Individual behavior and pairwise interactions between microswimmers in anisotropic liquid

Vortex Pattern Stabilization in Thin Films Resulting from Shear Thickening of Active Suspensions

The need for structuring on micrometer scales is abundant, for example, in view of phononic applications. We here outline a novel approach based on the phenomenon of active turbulence on the mesoscale. As we demonstrate, a shear-thickening carrier fluid of active microswimmers intrinsically stabilizes regular vortex patterns of otherwise turbulent active suspensions. The fluid self-organizes into a periodically structured nonequilibrium state. Introducing additional passive particles of intermediate size leads to regular spatial organization of these objects. Our approach opens a new path toward functionalization through patterning of thin films and membranes.

Taylor's swimming sheet in a smectic-A liquid crystal

We calculate the swimming speed of a Taylor sheet in a smectic-A liquid crystal. Assuming that the amplitude of the wave propagating on the sheet is much smaller than the wave number, we solve the governing equations using the method of series expansion up to the second order in amplitude. We find that the sheet can swim much faster in smectic-A liquid crystals than in Newtonian fluids. The elasticity associated with the layer compressibility is responsible for the enhanced speed. We also calculate the power dissipated in the fluid and the flux of the fluid. The fluid is pumped opposite to the direction of the wave propagation.

Motility of acoustically powered micro-swimmers in a liquid crystalline environment

Suspensions of microswimmers in liquid crystals demonstrate remarkably complex dynamics and serve as a model system for studying active nematics. So far, experimental realization of microswimmers suspended in liquid crystalline media has relied on biological microorganisms that impose strict limitations on the compatible media and makes it difficult to regulate activity. Here, we demonstrate that acoustically powered bubble microswimmers can efficiently self-propel in a lyotropic liquid crystal. The velocity of the swimmers is controlled by the amplitude of the acoustic field. Histograms of swimming directions with respect to the local nematic field reveal a bimodal distribution: the swimmers tend to either fully align with or swim perpendicular to the director field of the liquid crystal, occasionally switching between these two states. The bubble-induced streaming from a swimmer locally melts the liquid crystal and produces topological defects at the tail of the swimmer. We show that the defect proliferation rate increases with the angle between the swimmer's velocity and the local orientation of the director field.

Bacterial active matter

Bacteria are among the oldest and most abundant species on Earth. Bacteria successfully colonize diverse habitats and play a significant role in the oxygen, carbon, and nitrogen cycles. They also form human and animal microbiota and may become sources of pathogens and a cause of many infectious diseases. Suspensions of motile bacteria constitute one of the most studied examples of active matter: a broad class of non-equilibrium systems converting energy from the environment (e.g., chemical energy of the nutrient) into mechanical motion. Concentrated bacterial suspensions, often termed active fluids, exhibit complex collective behavior, such as large-scale turbulent-like motion (so-called bacterial turbulence) and swarming. The activity of bacteria also affects the effective viscosity and diffusivity of the suspension. This work reports on the progress in bacterial active matter from the physics viewpoint. It covers the key experimental results, provides a critical assessment of major theoretical approaches, and addresses the effects of visco-elasticity, liquid crystallinity, and external confinement on collective behavior in bacterial suspensions.

Anchoring-induced nonmonotonic velocity versus temperature dependence of motile bacteria in a lyotropic nematic liquid crystal

The elastic and viscous properties of lyotropic chromonic liquid crystals have a very sharp, often exponential temperature dependence. Self-propelled bacteria swimming in this viscoelastic medium induce director deformations which can strongly influence their velocity, and we study the temperature behavior of their motility in the whole range of the nematic phase. We observe experimentally that, with increasing temperature, while the viscosity drops exponentially and the frequency of the flagellum rotation grows linearly, the swimmers' speed first conventionally increases but then, above some crossover temperature, slows down and at the same time bacteria-induced director distortions become visible. It is shown that the physics behind this temperature-driven effect is in a sharp rise in the ability of the bacterium's flagellum to induce director deformations. As temperature increases, the splay and bend elastic constants sharply decrease and the anchoring extrapolation length of the flagellum surface gets shorter and shorter. At the crossover temperature the resulting effective anchoring effect dominates the fast dropping viscosity and the distortion strengthens. As a result, a fraction of the torque the flagellum applies for the propulsion is spent for the elastic degrees of freedom, which results in a bacterium slowdown. To find the director distortions, the flagellum is presented as a collection of anchoring-induced elastic monopoles, and the bacterium velocity is found from the balance of the energy spent for the propulsion and the viscous drag and nematodynamic dissipation.

