Speaker
Description
Active matter consists of many individual units that continuously consume energy to move or generate mechanical forces. These systems exhibit rich collective behaviors arising from interactions between their components. A particularly interesting class of interactions in active matter is non-reciprocal interactions, where the force exerted by particle i on particle j is not equal and opposite to the force exerted by j on i. This effectively breaks Newton’s third law at the coarse-grained level and gives rise to behaviors that are not possible in equilibrium systems. A well-known example of such non-reciprocal interaction is the predator–prey interaction, where the predator is attracted to the prey while the prey is repelled by the predator.
In this thesis, we investigate emergent interactions in active liquid crystal droplets. The experiments are conducted in a quasi-two-dimensional flow cell constructed using double-sided adhesive tape with a thickness of 50 µm. These liquid crystals are known to exhibit thermally reversible phase transitions from smectic to nematic to isotropic phases. Monodisperse droplets are generated using flow-focusing microfluidic devices, and epifluorescence microscopy is employed to track internal flows, interfacial behavior, and droplet shape dynamics.
We observe a predator–prey-like mechanism in liquid crystal droplets dispersed in Triton X surfactant solutions. Specifically, we work with 5CB (4-cyano-4′-pentylbiphenyl) and 8CB (4-cyano-4′-octylbiphenyl) droplets of approximately 50 µm diameter in non-ionic surfactant solutions. We first characterized the solubilization rates of individual droplets in both non-ionic and ionic surfactants. To identify material transfer between 5CB and 8CB droplets, one population of droplets was labeled with Nile Red dye. Over time, fluorescence was observed in initially unlabeled droplets, confirming inter-droplet material transfer. We then studied the solubilization dynamics in mixed droplet environments and compared them with the behavior of isolated droplets.
Our results show that 8CB droplets grow over time, while 5CB droplets shrink, establishing a predator–prey analogy in which 8CB acts as the predator and 5CB as the prey. We further investigate the effect of surfactant concentration on material transfer and interaction dynamics. Using a temperature-controlled stage (CherryTemp), we study the influence of temperature on the rate of interaction. Additionally, we analyze how the coordination number affects droplet assembly and the velocity of collective organization.
Future work will focus on further characterization of this system in Triton X surfactant, followed by extension to other surfactant systems. We aim to investigate whether the observed behavior is reversible and to understand the differences arising from the use of different surfactants, as well as to explore potential applications of these non-reciprocal active droplet systems