35,184 research outputs found
Energy transfer between colloids via critical interactions
We report the observation of a temperature-controlled synchronization of two
Brownian-particles in a binary mixture close to the critical point of the
demixing transition. The two beads are trapped by two optical tweezers whose
distance is periodically modulated. We notice that the motion synchronization
of the two beads appears when the critical temperature is approached. In
contrast, when the fluid is far from its critical temperature, the
displacements of the two beads are uncorrelated. Small changes in temperature
can radically change the global dynamics of the system. We show that the
synchronisation is induced by the critical Casimir forces. Finally, we present
the measure of the energy transfers inside the system produced by the critical
interaction.Comment: 8 pages, 7 figure
Field-control, phase-transitions, and life's emergence
Instances of critical-like characteristics in living systems at each
organizational level as well as the spontaneous emergence of computation
(Langton), indicate the relevance of self-organized criticality (SOC). But
extrapolating complex bio-systems to life's origins, brings up a paradox: how
could simple organics--lacking the 'soft matter' response properties of today's
bio-molecules--have dissipated energy from primordial reactions in a controlled
manner for their 'ordering'? Nevertheless, a causal link of life's macroscopic
irreversible dynamics to the microscopic reversible laws of statistical
mechanics is indicated via the 'functional-takeover' of a soft magnetic
scaffold by organics (c.f. Cairns-Smith's 'crystal-scaffold'). A
field-controlled structure offers a mechanism for bootstrapping--bottom-up
assembly with top-down control: its super-paramagnetic components obey
reversible dynamics, but its dissipation of H-field energy for aggregation
breaks time-reversal symmetry. The responsive adjustments of the controlled
(host) mineral system to environmental changes would bring about mutual
coupling between random organic sets supported by it; here the generation of
long-range correlations within organic (guest) networks could include SOC-like
mechanisms. And, such cooperative adjustments enable the selection of the
functional configuration by altering the inorganic network's capacity to assist
a spontaneous process. A non-equilibrium dynamics could now drive the
kinetically-oriented system towards a series of phase-transitions with
appropriate organic replacements 'taking-over' its functions.Comment: 54 pages, pdf fil
Deconstructing the glass transition through critical experiments on colloids
The glass transition is the most enduring grand-challenge problem in
contemporary condensed matter physics. Here, we review the contribution of
colloid experiments to our understanding of this problem. First, we briefly
outline the success of colloidal systems in yielding microscopic insights into
a wide range of condensed matter phenomena. In the context of the glass
transition, we demonstrate their utility in revealing the nature of spatial and
temporal dynamical heterogeneity. We then discuss the evidence from colloid
experiments in favor of various theories of glass formation that has
accumulated over the last two decades. In the next section, we expound on the
recent paradigm shift in colloid experiments from an exploratory approach to a
critical one aimed at distinguishing between predictions of competing
frameworks. We demonstrate how this critical approach is aided by the discovery
of novel dynamical crossovers within the range accessible to colloid
experiments. We also highlight the impact of alternate routes to glass
formation such as random pinning, trajectory space phase transitions and
replica coupling on current and future research on the glass transition. We
conclude our review by listing some key open challenges in glass physics such
as the comparison of growing static lengthscales and the preparation of
ultrastable glasses, that can be addressed using colloid experiments.Comment: 137 pages, 45 figure
Collective behavior of colloids due to critical Casimir interactions
If colloidal solute particles are suspended in a solvent close to its
critical point, they act as cavities in a fluctuating medium and thereby
restrict and modify the fluctuation spectrum in a way which depends on their
relative configuration. As a result effective, so-called critical Casimir
forces (CCFs) emerge between the colloids. The range and the amplitude of CCFs
depend sensitively on the temperature and the composition of the solvent as
well as on the boundary conditions of the order parameter of the solvent at the
particle surfaces. These remarkable, moreover universal features of the CCFs
provide the possibility for an active control over the assembly of colloids.
This has triggered a recent surge of experimental and theoretical interest in
these phenomena. We present an overview of current research activities in this
area. Various experiments demonstrate the occurrence of thermally reversible
self-assembly or aggregation or even equilibrium phase transitions of colloids
in the mixed phase below the lower consolute points of binary solvents. We
discuss the status of the theoretical description of these phenomena, in
particular the validity of a description in terms of effective, one-component
colloidal systems and the necessity of a full treatment of a ternary
solvent-colloid mixture. We suggest perspectives on the directions towards
which future research in this field might develop.Comment: review, 88 pages, 19 figure
Controlling the interfacial and bulk concentrations of spontaneously charged colloids in non-polar media
Stabilization and dispersion of electrical charge by colloids in non-polar
media, such as nano-particles or inverse micelles, is significant for a variety
of chemical and technological applications, ranging from drug delivery to
e-ink. Many applications require knowledge about concentrations near the
solid|liquid interface and the bulk, particularly in media where colloids
exhibit spontaneous charging properties. By modification of the mean field
equations to include the finite size effects that are typical in concentrated
electrolytes along with disproportionation kinetics, and by considering high
potentials, it is possible to evaluate the width of the condensed double layers
near planar electrodes and the bulk concentrations of colloids at steady state.
These quantities also provide an estimate of the minimum initial colloid
concentration that is required to support electroneutrality in the dispersion
bulk, and thus provide insights into the quasi-steady state currents that have
been observed in inverse micellar media.Comment: 13 pages, 5 figure
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