We study the non-equilibrium coarsening dynamics of a binary liquid solvent around a colloidal particle in the presence of a time-dependent temperature gradient that emerges after a temperature ...quench of a suitable 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 the binary solvent near the surface of the colloid as compared to the coarsening proceeding at a 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 confining walls.
Temperature gradient influences the coarsening patterns of a binary solvent around a heated colloid; under confinement a bridge forms connecting the colloid and the walls.
Using a fluid particle dynamics method we numerically investigate the motion of a spherical Janus particle suspended in a binary liquid mixture, which emerges under heating of one-half of a colloid ...surface. The method treats simultaneously the flow of the solvent and the motion of the particle, hence, the velocity of the particle can be computed directly. Our approach accounts for a phenomenon of critical adsorption, therefore, a particle that is adsorptionwise nonneutral is always completely covered by an adsorption layer (droplet). In order to establish the mechanism of self-propulsion, we study systematically various combinations of adsorption preference on both hemispheres of the Janus colloid as function of the heating power for symmetric and nonsymmetric binary solvents and for various particle sizes in three spatial dimensions. Only for a particle for which the heated hemisphere is neutral whereas the other hemisphere prefers one of the two components of the mixture does the reversal of the direction of motion occur. The particle self-propels much faster in nonsymmetric binary solvents. Self-propulsion originates from a gradient of mechanical stress, in a way similar to the Marangoni effect. This stress is not localized at the edge but distributed within the whole droplet. We compare our findings with the experimental observations and other theoretical results.
Self-propelled motion of a Janus particle suspended in a binary mixture with the lower critical solution temperature under local heating around a capped hemisphere.
A gold-capped Janus particle suspended in a near-critical binary liquid mixture can self-propel under illumination. We have immobilized such a particle in a narrow channel and carried out a combined ...experimental and theoretical study of the non-equilibrium dynamics of a binary solvent around it - lasting from the very moment of switching illumination on until the steady state is reached. In the theoretical study we use both a purely diffusive and a hydrodynamic model, which we solve numerically. Our results demonstrate a remarkable complexity of the time evolution of the concentration field around the colloid. This evolution is governed by the combined effects of the temperature gradient and the wettability, and crucially depends on whether the colloid is free to move or is trapped. For the trapped colloid, all approaches indicate that the early time dynamics is purely diffusive and characterized by composition layers travelling with constant speed from the surface of the colloid into the bulk of the solvent. Subsequently, hydrodynamic effects set in. Anomalously large nonequilibrium fluctuations, which result from the temperature gradient and the vicinity of the critical point of the binary liquid mixture, give rise to strong concentration fluctuations in the solvent and to permanently changing coarsening patterns not observed for a mobile particle. The early time dynamics around initially still Janus colloids produces a force which is able to set the Janus colloid into motion. The propulsion due to this transient dynamics is in the direction opposite to that observed after the steady state is attained.
We study experimentally and theoretically the non-equilibrium dynamics of a binary solvent around a gold-capped Janus particle, lasting from the very moment of switching illumination on until a steady state is reached.
Correction for 'Transient coarsening and the motility of optically heated Janus colloids in a binary liquid mixture' by Juan Ruben Gomez-Solano
et al.
,
Soft Matter
, 2020, DOI:
10.1039/d0sm00964d
.
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.
We show that hydrophilic ions present in a confined, near-critical aqueous mixture can lead to an attraction between like charge surfaces with opposing preferential adsorption of the two species of ...the mixture, even though the corresponding Casimir potential in uncharged systems is repulsive. This prediction agrees with a recent experiment Nellen
et al.
,
Soft Matter
, 2011,
7
, 5360. We also show that oppositely charged hydrophobic surfaces can repel each other, although the Casimir potential between uncharged surfaces with like preferential adsorption (selectivity) is attractive. This behavior is expected when the electrostatic screening length is larger than the correlation length, and one of the confining surfaces is strongly selective and weakly charged, whereas the other confining surface is weakly selective and strongly charged. The Casimir potential can change sign because the hydrophilic ions near the weakly hydrophobic surface can overcompensate the effect of hydrophobicity, and this surface can act as a hydrophilic one. We also predict a more attractive interaction between charged, hydrophilic surfaces and a more repulsive interaction between charged, hydrophobic surfaces than given by the sum of the Casimir and Debye-Hückel potentials. Our theory is derived systematically from a microscopic approach, and combines the Landau-type and Debye-Hückel theories with an additional contribution of an entropic origin.
Influence of the distribution of ions on the concentration profile can lead to the effective potential between charged and selective surfaces confining a near-critical mixture that has a sign different from the sum of the Casimir and Debye-Hückel potentials.
Near-critical binary mixtures containing ions and confined between two charged and selective surfaces are studied within a Ginzburg-Landau 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 the 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 with respect to the ones obtained from Debye-Hückel theory. As a result, the electrostatic contribution to the effective potential between the charged surfaces can exhibit an attractive well. Our calculations are based on the approximation scheme that is 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
, 2008,
451
, 172. Our analytical results are in quantitative agreement with the experimental ones.
An expression is derived for effective interactions between charged and selective surfaces confining a near-critical mixture, which agrees quantitatively with experiments.
PDF
