399 research outputs found
Bulk dynamics of Brownian hard disks: Dynamical density functional theory versus experiments on two-dimensional colloidal hard spheres
Using dynamical density functional theory (DDFT), we theoretically study
Brownian self-diffusion and structural relaxation of hard disks and compare to
experimental results on quasi two-dimensional colloidal hard spheres. To this
end, we calculate the self and distinct van Hove correlation functions by
extending a recently proposed DDFT-approach for three-dimensional systems to
two dimensions. We find that the theoretical results for both self- and
distinct part of the van Hove function are in very good quantitative agreement
with the experiments up to relatively high fluid packing fractions of roughly
0.60. However, at even higher densities, deviations between experiment and the
theoretical approach become clearly visible. Upon increasing packing fraction,
in experiments the short-time self diffusive behavior is strongly affected by
hydrodynamic effects and leads to a significant decrease in the respective
mean-squared displacement. In contrast, and in accordance with previous
simulation studies, the present DDFT which neglects hydrodynamic effects, shows
no dependence on the particle density for this quantity
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
Polyhedral colloidal `rocks': low-dimensional networks
We introduce a model system of anisotropic colloidal `rocks'. Due to their
shape, the bonding introduced via non-absorbing polymers is profoundly
different from spherical particles: bonds between rocks are rigid against
rotation, leading to strong frustration. We develop a geometric model which
captures the essence of the rocks. Experiments and simulations show that the
colloid geometry leads to structures of low fractal dimension. This is in stark
contrast to gels of spheres, whose rigidity results from locally dense regions.
At high density the rocks form a quasi one-component glass
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
The equilibrium intrinsic crystal-liquid interface of colloids
We use confocal microscopy to study an equilibrated crystal-liquid interface
in a colloidal suspension. Capillary waves roughen the surface, but locally the
intrinsic interface is sharply defined. We use local measurements of the
structure and dynamics to characterize the intrinsic interface, and different
measurements find slightly different widths of this interface. In terms of the
particle diameter , this width is either (based on structural
information) or (based on dynamics), both not much larger than the
particle size. This work is the first direct experimental visualization of an
equilibrated crystal-liquid interface.Comment: 6 pages; revised version, submitted to PNA
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
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
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