Liquid Crystals: Versatile Self-Organized Smart Soft Materials

Smart soft materials are envisioned to be the building blocks of the next generation of advanced devices and digitally augmented technologies. In this context, liquid crystals (LCs) owing to their responsive and adaptive attributes could serve as promising smart soft materials. LCs played a critical role in revolutionizing the information display industry in the 20th century. However, in the turn of the 21st century, numerous beyond-display applications of LCs have been demonstrated, which elegantly exploit their controllable stimuli-responsive and adaptive characteristics. For these applications, new LC materials have been rationally designed and developed. In this Review, we present the recent developments in light driven chiral LCs, i.e., cholesteric and blue phases, LC based smart windows that control the entrance of heat and light from outdoor to the interior of buildings and built environments depending on the weather conditions, LC elastomers for bioinspired, biological, and actuator applications, LC based biosensors for detection of proteins, nucleic acids, and viruses, LC based porous membranes for the separation of ions, molecules, and microbes, living LCs, and LCs under macro- and nanoscopic confinement. The Review concludes with a summary and perspectives on the challenges and opportunities for LCs as smart soft materials. This Review is anticipated to stimulate eclectic ideas toward the implementation of the nature's delicate phase of matter in future generations of smart and augmented devices and beyond.

Polar swimmers induce several phases in active nematics

Swimming bacteria in passive nematics in the form of lyotropic liquid crystals are defined as a new class of active matter known as living liquid crystals in recent studies. It has also been shown that liquid crystal solutions are promising candidates for trapping and detecting bacteria. We ask the question, can a similar class of matter be designed for background nematics which are also active? Hence, we developed a minimal model for the mixture of polar particles in active nematics. It is found that the active nematics in such a mixture are highly sensitive to the presence of polar particles and show the formation of large scale higher order structures for a relatively low polar particle density. Upon increasing the density of polar particles, different phases of active nematics are found and it is observed that the system shows two phase transitions. The first phase transition is a first order transition from quasi-long-ranged ordered active nematics to disordered active nematics with larger scale structures. On further increasing density of polar particles, the system transitions to a third phase, where polar particles form large, mutually aligned clusters. These clusters sweep the whole system and enforce local order in the nematics. The current study can be helpful for detecting the presence of very low densities of polar swimmers in active nematics and can be used to design and control different structures in active nematics.

Design of nematic liquid crystals to control microscale dynamics

The 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 the transformation of stored or environmental energy into systematic motion, with applications in micro-robotics, transport of matter, guided morphogenesis. This review presents an approach to command microscale dynamics by replacing an isotropic medium with a liquid crystal. Orientational order and associated properties, such as elasticity, surface anchoring, and bulk anisotropy, enable new dynamic effects, ranging from the appearance and propagation of particle-like solitary waves to self-locomotion of an active droplet. By using photoalignment, the liquid crystal can be patterned into predesigned structures. In the presence of the electric field, these patterns enable the transport of solid and fluid particles through nonlinear electrokinetics rooted in anisotropy of conductivity and permittivity. Director patterns command the dynamics of swimming bacteria, guiding their trajectories, polarity of swimming, and distribution in space. This guidance is of a higher level of complexity than a simple following of the director by rod-like microorganisms. Namely, the director gradients mediate hydrodynamic interactions of bacteria to produce an active force and collective polar modes of swimming. The patterned director could also be engraved in a liquid crystal elastomer. When an elastomer coating is activated by heat or light, these patterns produce a deterministic surface topography. The director gradients define an activation force that shapes the elastomer in a manner similar to the active stresses triggering flows in active nematics. The patterned elastomer substrates could be used to define the orientation of cells in living tissues. The liquid-crystal guidance holds a major promise in achieving the goal of commanding microscale active flows.

Multiparticle collision dynamics simulations of a squirmer in a nematic fluid

We study the dynamics of a squirmer in a nematic liquid crystal using the multiparticle collision dynamics (MPCD) method. A recently developed nematic MPCD method [Phys. Rev. E 99, 063319 (2019)] which employs a tensor order parameter to describe the spatial and temporal variations of the nematic order is used to simulate the suspending anisotropic fluid. Considering both nematodynamic effects (anisotropic viscosity and elasticity) and thermal fluctuations, in the present study, we couple the nematic MPCD algorithm with a molecular dynamics (MD) scheme for the squirmer. A unique feature of the proposed method is that the nematic order, the fluid, and the squirmer are all represented in a particle-based framework. To test the applicability of this nematic MPCD-MD method, we simulate the dynamics of a spherical squirmer with homeotropic surface anchoring conditions in a bulk domain. The importance of anisotropic viscosity and elasticity on the squirmer's speed and orientation is studied for different values of self-propulsion strength and squirmer type (pusher, puller or neutral). In sharp contrast to Newtonian fluids, the speed of the squirmer in a nematic fluid depends on the squirmer type. Interestingly, the speed of a strong pusher is smaller in the nematic fluid than for the Newtonian case. The orientational dynamics of the squirmer in the nematic fluid also shows a non-trivial dependence on the squirmer type. Our results compare well with existing experimental and numerical data. The full particle-based framework could be easily extended to model the dynamics of multiple squirmers in anisotropic fluids.

