Acousto-optic effects are demonstrated in polymer dispersed liquid crystal (PDLC) films, showing promise for applications in ultrasound sensing. The PDLC films are used to image two displacement profiles of an air-coupled flexural transducers resonant modes at 295 kHz and 730 kHz. Results are confirmed using laser vibrometry.  The regions on the transducers with the largest displacements are clearly imaged by the PDLC films, with the resolution agreeing well with laser vibrometry scanning. Imaging takes significantly less time than a scanning system (switching time of a few seconds, as compared to 8 hours for laser vibrometry). Heating effects are carefully monitored using thermal imaging, and are found not to be the main cause of PDLC clearing

Eriksson T. J. R.

Leissa A. W.

O. Trushkevych

R. S. Edwards

Rose J.

S. Dixon

S. N. Ramadas

T. J. R. Eriksson

Thayer P.

Trushkevych, Oksana

Eriksson, Tobias J. R.

Ramadas, Sivaram Nishal

Dixon, Steve M.

Edwards, R. S. (Rachel S.)

Warwick Research Archives Portal Repository

http://wrap.warwick.ac.uk           Original citation: Trushkevych, Oksana, Eriksson, Tobias J. R. , Ramadas, S. N, Dixon, S. and Edwards, R. S. (Rachel S.). (2015) Ultrasound sensing using the acousto-optic effect in polymer dispersed liquid crystals. Applied Physics Letters, 107 (5). 054102 .  Permanent WRAP url: http://wrap.warwick.ac.uk/71372                      Copyright and reuse: The Warwick Research Archive Portal (WRAP) makes this work by researchers of the University of Warwick available open access under the following conditions.  Copyright © and all moral rights to the version of the paper presented here belong to the individual author(s) and/or other copyright owners.  To the extent reasonable and practicable the material made available in WRAP has been checked for eligibility before being made available. Copies of full items can be used for personal research or study, educational, or not-for profit purposes without prior permission or charge.  Provided that the authors, title and full bibliographic details are credited, a hyperlink and/or URL is given for the original metadata page and the content is not changed in any way.  Publisher’s statement: Copyright (2015) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.  The following article appeared in Trushkevych, Oksana, Eriksson, Tobias J. R. , Ramadas, S. N, Dixon, S. and Edwards, R. S. (Rachel S.). (2015) Ultrasound sensing using the acousto-optic effect in polymer dispersed liquid crystals. Applied Physics Letters, 107 (5). 054102 and may be found at (http://dx.doi.org/10.1063/1.4928390).  A note on versions: The version presented here may differ from the published version or, version of record, if you wish to cite this item you are advised to consult the publisher’s version.  Please see the ‘permanent WRAP url’ above for details on accessing the published version and note that access may require a subscription.  For more information, please contact the WRAP Team at: publications@warwick.ac.uk  Ultrasound sensing using the acousto-optic eect in polymer dispersed liquid crystalsO. Trushkevych, T. J. R. Eriksson, S.N. Ramadas, S. Dixon, and R. S. EdwardsDepartment of Physics, University of Warwick, Coventry, CV4 7AL,UKa)(Dated: 29 July 2015)Acousto-optic eects are demonstrated in polymer dispersed liquid crystal (PDLC)lms, showing promise for applications in ultrasound sensing. The PDLC lms areused to image two displacement proles of an air-coupled exural transducers reso-nant modes at 295 kHz and 730 kHz. Results are conrmed using laser vibrometry.The regions on the transducers with the largest displacements are clearly imaged bythe PDLC lms, with the resolution agreeing well with laser vibrometry scanning.Imaging takes signicantly less time than a scanning system (switching time of a fewseconds, as compared to 8 hours for laser vibrometry). Heating eects are carefullymonitored using thermal imaging, and are found not to be the main cause of PDLCclearing.a)Electronic mail: r.s.edwards@warwick.ac.uk1The eld of acoustics is of fundamental importance, allowing extraction of material prop-erties such as elastic constants, aw and crack detection in components and structures, anddetection of phase transitions in single crystals1,2. Measurements can use a variety of dif-ferent acoustic wavemodes, including bulk (longitudinal, shear) and surface acoustic waves(SAWs)3. Ultrasonics is in common use in non-destructive testing (NDT), where waves withfrequencies typically between 50 kHz and 10 MHz are used for e.g. monitoring safety-criticalcomponents and structures such as rail tracks, pipelines and nuclear waste containers