245 research outputs found
Transport of Brownian particles confined to a weakly corrugated channel
We investigate the average velocity of Brownian particles driven by a
constant external force when constrained to move in two-dimensional,
weakly-corrugated channels. We consider both the geometric confinement of the
particles between solid walls as well as the soft confinement induced by a
periodic potential. Using perturbation methods we show that the leading order
correction to the marginal probability distribution of particles in the case of
soft confinement is equal to that obtained in the case of geometric
confinement, provided that the (configuration) integral over the cross-section
of the confining potential is equal to the width of the solid channel. We then
calculate the probability distribution and average velocity in the case of a
sinusoidal variation in the width of the channels. The reduction on the average
velocity is larger in the case of soft channels at small P\'eclet numbers and
for relatively narrow channels and the opposite is true at large P\'eclet
numbers and for wide channels. In the limit of large P\'eclet numbers the
convergence to bulk velocity is faster in the case of soft channels. The
leading order correction to the average velocity and marginal probability
distribution agree well with Brownian Dynamics simulations for the two types of
confinement and over a wide range of P\'eclet numbers
Nanoscale simulations of directional locking
When particles suspended in a fluid are driven through a regular lattice of
cylindrical obstacles, the particle motion is usually not simply in the
direction of the force, and in the high Peclet number limit particle
trajectories tend to lock along certain lattice directions. By means of
molecular dynamics simulations we show that this effect persists in the
presence of molecular diffusion for nanoparticle flows, provided the Peclet
number is not too small. We examine the effects of varying particle and
obstacle size, the method of forcing, solid roughness, and particle
concentration. While we observe trajectory locking in all cases, the degree of
locking varies with particle size and these flows may have application as a
separation technique
Deterministic and stochastic behaviour of non-Brownian spheres in sheared suspensions
The dynamics of macroscopically homogeneous sheared suspensions of neutrally
buoyant, non-Brownian spheres is investigated in the limit of vanishingly small
Reynolds numbers using Stokesian dynamics. We show that the complex dynamics of
sheared suspensions can be characterized as a chaotic motion in phase space and
determine the dependence of the largest Lyapunov exponent on the volume
fraction . The loss of memory at the microscopic level of individual
particles is also shown in terms of the autocorrelation functions for the two
transverse velocity components. Moreover, a negative correlation in the
transverse particle velocities is seen to exist at the lower concentrations, an
effect which we explain on the basis of the dynamics of two isolated spheres
undergoing simple shear. In addition, we calculate the probability distribution
function of the velocity fluctuations and observe, with increasing , a
transition from exponential to Gaussian distributions.
The simulations include a non-hydrodynamic repulsive interaction between the
spheres which qualitatively models the effects of surface roughness and other
irreversible effects, such as residual Brownian displacements, that become
particularly important whenever pairs of spheres are nearly touching. We
investigate the effects of such a non-hydrodynamic interparticle force on the
scaling of the particle tracer diffusion coefficient for very dilute
suspensions, and show that, when this force is very short-ranged, becomes
proportional to as . In contrast, when the range of the
non-hydrodynamic interaction is increased, we observe a crossover in the
dependence of on , from to as .Comment: Submitted to J. Fluid Mec
Hysteresis, force oscillations and non-equilibrium effects in the adhesion of spherical nanoparticles to atomically smooth surfaces
Molecular dynamics simulations are used to examine hysteretic effects and
distinctions between equilibrium and non-equilibrium aspects of particle
adsorption on the walls of nano-sized fluidfilled channels. The force on the
particle and the system's Helmholtz free energy are found to depend on the
particle's history as well as on its radial position and the wetting properties
of the fluid, even when the particle's motion occurs on time scales much longer
than the spontaneous adsorption time. The hysteresis is associated with changes
in the fluid density in the gap between the particle and the wall, and these
structural rearrangements persist over surprisingly long times. The force and
free energy exhibit large oscillations with distance when the lattice of the
structured nanoparticle is held in register with that of the tube wall, but not
if the particle is allowed to rotate freely. Adsorbed particles are trapped in
free energy minima in equilibrium, but if the particle is forced along the
channel the resulting stick-slip motion alters the fluid structure and allows
the particle to desorb
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