10 research outputs found
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
Crossover of Critical Casimir forces between different surface universality classes
In confined systems near a continuous phase transition the long-ranged
fluctuations of the corresponding order parameter are subject to boundary
conditions. These constraints result in so-called critical Casimir forces
acting as effective forces on the confining surfaces. For systems belonging to
the Ising bulk universality class corresponding to a scalar order parameter the
critical Casimir force is studied for the film geometry in the crossover regime
characterized by different surface fields at the two surfaces. The scaling
function of the critical Casimir force is calculated within mean field theory.
Within our approach, the scaling functions of the critical Casimir force and of
the order parameter profile for finite surface fields can be mapped by
rescaling, except for a narrow crossover regime, onto the corresponding scaling
function of the so-called normal fixed point of strong surface fields. In the
crossover regime, the critical Casimir force as function of temperature
exhibits more than one extremum and for certain ranges of surface field
strengths it changes sign twice upon varying temperature. Monte Carlo
simulation data obtained for a three-dimensional Ising film show similar
trends. The sign of the critical Casimir force can be inferred from the
comparison of the order parameter profiles in the film and in the semi-infinite
geometry
Phase behavior of colloidal suspensions with critical solvents in terms of effective interactions
We study the phase behavior of colloidal suspensions the solvents of which
are considered to be binary liquid mixtures undergoing phase segregation. We
focus on the thermodynamic region close to the critical point of the
accompanying miscibility gap. There, due to the colloidal particles acting as
cavities in the critical medium, the spatial confinements of the critical
fluctuations of the corresponding order parameter result in the effective,
so-called critical Casimir forces between the colloids. Employing an approach
in terms of effective, one-component colloidal systems, we explore the
possibility of phase coexistence between two phases of colloidal suspensions,
one being rich and the other being poor in colloidal particles. The reliability
of this effective approach is discussed
Structure and aggregation of colloids immersed in critical solvents
We consider an ensemble of spherical colloidal particles immersed in a
near-critical solvent such as a binary liquid mixture close to its critical
demixing point. The emerging long-ranged fluctuations of the corresponding
order parameter of the solvent drive the divergence of the correlation length.
Spatial confinements of these critical fluctuations by colloidal solute
particles, acting as cavities in the fluctuating medium, restrict and modify
the fluctuation spectrum in a way which depends on their relative
configuration. This results in effective, so-called critical Casimir forces
(CCFs) acting on the confining surfaces. Using the available knowledge about
CCFs we study the structure and stability of such colloidal suspensions by
employing an approach in terms of effective, one-component colloidal systems.
Applying the approximation of pairwise additive CCFs we calculate the radial
distribution function of the colloids, which is experimentally accessible. We
analyze colloidal aggregation due to CCFs and thus allude to previous
experimental studies which are still under debat
Tunability of Critical Casimir Interactions by Boundary Conditions
We experimentally demonstrate that critical Casimir forces in colloidal
systems can be continuously tuned by the choice of boundary conditions. The
interaction potential of a colloidal particle in a mixture of water and
2,6-lutidine has been measured above a substrate with a gradient in its
preferential adsorption properties for the mixture's components. We find that
the interaction potentials at constant temperature but different positions
relative to the gradient continuously change from attraction to repulsion. This
demonstrates that critical Casimir forces respond not only to minute
temperature changes but also to small changes in the surface properties.Comment: 4 figures;
http://www.iop.org/EJ/article/0295-5075/88/2/26001/epl_88_2_26001.htm
Critical Casimir forces and adsorption profiles in the presence of a chemically structured substrate
Motivated by recent experiments with confined binary liquid mixtures near
demixing, we study the universal critical properties of a system, which belongs
to the Ising universality class, in the film geometry. We employ periodic
boundary conditions in the two lateral directions and fixed boundary conditions
on the two confining surfaces, such that one of them has a spatially
homogeneous adsorption preference while the other one exhibits a laterally
alternating adsorption preference, resembling locally a single chemical step.
By means of Monte Carlo simulations of an improved Hamiltonian, so that the
leading scaling corrections are suppressed, numerical integration, and
finite-size scaling analysis we determine the critical Casimir force and its
universal scaling function for various values of the aspect ratio of the film.
In the limit of a vanishing aspect ratio the critical Casimir force of this
system reduces to the mean value of the critical Casimir force for laterally
homogeneous ++ and +- boundary conditions, corresponding to the surface spins
on the two surfaces being fixed to equal and opposite values, respectively. We
show that the universal scaling function of the critical Casimir force for
small but finite aspect ratios displays a linear dependence on the aspect ratio
which is solely due to the presence of the lateral inhomogeneity. We also
analyze the order-parameter profiles at criticality and their universal scaling
function which allows us to probe theoretical predictions and to compare with
experimental data.Comment: revised version, section 5.2 expanded; 53 pages, 12 figures, iopart
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