Session E61: Active Matter II

Defect loops in 3D active nematics

In 2D active nematics, internally driven chaotic flows are characterized by the continual production, motion, and annihilation of point defect pairs. We investigate the behavior of active nematics in 3D, for which we have developed an experimental model system of microtubules and molecular motors, as well as numerical modeling approaches. The defects characterizing chaotic flow are here curvilinear rather than point-like. We present a theoretical model predicting a certain class of closed disclination loops to be the system’s generic singularities. Through detailed analysis of experimental and numerically generated configurations, we show how our predictions of defect topology, geometry, and dynamics provide important insights into this highly complex 3D system.   

Synchronization and pattern formation in chiral active matter

The emergence of synchronized states in populations of mobile entities is widely observed in different contexts: from animal groups - like flocks of birds moving coherently or crowds of people walking cooperatively on the London's Millenium bridge - to synthetic colloidal systems. However, previous studies on synchronization have focused on, either immobile oscillators, or mobile oscillators which phase does not directly influence the way they move in space [1]. Here we focus on active oscillators - circle swimmers which rotate with an intrinsic frequency - and show that self-propulsion, induces a qualitatively new and generic synchronization scenario which generates two novel phases: (i) a phase where oscillators move collectively along a given direction; a phase showing long-range order in 2D (akin to the celebrated Toner-Tu flocking phase [2]). (ii) a second phase where particles of opposite chirality segregate into rotating clusters. Both phases feature activity-induced synchronization in 2D, which is not achievable for immobile oscillators in low dimensional systems [3].
[1] D Levis, I Pagonabarraga, A Díaz-Guilera, Phys. Rev. X 7 , 011028, 2017.
[2] J Toner and Y Tu, Phys. Rev. Lett. 75, 4326, 1995.
[3] D Levis, I Pagonabarraga, B Liebchen, arXiv:1802.02371, 2018.   

Active mixing, manifolds and barriers in imposed, laminar flows

We present experiments and simulations on the motion of self-propelled tracers in imposed laminar fluid flows. The flows used are hyperbolic and vortex-dominated flows, generated in microfluidic (PDMS) cells and in laboratory-scale, magnetohydrodynamically-driven systems. The tracers are either brine shrimp (for large-scale flows) or bacillus subtilis (for microfluidic flows). Two types of bacillus subtilis are studied: a wild-type -- characterized by run-and-tumble trajectories in the absence of a flow -- and a mutated "smooth swimmer" strain in which the tumbling is suppressed. We analyze the results in conjunction with a theory that predicts the existence of "Swimming Invariant Manifolds" (SwIMs) that act as one-way barriers that impede the trajectories of self-propelled tracers. We explore how the shape and location of the SwIMs vary with the imposed flow, along with the different ways in which the swimming behavior of the organism affect these SwIMs.   

Stability of interfaces in active fluids

We study the linear stability of an active nematic fluid at rest in its isotropic phase in the following geometries: (1) a membrane immersed in the fluid, (2) a fluid film of finite height, (3) a spherical droplet of fluid, and (4) a cylindrical thread of fluid. In all four cases, we observe two frequency modes due to the coupling between the dynamics of the interface of the fluid and the dynamics of the nematic molecules. Propagating waves are seen above a value of activity which is independent of surface tension and has the same value in all four cases. For the first three cases, the fluid becomes unstable as the activity is further increased. In cases (1) and (2) the critical activity for instability is the same as for an unconfined active fluid and independent of surface tension. For case (3), the critical activity is larger than that for an unconfined fluid, and increases with increasing surface tension. A cylindrical thread of radius R is always unstable against harmonic perturbations of wavenumber k if kR<1, but the growth rate can be controlled by varying the activity. Perturbations with k>1/R become unstable above a critical activity which changes with k and surface tension.   

1D condensation and onset to collective motion of swimming droplets

We observe that swimming droplets confined in a 1D channel spontaneously develop clustering and collective motion. A careful examination of the individual and interaction dynamics suggest that it can be described by effective inelastic collisions followed by a relaxation to the nominal velocity prescribed by activity. Starting from these experimentally observed features, and inspired by paradigmatic lattice models of interacting particles, we develop a theoretical framework in which alignment rules emerge from the microscopic interactions between the particles. This model reveals a rich phase diagram, in which the onset to collective motion results from the competition between inelasticity and activity, and can be preceded by very long-lived but transient macroscopic clustered states. We provide quantitative arguments that account for the formation of clusters in the system, and for the emergence of collective motion.   