Multiparticle collision dynamics for tensorial nematodynamics

Liquid crystals establish a nearly unique combination of thermodynamic, hydrodynamic, and topological behavior. This poses a challenge to their theoretical understanding and modeling. The arena where these effects come together is the mesoscopic (micron) scale. It is then important to develop models aimed at capturing this variety of dynamics. We have generalized the particle-based multiparticle collision dynamics (MPCD) method to model the dynamics of nematic liquid crystals. Following the Qian-Sheng theory [Phys. Rev. E 58, 7475 (1998)1063-651X10.1103/PhysRevE.58.7475] of nematics, the spatial and temporal variations of the nematic director field and order parameter are described by a tensor order parameter. The key idea is to assign tensorial degrees of freedom to each MPCD particle, whose mesoscopic average is the tensor order parameter. This nematic MPCD method includes backflow effect, velocity-orientation coupling, and thermal fluctuations. We validate the applicability of this method by testing (i) the nematic-isotropic phase transition, (ii) the flow alignment of the director in shear and Poiseuille flows, and (iii) the annihilation dynamics of a pair of line defects. We find excellent agreement with existing literature. We also investigate the flow field around a force dipole in a nematic liquid crystal, which represents the leading-order flow field around a force-free microswimmer. The anisotropy of the medium not only affects the magnitude of velocity field around the force dipole, but can also induce hydrodynamic torques depending on the orientation of dipole axis relative to director field. A force dipole experiences a hydrodynamic torque when the dipole axis is tilted with respect to the far-field director. The direction of hydrodynamic torque is such that the pusher- (or puller-) type force dipole tends to orient along (or perpendicular to) the director field. Our nematic MPCD method can have far-reaching implications not only in modeling of nematic flows, but also to study the motion of colloids and microswimmers immersed in an anisotropic medium.

Advances in Biological Liquid Crystals

Biological liquid crystals, a rich set of soft materials with rod-like structures widely existing in nature, possess typical lyotropic liquid crystalline phase properties both in vitro (e.g., cellulose, peptides, and protein assemblies) and in vivo (e.g., cellular lipid membrane, packed DNA in bacteria, and aligned fibroblasts). Given the ability to undergo phase transition in response to various stimuli, numerous practices are exercised to spatially arrange biological liquid crystals. Here, a fundamental understanding of interactions between rod-shaped biological building blocks and their orientational ordering across multiple length scales is addressed. Discussions are made with regard to the dependence of physical properties of nonmotile objects on the first-order phase transition and the coexistence of multi-phases in passive liquid crystalline systems. This work also focuses on how the applied physical stimuli drives the reorganization of constituent passive particles for a new steady-state alignment. A number of recent progresses in the dynamics behaviors of active liquid crystals are presented, and particular attention is given to those self-propelled animate elements, like the formation of motile topological defects, active turbulence, correlation of orientational ordering, and cellular functions. Finally, future implications and potential applications of the biological liquid crystalline materials are discussed.

