163 research outputs found
Solvent coarsening around colloids driven by temperature gradients
Using mesoscopic numerical simulations and analytical theory we investigate
the coarsening of the solvent structure around a colloidal particle emerging
after a temperature quench of the colloid surface. Qualitative differences in
the coarsening mechanisms are found, depending on the composition of the binary
liquid mixture forming the solvent and on the adsorption preferences of the
colloid. For an adsorptionwise neutral colloid, as function of time the phase
being next to its surface alternates. This behavior sets in on the scale of the
relaxation time of the solvent and is absent for colloids with strong
adsorption preferences. A Janus colloid, with a small temperature difference
between its two hemispheres, reveals an asymmetric structure formation and
surface enrichment around it, even if the solvent is within its one-phase
region and if the temperature of the colloid is above the critical demixing
temperature of the solvent. Our phenomenological model turns out to
capture recent experimental findings according to which, upon laser
illumination of a Janus colloid and due to the ensuing temperature gradient
between its two hemispheres, the surrounding binary liquid mixture develops a
concentration gradient.Comment: 8 pages, 4 figure
Phase diagram of a model for 3He-4He mixtures in three dimensions
A lattice model of 3He - 4He mixtures which takes into account the continuous
rotational symmetry O(2) of the superfluid degrees of freedom of 4He is studied
in the molecular-field approximation and by Monte Carlo simulations in three
dimensions. In contrast to its two-dimensional version, for reasonable values
of the interaction parameters the resulting phase diagram resembles that
observed experimentally for 3He - 4He mixtures, for which phase separation
occurs as a consequence of the superfluid transition. The corresponding
continuum Ginzburg-Landau model with two order parameters describing 3He- 4He
mixtures near tricriticality is derived from the considered lattice model. All
coupling constants appearing in the continuum model are explicitly expressed in
terms of the mean concentration of 4He, the temperature, and the microscopic
interaction parameters characterizing the lattice system.Comment: 32 pages, 12 figures, submitted to the Phys. Rev.
Critical Casimir interactions around the consolute point of a binary solvent
Spatial confinement of a near-critical medium changes its fluctuation
spectrum and modifies the corresponding order parameter distribution. These
effects result in effective, so-called critical Casimir forces (CCFs) acting on
the confining surfaces. These forces are attractive for like boundary
conditions of the order parameter at the opposing surfaces of the confinement.
For colloidal particles dissolved in a binary liquid mixture acting as a
solvent close to its critical point of demixing, one thus expects the emergence
of phase segregation into equilibrium colloidal liquid and gas phases. We
analyze how such phenomena occur asymmetrically in the whole thermodynamic
neighborhood of the consolute point of the binary solvent. By applying
field-theoretical methods within mean-field approximation and the
semi-empirical de Gennes-Fisher functional, we study the CCFs acting between
planar parallel walls as well as between two spherical colloids and their
dependence on temperature and on the composition of the near-critical binary
mixture. We find that for compositions slightly poor in the molecules
preferentially adsorbed at the surfaces, the CCFs are significantly stronger
than at the critical composition, thus leading to pronounced colloidal
segregation. The segregation phase diagram of the colloid solution following
from the calculated effective pair potential between the colloids agrees
surprisingly well with experiments and simulations
Current-mediated synchronization of a pair of beating non-identical flagella
The basic phenomenology of experimentally observed synchronization (i.e., a
stochastic phase locking) of identical, beating flagella of a biflagellate alga
is known to be captured well by a minimal model describing the dynamics of
coupled, limit-cycle, noisy oscillators (known as the noisy Kuramoto model). As
demonstrated experimentally, the amplitudes of the noise terms therein, which
stem from fluctuations of the rotary motors, depend on the flagella length.
