535 research outputs found
Use of Research Evidence: Social Services Portfolio
The William T. Grant Foundation intends that the emerging research evidence from its Use of Research Evidence (URE) portfolio be useful to those engaged in these (and other) diverse efforts. But broad and meaningful use of research evidence will require conversations that extend beyond researchers and expert forums. Indeed, URE findings suggest that policymakers and practitioners should not be viewed simply as "end users" of research evidence. To provide insight into how URE studies and the resulting evidence could be most relevant and useful to them, policymakers and practitioners at all levels in the social services system must have a voice in these conversations. This paper is intended to foster and inform dialogue among researchers, policymakers, and practitioners by reflecting on the Foundation's social services URE portfolio from the perspective of policy and practice and by identifying potential opportunities for the next generation of studies and considerations for those undertaking that work
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
Phase separation around heated colloid in bulk and under confinement
We study the non-equilibrium coarsening dynamics of a binary liquid solvent
around a colloidal particle in a presence of a time-dependent temperature
gradient that emerges after temperature quench of a suitably coated colloid
surface. The solvent is maintained at its critical concentration and the
colloid is fixed in space. The coarsening patterns near the surface are shown
to be strongly dependent on the colloid surface adsorption properties and on
the temperature evolution. The temperature gradient alters the morphology of a
binary solvent near the surface of a colloid as compared to the coarsening
proceeding at constant temperature everywhere. We also present results for the
evolution of coarsening in thin films with confining surfaces preferring one
species of the binary liquid mixture over the other. Confinement leads to a
faster phase segregation process and formation of a bridge connecting the
colloid and both the confining walls.Comment: 9 pages, 12 figure
Effect of ions on confined near-critical binary aqueous mixture
Near-critical binary mixtures containing ions and confined between two
charged and selective surfaces are studied within a Landau-Ginzburg theory
extended to include electrostatic interactions. Charge density profiles and the
effective interactions between the confining surfaces are calculated in the
case of chemical preference of ions for one of the solvent components. Close to
the consolute point of the binary solvent, the preferential solubility of ions
leads to the modification of the charge density profiles in respect to the ones
obtained from the Debye-H\"uckel theory. As a result, the electrostatic
contribution to the effective potential between the charged surface can exhibit
an attractive well. Our calculations are based on the approximation scheme
valid if the bulk correlation length of a solvent is much larger than the Debye
screening length; in this critical regime the effect of charge on the
concentration profiles of the solvent is subdominant. Such conditions are met
in the recent measurements of the effective forces acting between a substrate
and a spherical colloidal particle immersed in the near-critical water-lutidine
mixture [Nature V. 451, 172 (2008)]. Our analytical results are in a
quantitative agreement with the experimental ones.Comment: 37 pages, 8 figure
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.
Two-temperature Langevin dynamics in a parabolic potential
We study a planar two-temperature diffusion of a Brownian particle in a
parabolic potential. The diffusion process is defined in terms of two Langevin
equations with two different effective temperatures in the X and the Y
directions. In the stationary regime the system is described by a non-trivial
particle position distribution P(x,y), which we determine explicitly. We show
that this distribution corresponds to a non-equilibrium stationary state,
characterised by the presence of space-dependent particle currents which
exhibit a non-zero rotor. Theoretical results are confirmed by the numerical
simulations.Comment: 9 pages, 2 figure
Effects of Confinement on Critical Adsorption: Absence of Critical Depletion for Fluids in Slit Pores
The adsorption of a near-critical fluid confined in a slit pore is
investigated by means of density functional theory and by Monte Carlo
simulation for a Lennard-Jones fluid. Our work was stimulated by recent
experiments for SF_6 adsorbed in a mesoporous glass which showed the striking
phenomenon of critical depletion, i.e. the adsorption excess "Gamma" first
increases but then decreases very rapidly to negative values as the bulk
critical temperature T_c is approached from above along near-critical
isochores. By contrast, our density functional and simulation results, for a
range of strongly attractive wall-fluid potentials, show Gamma monotonically
increasing and eventually saturating as the temperature is lowered towards T_c
along both the critical (rho=rho_c) and sub-critical isochores (rho<\rho_c).
Such behaviour results from the increasingly slow decay of the density profile
away from the walls, into the middle of the slit, as T->T_c. For rho < rho_c we
find that in the fluid the effective bulk field, which is negative and which
favours desorption, is insufficient to dominate the effects of the surface
fields which favour adsorption. We compare this situation with earlier results
for the lattice gas model with a constant (negative) bulk field where critical
depletion was found. Qualitatively different behaviour of the density profiles
and adsorption is found in simulations for intermediate and weakly attractive
wall-fluid potentials but in no case do we observe the critical depletion found
in experiments. We conclude that the latter cannot be accounted for by a single
pore model.Comment: 21 pages Revtex. Submitted to Phys. Rev.
- …