269 research outputs found
Diffusive spreading and mixing of fluid monolayers
The use of ultra-thin, i.e., monolayer films plays an important role for the
emerging field of nano-fluidics. Since the dynamics of such films is governed
by the interplay between substrate-fluid and fluid-fluid interactions, the
transport of matter in nanoscale devices may be eventually efficiently
controlled by substrate engineering. For such films, the dynamics is expected
to be captured by two-dimensional lattice-gas models with interacting
particles. Using a lattice gas model and the non-linear diffusion equation
derived from the microscopic dynamics in the continuum limit, we study two
problems of relevance in the context of nano-fluidics. The first one is the
case in which along the spreading direction of a monolayer a mesoscopic-sized
obstacle is present, with a particular focus on the relaxation of the fluid
density profile upon encountering and passing the obstacle. The second one is
the mixing of two monolayers of different particle species which spread side by
side following the merger of two chemical lanes, here defined as domains of
high affinity for fluid adsorption surrounded by domains of low affinity for
fluid adsorption.Comment: 12 pages, 3 figure
Active colloids at fluid interfaces
If an active Janus particle is trapped at the interface between a liquid and
a fluid, its self-propelled motion along the interface is affected by a net
torque on the particle due to the viscosity contrast between the two adjacent
fluid phases. For a simple model of an active, spherical Janus colloid we
analyze the conditions under which translation occurs along the interface and
we provide estimates of the corresponding persistence length. We show that
under certain conditions the persistence length of such a particle is
significantly larger than the corresponding one in the bulk liquid, which is in
line with the trends observed in recent experimental studies
Collective dynamics of chemically active particles trapped at a fluid interface
Chemically active colloids generate changes in the chemical composition of
their surrounding solution and thereby induce flows in the ambient fluid which
affect their dynamical evolution. Here we study the many-body dynamics of a
monolayer of active particles trapped at a fluid-fluid interface. To this end
we consider a mean-field model which incorporates the direct pair interaction
(including also the capillary interaction which is caused specifically by the
interfacial trapping) as well as the effect of hydrodynamic interactions
(including the Marangoni flow induced by the response of the interface to the
chemical activity). The values of the relevant physical parameters for typical
experimental realizations of such systems are estimated and various scenarios,
which are predicted by our approach for the dynamics of the monolayer, are
discussed. In particular, we show that the chemically-induced Marangoni flow
can prevent the clustering instability driven by the capillary attraction.Comment: 8 pages, 2 figure
Precursor films in wetting phenomena
The spontaneous spreading of non-volatile liquid droplets on solid substrates
poses a classic problem in the context of wetting phenomena. It is well known
that the spreading of a macroscopic droplet is in many cases accompanied by a
thin film of macroscopic lateral extent, the so-called precursor film, which
emanates from the three-phase contact line region and spreads ahead of the
latter with a much higher speed. Such films have been usually associated with
liquid-on-solid systems, but in the last decade similar films have been
reported to occur in solid-on-solid systems. While the situations in which the
thickness of such films is of mesoscopic size are rather well understood, an
intriguing and yet to be fully understood aspect is the spreading of
microscopic, i.e., molecularly thin films. Here we review the available
experimental observations of such films in various liquid-on-solid and
solid-on-solid systems, as well as the corresponding theoretical models and
studies aimed at understanding their formation and spreading dynamics. Recent
developments and perspectives for future research are discussed.Comment: 51 pages, 10 figures; small typos correcte
Self-propulsion of a catalytically active particle near a planar wall: from reflection to sliding and hovering
Micron-sized particles moving through solution in response to self-generated
chemical gradients serve as model systems for studying active matter. Their
far-reaching potential applications will require the particles to sense and
respond to their local environment in a robust manner. The self-generated
hydrodynamic and chemical fields, which induce particle motion, probe and are
modified by that very environment, including confining boundaries. Focusing on
a catalytically active Janus particle as a paradigmatic example, we predict
that near a hard planar wall such a particle exhibits several scenarios of
motion: reflection from the wall, motion at a steady-state orientation and
height above the wall, or motionless, steady "hovering." Concerning the steady