Odd viscosity in a chiral active fluid

We spin a system of colloidal magnets in an external magnetic field, forming a cohesive material that behaves like a liquid. Along a boundary of the fluid, we observe lively dynamics, including unidirectional surface waves that propagate due to an interplay of viscous stresses and surface tension. Through broken time-reversal and parity symmetries, this system allows the emergence of an anomalous transport coefficient known as odd (or Hall) viscosity. Unlike ordinary viscosity, this coefficient is dissipationless and absent in simple fluids. By reducing substrate drag, we are able to experimentally observe odd viscosity through the decay of free surface waves.   

Universal scaling in defect-free active turbulence

Active fluids exhibit turbulent flows at low Reynolds numbers. In active liquid crystals, these flows are strongly constrained by topological defects, whose density defines a characteristic length that prevents the scale invariance typical of turbulence. Here we show that, at zero Reynolds number, defect-free active nematics exhibit a new type of turbulence with a distinctive scaling regime at large length scales. The system self-organizes into a disordered spatiotemporal pattern of orientation domains with a characteristic wavelength selected by the nonlinear dynamics, at which the active energy injection is maximal. In contrast to inertial turbulence, the energy is entirely dissipated at the scale where it is injected, without energy transfer to other scales. Nevertheless, arbitrarily large flow vortices are generated by the instantaneous, long-range kernel of Stokes hydrodynamics. Hence, instead of the Kolmogorov ∼q^-5/3 scaling of inertial turbulence, the velocity power spectrum scales as ∼q^-1, with q being the wave number. Thus, in the absence of the screening effects due to topological defects, active nematic fluids exhibit turbulence without energy cascades and with a new universal scaling.   

Polar active suspensions - stability, waves, turbulence

We study the hydrodynamics of self-propelled fluids with polar orientational order, using linear
stability analysis and direct numerical simulations. We show that inertia and system dimension lead
to a threshold for the instability of such systems to spontaneous distortion, with a diffusive growth
rate at small wavenumber, and we confirm numerically the prediction (Simha & Ramaswamy 2002)
of long-wavelength propagating modes. We illustrate the varieties of disturbance growth and
explore the effects of self-propulsion and fluid inertia on active turbulence.   

Shear-induced transitions in passive and active polar liquid crystals: A novel shear-elongation parameter

Polar order in molecular liquid crystals is relatively rare, hence, it was not thoroughly investigated as other liquid crystalline phases. Recently, it has been discovered that suspensions of magnetic platelets in nematic liquid crystals show polar order at room temperature. Furthermore, polar order is ubiquitous in biology. It is seen in cell migration, swimming bacteria, the cytoskeletal etc. The growing interest in active matter and these recent experimental advances call for further theoretical investigations of polar liquid crystals. In the presence of external shear, polar particles tend to align with the shear flow at the Leslie angle, much like nematics. However, unlike the nematic director, the polar order parameter is not of fixed size. In this talk I will introduce the shear elongation parameter. This often neglected parameter, give rise to new physics such as shear-induced first order transition and significantly changes the rheological properties of the fluid. Active fluids can further exhibit yield stress and negative apparent viscosity.   

Active fluid of self-rotating particles

Suspensions of self-propelled particles, such as bacteria, have received considerable attention. Recently there has been increased interest in suspensions of self-rotating particles, such as Quincke rotors in electric fields and ferromagnetic colloids in alternating magnetic fields. While the individual particles are governed by relatively simple dynamics, the interaction of the particles can result in incredibly complex and interesting phenomena. Experiments show phase separation, macroscopic directed motion, and structure formation (e.g. vortices and asters). Modeling these systems as discrete particles at the micro-scale is computational expensive and limits the study of the rotors collective dynamics. We develop a continuum model for such rotor systems based on derivation for dielectric fluids with internal rotation This model allows us to study properties of the fluid and the existance of active turbulence caused by the rotors. To study the effect of confinement, we include phase parameter to restrict the rotors inside a region with a defined diffuse interface. We then can study the interaction between the rotors and the interface for both a fixed and deformable interface.