determine a velocity of a sample whilst a force is applied to the sample by Doppler shift optical coherence tomography (OCT).

Embodiments of the invention provide a rheometer capable of velocity measurement by Doppler shift OCT. Embodiments of the invention also provide a kit for providing a rheometer with a velocity measurement by Doppler shift OCT functionality.

Some embodiments of the invention are described and shown which are based around a Michelson interferometer, as will be appreciated by the skilled person. However, it is understood that these embodiments may alternatively be based around a Mach-Zender interferometer. Similarly, embodiments described and shown based around a Mach-Zender interferometer may be based around a Michelson interferometer.

Referring to Figure 1, a first embodiment of velocity measurement apparatus 100 is shown which is based around a Michelson interferometer.

The apparatus 100 comprises a light source 110 which is a broadband light source. In some embodiments, the light source 110 is a super luminescent diode. However, another suitable light source may also be used, such as a swept titanium sapphire laser. A power supply 115 for the light source is also provided, although it will be realised that in some embodiments the power supply and light source are integrated.

A frequency of the light source may be 600nm, 677nm, 800nm, 1300 nm or other suitable frequency. In the embodiment shown in Figure 1, the light source 110 outputs near-infrared light at 1295nm, although it will be realised that this is not limiting.

Light emitted by the light source 110 is communicated along a fibre optical cable to a three-port circulator 120, counter-clockwise from ports one to two in the shown embodiment, The circulator 120 isolates the light source 110 from the effects of backscatter light to ensure optimal performance and to protect the light source 110 from damage.

Light is communicated from port two of the circulator 120 along a fibre optical cable to a 2:2 coupler 125. In the described embodiment the coupler 125 has a 90/10 coupling ratio. A first branch of the coupler 125 directs light from the light source 110 toward a reference mirror 140 and a second branch of the coupler 125 directs light output from the light source 110 toward a sample in a rheometer 160 for application of a stress/strain field and velocity measurement, as will be explained.

An output of the coupler 125 having the 10% coupling ratio is connected to a beam stop 165.

The first branch comprises a variable attenuator 130 and collimation lens 135 to prevent divergence of light directed towards the reference mirror 140. The reference mirror 140 is attached to a translation stage 141 for moving the reference mirror 140.

The translation stage 141 is moveable by a motor which is operated to control the location of the reference mirror 140. The reference mirror 140 is moved in order to change the length of the light path along the first branch. Movement of the reference mirror 140 controls the sample imaging depth, as will be appreciated by the skilled person. Movement of the reference mirror 140 allows a velocity of the sample material within a "slice" or section of the sample material to be determined in a non-invasive manner. The axial resolution of the slice' or section in which the velocity of the sample is determined is defined by the coherence length of the light source 110.

In some embodiments of the invention, the depth of the section in which the velocity of the sample is determined is 9tm. However, with other light sources, such as a swept laser light source, for example a Ti:Sapph laser, the depth of the section may be reduced to around 1 rim. The velocity of the sample material is determined in a sample volume of the material, the size of which is dependent on the depth of the sampling section and, transversely, a beam waist of a light beam projected into the sample material. As will be explained, some embodiments of the present invention have a beam waist of 22tm which results in the sample volume being approximately 3.4pL as determined by: beamwaist Sampvol = 1J( 2 * coherencelength It will of course be realised that other beam waist sizes and sample depths may be used, resulting in other sampling volume sizes.

The second branch of the coupler 125 directs light toward a polarisation controller which corrects for changes in polarisation of the light due to propagation of the light through the optical fibres.

The second branch further comprises a collimation and focussing optics 150 including a collimation lens and a focussing lens. In the described embodiment, the focussing lens has a focal length offrl 8.4mm, although it will be realised that lenses having other focal lengths may be used. The focussing lens in the described embodiment provides a depth of field of 1.2mm and a beam waist of 22tm.