for thepresence of defects. NDT is continually growing in importance as the existing infrastructureages, and as new challenges arise. Incremental improvements to NDT are insucient andnovel disruptive technologies and approaches must be introduced4.The acousto-optic eect in liquid crystals (LCs)5 shows promise for large area ultrasoundsensing and visualisation without the need for scanning6{10, oering the potential to bringthe step-change NDT requires. Ultrasound sensors using a thick layer of aligned nematic LCfor detecting longitudinal waves have been developed for holography and medical imaging(acoustography,11{13). However, they are optimised for use at a single frequency (3.3 MHz)and can only be used at oblique incidence. They also have large thickness (> 200 m),which is generally undesirable in LC devices as it leads to high cost and slow operation dueto backows during relaxation in the LC layer10.Recently, acoustic clearing (becoming transparent under the inuence of an acoustic eld)in polymer dispersed LC (PDLC) has been reported using SAWs at 18.74 MHz14,15. PDLCsare thin ( 20 m) lms in which LC droplets are held in a polymer matrix16,17, primarilyusing nematic LCs5. These are birefringent uids with ordinary and extraordinary refractiveindexes no and ne. The refractive index of the polymer matrix is usually chosen to be thesame as that of the LC in its aligned state, no. Without application of an external eld thedirection of alignment of the LC in these droplets varies randomly from droplet to dropletdue to the way the lms are produced (no preferential direction is set), and hence theeective refractive index of the droplets has random values between no and ne, producingstrong scattering of light and a `milky' appearance. The LC in the droplets can be aligned ifan external eld is applied, at which point the PDLC lm becomes optically homogeneousand transparent. Such clearing using acoustic waves is highly promising in a variety ofapplications, including visualisation of acoustic elds for ultrasonic imaging.For longitudinal acoustic (pressure) waves in aligned LC layers with thickness d, three2wavelength regimes (short ( d), long ( d) and intermediate (  d)) exist, dependingon the viscous wavelength6, = 2rf(1)where f is acoustic frequency and  is the kinematic viscosity of the LC. Within the longand short wavelength regimes the acousto-optic eect does not depend on the frequency ofthe exciting ultrasound, as long as this is higher than the relaxation of the LC layer6. Inaddition, the reorientation angle of the LC depends on the acoustic intensity.It is expected that PDLC lms will behave similarly. The dependence on acoustic in-tensity will lead to a \grey-scale" response to the acoustic eld, and the lack of frequencydependence in the short and long wavelength regimes will give a broadband sensor. ForPDLC, the LC droplet diameter  takes the role of the LC layer thickness d. The workin14,18 uses 70%wt of LC E7 in an optically curable monomer NOA65, with an expecteddroplet size of >1 m19. For the reported frequency of ultrasound, 18.74 MHz, the viscouswavelength is about 5 m, which is of the same order of magnitude as the droplet size, andhence the previous work is in the intermediate regime.This paper aims to demonstrate the practical importance of PDLC lms for ultrasoundsensing and their promise for applications. We work within the long wavelength regime inorder to operate at typical ultrasonic frequencies used in NDT, and to measure in the regimewhich is best understood in LCs, allowing prediction of the expected acousto-optic eect.Such a comparison cannot be direct, however, as in PDLC new considerations relating tosurface eects, anchoring strength, droplet size, shape and distribution are of importance.The chosen demonstration application is that of ultrasonic transducer characterisation. Atype of exural transducer, consisting of a piezoelectric element rigidly bonded to the backof of a metal cap, was used. The transducers operate at a resonant vibration mode of the cap(plate vibration20), and are routinely used for air-coupled ultrasonics 21. Their vibrationalbehaviour at resonance consists of areas with large displacement along with nodal pointsand/or lines (where there is no displacement), with regions of high displacement acting assources of longitudinal waves. Laser scanning measurements, currently used to understandthe resonance mode displacement proles, use a costly measurement system and can takeup to 8 hours to produce one image for a single resonance.PDLC lms were produced using LC E7 and UV curable Norland adhesive NOA74 at LCconcentrations of approximately 76%, with NOA74 chosen due to its