1,397 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
Hydrodynamically enforced entropic trapping of Brownian particles
We study the transport of Brownian particles through a corrugated channel
caused by a force field containing curl-free (scalar potential) and
divergence-free (vector potential) parts. We develop a generalized Fick-Jacobs
approach leading to an effective one-dimensional description involving the
potential of mean force. As an application, the interplay of a pressure-driven
flow and an oppositely oriented constant bias is considered. We show that for
certain parameters, the particle diffusion is significantly suppressed via the
property of hyrodynamically enforced entropic particle trapping.Comment: 5 pages, 4 figures, in press with Physical Review Letter
Giant enhancement of hydrodynamically enforced entropic trapping in thin channels
Using our generalized Fick-Jacobs approach [Martens et al., PRL 110, 010601
(2013); Martens et al., Eur. Phys. J. Spec. Topics 222, 2453-2463 (2013)] and
extensive Brownian dynamics simulations, we study particle transport through
three-dimensional periodic channels of different height. Directed motion is
caused by the interplay of constant bias acting along the channel axis and a
pressure-driven flow. The tremendous change of the flow profile shape in
channel direction with the channel height is reflected in a crucial dependence
of the mean particle velocity and the effective diffusion coefficient on the
channel height. In particular, we observe a giant suppression of the effective
diffusivity in thin channels; four orders of magnitude compared to the bulk
value.Comment: 16 pages, 8 figure
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
Pattern Formation Induced by Time-Dependent Advection
We study pattern-forming instabilities in reaction-advection-diffusion
systems. We develop an approach based on Lyapunov-Bloch exponents to figure out
the impact of a spatially periodic mixing flow on the stability of a spatially
homogeneous state. We deal with the flows periodic in space that may have
arbitrary time dependence. We propose a discrete in time model, where reaction,
advection, and diffusion act as successive operators, and show that a mixing
advection can lead to a pattern-forming instability in a two-component system
where only one of the species is advected. Physically, this can be explained as
crossing a threshold of Turing instability due to effective increase of one of
the diffusion constants
Sobre cinco compuestas errĂłneamente iconografiadas en publicaciones acerca de la biodiversidad de plantas vasculares andorranas
Five Compositae taxa which are illustrated in floristic publications for Andorra are questionable records which appear to have never been collected there in the wild.Se comentan cinco taxones de compuestas que aparecen confusamente ilustrados en publicaciones florĂsticas de Andorra y que nunca se han recolectado en territorio andorrano
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