Dynamics of inert spheres in active suspensions of micro-rotors.

Inert particles suspended in active fluids of self-propelled particles are known to often exhibit enhanced diffusion and novel coherent structures. Here we numerically investigate the dynamical behavior and self-organization in a system consisting of passive and actively rotating spheres of the same size. The particles interact through direct collisions and the fluid flows generated as they move. In the absence of passive particles, three states emerge in a binary mixture of spinning spheres depending on particle fraction: a dilute gas-like state where the rotors move chaotically, a phase-separated state where like-rotors move in lanes or vortices, and a jammed state where crystals continuously assemble, melt and move (K. Yeo, E. Lushi, and P. M. Vlahovska, Phys. Rev. Lett., 2015, 114, 188301). Passive particles added to the rotor suspension modify the system dynamics and pattern formation: while states identified in the pure active suspension still emerge, they occur at different densities and mixture proportions. The dynamical behavior of the inert particles is also non-trivially dependent on the system composition.

Collective dynamics in a binary mixture of hydrodynamically coupled microrotors.

We study, numerically, the collective dynamics of self-rotating nonaligning particles by considering a monolayer of spheres driven by constant clockwise or counterclockwise torques. We show that hydrodynamic interactions alter the emergence of large-scale dynamical patterns compared to those observed in dry systems. In dilute suspensions, the flow stirred by the rotors induces clustering of opposite-spin rotors, while at higher densities same-spin rotors phase separate. Above a critical rotor density, dynamic hexagonal crystals form. Our findings underscore the importance of inclusion of the many-body, long-range hydrodynamic interactions in predicting the phase behavior of active particles.

Ordering transition of non-Brownian suspensions in confined steady shear flow.

We report on ordering transitions of concentrated non-Brownian suspensions confined by two parallel walls under steady shear. At a volume fraction as low as ϕ=0.48, particles near the wall assemble into strings which are organized as a simple hexagonal array by hydrodynamic interactions. The suspension exhibits a complex phase behavior depending on the ratio of the channel height to the particle radius, Hy/a. In a strongly confined system Hy/a<12, the order state and rheology depend on the commensurability between particle layers and the channel height. At ϕ=0.60 , the order structure in the horizontal plane changes between hexagonal and rectangular structures depending on Hy/a. It is shown that the relative viscosity is a function of both the volume fraction and the ordered state.

Rheology and ordering transitions of non-Brownian suspensions in a confined shear flow: effects of external torques.

We investigate the effect of an external torque, applied in the vorticity direction, to particles in a sheared non-Brownian suspension confined by rigid walls. At volume fractions of ϕ=0.48-0.52 such suspension flows undergo an ordering transition, developing a hexagonal structure of particle strings in the velocity gradient-vorticity plane. The hexagonal structure is disturbed by negative torques, leading to an increase in the shear viscosity. Positive torque has a favorable effect on the ordered state. However, if the magnitude of the positive torque exceeds a certain threshold, the hexagonal order begins to be weakened. Due to the significant changes in suspension microstructures, rheological parameters such as the shear and vortex viscosities exhibit nonlinear responses to the external torques. On the other hand, at lower volume fractions ϕ≤0.40, where ordered structures are not developed, suspension microstructure is not sensitive to an external torque and the apparent viscosity is a linear function of the torque.

Behavior of heavy particles in isotropic turbulence.

The motion of heavy particles in isotropic turbulence is investigated using direct numerical simulation. The statistics related to the velocity and acceleration of heavy particles for a wide range of Stokes numbers, defined as the ratio of the particle response time to the Kolmogorov time scale of turbulence (St=tau_{p}/tau_{eta}) , are investigated. A particular emphasis is placed on the statistics of the fluid experienced by heavy particles, which provide essential information on the dispersion of these particles. The integral time scale of the velocity correlation of fluid seen by the particles T_{f} , which determines the diffusivity of heavy particles, displays a complex behavior different from the Lagrangian integral time scale of fluid T_{L} . A plausible physical explanation for the behavior of the time scale is provided.

Intermittent nature of acceleration in near wall turbulence.

Using direct numerical simulation of a fully developed turbulent channel flow, we investigate the behavior of acceleration near a solid wall. We find that acceleration near the wall is highly intermittent and the intermittency is in large part associated with the near wall organized coherent turbulence structures. We also find that acceleration of large magnitude is mostly directed towards the rotation axis of the coherent vortical structures, indicating that the source of the intermittent acceleration is the rotational motion associated with the vortices that causes centripetal acceleration.