Harnessing Medium Anisotropy To Control Active Matter

Active matter is a wide class of nonequilibrium systems consisting of interacting self-propelled agents transducing the energy stored in the environment into mechanical motion. Numerous examples range from microscopic cytoskeletal filaments and swimming organisms (bacteria and unicellular algae), synthetic catalytic nanomotors, colloidal self-propelled Janus particles, to macroscopic bird flocks, fish schools, and even human crowds. Active matter demonstrates a remarkable tendency toward self-organization and development of collective states with the long-range spatial order. Furthermore, active materials exhibit properties that are not present in traditional materials like plastics or ceramics: self-repair, shape change, and adaptation. A suspension of microscopic swimmers, such as motile bacteria or self-propelled colloids (active suspensions), is possibly the simplest and the most explored realization of active matter. Recent studies of active suspensions revealed a wealth of unexpected behaviors, from a dramatic reduction of the effective viscosity, enhanced mixing and self-diffusion, rectification of chaotic motion, to artificial rheotaxis (drift against the imposed flow) and cross-stream migration. To date, most of the studies of active matter are performed in isotropic suspending medium, like water with the addition of some "fuel", e.g., nutrient for bacteria or H₂O₂ for catalytic bimetallic AuPt nanorods. A highly structured anisotropic suspending medium represented by lyotropic liquid crystal (water-soluble) opens enormous opportunities to control and manipulate active matter. Liquid crystals exhibit properties intermediate between solid and liquids; they may flow like a liquid but respond to deformations as a solid due to a crystal-like orientation of molecules. Liquid crystals doped by a small amount of active component represent a new class of composite materials (living liquid crystals or LLCs) with unusual mechanical and optical properties. LLCs demonstrate a variety of highly organized dynamic collective states, spontaneous formation of dynamic textures of topological defects (singularities of local molecular orientation), controlled and reconfigurable transport of cargo particles, manipulation of individual trajectories of microswimmers, and many others. Besides insights into fundamental mechanisms governing active materials, living liquid crystals may have intriguing applications, such as the design of new classes of soft adaptive bioinspired materials capable to respond to physical and chemical stimuli, such as light, magnetic, and electric fields, mechanical shear, airborne pollutants, and bacterial toxins. This Account details the most recent developments in the field of LLCs and discusses how the anisotropy of liquid crystals can be harnessed to control and manipulate active materials.

Order parameters and time evolution of mesophases in the lyotropic chromonic liquid crystal Sunset Yellow FCF by DNMR

Uniaxial order parameters of the nematic and columnar mesophases in the lyotropic chromonic liquid crystal Sunset Yellow FCF have been determined from deuteron nuclear magnetic resonance, where random confinement of the system by the dispersion of aerosil nanoparticles has been performed to help obtain the angular dependent spectra. The long-time evolution study of the order parameters shows that the system requires tens of hours to stabilize after a deep change in temperature, in contrast with the very fast assembly process of the aggregates. Finally, the degree of order of the water molecules, forced by the uniaxial environment, has been determined.

Cromoglycate mesogen forms isodesmic assemblies promoted by peptides and induces aggregation of a range of proteins

Disodium cromoglycate (5′DSCG) belongs to a class of nonamphiphilic molecules that form nematic chromonic liquid crystals in aqueous solutions. As the concentration increases, it is believed that the molecules first form isodesmic assemblies in water, which further align to form liquid crystal phases. However, the reports on isodesmic assemblies of 5′DSCG have been scarce. Herein, we show that the presence of peptides can promote the isodesmic assembly of 5′DSCG over a broad range of concentrations before reaching the liquid crystal phase. The presence of peptides can lower the 5′DSCG concentration in the aqueous solution to ∼1.5 wt% (from 11–12 wt%, forming a nematic liquid crystal phase) for isodesmic assembly formation. This result indicates a demixing between 5′DSCG and peptides in aqueous solution. We further explored this demixing mechanism to precipitate a wide range of proteins, namely, lectin A, esterase, lipase, bovine serum albumin, trypsin, and a pilin protein from bacterium Pseudomonas aeruginosa. We found that 5′DSCG caused the aggregation of all these proteins except trypsin. These results, along with past findings, suggest that 5′DSCG isodesmic assemblies have the potential to assist in protein purification and crystallization.

5′DSCG molecules form isodesmic assembly in the presence of peptides, and cause a wide range of proteins to aggregate.

Lane formation in a driven attractive fluid

We investigate nonequilibrium lane formation in a generic model of a fluid with attractive interactions, that is, a two-dimensional Lennard-Jones fluid composed of two particle species driven in opposite directions. Performing Brownian dynamics simulations for a wide range of parameters, supplemented by a stability analysis based on dynamical density functional theory, we identify generic features of lane formation in the presence of attraction, including structural properties. In fact, we find a variety of states (as compared to purely repulsive systems), as well as a close relation between laning and long-wavelength instabilities of the homogeneous phase such as demixing and condensation.

Rapid expulsion of microswimmers by a vortical flow

Interactions of microswimmers with their fluid environment are exceptionally complex. Macroscopic shear flow alters swimming trajectories in a highly nontrivial way and results in dramatic reduction of viscosity and heterogeneous bacterial distributions. Here we report on experimental and theoretical studies of rapid expulsion of microswimmers, such as motile bacteria, by a vortical flow created by a rotating microparticle. We observe a formation of a macroscopic depletion area in a high-shear region, in the vicinity of a microparticle. The rapid migration of bacteria from the shear-rich area is caused by a vortical structure of the flow rather than intrinsic random fluctuations of bacteria orientations, in stark contrast to planar shear flow. Our mathematical model reveals that expulsion is a combined effect of motility and alignment by a vortical flow. Our findings offer a novel approach for manipulation of motile microorganisms and shed light on bacteria–flow interactions.

