Adsorption and wetting phenomena for colloids in liquid mixtures

Critical adsorption profiles around a sphere and a cylinder in a fluid at criticality: Local functional theory

We study universal critical adsorption on a solid sphere and a solid cylinder in a fluid at bulk criticality, where preferential adsorption occurs. We use a local functional theory proposed by Fisher et al. [M. E. Fisher and P. G. de Gennes, C. R. Acad. Sci. Paris Ser. B 287, 207 (1978); M. E. Fisher and H. Au-Yang, Physica A 101, 255 (1980)PHYADX0378-437110.1016/0378-4371(80)90112-0]. We calculate the mean order parameter profile ψ(r), where r is the distance from the sphere center and the cylinder axis, respectively. The resultant differential equation for ψ(r) is solved exactly around a sphere and numerically around a cylinder. A strong adsorption regime is realized except for very small surface field h_{1}, where the surface order parameter ψ(a) is determined by h_{1} and is independent of the radius a. If r considerably exceeds a, ψ(r) decays as r^{-(1+η)} for a sphere and r^{-(1+η)/2} for a cylinder in three dimensions, where η is the critical exponent in the order parameter correlation at bulk criticality.

Hydrodynamics in bridging and aggregation of two colloidal particles in a near-critical binary mixture

We investigate bridging and aggregation of two colloidal particles in a near-critical binary mixture when the fluid far from the particles is outside the coexistence (CX) curve and is rich in the component disfavored by the colloid surfaces. In such situations, the adsorption-induced interaction is enhanced, leading to bridging and aggregation of the particles. We realize bridging firstly by changing the temperature with a fixed interparticle separation and secondly by letting the two particles aggregate. The interparticle attractive force dramatically increases upon bridging. The dynamics is governed by hydrodynamic flow around the colloid surfaces. In aggregation, the adsorption layers move with the particles and squeezing occurs at narrow separation. These results suggest relevance of bridging in the reversible colloid aggregation observed so far. We use the local functional theory [J. Chem. Phys., 2012, 136, 114704] to take into account the renormalization effect and the simulation method [Phys. Rev. Lett., 2000, 85, 1338] to calculate the hydrodynamic flow around the colloidal particles.

Critical Casimir forces along the isofields

Using quasiexact numerical density-matrix renormalization-group techniques we calculate the critical Casimir force for a two-dimensional (2D) Ising strip with equal strong surface fields, along the thermodynamic paths corresponding to the fixed nonzero bulk field h≠0. Using the Derjaguin approximation we also determine the critical Casimir force and its potential for two disks. We find that varying the temperature along the isofields lying between the bulk coexistence and the capillary condensation critical point leads to a dramatic increase of the critical Casimir interactions with a qualitatively different functional dependence on the temperature than along h=0. These findings might be of relevance for biomembranes, whose heterogeneity is recently interpreted as being connected with a critical behavior belonging to the 2D Ising universality class.

Attractive interaction and bridging transition between neutral colloidal particles due to preferential adsorption in a near-critical binary mixture

We examine the solvent-mediated interaction between two neutral colloidal particles due to preferential adsorption in a near-critical binary mixture. We take into account the renormalization effect due to the critical fluctuations using the recent local functional theory [J. Chem. Phys. 136, 114704 (2012)]. We calculate the free energy and the force between two colloidal particles as functions of the temperature T, the composition far from the colloidal particles c(∞), and the colloid separation ℓ. The interaction is much enhanced when the component favored by the colloid surfaces is poor in the reservoir. For such off-critical compositions, we find a surface of a first-order bridging transition ℓ=ℓ(cx)(T,c(∞)) in the T-c(∞)-ℓ space in a universal, scaled form, across which a discontinuous change occurs between separated and bridged states. This surface starts from the bulk coexistence surface (CX) and ends at a bridging critical line where ℓ is determined by T as ℓ=ℓ(c)(T). On approaching the critical line, the discontinuity vanishes and the derivatives of the force with respect to T and ℓ both diverge. Furthermore, bridged states continuously change into separated states if c(∞) (or T) is varied from a value on CX to a value far from CX with ℓ kept smaller than ℓ(c)(T).

