Research reveals that nonchiral systems can give rise to chiral structures with mirror-image configurations, opening up new possibilities for the engineering of these materials.
MIT engineers observed that a liquid crystal’s orderly microstructures will spontaneously assemble into large, twisted structures (pictured) when the liquid is made to slowly flow. Image Credit: Researchers
The hands cannot be superimposed over the other when holding the hands out in front of them, no matter how the hands are rotated. Chirality, the geometric property that prevents an object from being superimposed on its mirror image, is exemplified by our hands.
Nature exhibits chirality in many forms, including the spiral structure of DNA, the hands, and the arrangement of our internal organs. Functional metamaterials, optical devices, and numerous drug therapies have benefited greatly from the use of chiral molecules and materials.
Up until now, scientists have believed that chirality breeds chirality or that chiral forces and building blocks produce chiral structures. However, that presumption might require revision.
It was recently discovered by MIT engineers that chirality can also arise through nonchiral means in a completely nonchiral material. The team reports discovering chirality in a liquid crystal, which is a material with a non-ordered, crystal-like microstructure similar to that of a solid and flows like a liquid. The study will be published in Nature Communications.
The team discovered that the fluid’s normally nonchiral microstructures spontaneously assemble into massive, twisted, chiral structures when it flows slowly. The effect is similar to what would happen if a conveyor belt full of symmetrically aligned crayons were to abruptly rearrange into massive spiral patterns after reaching a particular speed.
Considering that the liquid crystal is inherently nonchiral, or “achiral,” the geometric transformation is surprising. Thus, the study by the team provides a new avenue for chiral structure generation.
Once formed, the structures could act as spiral scaffolds for the assembly of complex molecular structures, according to the researchers’ vision. Since the chiral liquid crystals’ structural transformation would alter how the liquid interacts with light, the liquid could also be employed as optical sensors.
This is exciting, because this gives us an easy way to structure these kinds of fluids. And from a fundamental level, this is a new way in which chirality can emerge.
Irmgard Bischofberger, Associate Professor and Co-Author, Mechanical Engineering, Massachusetts Institute of Technology
The study’s co-authors include lead author Qing Zhang PhD ’22, Weiqiang Wang and Rui Zhang of Hong Kong University of Science and Technology, and Shuang Zhou of the University of Massachusetts at Amherst.
Striking Stripes
A phase of matter that combines characteristics of a liquid and a solid is called a liquid crystal. These intermediate materials have the molecular structure of solids and flow like liquids. Because the symmetric alignment of their molecules can be uniformly switched with voltage to collectively create high-resolution images, liquid crystals are used as the primary component of the pixels that make up LCD displays.
The MIT group of Bischofberger investigates the spontaneous pattern formation in nature and the laboratory of fluids and soft materials. The group’s goal is to comprehend the mechanics of fluid transformations in order to develop new, reconfigurable materials.
The researchers’ latest work concentrated on a particular kind of nematic liquid crystal, which is a water-based fluid with microscopic molecular structures resembling rods. Normally, the rods in the fluid align in the same direction. At first, Zhang was interested in how the fluid would respond to different flow circumstances.
I tried this experiment for the first time at home, in 2020. I had samples of the fluid, and a small microscope, and one day I just set it to a low flow. When I came back, I saw this really striking pattern.
Rui Zhang, Study Lead Author, Hong Kong University of Science and Technology
In the lab, Zhang conducted initial experiments once more with her colleagues. Two glass slides were used to create a microfluidic channel, which was then connected to a main reservoir and divided by an extremely thin gap. After carefully pumping liquid crystal samples through the reservoir and into the gap between the plates, the team captured images of the fluid with a microscope.
Similar to Zhang’s first experiments, the group noticed an unexpected change: as the normally uniform fluid moved slowly through the channel, it started to form stripes resembling tigers.
Bischofberger says, “It was surprising that it formed any structure, but even more surprising once we actually knew what type of structure it formed. That’s where chirality comes in.”
Twist and Flow
By successfully retracing the fluid’s flow using a variety of optical and modeling techniques, the team discovered that the fluid’s stripes were unexpectedly chiral. The team noticed that the fluid’s microscopic rods are typically arranged in almost perfect formation when it is stationary.
Upon swiftly pumping the fluid through the channel, the rods become completely disorganized. However, the structures begin to wiggle and then gradually twist like tiny propellers, with each one turning slightly more than the previous, at a slower, in-between flow.
Under a microscope, the twisting crystals assemble into large spiral structures that resemble stripes if the fluid keeps flowing slowly.
Zhang says, “There’s this magic region, where if you just gently make them flow, they form these large spiral structures.”
After simulating the fluid's dynamics, the researchers discovered that the large spiral patterns appeared when the fluid reached a state of equilibrium between its viscosity and elasticity. Elasticity is essentially a material’s propensity to deform (for example, how easily the fluid’s rods wiggle and twist), whereas viscosity describes how easily a material flows.
Bischofberger explains, “When these two forces are about the same, that’s when we see these spiral structures. It’s kind of amazing that individual structures, on the order of nanometers, can assemble into much larger, millimeter-scale structures that are very ordered, just by pushing them a little bit out of equilibrium.”
The group discovered that the twisted assemblages have chiral geometry, meaning that no matter how the spirals were rearranged, the spirals could not be superimposed over one another if a mirror image of that spiral was created. The chiral spirals’ emergence from a nonchiral material via nonchiral processes is a first and suggests a reasonably easy method for creating structured fluids.
The results are indeed surprising and intriguing. “It would be interesting to explore the boundaries of this phenomenon. I would see the reported chiral patterns as a promising way to periodically modulate optical properties at the microscale.
Giuliano Zanchetta, Associate Professor, University of Milan
Zanchetta was not involved in the study.
Bischofberger says, “We now have some knobs to tune this structure. This might give us a new optical sensor that interacts with light in certain ways. It could also be used as scaffolds to grow and transport molecules for drug delivery. We’re excited to explore this whole new phase space.”
This research was supported, in part, by the US National Science Foundation.
Journal Reference:
Zhang, Q., et al. (2024). Flow-induced periodic chiral structures in an achiral nematic liquid crystal. Nature Communications. doi.org/10.1038/s41467-023-43978-6.