Henk N. W. Lekkerkerker

Insights into phase transition kinetics from colloid science

Colloids display intriguing transitions between gas, liquid, solid and liquid crystalline phases. Such phase transitions are ubiquitous in nature and have been studied for decades. However, the predictions of phase diagrams are not always realized; systems often become undercooled, supersaturated, or trapped in gel-like states. In many cases the end products strongly depend on the starting position in the phase diagram and discrepancies between predictions and actual observations are due to the intricacies of the dynamics of phase transitions. Colloid science aims to understand the underlying mechanisms of these transitions. Important advances have been made, for example, with new imaging techniques that allow direct observation of individual colloidal particles undergoing phase transitions, revealing some of the secrets of the complex pathways involved.

Phase Behaviour of Colloid + Polymer Mixtures

A new treatment of the phase behaviour of a colloid + nonadsorbing polymer mixture is described. The calculated phase diagrams show marked polymer partitioning between coexisting phases, an effect not considered in the usual effective-potential approaches to this problem. We also predict that under certain conditions an area of three-phase coexistence should appear in the phase diagram.

Liquid crystal phase transitions in suspensions of polydisperse plate-like particles

Colloidal suspensions that form periodic self-assembling structures on sub-micrometre scales are of potential technological interest; for example, three-dimensional arrangements of spheres in colloidal crystals might serve as photonic materials, intended to manipulate light. Colloidal particles with non-spherical shapes (such as rods and plates) are of particular interest because of their ability to form liquid crystals. Nematic liquid crystals possess orientational order; smectic and columnar liquid crystals additionally exhibit positional order (in one or two dimensions respectively). However, such positional ordering may be inhibited in polydisperse colloidal suspensions. Here we describe a suspension of plate-like colloids that shows isotropic, nematic and columnar phases on increasing the particle concentration. We find that the columnar two-dimensional crystal persists for a polydispersity of up to 25%, with a cross-over to smectic-like ordering at very high particle concentrations. Our results imply that liquid crystalline order in synthetic mesoscopic materials may be easier to achieve than previously thought.

Direct Visual Observation of Thermal Capillary Waves

We studied the free fluid-fluid interface in a phase-separated colloid-polymer dispersion with laser scanning confocal microscopy and directly observed thermally induced capillary waves at the interface in real space. Experimental results for static and dynamic correlation functions validate the capillary wave model down to almost the particle level. The ultralow interfacial tension, the capillary length, and the capillary time are found to be in agreement with independent measurements. Furthermore, we show that capillary waves induce the spontaneous breakup of thin liquid films and thus are of key importance in the process of droplet coalescence.

Numerical study of the phase behavior of rodlike colloids with attractive interactions

We examine the influence of attractive interactions on the phase behavior of rodlike colloids. We model the rodlike particles by spherocylinders, for which the phase diagram, in the absence of attraction, is known for arbitrary aspect ratio. We consider the case that the attraction is due to depletion forces caused by the addition of nonadsorbing polymer. The range of this attraction is determined by the size of the polymer. If the radius of gyration of the polymer is small compared to the diameter of the rods, we can model the polymer-induced attraction by a suitable generalization of the square-well model for spherical particles. However, for longer ranged attractions, pairwise additive attractions lead to phase behavior that is very different from what is found when the nonadditivity of depletion forces is taken into account. In our simulations, we find evidence for demixing transitions in the isotropic, nematic and solid phases. We compare our simulation results with predictions based on the perturbation theory of Lekkerkerker and Stroobants [Nuovo Cimento D 16, 949 (1994)]. A crucial input in this theory is the so-called free-volume fraction of the hard spherocylinder reference system. In the work of Lekkerkerker and Stroobants, this quantity is estimated using scaled-particle theory. We test the validity of this approach by comparing it to numerical results for the free-volume fraction.

A Time Resolved Static Light Scattering Study on Nucleation and Crystallization in a Colloidal System

Abstract The crystallization process in a colloidal system of slightly charged spherical particles is studied with time-resolved static light scattering. The induction time, the crystallization rate, the scattered intensity after completion of the crystallization process, and the width of the Bragg peaks are found to be strongly dependent on the concentration of the initially metastable colloidal fluid. Assuming a simple crystal geometry, quantities such as the size of the crystallites, the number concentration of the crystallites, and nucleation and crystallite growth rates, as functions of the concentration, are calculated from those experimental quantities.

On understanding microemulsions : II. Thermodynamics of droplet-type microemulsions

A thermodynamic theory of microemulsions containing brine, oil, an ionic surfactant, and a nonionic cosurfactant is given. Conditions for phase equilibria are derived. The treatment is limited to droplettype microemulsions with emphasis on W/O + W systems. Important features are saturation adsorption of surfactant and cosurfactant, the interfacial bending stress, the standard chemical potential of the droplets in the free energy of mixing, and a quantitative treatment of the contribution of the electric double layer to the bending stress. Numerical illustrations show good agreement with experimental data on the influence of salt and cosurfactant on droplet size and interfacial tension in W/O + W equilibria.

Transient gelation by spinodal decomposition in colloid-polymer mixtures

We have investigated with small angle light scattering and optical microscopy transient gelation phenomena which occur in phase-separating colloid-polymer mixtures. The scattering intensity distribution shows a peak at non-zero wave vector and satisfies the asymptotic q−4 Porod behaviour. Consistent with these observations, optical micrographs show an alternating pattern of dark and bright domains. These findings suggest that the polymer-induced depletion forces lead to the formation of a bicontinuous network of colloid-rich and colloid-poor domains, via a spinodal decomposition process. This bicontinuous network rapidly attains a gel-like character as indicated by the arrest of speckle fluctuations. The occurrence of the gel is ascribed to polymer-induced aggregation between the colloids in the colloid-rich phase. Due to the reversible nature of the aggregation the network restructures and eventually the gel collapses, as is manifested by the rapid separation of the colloid-rich phase from the colloid-poor phase.

Depletion interaction between spheres immersed in a solution of ideal polymer chains

The depletion interaction between two spheres due to nonadsorbing ideal polymers is calculated from the polymer concentration profile using the excess (negative) adsorption. Computer simulations show that the polymer concentration profiles around two spheres are well described by the product function of the concentration profile around a single sphere. From the interaction potential between two spheres the second osmotic virial coefficient, B2 , is calculated for various polymer-colloid size ratios. We find that when the polymers become smaller than the spheres, B2 remains positive in the dilute regime. This shows that the depletion interaction is ineffective for relatively small spheres.

Strong, weak and metastable liquids. Structural and dynamical aspects of the liquid state

Structural and dynamical properties of a model liquid system were studied as a function of the range of the interaction using computer simulations. We observed the usual strong dependency of the global phase diagram on the range of attraction. In systems with long range attractive interactions the structure of the liquid state down to the triple point is determined by the short range repulsive forces. For short ranged attractive interactions however the attraction has a noticeable effect. Although the gas-liquid transition becomes metastable as the attractive range becomes shorter, the change in dynamics in these systems when passing the hidden binodal is gradual rather then abrupt.