1,537 research outputs found
Hydrodynamic oscillations and variable swimming speed in squirmers close to repulsive walls
We present a lattice Boltzmann study of the hydrodynamics of a fully resolved
squirmer, radius R, confined in a slab of fluid between two no-slip walls. We
show that the coupling between hydrodynamics and short-range repulsive
interactions between the swimmer and the surface can lead to hydrodynamic
trapping of both pushers and pullers at the wall, and to hydrodynamic
oscillations in the case of a pusher. We further show that a pusher moves
significantly faster when close to a surface than in the bulk, whereas a puller
undergoes a transition between fast motion and a dynamical standstill according
to the range of the repulsive interaction. Our results critically require
near-field hydrodynamics; they further suggest that it should be possible to
control density and speed of squirmers at a surface by tuning the range of
steric and electrostatic swimmer-wall interactions.Comment: 5 + 8 pages, 4 + 4 Figure
Field-Induced Breakup of Emulsion Droplets Stabilized by Colloidal Particles
We simulate the response of a particle-stabilized emulsion droplet in an
external force field, such as gravity, acting equally on all particles. We
show that the field strength required for breakup (at fixed initial area
fraction) decreases markedly with droplet size, because the forces act
cumulatively, not individually, to detach the interfacial particles. The
breakup mode involves the collective destabilization of a solidified particle
raft occupying the lower part of the droplet, leading to a critical force per
particle that scales approximately as .Comment: 4 pages, plus 3 pages of supplementary materia
Colloidal templating at a cholesteric - oil interface: Assembly guided by an array of disclination lines
We simulate colloids (radius m) trapped at the interface between
a cholesteric liquid crystal and an immiscible oil, at which the helical order
(pitch p) in the bulk conflicts with the orientation induced at the interface,
stabilizing an ordered array of disclinations. For weak anchoring strength W of
the director field at the colloidal surface, this creates a template, favoring
particle positions eitheron top of or midway between defect lines, depending on
. For small , optical microscopy experiments confirm this
picture, but for larger no templating is seen. This may stem from the
emergence at moderate W of a rugged energy landscape associated with defect
reconnections.Comment: 5 pages, 4 figure
Bulk rheology and microrheology of active fluids
We simulate macroscopic shear experiments in active nematics and compare them
with microrheology simulations where a spherical probe particle is dragged
through an active fluid. In both cases we define an effective viscosity: in the
case of bulk shear simulations this is the ratio between shear stress and shear
rate, whereas in the microrheology case it involves the ratio between the
friction coefficient and the particle size. We show that this effective
viscosity, rather than being solely a property of the active fluid, is affected
by the way chosen to measure it, and strongly depends on details such as the
anchoring conditions at the probe surface and on both the system size and the
size of the probe particle.Comment: 12 pages, 10 figure
Colloids in active fluids: Anomalous micro-rheology and negative drag
We simulate an experiment in which a colloidal probe is pulled through an
active nematic fluid. We find that the drag on the particle is non-Stokesian
(not proportional to its radius). Strikingly, a large enough particle in
contractile fluid (such as an actomyosin gel) can show negative viscous drag in
steady state: the particle moves in the opposite direction to the externally
applied force. We explain this, and the qualitative trends seen in our
simulations, in terms of the disruption of orientational order around the probe
particle and the resulting modifications to the active stress.Comment: 5 pages, 3 figure
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