states, the height and the orientation are determined both by the proportion of
catalyst coverage and the interactions of the solutes with the different
"faces" of the particle. Accordingly, we propose that a desired behavior can be
selected by tuning these parameters via a judicious design of the particle
surface chemistry
Effective squirmer models for self-phoretic chemically active spherical colloids
Various aspects of self-motility of chemically active colloids in Newtonian
fluids can be captured by simple models for their chemical activity plus a
phoretic slip hydrodynamic boundary condition on their surface. For particles
of simple shapes (e.g., spheres) -- as employed in many experimental studies --
which move at very low Reynolds numbers in an unbounded fluid, such models of
chemically active particles effectively map onto the well studied so-called
hydrodynamic squirmers [S. Michelin and E. Lauga, J. Fluid Mech. \textbf{747},
572 (2014)]. Accordingly, intuitively appealing analogies of
"pusher/puller/neutral" squirmers arise naturally. Within the framework of
self-diffusiophoresis we illustrate the above mentioned mapping and the
corresponding flows in an unbounded fluid for a number of choices of the
activity function (i.e., the spatial distribution and the type of chemical
reactions across the surface of the particle). We use the central collision of
two active particles as a simple, paradigmatic case for demonstrating that in
the presence of other particles or boundaries the behavior of chemically active
colloids may be \textit{qualitatively} different, even in the far field, from
the one exhibited by the corresponding "effective squirmer", obtained from the
mapping in an unbounded fluid. This emphasizes that understanding the
collective behavior and the dynamics under geometrical confinement of
chemically active particles necessarily requires to explicitly account for the
dependence of the hydrodynamic interactions on the distribution of chemical
species resulting from the activity of the particles.Comment: 26 pages, 11 figure
Self-diffusiophoresis induced by fluid interfaces
The influence of a fluid-fluid interface on self-phoresis of chemically
active, axially symmetric, spherical colloids is analyzed. Distinct from the
studies of self-phoresis for colloids trapped at fluid interfaces or in the
vicinity of hard walls, here we focus on the issue of self-phoresis close to a
fluid-fluid interface. In order to provide physically intuitive results
highlighting the role played by the interface, the analysis is carried out for
the case that the symmetry axis of the colloid is normal to the interface;
moreover, thermal fluctuations are not taken into account. Similarly to what
has been observed near hard walls, we find that such colloids can be set into
motion even if their whole surface is homogeneously active. This is due to the
anisotropy along the direction normal to the interface owing to the
partitioning by diffusion, among the coexisting fluid phases, of the product of
the chemical reaction taking place at the colloid surface. Different from
results corresponding to hard walls, in the case of a fluid interface the
direction of motion, i.e., towards the interface or away from it, can be
controlled by tuning the physical properties of one of the two fluid phases.
This effect is analyzed qualitatively and quantitatively, both by resorting to
a far-field approximation and via an exact, analytical calculation which
provides the means for a critical assessment of the approximate analysis
Confinement effects on diffusiophoretic self-propellers
We study theoretically the effects of spatial confinement on the phoretic
motion of a dissolved particle driven by composition gradients generated by
chemical reactions of its solvent, which are active only on certain parts of
the particle surface. We show that the presence of confining walls increases in
a similar way both the composition gradients and the viscous friction, and the
overall result of these competing effects is an increase in the phoretic
velocity of the particle. For the case of steric repulsion only between the
particle and the product molecules of the chemical reactions, the absolute
value of the velocity remains nonetheless rather small.Comment: 18 pages, 4 figures, J. Chem. Phys. (in print; full bibliographic
info and DOI to be added once available
Self-motility of an active particle induced by correlations in the surrounding solution
Current models of phoretic transport rely on molecular forces creating a
"diffuse" particle-fluid interface. We investigate theoretically an alternative
mechanism, in which a diffuse interface emerges solely due to a non-vanishing
correlation length of the surrounding solution. This mechanism can drive
self-motility of a chemically active particle. Numerical estimates indicate
that the velocity can reach micrometers per second. The predicted phenomenology
includes a bilinear dependence of the velocity on the activity and a possible
double velocity reversal upon varying the correlation length.Comment: 6 pages, 2 figures, and 22 pages of supplemental material. To be
published as Phys. Rev. Let
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