Under suitable conditions, liquid crystals can form incredible structures, creating filaments and flat discs that can transport materials similar to complex biological systems. This discovery could pave the way for self-organizing materials, alternative methods for modeling cellular processes, and much more.
Liquid crystals are present in various everyday items, including smartphone screens, gaming consoles, vehicle dashboards, and medical equipment. When an electric current runs through liquid crystal displays (LCDs), they produce colors because of these substances’ unique characteristics: when their shape changes, they reflect different light wavelengths.
Recent findings from the lab of Chinedum Osuji, the Eduardo D. Glandt Presidential Professor and Chair of Chemical and Biomolecular Engineering, reveal that liquid crystals may have more uses than previously thought. Under optimal conditions, they can coalesce into remarkable structures, forming filaments and flattened discs capable of moving materials like complex biological systems.
This discovery has the potential to lead to materials that self-assemble, innovative techniques to model cellular behavior, and additional advancements. “It’s akin to a series of conveyor belts,” explains Christopher Browne, a postdoctoral researcher in Osuji’s lab and one of the co-first authors of a recent paper published in Proceedings of the National Academy of Sciences (PNAS) detailing this finding. “This happened serendipitously, as we observed something that superficially appears lifelike, which prompted us to explore whether it could be part of a broader, more fascinating phenomenon.”
Browne and Osuji are now collaborating in an NSF-funded interdisciplinary group at the Laboratory for Research on the Structure of Matter (LRSM), under the leadership of Matthew Good, Associate Professor of Cell and Developmental Biology at the Perelman School of Medicine, and Elizabeth Rhoades, Professor of Chemistry in the School of Arts & Sciences. They are examining the formation of condensates in both biological and non-biological systems.
Initially, Osuji’s lab collaborated with ExxonMobil to investigate mesophase pitch, a component used to create high-strength carbon fibers utilized in Formula 1 cars and premium tennis rackets. “Those materials are liquid crystals,” states Osuji regarding the chemical precursors to carbon fibers. “Or more accurately, they exhibit liquid crystalline properties during certain processing stages.” While experimenting with condensing materials at varying temperatures, Yuma Morimitsu, another postdoctoral researcher in Osuji’s lab and the paper’s other co-first author, made an unexpected observation regarding the material’s behavior.
Typically, combining two immiscible liquids — those that do not naturally mix — and heating them sufficiently to encourage mixing, followed by cooling, results in separation or “demixing.” This process usually leads to the formation of droplets that join to create distinct layers, similar to how oil and water separate, with oil resting on top.
In an intriguing deviation, the liquid crystal 4′-cyano 4-dodecyloxybiphenyl, or 12OCB, spontaneously formed irregular structures when separating from squalane, a clear oil. “Instead of droplet formation,” Osuji explains, “the phase separation of the liquid crystal from the other system components resulted in cascaded structures, starting with these rapidly growing filaments that subsequently developed into what we term bulged discs or flat droplets.”
To analyze this behavior, the researchers employed advanced microscopes to monitor the movement of the liquid crystals at the micrometer scale — a measurement tantamount to millionths of a meter, comparable to the diameter of a human hair. “When we first observed these structures, we were examining them at an excessively rapid cooling rate,” Osuji recalls, leading to the liquid crystals clustering. It was only by reducing the cooling rate and examining more closely that the researchers discerned the spontaneous formation of structures resembling those found in living systems.
Interestingly, Browne noted that several researchers had nearly documented similar behavior in past decades, but they either studied systems in which the behavior was less pronounced or lacked the microscopy tools to visualize the events accurately.
Browne believes the most thrilling aspect of this discovery is its ability to bridge different fields: the active matter research area focused on biological systems that facilitate material transport and movement, along with fields examining self-assembly and phase behavior, which explore materials capable of forming new structures independently and exhibiting distinct behaviors in different phases. “This represents a new category of active matter systems,” Browne asserts.
Additionally, he and Osuji highlight the potential of utilizing this discovery to replicate biological systems, either to deepen our understanding of these systems or to create new materials. “Molecules are getting absorbed into the filaments and are then continuously transported into the flat droplets,” Osuji explains, “even though the system’s activity isn’t obvious upon visual inspection.” Essentially, these flat droplets could act like miniature reactors, generating molecules which the filaments transport to other droplets for storage or additional chemical reactions.
The researchers also propose that their findings could breathe new life into liquid crystal research. “When a field gets industrialized,” Browne states, “the foundational research often diminishes. However, there can be unresolved questions that linger on.”
This research took place at the University of Pennsylvania, within the School of Engineering and Applied Science’s Department of Chemical and Biomolecular Engineering, and the Department of Physics and Astronomy in the School of Arts & Sciences, alongside ExxonMobil’s Research Division. The study received support from a grant provided by ExxonMobil and from the U.S. National Science Foundation (DMR-2309043).
Additional co-authors include Zhe Liu from Penn Engineering; Paul G. Severino from the School of Arts & Sciences; and Manesh Gopinadhan, Eric B. Sirota, Ozcan Altintas, and Kazem V. Edmond from ExxonMobil.