The light beam output from the focussing lens 155 is directed onto and through a stationary transparent lower plate 161 forming a lower boundary or confinement of a sample chamber of the rheometer 160. In some embodiments, the sample region defined, by a distance between the lower 161 and upper 162 plates of the rheometer 160, has a depth x of 200 tm, although it will be realised that other depth sample chambers may be used. In some embodiments, the sample region or sample chamber is between 50tm and 1200 Jtm, or more preferably between 600-1000pm. A diameter y of the lower plate may be 40 0mm, although this is merely exemplary and the lower plate may have any diameter. The lower plate 161 allows the light beam to be communicated into the sample chamber of the rheometer 160. As will be appreciated from Figure 12, in some rheometer geometries, the sample chamber is only defined by the upper and lower plates 161, 162 i.e. there are no side-walls to the sample chamber.

However, in other rheometers the sample chamber is defined as an at least partial or complete enclosure i.e. the sample chamber may be enclosed by side-walls.

The focussing lens is arranged at a relatively small angle, such as 100, from perpendicular to the lower plate 161 i.e. 100 from vertical, although it will be realised that other angles may be used. The angle of the focussing lens with respect to the lower plate 161 allows a velocity component of a material in the sample chamber in the direction of a shear force applied to the material by the rheometer to be measured.

Furthermore, the angling of the lens 155 reduces an effect of backscatter from the lower plate 161 which could increase a signal to noise ratio of the velocity measurement.

An upper plate 162 defines an upper boundary of the sample chamber of the rheometer. In some embodiments the upper plate 162 is also transparent to reduce background reflection of the velocity measurement light. As will be explained, in some embodiments, an upper plate 162 formed from a polished transparent material, such as polished Perspex, has been found to be particularly useful at reducing backscatter and thus reducing the signal to noise ratio. However, in some embodiments, the upper plate 162 may be metal or other material, such as with a Couette geometry rheometer.

Whilst embodiments of the invention may be envisaged using a variety of rheometer designs, the rheometer shown in Figure 1 is a shear rheometer designed to operate with cone and plate, parallel plate or cup and bob geometries, as will be appreciated by the skilled person. In some rheometer geometries, one plate remains fixed while a rotational force is applied to the other plate by a motor. The stress and strain are then calculated by measuring the required motor torque. However, other types of rheometer geometries may be envisaged. For example, a rotational cylinder rheometer may be used in which the sample is placed in an annular sample volume formed by two concentric cylinders. One of the cylinders is rotated to apply force to the sample under test. Shear stress may be determined by measurement of the torque applied to the other (non-rotated) cylinder.

In some embodiments, the location of the beam of light output by the focussing lens is moveable with respect to the sample chamber of the rheometer. The collimation and focussing optics 150 are mounted on a motor stage i.e. are move able in response to control signals. The motor stage is, in some embodiments, arranged to move the collimation and focussing optics radially outward from a centre of the sample chamber of the rheometer.

Light returning from the reference mirror 140 and rheometer 160 is directed from port 2 to port 3 of the circulator 120 to be output via fibre optic cable to a detector 170. In some embodiments the detector 170 is a photodiode detector, although it will be realised that other types of detector may be used. An electronic signal corresponding to the light received by the detector 170 is output from the detector 170 to a processing unit, which in the described embodiment is a computer 175. The computer is arranged to execute analysis software as will be described. However, in other embodiments, the processing unit may be a dedicated hardware processing unit, or a combination of hardware and software. The computer 175 may also provide control signals to control a location of the reference mirror 140 i.e. to control the reference mirror motor stage and/or the collimation and focussing optics 150 motor stage.

Figure 2 shows a second embodiment 200 of velocity measurement apparatus. The second embodiment 200 is identical to the first embodiment 100 described with reference to Figure 1 unless otherwise described and repetition thereof will be omitted for clarity.

As with the first embodiment, the apparatus 200 comprises a coupler 225 which in the described embodiment is a 50/50 coupler. The apparatus 200 further comprises a balanced detector 270 in place of the detector 170 of Figure 1. The balanced detector is arranged to receive an output from a port circulator 220, as in the embodiment of Figure 1. However, the balanced detector 270 also receives an output from the coupler 225 which was previously provided to the beam stop 165 in Figure 1.

The balanced detector 270 produces an electrical output which corresponds to a difference between the inputs from the port circulator 220 and the coupler 225. The embodiment of Figure 2 may provide an increased signal to noise ratio over the embodiment of Figure 1.

