Polymer-dispersed liquid-crystal coatings for ultrasound visualization: Experiment and theory.

Polymer dispersed liquid crystal (PDLC) films are formed of droplets of liquid crystal (LC) held in a polymer matrix. Similar to aligned LC films, PDLCs exhibit the acousto-optic (AO) effect when excited by acoustic waves of sufficient amplitude, whereby the PDLC film becomes transparent in the excited regions (acoustic clearing). Despite decades of research there is still debate over the mechanisms of the AO effect for the case of LC films, with several competing theories, and AO effects in PDLC have not been studied theoretically. This paper explores the AO effect in PDLC both experimentally and theoretically, and attempts a theoretical description of the observed phenomena based on the theoretical approach by Selinger et al. for aligned LC films. The acousto-optic effect in PDLC is shown to be due to direct interaction of acoustic waves with LC droplets, rather than due to compression of the droplet itself. Polarizing microscopy revealed changes in droplet shape at excited points. This is consistent with reorientation as a contributing factor, possibly coexisting with flows at higher excitation powers. In previous experimental studies PDLC films were prepared with cover slides, in the same way as LC AO cells, significantly limiting applications by adding complexity to the design. Also, to exhibit AO clearing it was considered that the PDLC needed to be prepared with high LC concentrations (over 75% by weight). We demonstrate that no cover slide is necessary, and that PDLC coatings without a cover have improved sensitivity to acoustic waves. We demonstrate the AO effect for LC concentrations as low as 40% by weight. The ability to use standard composition PDLC, with no top cover, is paving the way to paint-on visual ultrasound sensors.

Ultrasonic displacement measurements from local absorption of ultrasound in thermochromic liquid crystal sensors.

Wavefield imaging can be used for measuring the wavefield produced by an ultrasound transducer for medical and industrial applications, or for the detection and monitoring of defects in non-destructive testing. Typical wavefield imaging methods include interferometry/vibrometry, and the use of microphones and hydrophones. These involve scanning, making them time consuming, and microphones have limited resolution. An alternative method presented here uses thermochromic liquid crystal sensors which react to heat generated due to absorption of ultrasonic waves. The result is a colour scale that varies with temperature, with the temperature change dependent on ultrasonic displacement. Measurements of the resonant modes of a flexural ultrasonic transducer were taken between 320 kHz and 6.77 MHz. Temperature maps were obtained from photographs of the TLC sensor using the true-colour image processing method. The obtained temperature change across the transducer face was compared with displacement measurements taken using interferometry, showing excellent agreement in the position of the mode features and good resolution at lower frequencies. Thermal measurements were also taken to directly observe the heating of the transducer cap, showing the effect of the thermal conductivity of the transducer along with confirming the increased heat generated by the ultrasound absorption when a backing layer is used. The sensors show promise for fast transducer characterisation, with further potential applications in structural health monitoring and defect detection.

Acoustic field visualisation using local absorption of ultrasound and thermochromic liquid crystals.

Acoustic field and vibration visualisation is important in a wide range of applications. Laser vibrometry is often used for such visualisation, however, the equipment has a high cost and requires significant user expertise, and the method can be slow, as it requires scanning point by point. Here we suggest a different approach to visualisation of acoustic fields in the kHz - MHz range, using paint-on or removable film sensors, which produce a direct visual map of ultrasound displacement. The sensors are based on a film containing thermochromic liquid crystals (TLC), along with a backing/underlay layer which improves absorption of ultrasound. The absorption generates heat, which can be seen by a change in colour of the TLC film. A removable sensor is used to visualise the resonant modes of an air-coupled flexural transducer operated from 410 kHz to 7.23 MHz, and to visualise 40 kHz standing waves in a Perspex plate. The thermal basis of the visualisation is confirmed using thermal imaging. The speed and cost of visualisation makes the new sensor attractive for use in condition monitoring, for fast assessment of transducer quality, or for analysis of acoustic field distribution in power ultrasonic systems.