The control of microswimmers such as bacteria is important for emerging applications of active bioinspired materials. Here, the authors demonstrate the use of vortical shear to expel suspended motile bacteria from the vicinity of a rotating microparticle.

Design of Responsive and Active (Soft) Materials Using Liquid Crystals

Liquid crystals (LCs) are widely known for their use in liquid crystal displays (LCDs). Indeed, LCDs represent one of the most successful technologies developed to date using a responsive soft material: An electric field is used to induce a change in ordering of the LC and thus a change in optical appearance. Over the past decade, however, research has revealed the fundamental underpinnings of potentially far broader and more pervasive uses of LCs for the design of responsive soft material systems. These systems involve a delicate interplay of the effects of surface-induced ordering, elastic strain of LCs, and formation of topological defects and are characterized by a chemical complexity and diversity of nano- and micrometer-scale geometry that goes well beyond that previously investigated. As a reflection of this evolution, the community investigating LC-based materials now relies heavily on concepts from colloid and interface science. In this context, this review describes recent advances in colloidal and interfacial phenomena involving LCs that are enabling the design of new classes of soft matter that respond to stimuli as broad as light, airborne pollutants, bacterial toxins in water, mechanical interactions with living cells, molecular chirality, and more. Ongoing efforts hint also that the collective properties of LCs (e.g., LC-dispersed colloids) will, over the coming decade, yield exciting new classes of driven or active soft material systems in which organization (and useful properties) emerges during the dissipation of energy.

Microscale locomotion in a nematic liquid crystal

Microorganisms often encounter anisotropy, for example in mucus and biofilms. We study how anisotropy and elasticity of the ambient fluid affects the speed of a swimming microorganism with a prescribed stroke. Motivated by recent experiments on swimming bacteria in anisotropic environments, we extend a classical model for swimming microorganisms, the Taylor swimming sheet, actuated by small-amplitude traveling waves in a three-dimensional nematic liquid crystal without twist. We calculate the swimming speed and entrained volumetric flux as a function of the swimmer's stroke properties as well as the elastic and rheological properties of the liquid crystal. These results are then compared to previous results on an analogous swimmer in a hexatic liquid crystal, indicating large differences in the cases of small Ericksen number and in a nematic fluid when the tumbling parameter is near the transition to a shear-aligning nematic. We also propose a novel method of swimming or pumping in a nematic fluid by passing a traveling wave of director oscillation along a rigid wall.

Bacterial transport of colloids in liquid crystalline environments

We describe the controlled transport and delivery of non-motile eukaryotic cells and polymer microparticles by swimming bacteria suspended in nematic liquid crystals. The bacteria push reversibly attached cargo in a stable, unidirectional path (or along a complex patterned director field) over exceptionally long distances. Numerical simulations and analytical predictions for swimming speeds provide a mechanistic insight into the hydrodynamics of the system. This study lays the foundation for using cargo-carrying bacteria in engineering applications and for understanding interspecies interactions in polymicrobial communities.

Effects of confinement, surface-induced orientations and strain on dynamical behaviors of bacteria in thin liquid crystalline films

We report on the organization and dynamics of bacteria (Proteus mirabilis) dispersed within lyotropic liquid crystal (LC) films confined by pairs of surfaces that induce homeotropic (perpendicular) or hybrid (homeotropic and parallel orientations at each surface) anchoring of the LC. By using motile vegetative bacteria (3 µm in length) and homeotropically aligned LC films with thicknesses that exceed the length of the rod-shaped cells, a key finding reported in this paper is that elastic torques generated by the LC are sufficiently large to overcome wall-induced hydrodynamic torques acting on the cells, thus leading to LC-guided bacterial motion near surfaces that orient LCs. This result extends to bacteria within LC films with hybrid anchoring, and leads to the observation that asymmetric strain within a hybrid aligned LC rectifies motions of motile cells. In contrast, when the LC film thickness is sufficiently small that confinement prevents alignment of the bacteria cells along a homeotropically aligned LC director (achieved using swarm cells of length 10-60 µm), the bacterial cells propel in directions orthogonal to the director, generating transient distortions in the LC that have striking "comet-like" optical signatures. In this limit, for hybrid LC films, we find LC elastic stresses deform the bodies of swarm cells into bent configurations that follow the LC director, thus unmasking a coupling between bacterial shape and LC strain. Overall, these results provide new insight into the influence of surface-oriented LCs on dynamical bacterial behaviors and hint at novel ways to manipulate bacteria using confined LC phases that are not possible in isotropic solutions.