Figure 3 illustrates a velocity measurement apparatus 300 according to a third embodiment of the invention. Parts of the apparatus 300 in common with the second embodiment 200 described with reference to Figure 2 will not be discussed again for clarity.

In addition to the embodiment shown in Figure 2, the apparatus 300 of the third embodiment includes an oscillation means 390 arranged to oscillate the reference mirror 340. The oscillation means 390 is, in some embodiments, a piezo oscillator, although it will be realised that other types of oscillator may be used, such as an electro-optical phase modulator. Other types of oscillation means are an acousto-optical modulator or an electro-optical modulator which may provide faster phase modulation. Advantageously, the oscillation of the reference mirror 340 shifts the measured Doppler signal to a higher frequency i.e. introduces a carrier frequency, and may increase an acquisition rate, improve a signal to noise ratio of the apparatus and make the apparatus less susceptible to low frequency vibrations.

Figure 4 illustrates a further embodiment of velocity measurement apparatus 400.

Unless otherwise described, the apparatus 400 comprises like parts and operates as described with reference to Figure 3 and a repetition of discussion will be omitted for clarity. In the fourth embodiment, a magnetically-induced force is applied to the sample.

A plurality of superparamagnetic beads 410 are placed into the sample material. The sample material is arranged within a sample cell or holder, which may be a cuvette or glass vial, although it will be appreciated that other sample cells may be envisaged.

Two Helmhotz coils 420 are arranged around the sample cell to induce a magnetic field in the sample cell. For example, the coils 420 may be arranged around the cuvette or glass vial forming the sample cell. It will be realised that other types of coil may be used. The coils 420 are driven by a current which is generated by a current generator 430 in response to a control signal from the computer 475. The coils 420 are driven by a sinusoidally varying current to produce a sinusoidally oscillating magnetic field controlled in such a way that a magnetic field gradient exists across the sample. The beads 410 within the sample are caused to move in response to the magnetic field to thereby apply a sinusoidally varying force on the sample, proportional to the gradient of the applied magnetic field. Collimation and focussing optics 450 are arranged to focus light into the sample cell, for example at a bottom of the cuvette or glass vial to direct light therein.

A complex shear modulus of the sample may be determined by phase-locked detection of the bead 410 displacement compared with the current output by the current generator 430. The complex shear modulus (G*) of the sample may be determined over a wide frequency range by equation 1: G* = so Equation 1 Where c is stress, EO is strain and 6 is phase angle. By averaging over a plurality, possibly hundreds, of cycles, a signal to noise ratio may be improved. The apparatus may, in some embodiments although not in all, comprise an oscillation means 490 to oscillate the reference mirror 440, as in the embodiment described with reference to Figure 3. The oscillation is used to phase modulate the reference light beam. This could also be achieved by using an acousto-optical modulator (AOM) or an electro-optical modulator (EOM). Furthermore, in some embodiments, the coils 420 may include pole pieces to increase the force applied to the beads 410.

Figure 5 illustrates an apparatus 500 according to a fifth embodiment of the invention.

In the fifth embodiment, a force is acoustically applied to the sample. As in the embodiment described with reference to Figure 4, a signal generator 530 outputs a sinusoidally oscillating signal in response to control signals provided from a computer 575. However, in contrast to the embodiment of Figure 4, the current output by the current generator 530 is provided to a piezoelectric transducer 540 connected to the sample which generates sinusoidal acoustic shear waves in response thereto. The signal generator may be a data acquisition card in the computer 575 which is arranged to provide the output signal to a piezo driver. The piezoelectric transducer may be a piezoelectric stage 540 on which the sample is mounted, although other arrangements may be envisaged. Phase-locked detection of particle displacement of the sample compared with the current output by the current generator 530 enables determination of the complex shear modulus G*. The complex shear modulus may be determined with reference to Equation 1.

Figure 6 shows an apparatus 600 according to a sixth embodiment of the invention.

The embodiment of Figure 6 is appreciably similar to the second embodiment described with reference to Figure 2 and repeated discussion of like parts is omitted for clarity. In the sixth embodiment, collimation and focussing optics 650 are divided such that a collimation lens 651 and a focussing lens are interposed by a moveable scanning mirror 641 and a mirror 642. The scanning mirror 641 can be used to adjust the angle at which the probe beam passes through the sample. This allows further velocity components (in some embodiments all of x, y and z) to be probed in addition to that in the plane of the shear rotation.