low viscosity and good3FIG. 1. Laser vibrometry results on exural transducer with varnish layer at (a) & (b) mode 6:0at 300 kHz. (c) & (d) 11:0 mode at 730 kHz. The bottom images show the amplitudes of vibrationat the scanned points in a radial direction. Displacements are in nm.FIG. 2. PDLC lm on top of a exural transducer, 11:0 mode at 730 kHz, (a)-(d) cap clearing onincreasing power/time. The machining marks on the cap are visible. The circular cap (light circle)is 25 mm in diameter.mixing with E7. The resulting PDLC droplet size was less than 3 m, conrmed by opticalmicroscopy. The PDLC lm thickness was set to 100 m using spacers to obtain optimaloptical contrast. The lms were placed on the front faces of a set of exural transducers(25 mm diameter aluminium caps21). Two resonant modes were chosen, one a 6:0 axi-symmetric mode, with frequency close to 300 kHz ( = 41 m) depending on the transducer,and one corresponding to the 11:0 axi-symmetric mode, with a frequency around 730 kHz( = 26 m). As the viscous wavelength is much larger than the droplet size of PDLC,operation was in the intermediate to long wavelength regime. The optical contrast betweenscattering and transparent states was optimised for some caps by placing a thin layer of UV-4curable varnish (used in the cosmetics industry) underneath the PDLC lm. The excitationof the transducers was done using a continuous sine wave generated by a function generatorand a 25W RF power amplier. The PDLC lms were covered by glass slides to conne thePDLC and improve the coupling of ultrasound into the PDLC lm.The central frequency for each mode was determined separately for each transducer beforeand after placing the PDLC lm and cover slide (with and without the contrast enhancinglayer) using impedance spectroscopy. The PDLC lms and contrast enhancing layers did notalter the cap vibration signicantly. The mode number and structure were conrmed by laservibrometry on the transducer surface (using a Polytec vibrometer and a 2D scanning stage,measured without the PDLC lm), with the full scans, of 8 hour duration, shown in gure 1.The colour scale shows the surface displacement, and (c) and (d) show the amplitudes acrossthe centre scan line. For the 6:0 mode, the area of the central (strongest) displacement of thecap is 3 0:2 mm in diameter, and the concentric nodal lines are spaced at  2:4 0:1 mmapart. For the 11:0 mode, the area of the central (strongest) displacement of the cap is1:8  0:2 mm in diameter, and the concentric nodal lines are spaced at  1:1  0:1 mmapart. The rings have uneven amplitude and are modulated, due to tiny variations in theshape and thickness prole of the cap.When the power supplied to the transducer exceeded 2 W, a characteristic clearing pat-tern for each mode appeared, with images of the clearing as time progressed shown in gure 2.The clearing pattern coincided with the areas of maximum displacement as measured usinglaser vibrometry, with the clearing starting to appear at the central region with the largestdisplacement (gure 2(b)). As the power to the vibrating cap was increased and the am-plitude of vibration became larger, the PDLC lms cleared in a series of concentric rings(gure 2(c)). The relaxation time associated with these patterns was 1-2 s. As ultrasonicgeneration is accompanied by heating, prolonged operating times and high powers led tothe PDLC lm melting and becoming fully transparent. The relaxation time then becameof the order of 30-60 s, conrming clearing due to heating.The PDLC lm reproduces the modulation of the displacement intensity in the ringsrelatively accurately (gure 2(c)). Figure 3 shows the comparison when a contrasting back-ground is used, removing the appearance of the cap machining marks. Mode 6:0 is shownin (a) and (b), with the larger central clearing area much clearer in the contrast image.Here the clearing in the central area is 3mm in diameter, closely correlating with the laser5FIG. 3. PDLC lms on top of vibrating caps placed directly and with enhanced contrast usingvarnish layer; (a) mode 6:0 at 320 kHz, (b) mode 6:0 at 296 kHz, (c)11:0 mode at 730 kHz, (d)same mode 11:0, o resonance, 690 kHz - with dierent amplitude distribution within the peaks.vibrometry result. Figure 3(c) and (d) show the 11:0 mode, at resonance and at a sideresonance, giving dierent amplitude distribution between the peaks. The clearing in thecentral area is 1.8 mm in diameter, within very good agreement with laser vibrometry. Inaddition, concentric rings  0.7 mm thick are visible. This correlates very well with the ringthickness measured by laser vibrometry. These results suggest that the resolution of PDLClms in relation to