Liquid Crystals: A New State of Matter That’s Making Waves
By Gabby Escalante
We interact with solids, liquids, and gases every day. Whether we’re filling a balloon, drinking a cup of coffee, or scrolling through memes on the Internet, there is no escaping the three most prevalent states of matter. Although solids, liquids, and gases make up the majority of the naturally occurring phases on Earth, scientists have been keen to explore other states of matter; plasma and Bose-Einstein condensates, for example, have been subjects of scientific research for decades. But more recently, a new phase of matter has stirred excitement within the scientific community. This phase, a blend between a solid and a liquid, is known as liquid crystal1.
Liquid crystals are long, thin molecules consisting of hydrocarbon chains and several hexagonally-shaped aromatic compounds. The unique properties of liquid crystal molecules are best seen when multiple molecules are linked together into sheets known as liquid-crystal networks (LCN). LCNs are low in density but very strong, so they have been used to fortify armor, wind turbines, and electrical cables. Due to their chemical composition, LCNs change shape when exposed to certain stimuli2, such as temperature, pH, and light1. For instance, if coupled with light-sensitive molecules such as azobenzene—a compound consisting of two carbon phenyl rings linked by nitrogen double bonds1—LCNs can change shape in response to UV light. When hit by UV light, azobenzene’s structure bends and assumes a smaller size. This shrinkage of azobenzene increases the disorder among LCN molecules, causing the LCN to change shape on a large scale that is visible to the naked eye. After an azobenzene molecule changes shape and UV light is removed, the azobenzene molecule and its accompanying LCN slowly return to their original conformations2.
Previously, waves had been difficult to create in LCNs because the time required for a bent azobenzene molecule to return to its original shape had been too slow. When exposed to UV light, the azobenzene would immediately change shape but would take much longer to relax, resulting in a slowly moving LCN. A useful analogy is the human wave made by the audience members at a baseball game. This human wave works because each individual stands up when it’s his or her turn, and then sits back down within a few seconds. If, instead of seconds, sitting back down took ten minutes, the wave could not possibly circle the stadium more than once because baseball fans would still be in the process of sitting down when their next turn came around. Similarly, since previous versions of LCNs took too long to relax, a fluid wave motion was challenging to create. For this reason, technologies incorporating light-sensitive LCNs were usually only able to harness the initial conformational change (the bending phase) rather than the relaxation phase of azobenzene.
How were researchers ultimately able to overcome this challenge? In June of 2017, a team of researchers from the Netherlands and the United States engineered a new and improved LCN, one that generates perpetual mechanical waves when lit by UV light3. The accomplished team of chemists and physicists altered the azobenzene molecules so that they quickly return to their original conformation. They designed and mixed two different azobenzene derivatives, one that includes a third carbon ring and one that has a hydroxyl group bonded to a specific location on a carbon ring3. These azobenzene derivatives rapidly transition between a straight to a bent form, causing the LCNs to quickly bend and relax.
This change in structure enabled the scientists to create oscillatory motion in the LCN polymers. First, they loosely stretched out the LCN film over an open surface, so that the LCNs would be able to bend. Next, they set up a fixed UV light source at an angle to the LCN film3. Then, they set up a few cameras, and observed with the naked eye. Remarkably, the molecules nearest the light source rose and fell, briefly shadowing and then exposing neighboring molecules to the UV light. These neighboring molecules subsequently rose and fell in response to the light, shadowing and then exposing their neighbors to the light, initiating a chain of rising-and-falling down the length of the polymer film. This self-shadowing phenomenon, not unlike the human wave at the baseball game, resulted in a continuous wave pattern3 with constant amplitude and wavelength.
The waves produced on LCNs show promise for future technologies, including self-cleaning surfaces, transportation of miniature species, medical appliances, and energy harvesting. In one of their experiments, the scientists placed sand on the LCN film, and the film was able to (1) carry sand from one side of the film to the other, and (2) eject the sand off of its surface3. If expanded to cover counters or lab benches, LCN films could contribute to cleaner workspaces. In another experiment, the scientists demonstrated that a heavy object could be carried down the length of the polymer film3, which could be useful for small-scale transportation. Currently, scientists are investigating ways to incorporate the technology into medical appliances, since the nanoscopic movements and strength of these tools are ideal for surgical procedures and bionic technologies that require extreme precision. With continued creativity and effort from scientists, liquid crystal networks may feature more prominently in our daily lives, waving in a new era for LCNs to join the ranks of solids, liquids, and gases.
Iqbal, Danish, and Muhammad Haris Samiullah. “Photo-responsive shape-memory and shape-changing liquid-crystal polymer networks.” Materials 6.1 (2013): 116-142.
Yu, Yanlei. “Materials science: A light-fuelled wave machine.” Nature 546.7660
Gelebart, Anne Helene, et al. “Making waves in a photoactive polymer film.” Nature
546.7660 (2017): 632-636.