Liquid Crystals to Command Dynamics of Bacteria and Colloids
Abstract: The dynamics of small particles in fluids has fascinated scientists for centuries, since 1674 when van Leeuwenhoek observed 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 dynamic. If one could control and streamline the chaotic motion of particles such as bacteria and colloids at the microscale, it would open endless technological opportunities in areas such as energy harvesting, transformation of stored or environmental energy into systematic motion, development of new principles of bio-engineering, micro-robotics, and transport of matter at microscale.
Remarkably, bacteria and colloids driven by an external field do not obey the laws of thermodynamics and can be used to extract 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. The orientational order of the medium leads to new dynamic effects, such as anomalous diffusion.
By using a newly developed technique of nanophotonic photoalignment, the liquid crystal director can be patterned into any predesigned structure. We demonstrate that the patterned liquid crystals can control microscale dynamics of inanimate particles, such as solid colloids or fluid droplets, through the effects of nonlinear electrophoresis and electro-osmosis. Moreover, the plasmonic patterning of liquid crystals allows one to command the dynamics of swimming bacteria, guiding their trajectories, polarity of swimming, and concentration in space.