104 research outputs found
Anisotropic thermophoresis
Colloidal migration in temperature gradient is referred to as thermophoresis.
In contrast to particles with spherical shape, we show that elongated colloids
may have a thermophoretic response that varies with the colloid orientation.
Remarkably, this can translate into a non-vanishing thermophoretic force in the
direction perpendicular to the temperature gradient. Oppositely to the friction
force, the thermophoretic force of a rod oriented with the temperature gradient
can be larger or smaller than when oriented perpendicular to it. The precise
anisotropic thermophoretic behavior clearly depends on the colloidal rod aspect
ratio, and also on its surface details, which provides an interesting
tunability to the devices constructed based on this principle. By means of
mesoscale hydrodynamic simulations, we characterize this effect for different
types of rod-like colloids.Comment: 8 pages, 10 figure
Hydrodynamic simulations of self-phoretic microswimmers
A mesoscopic hydrodynamic model to simulate synthetic self-propelled Janus
particles which is thermophoretically or diffusiophoretically driven is here
developed. We first propose a model for a passive colloidal sphere which
reproduces the correct rotational dynamics together with strong phoretic
effect. This colloid solution model employs a multiparticle collision dynamics
description of the solvent, and combines potential interactions with the
solvent, with stick boundary conditions. Asymmetric and specific colloidal
surface is introduced to produce the properties of self-phoretic Janus
particles. A comparative study of Janus and microdimer phoretic swimmers is
performed in terms of their swimming velocities and induced flow behavior.
Self-phoretic microdimers display long range hydrodynamic interactions and can
be characterized as pullers or pushers. In contrast, Janus particles are
characterized by short range hydrodynamic interactions and behave as neutral
swimmers. Our model nicely mimics those recent experimental realization of the
self-phoretic Janus particles.Comment: 11pages, 12figures, 2table
Modelling the Mechanics and Hydrodynamics of Swimming E. coli
The swimming properties of an E. coli-type model bacterium are investigated
by mesoscale hy- drodynamic simulations, combining molecular dynamics
simulations of the bacterium with the multiparticle particle collision dynamics
method for the embedding fluid. The bacterium is com- posed of a
spherocylindrical body with attached helical flagella, built up from discrete
particles for an efficient coupling with the fluid. We measure the hydrodynamic
friction coefficients of the bacterium and find quantitative agreement with
experimental results of swimming E. coli. The flow field of the bacterium shows
a force-dipole-like pattern in the swimming plane and two vor- tices
perpendicular to its swimming direction arising from counterrotation of the
cell body and the flagella. By comparison with the flow field of a force dipole
and rotlet dipole, we extract the force- dipole and rotlet-dipole strengths for
the bacterium and find that counterrotation of the cell body and the flagella
is essential for describing the near-field hydrodynamics of the bacterium
Microfluidic pump driven by anisotropic phoresis
Fluid flow along microchannels can be induced by keeping opposite walls at
different temperatures, and placing elongated tilted pillars inside the
channel. The driving force for this fluid motion arises from the anisotropic
thermophoretic effect of the elongated pillars that generates a force parallel
to the walls, and perpendicular to the temperature gradient. The force is not
determined by the thermophilic or thermophobic character of the obstacle
surface, but by the geometry and the thermophoretic anisotropy of the obstacle.
Via mesoscale hydrodynamic simulations, we investigate the pumping properties
of the device as a function of the channel geometry, and pillar surface
properties. Applications as fluidic mixers, and fluid alternators are also
outlined, together with the potential use of all these devices to harvest waste
heat energy. Furthermore, similar devices can be also built employing
diffusiophoresis or electrophoresis.Comment: 12 pages, 14 figure
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