Living Liquid Crystals
We use lyotropic chromonic liquid crystals to host and manipulate biological microswimmers, such as bacteria Bacillus Subtilis. In this anisotropic environment, bacteria no longer run-and-tumble, but swim along prescribed trajectories defined by the LC director. Their flow field is concentrated along the director, giving them new ability to transport cargo particles. At right condition, their motion can melt LC into isotropic phase, creating a bacteria version of Wilson cloud chamber. Meanwhile, the activity brought in by swimmers sets the LC into new steady states: the director field adopts bend stripes and topological turbulence state at zero Reynolds number. Read more about the works.
Pattern formations in liquid crystal
In the classic Hele-Shaw setup, we inject less viscous fluid into a liquid crystal environment. By changing the viscosity of inner fluid, injection rate, and cell gap, the interfacial instability changes from finger splitting to dendritic growth. The intrinsically anisotropic viscosity of the LC made it possible. This work is in collaboration with Irmgard Bischofberger and Qing Zhang at MIT.
Defects in liquid crystals
Topological defects put the liquid crystal into a harsh test: within a small space, both the magnitude and the phase of order parameter changes dramatically to accommodate the singular points or lines; as a result, the liquid crystal melts into a less-ordered phase. Due to the small size, typically on the order of 10nm, the fine structures of the defect cores are hard to explore. In lyotropic chromonic liquid crystal, however, we observed large disclination defects on the order of 10μm in radius, where the nematic gradually melt into isotropic phase. The coupling between director field and scalar order parameter deviates the cores from a cylindrical shape, different from theoretical predictions with simplified material constants. We attribute this unique feature to the aggregate-nature of lyotropic chromonic liquid crystals, where the basic building units are weakly-combined molecular stacks that easy to break. Read more about this work.
Viscoelastic properties of lyotropic liquid crystals
In a lyotropic LC formed by thin rods, the viscoelastic properties are determined by both the entropic condensation and the intrinsic mechanical properties of the rods themselves. Using magnetic Fredriks transition and dynamic light scattering (DLS), we explored two model lyotropic chromonic LCs under different concentration, temperature, and ionic contents. The findings are astonishing: the three Frank elastic constants are very different from each other and from thermotropic LCs; the anisotropic viscosity for different deformation modes can be 4-5 orders of magnitude different. We attribute these unique features to the aggregation nature of the chromonic aggregates that are only weakly combined by non-covalent interactions. There are more to be explored in a the large family of lyotropic LCs. Read more about these works.