Figure 7 shows an apparatus 700 according to a seventh embodiment of the invention.

Fiogure 7 illustrates the apparatus from a top-down view into the sample chamber of the rheometer. This embodiment includes a plurality of interferometers arranged to each provide a respective OCT probe beam within the sample chamber to measure a velocity of the sample. In the shown embodiment, three interferometers are used, although it will be realised that other numbers of interferometers may be used, in order to provide three independent probe beams. The probe beams are positioned such that they are orthogonal to each other and intersect the same region. The orthogonality of the beams will allow the full x, y, z velocity components to be simultaneously measured.

Figure 8 illustrates an apparatus 800 according to an eighth embodiment of the invention. The apparatus 800 includes a Mach-Zender based interferometer, otherwise the eighth embodiment is as previously described with reference to Figure 2.

The apparatus 800 comprises a light source 810 and a power supply 115 for the light source. Light emitted by the light source 810 is communicated along a fibre optical cable to a 2:2 coupler 825 having a 50/50 coupling ratio. A first output of the coupler 825 directs light from the light source 810 toward a reference mirror 840 via a variable attenuator 830 and collimation lens 835. The reference mirror 840 is attached to a translation stage 841 for moving the reference mirror 140. A second output from the couple 825 directs light toward port one of a three-port circulator 820.

Light is communicated from port two of the circulator 820 along a fibre optical cable to a polarisation controller 845, collimation and focussing optics 150 including a collimation lens and a focussing lens, and a sample in a rheometer 860 for application of a stress/strain field and velocity measurement. The light beam output from the focus sing lens is directed onto and through a stationary transparent lower plate 861 forming a lower boundary or confinement of a sample chamber of the rheometer 860.

An upper plate 862 defines an upper boundary of the sample chamber of the rheometer. Port three of the circulator 820 directs light to a second coupler 880 having a 50/50 coupling ratio. Light is also communicated to the second coupler 880 from the first coupler 825. Outputs of the second coupler 880 are provided to a balanced detector which outputs an electrical signal corresponding to a difference between the inputs provided to the second coupler 880. A computer 875 receives the signal output from the detector 870 and determines the velocity of the sample subjected to the force applied by the rheometer 860.

A method of determining a velocity of same according to an embodiment of the invention will now be described. The computer 175 may determine a velocity of a sample subjected to force by the rheometer.

Figure 9 illustrates a method 900 according to an embodiment of the invention. The method may be implemented by any of the apparatus 100-800 shown in Figures 1-8.

In step 910 of the method 900 force is applied to a sample of material. The sample is present in the sample chamber of a rheometer. The force may be an applied shear of a sample with a shear rheometer or the application of a magnetically or acoustically induced force to the sample, as previously described.

In step 920 a correlation function for the sample is determined. Figure 10 shows a correlation function for a material subjected to a shear force in a rheometer. The material is 0.1%w/w 0.5jim polystyrene spheres in water. A sinusoidal wave is produced due to the velocity of the sample in the OCT detection volume. As the sample moves relative to the light beam inside the detection volume, an interference signal between the light directed to the reference mirror 140 and that directed to the rheometer changes 160. When a distance between a particle or probe sphere in the sample material and the coupler is the same as a distance between the reference mirror and the coupler the two light path lengths of the sample and reference branches of the OCT system are equal. In this case, light from both arms interferes constructively resulting in a maximum light intensity at the detector. However, when the particle or probe sphere has moved a distance of X/2, where is a wavelength of the light source, the path lengths have changed by the distance of 212 and destructive interference results in a minimum intensity at the detector. Further movement of the particle results in a return to constructive interference. Autocorrelating the intensity signal output by the detector allows a frequency of the interference fringes to be determined.

If the sample material is substantially transparent to the probe light then the introduction of probe spheres to the sample allow the velocity of the sample to be determined by OCT. However, if the sample is opaque to the light source then it is possible to determine the velocity of the sample without the use of probe spheres. For opaque sample materials such as margarine, humus, chocolate, polymer melts etc, probe spheres are unnecessary since these materials contain enough features which scatter the probe light beam allow the velocity to be determined.

