291 research outputs found
Stirring by Periodic Arrays of Microswimmers
The interaction between swimming microorganisms or artificial self-propelled
colloids and passive (tracer) particles in a fluid leads to enhanced diffusion
of the tracers. This enhancement has attracted strong interest, as it could
lead to new strategies to tackle the difficult problem of mixing on a
microfluidic scale. Most of the theoretical work on this topic has focused on
hydrodynamic interactions between the tracers and swimmers in a bulk fluid.
However, in simulations, periodic boundary conditions (PBCs) are often imposed
on the sample and the fluid. Here, we theoretically analyze the effect of PBCs
on the hydrodynamic interactions between tracer particles and microswimmers. We
formulate an Ewald sum for the leading-order stresslet singularity produced by
a swimmer to probe the effect of PBCs on tracer trajectories. We find that
introducing periodicity into the system has a surprisingly significant effect,
even for relatively small swimmer-tracer separations. We also find that the
bulk limit is only reached for very large system sizes, which are challenging
to simulate with most hydrodynamic solvers.Comment: 11 pages, 4 figure
Electrostatic Interactions between Janus Particles
In this paper we study the electrostatic properties of `Janus' spheres with
unequal charge densities on both hemispheres. We introduce a method to compare
primitive-model Monte Carlo simulations of the ionic double layer with
predictions of (mean-field) nonlinear Poisson-Boltzmann theory. We also derive
practical DLVO-like expressions that describe the Janus-particle pair
interactions by mean-field theory. Using a large set of parameters, we are able
to probe the range of validity of the Poisson-Boltzmann approximation, and thus
of DLVO-like theories, for such particles. For homogeneously charged spheres
this range corresponds well to the range that was predicted by
field-theoretical studies of homogeneously charged flat surfaces. Moreover, we
find similar ranges for colloids with a Janus-type charge distribution. The
techniques and parameters we introduce show promise for future studies of an
even wider class of charged-patterned particles.Comment: 14 pages, 6 figure
Lattice-Boltzmann simulations of microswimmer-tracer interactions
Hydrodynamic interactions in systems composed of self-propelled particles, such as swimming microorganisms and passive tracers, have a significant impact on the tracer dynamics compared to the equivalent "dry" sample. However, such interactions are often difficult to take into account in simulations due to their computational cost. Here, we perform a systematic investigation of swimmer-tracer interaction using an efficient force-counterforce-based lattice-Boltzmann (LB) algorithm [De Graaf et al., J. Chem. Phys. 144, 134106 (2016)JCPSA60021-960610.1063/1.4944962] in order to validate its ability to capture the relevant low-Reynolds-number physics. We show that the LB algorithm reproduces far-field theoretical results well, both in a system with periodic boundary conditions and in a spherical cavity with no-slip walls, for which we derive expressions here. The force-lattice coupling of the LB algorithm leads to a "smearing out" of the flow field, which strongly perturbs the tracer trajectories at close swimmer-tracer separations, and we analyze how this effect can be accurately captured using a simple renormalized hydrodynamic theory. Finally, we show that care must be taken when using LB algorithms to simulate systems of self-propelled particles, since its finite momentum transport time can lead to significant deviations from theoretical predictions based on Stokes flow. These insights should prove relevant to the future study of large-scale microswimmer suspensions using these methods
Structuring Colloidal Gels via Micro-Bubble Oscillations
Locally (re)structuring colloidal gels \unicode{x2013} micron-sized
particles forming a connected network with arrested dynamics \unicode{x2013}
enables precise tuning of the micromechanical and -rheological properties of
the system. A recent experimental study [B. Saint-Michel, G. Petekidis, and V.
Garbin, Soft Matter , 2092 (2022)] showed that rapid
restructuring can occur by acoustically modulating an embedded microbubble.
Here, we perform Brownian dynamics simulations to understand the mechanical
effect of an oscillating microbubble on the structure of the embedding
colloidal gel. Our simulations reveal a hexagonal-close-packed restructuring in
a range that is comparable to the amplitude of the oscillations. However, we
were unable to reproduce the unexpectedly long-ranged modification of the gel
structure \unicode{x2013} dozens of amplitudes \unicode{x2013} observed in
experiment. This suggests including long-ranged effects, such as fluid flow,
should be considered in future work.Comment: 7 pages, 6 figure
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