Here we address the conceptually important question which kind of synchrony
occurs if the two flagella have different lengths such that the noises acting
on each of them have different amplitudes. On the basis of a minimal model,
too, we show that a different kind of synchrony emerges, and here it is
mediated by a current carrying, steady-state; it manifests itself via
correlated "drifts" of phases. We quantify such a synchronization mechanism in
terms of appropriate order parameters and - for an ensemble of
trajectories and for a single realization of noises of duration ,
respectively. Via numerical simulations we show that both approaches become
identical for long observation times . This reveals an ergodic
behavior and implies that a single-realization order parameter is
suitable for experimental analysis for which ensemble averaging is not always
possible.Comment: 10 pages, 2 figure
Sensitivity of the thermodynamics of two-dimensional systems towards the topological classes of their surfaces
Using Monte Carlo simulations we study the two-dimensional Ising model on
triangular, square, and hexagonal lattices with various topologies. We focus on
the behavior of the magnetic susceptibility and of the specific heat near the
critical point of the planar bulk system. We find that scaling functions of
these quantities on the spherical surface (Euler characteristic K = 2) differ
from the scaling functions on the projective plane (K = 1) which, in turn,
differ from the scaling functions on the torus and on the Klein bottle (both K
= 0). This provides strong evidence that phase transitions of the Ising model
on two-dimensional surfaces depend on their topologies.Comment: 25 pages, 10 figure
Correlations and forces in sheared fluids with or without quenching
Spatial correlations play an important role in characterizing material
properties related to non-local effects. Inter alia, they can give rise to
fluctuation-induced forces. Equilibrium correlations in fluids provide an
extensively studied paradigmatic case, in which their range is typically
bounded by the correlation length. Out of equilibrium, conservation laws have
been found to extend correlations beyond this length, leading, instead, to
algebraic decays. In this context, here we present a systematic study of the
correlations and forces in fluids driven out of equilibrium simultaneously by
quenching and shearing, both for non-conserved as well as for conserved
Langevin-type dynamics. We identify which aspects of the correlations are due
to shear, due to quenching, and due to simultaneously applying both, and how
these properties depend on the correlation length of the system and its
compressibility. Both shearing and quenching lead to long-ranged correlations,
which, however, differ in their nature as well as in their prefactors, and
which are mixed up by applying both perturbations. These correlations are
employed to compute non-equilibrium fluctuation-induced forces in the presence
of shear, with or without quenching, thereby generalizing the framework set out
by Dean and Gopinathan. These forces can be stronger or weaker compared to
their counterparts in unsheared systems. In general, they do not point along
the axis connecting the centers of the small inclusions considered to be
embedded in the fluctuating medium. Since quenches or shearing appear to be
realizable in a variety of systems with conserved particle number, including
active matter, we expect these findings to be relevant for experimental
investigations.Comment: 19 pgs (15 main text + 4 appendices); 7 figure
Silicon on ceramic process. Silicon sheet growth development for the large-area silicon sheet task of the low-cost silicon solar array project
The technical and economic feasibility of producing solar-cell-quality sheet silicon was investigated. The sheets were made by coating one surface of carbonized ceramic substrates with a thin layer of large-grain polycrystalline silicon from the melt. Significant progress was made in all areas of the program
Dip-coating process: Silicon sheet growth development for the large-area silicon sheet task of the low-cost silicon solar array project
The objective of this research program is to investigate the technical and economic feasibility of producing solar-cell-quality sheet silicon by coating one surface of carbonized ceramic substrates with a thin layer of large-grain polycrystalline silicon from the melt. The past quarter demonstrated significant progress in several areas. Seeded growth of silicon-on-ceramic (SOC) with an EFG ribbon seed was demonstrated. Different types of mullite were successfully coated with silicon. A new method of deriving minority carrier diffusion length, L sub n from spectral response measurements was evaluated. ECOMOD cost projections were found to be in good agreement with the interim SAMIS method proposed by JPL. On the less positive side, there was a decrease in cell performance which we believe to be due to an unidentified source of impurities
Origin of attraction between likely charged hydrophobic and hydrophilic walls confining near-critical binary aquaeous mixture with ions
Effect of ionic solute on a near-critical binary aqueous mixture confined
between charged walls with different adsorption preferences is considered
within a simple density functional theory. For the near-critical system
containing small amount of ions a Landau-type functional is derived based on
the assumption that the correlation, , and the Debye screening length,
, are both much larger than the molecular size. The corresponding
approximate Euler-Lagrange equations aresolved analytically for ions insoluble
in the organic solvent. Nontrivial concentration profile of the solvent is
found near the charged hydrophobic wall as a result of the competition between
the short-range attraction of the organic solvent and the electrostatic
attraction of the hydrated ions. Excess of water may be present near the
hydrophobic surface for some range of the surface charge and . As a
result, the effective potential between the hydrophilic and the hydrophobic
surface can be repulsive far from the critical point, then attractive and again
repulsive when the critical temperature is approached, in agreement with the
recent experiment [Nellen at.al., Soft Matter {\bf 7}, 5360 (2011)]
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