Liquid Crystal Institute, Kent State University, Kent, Ohio, USA
Dynamics of small particles in fluids has fascinated scientists for centuries, since van Leeuwenhoek observed in 1674 tiny creatures, nowadays known as “bacteria”, swimming chaotically in a droplet of water. Much later, Brown found that even inanimate small particles, when placed in water, engage in a similar chaotic dynamics. If one could learn how to control and streamline the chaotic motion of particles such as bacteria and colloids at the microscale, that would open technological opportunities in areas such as transformation of stored or environmental energy into systematic motion, micro-robotics, transport of matter at microscale, etc. Remarkably, bacteria and colloids driven by an external field do not obey the laws of thermodynamics and can be used to extract a useful work. This lecture presents an approach to command microscale dynamics in which the isotropic medium such as water is replaced with an anisotropic fluid, a liquid crystal. The liquid crystals are formed by elongated molecules that tend to align parallel to each other along a common direction called the director. As a result, physical properties such as electric conductivity or viscosity depend on the direction of measurement, whether it is parallel or perpendicular to the director. Orientational order of the medium leads to new dynamic effects, such as anomalous diffusion (1). By using a newly developed technique of nano-photonic photoalignment, the liquid crystal director can be patterned into any predesigned structure (2). We demonstrate that the patterned liquid crystals can control microscale dynamics of inanimate particles such as solid colloids, fluid droplets, through the effects of nonlinear electrophoresis (3) and electro-osmosis (4). Moreover, plasmonic patterning of liquid crystals allows one to command the dynamics of swimming bacteria, guiding their trajectories, polarity of swimming and concentration in space (5). The work is supported by NSF DMR-1507637, NSF DMREF DMS-1729509 and Petroleum Research grant PRF#56046-ND7.
1.T. Turiv, V.G. Nazareno et al., Effect of Collective Molecular Reorientations on Brownian Motion of Colloids in Nematic Liquid Crystal. Science 342, 1351-1354 (2013).
2.Y. Guo et al., High-Resolution and High-Throughput Plasmonic Photopatterning of Complex Molecular Orientations in Liquid Crystals Adv Mater 28, 2353-2358 (2016).
3.O. D. Lavrentovich, I. Lazo, O. P. Pishnyak, Nonlinear electrophoresis of dielectric and metal spheres in a nematic liquid crystal. Nature 467, 947-950 (2010).
4.I. Lazo, C. H. Peng, J. Xiang, S. V. Shiyanovskii, O. D. Lavrentovich, Liquid crystal-enabled electro-osmosis through spatial charge separation in distorted regions as a novel mechanism of electrokinetics. Nature Communications 5, 5033 (2014).
5.C. Peng, T. Turiv, Y. Guo, Q.-H. Wei, O. D. Lavrentovich, Command of active matter by topological defects and patterns. Science 354, 882-885 (2016).
Abut the speaker: Prof. Oleg D. Lavrentovich received his Ph.D. (1984) and Doctor of Science (1990) degrees in Physics and Mathematics from the Ukrainian Academy of Sciences. In 1992 he joined the Liquid Crystal Institute of Kent State University as a Senior Research Fellow. He served as the director of institute in 2003-2011, and he is now a Trustees Research Professor of Kent State. Lavrentovich held visiting appointments at the University P. et M. Curie, University Denis Diderot in France and other universities. He is the editor of Liquid Crystals Reviews (Taylor & Francis), member of the Editorial Boards of Liquid Crystals, Condensed Matter Physics, Ukrainian Journal of Physics and Advisor of the Kent State University SPIE Student Chapter. His research focuses on soft matter formed by weakly interacting organic molecules. The emphasis is on systems with orientational order, known as liquid crystals, and their composites, such as dispersions of colloidal particles in liquid crystals, water dispersions of self-organizing chromonic aggregates, living liquid crystals.