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Liquid Crystal Sensors for Ultrasound Visualisation: Seeing Sound

Introduction

Ultrasonic testing and wavefield visualisation play a large part in non-destructive testing (NDT) and structural health monitoring (SHM). The wavefield can be used to see where defects form in structures and components, tracking how these defects might develop over their lifetimes. To test a sample, pulses of ultrasound are sent through the bulk or along the surface of a sample, with any cracks or other defects partially/fully reflecting or blocking the waves. Typically, scanning methods such as vibrometry and interferometry, or the use of microphones and hydrophones are used to inspect an area on the surface of the sample. This point-by-point scanning can create a very slow inspection process, which can cause delays where large scale testing and monitoring is necessary.

Could we instead create a paint-on to removable 2D sensor, that visually changes under the influence of ultrasound? Provided the sensor has a broadband response and is sensitive to different types of ultrasonic waves, it could visually show when the waves reach or interact with defects at the surface sample. This would open up a wide variety of opportunities such as embedded sensors or smart coatings that help visually identify problems. Important structural safety data could be collected by anyone with a mobile phone camera and sent to a monitoring center, instead of having to train people in rather complex testing procedures.

The answer may be liquid crystals. These materials are used in displays, such as phones and computers, and are very sensitive to external fields such as electric or magnetic fields. They are also sensitive to displacement, try pushing a liquid crystal display with your finger. This means that they are sensitive to ultrasound. Whilst liquid crystal films require complex technological processes to be made into displays, there are more simple low-cost alternatives. Small droplets of liquid crystal can be mixed into a polymer, which can be painted directly onto a sample or made into flexible large area films.

Liquid Crystals: The Basics

Liquid crystal (LC) is a phase of matter between solid and liquid. It can flow like fluids but also has order and anisotropy (where material properties are dependent on measurement direction) like crystals. Liquid crystals contain rod-like molecules that are generally aligned along the same axis, known as the director or optical axis, n̂. This orientational ordering is defined by the order parameter, $S equals a half times the expectation value of bracket open 3 cos squared theta minus 1 bracket close$ , see figure 1.

Figure 1. From left to right, a rod-like molecule slightly rotated to define theta, rod-like molecules in an even grid pattern to depict a crystalline solid with order parameter S equals 1, rod-like molecules generally pointing in the same upwards direction with some variation to depict liquid crystal with order parameter S equals 0.3 to 0.9, and finally rod-like molecules randomly oriented to depict liquid with order parameter S equals 0.

Figure 1: The orientational ordering of rod-like molecules in various phases of matter, with order parameter S.

Change in the positional ordering of the molecules creates different structures of liquid crystal materials. The most commonly used is nematic LC where there is no positional ordering, see figure 2a. Cholesteric or chiral nematic LCs are a sub-structure of nematics, where layers of nematics are helically arranged and the distance between layers with parallel director axes is known as the pitch, see figure 2b.

Figure 2. Part A nematic liquid crystal structure and part B cholesteric slash chiral nematic liquid crystal structure.

Figure 2: (a) Nematic and (b) cholesteric/chiral nematic liquid crystal structures.

Liquid Crystal Sensors

Polymer-Dispersed Liquid Crystal (PDLC) Sensor

Polymer-dispersed liquid crystals are made by mixing droplets of nematic liquid crystal into a polymer matrix. The refractive index of the polymer and the droplets when their director axes are aligned, are chosen to match. When there is no field present, light scatters off the unaligned droplets and the sensor appears cloudy. In the presence of an ultrasonic field, the LC droplets in the polymer align allowing light to pass through and the sensor become transparent. A schematic of this acousto-optic effect is shown in figure 3a.

Figure 3. Part A the acousto-optic effect in polymer-dispersed liquid crystals showing how the liquid crystal droplets align when a field is applied resulting in clearing and part B a photo of a flexible polymer-dispersed liquid crystal film.

Figure 3: (a) The acousto-optic effect in PDLCs showing how LC droplet alignment in an applied field results in clearing and (b) an example of a PDLC film.

An initial prototype polymer dispersed liquid crystal sensor has been developed and used to visualise the vibrational modes of a flexural transducer. A contrasting layer was painted onto the transducer with a 100 μm thick layer of the polymer dispersed liquid crystal painted on top. A laser vibrometry scan of the mode at 720 kHz is shown below in figure 4a, which gives excellent resolution but took around 8 hours to produce. The central image, figure 4b, shows the mode obtained by the PDLC sensor in just a few seconds, following a warm up period of around 30 seconds. This means that it is now possible to test vibrational patterns of transducers over a very wide range of frequencies in less than 5 minutes, something unfeasible using other methods. The third image in figure 4c, shows a resonance mode at 6.7 MHz, demonstrating the broadband nature of the sensor, as well as the interesting shape of one of the higher-order resonance modes.

Figure 4. Part A laser vibrometry scan of the mode at 720kHz, part B polymer-dispersed liquid crystal at 720kHz, and part C polymer-dispersed liquid crystal at 6.7MHz.

Figure 4: (a) Laser vibrometry scan of the mode on a flexural transducer at 720kHz, (b) PDLC at 720kHz, and (c) PDLC at 6.7MHz.