Living liquid crystals

Abstract

Collective motion of self-propelled organisms or synthetic particles, often termed "active fluid," has attracted enormous attention in the broad scientific community because of its fundamentally nonequilibrium nature. Energy input and interactions among the moving units and the medium lead to complex dynamics. Here, we introduce a class of active matter--living liquid crystals (LLCs)--that combines 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 intriguing dynamic phenomena, caused by the coupling between the activity-triggered flow and long-range orientational order of the medium. Among these are (i) nonlinear trajectories of bacterial motion guided by nonuniform director, (ii) local melting of the liquid crystal caused by the bacteria-produced shear flows, (iii) activity-triggered transition from a nonflowing uniform state into a flowing one-dimensional periodic pattern and its evolution into a turbulent array of topological defects, and (iv) birefringenceenabled visualization of microflow generated by the nanometersthick 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.motile bacteria | self-organization | cromonic liquid crystals A ctive matter has recently emerged as an important physical model of living systems that can be described by the methods of nonequilibrium statistical mechanics and hydrodynamics (1-3). Active matter is driven by the internal sources of energy, associated with the self-propelled particles such as bacteria or synthetic swimmers. The interaction of these active particles among themselves and with the medium produces a rich variety of dynamic effects and patterns. Most of the studies deal with active particles embedded into a Newtonian isotropic fluid. In this case the interactions among particles are caused by longrange hydrodynamic and short-range excluded volume effects (4-13). In this work, we conceive a general class of active fluids, termed living liquid crystals (LLCs). The suspending medium is a nontoxic liquid crystal (LC) that supports the activity of selfpropelled particles, namely bacteria. At the very same time, the medium imposes long-range anisotropic interactions onto bacteria, thanks to the intrinsic orientational order that persists even when the bacteria are not active. The importance of this system is twofold. Firstly, the bacterial activity modifies the orientational order of the system, by producing well-defined and reproducible patterns with or without topological defects. Secondly, the orientational order of the suspending medium reveals facets of bacterial behavior, allowing one t...

Geometry of thresholdless active flow in nematic microfluidics

“…Many experimental realizations of active nematics, such as the living liquid crystals [13], are multicomponent systems. This leads to several differences between their hydrodynamic theories and that embodied by Eqs.…”

“…This may be of practical importance, since controlling boundaries and boundary conditions for liquid crystals is a highly developed technology that has long been used for the construction of liquid crystal displays. Efforts are under way to extend such control to the active regime [13,45,46]. Consider nonplanar alignment of the director to the walls of a three-dimensional channel with torsional symmetry [by which we mean equivalently that the sample is bounded by a surface of revolution about the z axis as shown in Fig.…”

“…These active components can be subcellular (such as microtubules powered by molecular motors, and actomyosin networks [2,3]), synthetic (e.g., self-propelled colloids [4] or interacting microrobots), or, alternatively, living organisms [5][6][7][8], such as birds, fish [9], microorganisms [10,11], or insects [12]. Hybrid systems composed of motile rod-shaped bacteria placed in nontoxic liquid crystals have also been realized recently [13]. All of these systems blur the line between the living and synthetic world, thereby opening up unprecedented opportunities for the design of novel smart materials and technology.…”

“…Moreover, these solutions provide qualitative insights into the nature of the flow even in systems in which the frozen director approximation is not quantitatively accurate. Our ideas can also be applied with some modifications to the recently discovered living liquid crystals [13]. These systems are a mixture of two components: living bacteria, which provide the activity, and a background medium composed of nematically ordered nonactive molecules.…”

Viscoelastic and elastomeric active matter: Linear instability and nonlinear dynamics

“…This suggests that, in some instances, the choice of boundary conditions can have an important effect on the system's behavior. Indeed, recent experiments demonstrate that living liquid crystals, in which conventional liquid crystals are made active by the addition of swimming bacteria, can be influenced by anchoring boundary conditions [42].…”