Critical Casimir effect in classical binary liquid mixtures

If a fluctuating medium is confined, the ensuing perturbation of its fluctuation spectrum generates Casimir-like effective forces acting on its confining surfaces. Near a continuous phase transition of such a medium the corresponding order parameter fluctuations occur on all length scales and therefore close to the critical point this effect acquires a universal character, i.e., to a large extent it is independent of the microscopic details of the actual system. Accordingly it can be calculated theoretically by studying suitable representative model systems. We report on the direct measurement of critical Casimir forces by total internal reflection microscopy with femtonewton resolution. The corresponding potentials are determined for individual colloidal particles floating above a substrate under the action of the critical thermal noise in the solvent medium, constituted by a binary liquid mixture of water and 2,6-lutidine near its lower consolute point. Depending on the relative adsorption preferences of the colloid and substrate surfaces with respect to the two components of the binary liquid mixture, we observe that, upon approaching the critical point of the solvent, attractive or repulsive forces emerge and supersede those prevailing away from it. Based on the knowledge of the critical Casimir forces acting in film geometries within the Ising universality class and with equal or opposing boundary conditions, we provide the corresponding theoretical predictions for the sphere-planar wall geometry of the experiment. The experimental data for the effective potential can be interpreted consistently in terms of these predictions and a remarkable quantitative agreement is observed.

Hard-sphere fluids in contact with curved substrates

The properties of a hard-sphere fluid in contact with hard-spherical and cylindrical walls are studied. Rosenfeld's density functional theory (DFT) is applied to determine the density profile and surface tension gamma for wide ranges of radii of the curved walls and densities of the hard-sphere fluid. Particular attention is paid to investigate the curvature dependence and the possible existence of a contribution to gamma which is proportional to the logarithm of the radius of curvature. Moreover, by treating the curved wall as a second component at infinite dilution, we provide an analytical expression for the surface tension of a hard-sphere fluid close to arbitrary hard convex walls. The agreement between the analytical expression and DFT is good. Our results show no signs for the existence of a logarithmic term in the curvature dependence of gamma.

Wetting-induced effective interaction potential between spherical particles

Using a density-functional-based interface displacement model, we determine the effective interaction potential between two spherical particles which are immersed in a homogeneous fluid such as the vapor phase of a one-component substance or the A-rich liquid phase of a binary liquid mixture composed of A and B particles. If this solvent is thermodynamically close to a first-order fluid-fluid phase transition, the spheres are covered with wetting films of the incipient bulk phase, i.e., the liquid phase or the B-rich liquid, respectively. Below a critical distance between the spheres their wetting films snap to a bridgelike configuration. We determine phase diagrams for this morphological transition, and analyze its repercussions on the effective interaction potential. Our results are accessible to various types of force microscopy and scattering experiments, and may be relevant to flocculation in colloidal suspensions.

Critical adsorption on curved objects

A systematic field-theoretical description of critical adsorption on curved objects such as spherical or rodlike colloidal particles immersed in a fluid near criticality is presented. The temperature dependence of the corresponding order parameter profiles and of the excess adsorption are calculated explicitly. Critical adsorption on elongated rods is substantially more pronounced than on spherical particles. It turns out that, within the context of critical phenomena in confined geometries, critical adsorption on a microscopically thin "needle" represents a distinct universality class of its own. Under favorable conditions the results are relevant for the flocculation of colloidal particles.

Stability of Dilute Colloidal Silica Suspensions in the Vicinity of the Binodal Curve of the System 2-Butoxyethanol/Water

In the temperature-composition diagram of the system 2-butoxyethanol (abbreviated C4E1)/water, SiO2 [mass fraction of SiO2, y(SiO2) approximately 0.01; volume fraction phi(SiO2) approximately 10(-3)] there exists a flocculation temperature-composition curve running below the binodal curve for x > xc (x, mole fraction of C4E1; xc = 0.0598). The concentration of the colloidal particles is low enough to consider them an "impurity." In the region of the phase diagram bound by the binodal and the flocculation curve the suspension of the colloidal SiO2 particles is unstable and the particles flocculate reversibly. The difference between the temperature of phase separation and the flocculation temperature increases with increasing values of x - xc up to a maximum value of x (xmax). For x > xmax the suspensions are unstable at all temperatures studied. For x < xc the suspension of the SiO2 particles is stable up to the temperature of phase separation of the C4E1/water mixture. The flocculation curve is assumed to reflect the influence of local concentration fluctuations with long range correlations on the stability of the SiO2 suspensions. For x > xc these concentration fluctuations are water rich and interact with the hydrophilic surface of the colloidal SiO2 particles by forming an adsorption layer. This layer is assumed to modify the interparticle potential energy-distance curve and to trigger flocculation. For x < xc the concentration fluctuations are C4E1 rich and no adsorption layer is formed at the hydrophilic surface of the colloidal particles. Copyright 1997Academic Press