Design of nematic liquid crystals to control microscale dynamics

Dynamics of small particles, both living such as swimming bacteria and inanimate, such as colloidal spheres, has fascinated scientists for centuries. If one could learn how to control and streamline their chaotic motion, that would open technological opportunities in areas such as the transformation of stored or environmental energy into systematic motion, micro-robotics, and transport of matter at the microscale. This overview presents an approach to command microscale dynamics by replacing an isotropic medium such as water with an anisotropic fluid, a nematic liquid crystal. Orientational order leads to new dynamic effects, such as propagation of particle-like solitary waves. Many of these effects are still awaiting their detailed mathematical description. By using plasmonic metamask photoalignment, the nematic director can be patterned into predesigned structures that control dynamics of inanimate particles through the liquid crystal enabled nonlinear electrokinetics. 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. The patterned director design can also be extended to liquid crystal elastomers, in which case the director gradients define the dynamic profile of elastomer coatings. Some of these systems form an experimental playground for the exploration of out-of-equilibrium active matter, in which the levels of activity, degree of orientational order and patterns of alignment can all be controlled independently of each other.Comment: 35 pages, 9 figures, a review based on a lectur

Morphogenesis of defects and tactoids during isotropic-nematic phase transition in self-assembled lyotropic chromonic liquid crystals

We explore the structure of nuclei and topological defects in the first-order phase transition between the nematic (N) and isotropic (I) phases in lyotropic chromonic liquid crystals (LCLCs). The LCLCs are formed by self-assembled molecular aggregates of various lengths and show a broad biphasic region. The defects emerge as a result of two mechanisms. 1) Surface anisotropy mechanism that endows each N nucleus (tactoid) with topological defects thanks to preferential (tangential) orientation of the director at the closed I-N interface, and 2) Kibble mechanism with defects forming when differently oriented N tactoids merge with each other. Different scenarios of phase transition involve positive (N-in-I) and negative (I-in-N) tactoids with non-trivial topology of the director field and also multiply connected tactoids-in-tactoids configurations. The closed I-N interface limiting a tactoid shows a certain number of cusps; the lips of the interface on the opposite sides of the cusp make an angle different from pi. The N side of each cusp contains a point defect-boojum. The number of cusps shows how many times the director becomes perpendicular to the I-N interface when one circumnavigates the closed boundary of the tactoid. We derive conservation laws that connect the number of cusps c to the topological strength m of defects in the N part of the simply-connected and multiply-connected tactoids. We demonstrate how the elastic anisotropy of the N phase results in non-circular shape of the disclination cores. A generalized Wulff construction is used to derive the shape of I and N tactoids as the function of I-N interfacial tension anisotropy in the frozen director field of various topological charges m. The complex shapes and structures of tactoids and topological defects demonstrate an important role of surface anisotropy in morphogenesis of phase transitions in liquid crystals.Comment: 31 pages, 13 figure

Chirality Amplification and Detection by Tactoids of Lyotropic Chromonic Liquid Crystals

Detection of chiral molecules requires amplification of chirality to measurable levels. Typically, amplification mechanisms are considered at the microscopic scales of individual molecules and their aggregates. Here we demonstrate chirality amplification and visualization of structural handedness in water solutions of organic molecules that extends over the scale of many micrometers. The mechanism is rooted in the long-range elastic nature of orientational order in lyotropic chromonic liquid crystals (LCLCs) formed in water solutions of achiral disc-like molecules. The nematic LCLC coexists with its isotropic counterpart, forming elongated tactoids; spatial confinement causes structural twist even when the material is nonchiral. Minute quantities of chiral molecules such as amino acid L-alanine and limonene transform the racemic array of left- and right-twisted tactoids into a homochiral set. The left and right chiral enantiomers are readily distinguished from each other as the induced structural handedness is visualized through a simple polarizing microscope observation. The effect is important for developing our understanding of chirality amplification mechanisms; it also might open new possibilities in biosensing.Comment: 10 pages, 6 figure

Chiral ground states of ferroelectric liquid crystals

Ferroelectric nematic liquid crystals are formed by achiral molecules with large dipole moments. Its three-dimensional orientational order is universally described as unidirectionally polar. We demonstrate that the ground state of ferroelectric nematic unconstrained by externally imposed alignment directions is chiral, with left- and right-hand twists of polarization. Although the helicoidal deformations and defect walls separating domains of opposite handedness increase the elastic energy, the twists reduce the electrostatic energy and become weaker when the material is doped with ions. The study shows that the polar orientational order of molecules could trigger chirality in the soft matter with no chemically induced chiral centers.Comment: 24 pages, 13 figure

Living Liquid Crystals

Collective motion of self-propelled organisms or synthetic particles often termed active fluid has attracted enormous attention in broad scientific community because of it fundamentally non-equilibrium nature. Energy input and interactions among the moving units and the medium lead to complex dynamics. Here we introduce a new class of active matter, living liquid crystals (LLCs) that combine living swimming bacteria with a lyotropic liquid crystal. The physical properties of LLCs can be controlled by the amount of oxygen available to bacteria, by concentration of ingredients, or by temperature. Our studies reveal a wealth of new intriguing dynamic phenomena, caused by the coupling between the activity-triggered flow and long-range orientational order of the medium. Among these are (a) non-linear trajectories of bacterial motion guided by non-uniform director, (b) local melting of the liquid crystal caused by the bacteria-produced shear flows, (c) activity-triggered transition from a non-flowing uniform state into a flowing one-dimensional periodic pattern and its evolution into a turbulent array of topological defects, (d) birefringence-enabled visualization of microflow generated by the nanometers-thick bacterial flagella. Unlike their isotropic counterpart, the LLCs show collective dynamic effects at very low volume fraction of bacteria, on the order of 0.2%. Our work suggests an unorthodox design concept to control and manipulate the dynamic behavior of soft active matter and opens the door for potential biosensing and biomedical applications.Comment: 32 pages, 8 figures, Supporting Information include