Vesicle shape transformations driven by confined active filaments

In active matter systems, deformable boundaries provide a mechanism to organize internal active stresses. To study a minimal model of such a system, we perform particle-based simulations of an elastic vesicle containing a collection of polar active filaments. The interplay between the active stress organization due to interparticle interactions and that due to the deformability of the confinement leads to a variety of filament spatiotemporal organizations that have not been observed in bulk systems or under rigid confinement, including highly-aligned rings and caps. In turn, these filament assemblies drive dramatic and tunable transformations of the vesicle shape and its dynamics. We present simple scaling models that reveal the mechanisms underlying these emergent behaviors and yield design principles for engineering active materials with targeted shape dynamics.

Spontaneous Self-propulsion and Nonequillibrium Shape Fluctuations of a Droplet Enclosing Active Particles

Active particles, such as swimming bacteria or self-propelled colloids, spontaneously assemble into large-scale dynamic structures. Geometric boundaries often enforce different spatio-temporal patterns compared to unconfined environment and thus provide a platform to control the behavior of active matter. Here, we report collective dynamics of active particles enclosed by soft, deformable boundaries, that is responsive to the particles' activity. We reveal that a fluid droplet enclosing motile colloids powered by the Quincke effect (Quincke rollers) exhibits strong shape fluctuations, and while the rollers do self-organize into a single vortex, it fills the droplet interior. We demonstrate that the shape fluctuations have a power spectrum consistent with active fluctuations driven by particle-interface collisions, and a broken detailed balance confirms the nonequilibrium nature of the shape dynamics. We further find that the rollers activity coupled to soft boundary fluctuations can result in a spontaneous symmetry breaking and vortex splitting. The droplet acquires motility while the vortex doublet exists. Our findings provide insights into the complex collective behavior of active colloidal suspensions in soft confinement.

A Moving-Least-Square Immersed Boundary Method for Rigid and Deformable Boundaries in Viscous Flow

We present a moving-least-square immersed boundary method for solving viscous incompressible flow involving deformable and rigid boundaries on a uniform Cartesian grid. For rigid boundaries, noslip conditions at the rigid interfaces are enforced using the immersed-boundary direct-forcing method. We propose a reconstruction approach that utilizes moving least squares (MLS) method to reconstruct the velocity at the forcing points in the vicinity of the rigid boundaries. For deformable boundaries, MLS method is employed to construct the interpolation and distribution operators for the immersed boundary points in the vicinity of the rigid boundaries instead of using discrete delta functions. The MLS approach allows us to avoid distributing the Lagrangian forces into the solid domains as well as to avoid using the velocity of points inside the solid domains to compute the velocity of the deformable boundaries. The present numerical technique has been validated by several examples including a Poiseuille flow in a tube, deformations of elastic capsules in shear flow and dynamics of red-blood cell in microfluidic devices.

Numerical Simulation of Underwater Explosion Based on Material Point Method

Underwater explosive load calculation and numerical simulation is the key issues of the design of underwater conventional weapons and protection of ships and submarine. Underwater explosion involves strong nonliner problems and multi-material coupling problems. The method of underwater explosion calculation base on the material point method (MPM) is presented. The MPM takes the advantages of the both Euler and Lagrangian methods and overcomes the shortcomings of them. The problems in the underwater explosive simulation such as large deformation, moving material interfaces and deformable boundaries can be solved effectively by the MPM. At last, blast wave produced by TNT exploding under similar infinite water region is computed. The calculated results are in good agreement with the results of empirical formula of Cole and SPH. The simulation results show that the MPM is an effective tool for underwater explosion calculation.