64 research outputs found
Transport of a colloidal particle driven across a temporally oscillating optical potential energy landscape
A colloidal particle is driven across a temporally oscillating one-dimensional optical potential energy landscape and its particle motion is analysed. Different modes of dynamic mode locking are observed and are confirmed with the use of phase portraits. The effect of the oscillation frequency on the mode locked step width is addressed and the results are discussed in light of a high-frequency theory and compared to simulations. Furthermore, the influence of the coupling between the particle and the optical landscape on mode locking is probed by increasing the maximum depth of the optical landscape. Stronger coupling is seen to increase the width of mode locked steps. Finally, transport across the temporally oscillating landscape is studied by measuring the effective diffusion coefficient of a mobile particle, which is seen to be highly sensitive to the driving velocity and mode locking
Localization dynamics of fluids in random confinement
The dynamics of two-dimensional fluids confined within a random matrix of
obstacles is investigated using both colloidal model experiments and molecular
dynamics simulations. By varying fluid and matrix area fractions in the
experiment, we find delocalized tracer particle dynamics at small matrix area
fractions and localized motion of the tracers at high matrix area fractions. In
the delocalized region, the dynamics is subdiffusive at intermediate times, and
diffusive at long times, while in the localized regime, trapping in finite
pockets of the matrix is observed. These observations are found to agree with
the simulation of an ideal gas confined in a weakly correlated matrix. Our
results show that Lorentz gas systems with soft interactions are exhibiting a
smoothening of the critical dynamics and consequently a rounded
delocalization-to-localization transition.Comment: 5 pages, 3 figure
Self-assembly and crystallisation of indented colloids at a planar wall
We report experimental and simulation studies of the structure of a monolayer
of indented ("lock and key") colloids, on a planar surface. On adding a
non-absorbing polymer with prescribed radius and volume fraction, depletion
interactions are induced between the colloids, with controlled range and
strength. For spherical particles, this leads to crystallisation, but the
indented colloids crystallise less easily than spheres, in both simulation and
experiment. Nevertheless, simulations show that indented colloids do form
plastic (rotator) crystals. We discuss the conditions under which this occurs,
and the possibilities of lower-symmetry crystal states. We also comment on the
kinetic accessibility of these states.Comment: 8 pages, 8 figure
Model-free measurement of the pair potential in colloidal fluids using optical microscopy
We report a straightforward, model-free approach for measuring pair
potentials from particle-coordinate data, based on enforcing consistency
between the pair distribution function measured separately by the
distance-histogram and test-particle insertion routes. We demonstrate the
method's accuracy and versatility in simulations of simple fluids, before
applying it to an experimental system composed of superparamagnetic colloidal
particles. The method will enable experimental investigations into many-body
interactions and allow for effective coarse-graining of interactions from
simulations.Comment: 8 pages, 3 figures including supplemental materia
Dynamics of individual Brownian rods in a microchannel flow
We study the orientational dynamics of heavy silica microrods flowing through
a microfluidic channel. Comparing experiments and Brownian dynamics simulations
we identify different particle orbits, in particular in-plane tumbling
behavior, which cannot be explained by classical Jeffery theory, and we relate
this behavior to the rotational diffusion of the rods. By constructing the
full, three-dimensional, orientation distribution, we describe the rod
trajectories and quantify the persistence of Jeffery orbits using temporal
correlation functions of the Jeffery constant. We find that our colloidal rods
lose memory of their initial configuration in about a second, corresponding to
half a Jeffery period.Comment: 5 pages, 4 figure
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