By determining time between corresponding portions of the intensity waveform, the velocity of the particle may be determined. As indicated in Figure 10, a peak 1010 exists in the intensity at a time t1=0. A second peak 1020 exists in the intensity at a time t2=t. Further peaks exist at nt where n is an integer. As the sample material moves by d=-2/2n, where 2 is the wavelength of the light source and n is the refractive index of the sample, the sinusoidal reference intensity will have moved through one whole cycle. The factor of 2 arises from the probe beam path length change being twice that of the sample displacement as the probe beam is back-scattered from the sample.

The velocity amplitude v is given by Equation 2: d 2 v=-= t 2nta Equation 2 Where a is a correction factor for the probe light beam being incident at an angle relative to motion of the sample. For example, as discussed above, the probe light beam may be inclined at an angle of 80° (10° from perpendicular) relative to the motion of the sample. The velocity may also be determined from a peak frequency in a power spectrum of the intensity signal.

Figure 11 shows velocity against depth profiles for four different materials. Figure 11(a) is for a dilute colloid material (0.003% solid), Figure 11(b) is for a concentrated colloid (0.8% solid), Figure 11(c) is for margarine and Figure 11(d) is for tomato puree. Figures 11(a) & (b) were obtained by including probe spheres within the sample materials. Figure 11(a) shows that embodiments of the invention are sensitive to very dilute concentrations of probe spheres. This concentration is typical for dynamic light scattering experiments. Fig llb shows that embodiments of the invention are also sensitive to very opaque materials. Dynamic light experiments would not be able to study this type of sample material.

Figures 11(c) & (d) were obtained without the use of probe spheres since these sample materials have a scattering cross-section large enough to produce enough speckle for velocity measurement without added probe spheres. Figures 11(c) & (d) demonstrate that embodiments of the invention allow velocity measurement of a wide variety of materials.

Figure 12 illustrates the sample chamber of some embodiments of rheometer, such as that described with reference particularly to Figures 1-3 & 6-8. As explained previously, in some embodiments the sample chamber is not constrained by side walls in some rheometer geometries. Lower 1210 and upper plates 1220 form respective lower and upper confinements of a sample material 1230. The sample material is sandwiched between the plates 1210, 1220. A vertical spacing of the plates is between 1 00tm and 1 000tm typically, although embodiments of the invention may have a spacing of up to a depth of field of the focussing lens i.e. 1.2mm in some embodiments of the invention. A preferred spacing is 200j.im although it will be realised that other spacings may be used.

One of the plates 1210, 1220 typically the lower plate 1210 is held stationary, whilst the other plate i.e. upper plate 1220 is connected to a tool 1240 for applying a rotational force to the plate 1220. In some embodiments, the upper plate 1220 and tool 1240 are integrated i.e. formed as a unitary component. The stationary plate i.e. lower plate 1210 is transparent and at least the focussing lens 1260 is proximal to the plate to allow the OCT light beam to project into the sample chamber to determine a velocity of the sample 1230. The rotated plate i.e. upper plate 1220 may be transparent although this is not essential. In some embodiments, however, the upper plate 1220 is polished and includes an angled upper surface to direct backscattered light away from the focussing lens 1260 as shown in Figure 12.

Embodiments of the invention may be provided as apparatus for determining a velocity of a sample material using an OCT technique whilst the sample material is subjected to force, such as a shear stress, whilst strain measurement occurs, or visa-versa. However, embodiments of the invention may be provides as a kit for adapting a rheometer to have an OCT velocity measurement capability. The kit may include an OCT system comprising a light source, a circulator one or more couplers, a photo-detector, a reference optical branch and optical components for projecting a sample light beam into the sample chamber of the rheometer, including at least one transparent plate and focussing lens.

It will be appreciated that embodiments of the present invention can be realised in the form of hardware, software or a combination of hardware and software. Any such software may be stored in the form of volatile or non-volatile storage such as, for example, a storage device like a ROM, whether erasable or rewritable or not, or in the form of memory such as, for example, RAM, memory chips, device or integrated circuits or on an optically or magnetically readable medium such as, for example, a CD, DVD, magnetic disk or magnetic tape. It will be appreciated that the storage devices and storage media are embodiments of machine-readable storage that are suitable for storing a program or programs that, when executed, implement embodiments of the present invention. Accordingly, embodiments provide a program comprising code for implementing a system or method as claimed in any preceding claim and a machine readable storage storing such a program. Still further, embodiments of the present invention may be conveyed electronically via any medium such as a communication signal carried over a wired or wireless connection and embodiments suitably encompass the same.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. The claims should not be construed to cover merely the foregoing embodiments, but also any embodiments which fall within the scope of the claims.

