New Technique to Develop Color-Changing Sensing System for Sensors

Chameleons are well-known for their abilities to change colors. Based on their mood or body temperature, their nervous system instructs skin tissue containing nanocrystals to expand or contract, modifying how the nanocrystals reflect light and make the skin of the reptile turn into rainbow colors.

Taking a cue from this, researchers from the Pritzker School of Molecular Engineering (PME) at the University of Chicago have designed a method to stretch and strain liquid crystals to produce different colors.

By making a thin film of polymer loaded with liquid crystal droplets and further controlling it, the researchers have identified the basics for a color-changing sensing system that can be utilized for sensors, smart coatings, and also wearable electronics.

The study, headed by Juan de Pablo, Liew Family Professor of Molecular Engineering, was reported in the Science Advances journal on July 10^th, 2020.

Stretching Liquid Using Thin Films

For several display technologies, liquid crystals, which show distinct molecular orientations, form the basis. However, de Pablo and his research group showed interest in chiral liquid crystals, which exhibit twists and turns and a specific asymmetrical “handedness”—such as left-handedness or right-handedness—that enables them to have more fascinating optical properties.

Moreover, these crystals can form what are called the “blue phase crystals,” which display properties of both liquids and crystals and can, in certain cases, transmit or reflect visible light better compared to liquid crystals themselves.

The scientists were aware that such crystals could be manipulated to generate an extensive range of optical effects if stretched or strained. However, they also knew that it is not feasible to stretch or strain a liquid instantly. Rather, they positioned small liquid crystal droplets into a polymer film.

That way we could encapsulate chiral liquid crystals and deform them in very specific, highly controlled ways. That allows you to understand the properties they can have and what behaviors they exhibit.

Juan de Pablo, Professor, The University of Chicago

Creating Temperature and Strain Sensors

Therefore, the team identified several more different phases—crystal’s molecular configurations—than had been known earlier. Such phases generate various colors based on how they are stretched or strained, or even when they experience changes in temperature.

Now the possibilities are really open to the imagination. Imagine using these crystals in a textile that changes color based on your temperature, or changes color where you bend your elbow.

Juan de Pablo, Professor, The University of Chicago

Also, a system of this kind could be utilized to quantify strain in airplane wings, for instance, or to perceive minute variations in temperature inside a room or system.

According to de Pablo, variations in color offer an outstanding method to quantify something remotely, without requiring any kind of contact.

You could just look at the color of your device and know how much strain that material or device is under and take corrective action as needed. For example, if a structure is under too much stress, you could see the color change right away and close it down to repair it. Or if a patient or an athlete placed too much strain on a particular body part as they move, they could wear a fabric to measure it and then try to correct it.

Juan de Pablo, Professor, The University of Chicago

De Pablo added, although scientists have manipulated the materials with temperature and strain, it is also possible to manipulate them using acoustic fields, magnetic fields, and voltage, which could result in new kinds of electronic devices that are made from such crystals.

“Now that we have the fundamental science to understand how these materials behave, we can start applying them to different technologies,” noted de Pablo.

The other authors of the study are Monirosadat Sadati and Nader Taheri Qazvini from the University of South Carolina, postdoctoral researchers Jose A. Martinez-Gonzalez and Rui Zhang, graduate student Ye Zhou, alumna Khia Kurtenbach, research scientist Xiao Li, Emre Bukusoglu from Middle East Technical University, Nicholas L. Abbott from Cornell University, and Juan Pablo Hernandez-Ortiz of the Universidad Nacional de Colombia.

This study was funded by the Department of Energy and the National Science Foundation.

Journal Reference:

Sadati, M., et al. (2020) Prolate and oblate chiral liquid crystal spheroids. Science Advances. doi.org/10.1126/sciadv.aba6728.