Active fluids such as swarming bacteria and motile colloids exhibit exotic properties different from conventional equilibrium materials. As a peculiar example, a spherical tracer immersed inside active fluids shows an enhanced translational diffusion, orders of magnitude stronger than its intrinsic Brownian motion. Here, rather than spherical tracers, we investigate the diffusion of isolated ellipsoids in a quasi-two-dimensional bacterial bath. Our study shows a nonlinear enhancement of both translational and rotational diffusions of ellipsoids. More importantly, we uncover an anomalous coupling between particles' translation and rotation that is strictly prohibited in Brownian diffusion. The coupling reveals a counterintuitive anisotropic particle diffusion, where an ellipsoid diffuses fastest along its minor axis in its body frame. Combining experiments with theoretical modeling, we show that such an anomalous diffusive behavior arises from the generic straining flow of swimming bacteria. Our work illustrates an unexpected feature of active fluids and deepens our understanding of transport processes in microbiological systems.

Colloidal suspensions self-assemble into equilibrium structures ranging from face-and body-centered cubic crystals to binary ionic crystals, and even kagome lattices. When driven out of equilibrium by hydrodynamic interactions, even more diverse structures can be accessed. However, mechanisms underlying out-of-equilibrium assembly are much less understood, though such processes are clearly relevant in many natural and industrial systems. Even in the simple case of hard-sphere colloidal particles under shear, there are conflicting predictions about whether particles link up into string-like structures along the shear flow direction. Here, using confocal microscopy, we measure the shear-induced suspension structure. Surprisingly, rather than flow-aligned strings, we observe log-rolling strings of particles normal to the plane of shear. By employing Stokesian dynamics simulations, we address the mechanism leading to this out-of-equilibrium structure and show that it emerges from a delicate balance between hydrodynamic and interparticle interactions. These results demonstrate a method for assembling large-scale particle structures using shear flows.colloids | shear-induced structure T he study of ordered structures and symmetries of equilibrium phases in condensed matter is central to our understanding of its various material properties (1). Once driven out of equilibrium, a material can exhibit richer phases with many unexpected structures (2-6). However, the dynamics of these nonequilibrium phases are much less understood relative to their equilibrium counterparts (7-9). Sheared hard-sphere colloidal suspensions provide a simple system for probing many out-of-equilibrium structures. For example, sliding layers in sheared colloidal crystals (10, 11), shear-induced crystallization of colloidal fluids (5), and formation of shear transformation zones-localized regimes of shear-induced structural rearrangement-in colloidal glasses (12) have been observed in such systems.The simplest and lowest ordered structure predicted to arise from sheared hard-sphere colloidal suspensions is a one-dimensional (1D) string structure, where particles link into strings under shear. This structure has received much attention since it was first found in a numerical simulation (13). Despite intensive study (13-23), however, there is still heated debate over whether such a structure exists in experiments (10,23,24) or is an artifact of certain numerical algorithms (20-23). In part, this controversy persists due to the lack of experimental techniques that provide direct visualization of the suspension structure. Since the string structure is predicted to exist in less concentrated suspensions below the crystallization threshold, previous scattering experiments, which average over large sample volumes, have not provided detailed information to unambiguously confirm or disprove the existence of this less regular structure (10,24). Here, by employing fast confocal microscopy, we show that sheared hardsphere colloidal suspensions form strings a...

Bacterial suspensions-a premier example of active fluids-show an unusual response to shear stresses. Instead of increasing the viscosity of the suspending fluid, the emergent collective motions of swimming bacteria can turn a suspension into a superfluid with zero apparent viscosity. Although the existence of active superfluids has been demonstrated in bulk rheological measurements, the microscopic origin and dynamics of such an exotic phase have not been experimentally probed. Here, using high-speed confocal rheometry, we study the dynamics of concentrated bacterial suspensions under simple planar shear. We find that bacterial superfluids under shear exhibit unusual symmetric shear bands, defying the conventional wisdom on shear banding of complex fluids, where the formation of steady shear bands necessarily breaks the symmetry of unsheared samples. We propose a simple hydrodynamic model based on the local stress balance and the ergodic sampling of nonequilibrium shear configurations, which quantitatively describes the observed symmetric shear-banding structure. The model also successfully predicts various interesting features of swarming vortices in stationary bacterial suspensions. Our study provides insights into the physical properties of collective swarming in active fluids and illustrates their profound influences on transport processes.

Colloidal suspensions exhibit shear thinning and shear thickening. The most common interpretation of these phenomena identifies layering of the fluid perpendicular to the shear gradient as the driver for the observed behavior. However, studies of the particle configurations associated with shear thinning and thickening cast doubt on that conclusion and leave unsettled whether these nonequilibrium phenomena are caused primarily by correlated particle motions or by changes in particle packing structure. We report the results of Stokesian dynamics simulations of suspensions of hard spheres that illuminate the relation among the suspension viscosity, shear rate, and particle configuration. Using a recently introduced sampling technique for nonequilibrium systems, we show that shear thinning can be decoupled from layering, thereby eliminating layering as the driver for shear thinning. In contrast, we find that there is a strong correlation between shear thinning and a two-particle measure of the shear stress. Our results are consistent with a recent experimental study.colloids | rare-event sampling | Nonequilibrium Umbrella Sampling | rheology F lowing colloidal suspensions exhibit many nonlinear response phenomena, prominent among which are shear thinning and thickening, i.e., the decrease and increase, respectively, of the suspension viscosity with increasing shear strength (1-5). In the last four decades, many experimental studies and computer simulations have attempted to identify changes in particle configurations responsible for these phenomena (6-16), but no conclusive signature has yet been identified. In early experimental studies of dense colloidal suspensions of hard spheres (8, 17) (solid or glass at equilibrium) and of dilute charge-stabilized suspensions that crystallize at equilibrium (18, 19), sliding layers were observed when the systems were subjected to shear flow. This layering structure, characterized by a nonuniform density distribution along the shear gradient direction, was independently predicted to occur by nonequilibrium molecular dynamics (NEMD) simulations without hydrodynamic interactions (20,21). The organization of the suspension into layers was thought to explain the shear-thinning behavior observed, but later experimental studies of less dense systems (with liquid-like order at equilibrium) using light scattering (22, 23) or small-angle neutron scattering (24-27) yielded mixed signals relevant to the existence of this layering structure in the shear-thinning regime, thereby implying that layering might not be necessary for shear thinning. This implication is supported by Stokesian dynamics simulations (28, 29), which differ from earlier NEMD simulations by incorporating hydrodynamic interactions between the particles. The Stokesian dynamics simulations find no shear-induced layering of hard spheres in the shear-thinning regime in bulk systems that are liquid-like at equilibrium (9, 30).It is certainly the case that diffraction from an assembly of particles is sensitive to ordering an...