Claims

CLAIMS1. A measurement apparatus, comprising: force application means for applying a known force to a sample material; and velocity determining means for determining a velocity of a portion of the sample material using a Doppler-shift optical coherence tomography technique.
2. The measurement apparatus of claim 1, comprising force measurement means for measuring a force exerted by the sample material in response to the applied force.
3. The measurement apparatus of claim 1 or 2, wherein the force applications means is a rheometer.
4. The measurement apparatus of any preceding claim, wherein force application means includes at least one transparent component forming a wall of a sample chamber for receiving the sample material.
5. The measurement apparatus of claim 4, wherein the at least one transparent component includes a plate for contacting the sample material.
6. The measurement apparatus of claim 5, wherein the plate is a lower plate for supporting the sample material.
7. The measurement apparatus of claim 5, wherein the at least one transparent component includes an upper plate for contacting the sample material.
8. The measurement apparatus of claim 7, wherein the upper plate has an angled upper surface for redirecting incident light at an angle.9. The measurement apparatus of claims 5, 6, 7 or 8, wherein the force application means is arranged to rotate one of the plates contacting the sample material.
9. The measurement apparatus of claim 2 and claim 9, wherein the force measurement means is arranged to measure the force exerted on the plate by the sample.
10. The measurement apparatus of any preceding claim, wherein the velocity measurement apparatus includes an interferometer.
11. The measurement apparatus of any preceding claim wherein the means for determining a velocity of a portion of the sample material includes a Michelson or a Mach-Zender interferometer.
12. The measurement apparatus of claim 10 or 11, wherein a reference arm of the interferometer is arranged to direct light toward a reference mirror.
13. The measurement apparatus of claim 10, 11 or 12, wherein a sample arm of the interferometer is arranged to direct light toward the sample material.
14. The measurement apparatus of claim 4 or any claim dependent thereon, wherein the velocity measurement means is arranged to project a light beam through the transparent component for measuring the velocity of the sample.
15. The apparatus of claims 10 to 14, comprising an autocorrelator for determining a frequency of interference fringes.
16. The apparatus of claim 15, wherein the velocity of the sample is determined based on the frequency of the interference fringes.
17. The apparatus of any of claims 1 to 4, wherein the force application means is arranged for magnetically or acoustically applying the force to the sample material.
18. A measurement method, comprising: applying a known force to a sample material; and determining a velocity of a portion of the sample material by a Doppler-shift optical coherence tomography technique.
19. The method of claim 18, comprising determining a force exerted by the sample in response to the applied force simultaneously whilst determining the velocity of the sample.
20. The method of claim 18 or 19, comprising projecting a beam of light into the sample material to determine the velocity of the sample material.
21. The method of claiml8, 19 or 20, comprising controlling a position of a reference mirror in order to control a depth at which the velocity of the sample material is detennined.
22. The method of any of claims 18 to 21, comprising determining a correlation function of light directed to the sample material and light directed to a reference mirror.
23. The method of claim 22, wherein the velocity of the sample material is determined from the correlation function.
24. A kit of parts for providing a rheometer with a velocity measurement capability, comprising: a component for forming a wall of a sample chamber of the rheometer, at least a portion of the component being transparent; a Doppler-shift optical coherence tomography apparatus for determining a velocity of a sample material in the sample chamber of the rheometer.
25. The kit of claim 24, wherein the component is a first plate for contacting a sample.
26. The kit of claim 25, wherein the first plate is substantially transparent.
27. The kit of claim 25 or 26, comprising a second transparent plate for contacting the sample.
28. An apparatus substantially as described hereinbefore with reference to the drawings.
29. A method substantially as described hereinbefore with reference to the drawings.
30. A kit of parts substantially as described hereinbefore with reference to the drawings.