58 research outputs found
Zigzag transitions and nonequilibrium pattern formation in colloidal chains
Paramagnetic colloidal particles that are optically trapped in a linear array
can form a zigzag pattern when an external magnetic field induces repulsive
interparticle interactions. When the traps are abruptly turned off, the
particles form a nonequilibrium expanding pattern with a zigzag symmetry, even
when the strength of the magnetic interaction is weaker than that required to
break the linear symmetry of the equilibrium state. We show that the transition
to the equilibrium zigzag state is always potentially possible for purely
harmonic traps. For anharmonic traps that have a finite height, the equilibrium
zigzag state becomes unstable above a critical anharmonicity. A normal mode
analysis of the equilibrium line configuration demonstrates that increasing the
magnetic field leads to a hardening and softening of the spring constants in
the longitudinal and transverse directions, respectively. The mode that first
becomes unstable is the mode with the zigzag symmetry, which explains the
symmetry of nonequilibrium patterns. Our analytically tractable models help to
give further insight into the way that the interplay of such factors as the
length of the chain, hydrodynamic interactions, thermal fluctuations affect the
formation and evolution of the experimentally observed nonequilibrium patterns.Comment: 16 pages, 8 figures; to appear in the Journal of Chemical Physic
Pattern formation in colloidal explosions
We study the non-equilibrium pattern formation that emerges when magnetically
repelling colloids, trapped by optical tweezers, are abruptly released, forming
colloidal explosions. For multiple colloids in a single trap we observe a
pattern of expanding concentric rings. For colloids individually trapped in a
line, we observe explosions with a zigzag pattern that persists even when
magnetic interactions are much weaker than those that break the linear symmetry
in equilibrium. Theory and computer simulations quantitatively describe these
phenomena both in and out of equilibrium. An analysis of the mode spectrum
allows us to accurately quantify the non-harmonic nature of the optical traps.
Colloidal explosions provide a new way to generate well-characterized
non-equilibrium behaviour in colloidal systems.Comment: New restructured version (supplementary material goes into main text,
no change of content), added journal reference and DOI information; 6 pages,
6 figures, published in Europhysics Letters (EPL
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
From ultra-fast growth to avalanche growth in devitrifying glasses
During devitrification, pre-existing crystallites grow by adding particles to
their surface via a process which is either thermally activated (diffusive
mode), or happens without kinetic barriers (fast crystal growth mode). It is
yet unclear what factors determine the crystal growth mode, and how to predict
it. With simulations of repulsive hard-sphere glasses, we show for the first
time that the same system at the same volume fraction and temperature can
devitrify via both modes depending on the preparation protocol of the glass. We
prepare two types of glass, a conventional glass (CG) via fast quenching and a
uniform glass (UG) via density homogenization. Firstly, we bring either glass
into contact with a crystal (X) and find the inherent structure (CGX/UGX).
During energy minimization, the crystal front grows deep into the CG interface,
while the growth is minimal for UG. When thermal noise is added, this behavior
is reflected in different crystallization dynamics. CGX exhibits a density drop
at the crystal growth front, leading to enhanced dynamics at the interface and
a fast growth mode. This mechanism may explain the faster crystal growth
observed below the glass transition experimentally. In contrast, UGX grows via
intermittent avalanche-like dynamics localized at the interface, a combination
of localized mechanical defects and the exceptional mechanical stability
imposed by the UG glass phase.Comment: 23 pages, 8 figure
Towards glasses with permanent stability
Unlike crystals, glasses age or devitrify over time, reflecting their
non-equilibrium nature. This lack of stability is a serious issue in many
industrial applications. Here, we show by numerical simulations that the
devitrification of quasi-hard-sphere glasses is prevented by suppressing volume
fraction inhomogeneities. A monodisperse glass known to devitrify with
`avalanche'-like intermittent dynamics is subjected to small iterative
adjustments to particle sizes to make the local volume fractions spatially
uniform. We find that this entirely prevents structural relaxation and
devitrification over aging time scales, even in the presence of crystallites.
There is a dramatic homogenization in the number of load-bearing nearest
neighbors each particle has, indicating that ultra-stable glasses may be formed
via `mechanical homogenization'. Our finding provides a physical principle for
glass stabilization and opens a novel route to the formation of mechanically
stabilized glasses.Comment: 6 pages, 4 figures, 1 ancillary video file, 1 supplementary PD
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
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
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