ultrasound waves is excellent.Piezoelectric generation of ultrasound is inevitably accompanied by heating of transduc-ers and this demands careful consideration. Heating eects were closely monitored usinga \Titanium" thermal imaging camera from Cedip/Flir, with 25 mK sensitivity, with im-ages during operation shown in gure 4. Measurements without cover glass, to facilitatemeasurement of the actual lm temperature, showed that the PDLC lms have a clearingtemperature of 52C. This is lower than that of pure E7, due to the residual dissolution ofmonomer and LC in each other17. In the acousto-optic experiments described in this papercover glass is necessary for ecient transfer of ultrasound into the PDLC. However, the glassacts as an additional convection-cooled heat sink, and the temperature of the PDLC lm isslightly higher than measured at the surface of the cover glass.Figure 4 shows thermal images during operation in the 11:0 mode. For operation at2-3 W, the PDLC lm clears in the pattern shown in gure 3(c), while the temperaturedistribution is clearly radial (gure 4(a)). When the power was increased further, or thecap left to operate for a long time, the PDLC started to melt, manifested by radial clearingmatching the heating prole in gure 4(b) and (c). This clearing takes signicantly longerto relax back into the opaque state, of the order of 30-60 s.This heating eect does have some advantages. The initial switch on time for the acousto-6FIG. 4. Thermal maps of the PDLC lm with cover glass, placed on top of a cap, vibrating at730 MHz. The dashed lines indicate cap edges.optic eect was 30-60 s, depending on the power used, during which the temperature of thecap and the PDLC lm increased. After initial warming the switching and relaxation took1-2 s. The need to raise the temperature of the sample to observe the acousto-optic eectis due to the fact that, as the PDLC temperature is brought close to the temperature ofthe transition into the isotropic phase, the LC can change optical properties signicantly inresponse to small disturbances, and sensitivity is dramatically increased.The long wavelength regime acousto-optic eect in PDLC lms has been used to visu-alise vibration of plates (exural transducers) for two modes; the 6:0 and 11:0 axi-symmetricmodes at 300 kHz and 730 kHz respectively, at power levels of 2-3 W. The PDLC lms pro-duced greyscale images of cap displacement amplitude, with lateral resolution better than0.7 mm, with switching times as short as 1 s after the initial period of pre-heating. Heatingcan improve switching times due to the increased LC sensitivity near the temperature oftransition into the isotropic phase. The eect shows promise for ultrasound sensing appli-cations for transducer characterisation, NDT, and medical applications.ACKNOWLEDGEMENTSThis work was funded by the University of Warwick Energy Global Research Priority(GRP), which seeks to address the global challenges presented by the rise in the demandfor energy via it's world-class multi-disciplinary research, and supported by Merck KGaAwho provided LC materials. We thank Dr A. Dyadyusha for advice on PDLC preparation,and Dr Y. Fan and Mr R. Day for discussions and help with electronics. We thank Mr D.McKnight for photography.7REFERENCESREFERENCES1O. Trushkevych, Y. Fan, R. Perry and R.S. Edwards, Journal of Physics D: Applied Physics46(10):105005 (2013)2R.S. Edwards, B. Dutton, A.R. Clough and M.H. Rosli, Applied Physics Letters99(9):094104 (2011)3J. 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Weber, J.Tawiah, V.Kunin and M.A. Anastasio, Ad-vances in acoustics and vibration 275858 (2012)14Y.J. Liu, X. Ding, S. Lin, J. Shi, I-K. Chiang and T.J. Huang, Advanced Materials23(14):1656{1659 (2011)15Y.J. Liu, M. Lu, X. Ding, E.S.P Leong, S. Lin, J. Shi, J.H. Teng, L. Want, T.J. Bunningand T.J. Huang, Journal of Laboratory Automation 18:291{295 (2013)16J.W. Doane, N.A. Vaz, B.G. Wu, and S. umer, Applied Physics Letters 48(4) (1986)17D, Coates, J. Mater. Chem. 5:2063{2072 (1995)18Prof. Tony Jun Huang, Private communication.819J. Kelly, W. Wu and P. Paly-Muhoray, Molecular Crystals and Liquid Crystals Scienceand Technology; Section A. Molecular Crystals and Liquid Crystals 223(1):251{261 (1992)20A.W. Leissa, Vibration of Plates, US government printing oce, Washington, DC (1969)21T.J.R. Eriksson, S.N. Ramadas, and S. Dixon, Ultrasonics, submitted 08/06/20159

Ultrasound sensing using the acousto-optic effect in polymer dispersed liquid crystals

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Ultrasound sensing using the acousto-optic effect in polymer dispersed liquid crystals

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