V. W. A. de Villeneuve

Hard sphere crystal nucleation and growth near large spherical impurities

We report how large spherical impurities affect the nucleation and growth of hard sphere colloidal crystals. Both the impurities and the colloids are fluorescently labelled polymethylmetacrylate particles and are dispersed in an optically and density matching solvent mixture. Crystal growth, initiated either at the impurity surface, or at the sample bottom, was studied by imaging sequences of two-dimensional xy-slices in the plane of the impuritys centre of mass with a laser scanning confocal microscope. At least two factors determine whether a large impurity can function as a seed for heterogeneous nucleation: timescales and impurity curvature. The curvature needs to be sufficiently low for crystal nuclei to form on the impurity surface. If bulk crystal growth has already approached the impurity, bulk growth is dominant over growth of crystallites on the impurity surface. Such surface crystallites eventually reorient to adapt to the overall bulk crystal symmetry.

Orientation dependent Stokes drag in a colloidal liquid crystal

Stokes drag on the (sub)micrometre scale plays a key role in phenomena ranging from Brownian motion to the rheology of particulate suspensions. We report the first measurement of the direction dependent Stokes drag in a nematic liquid crystal of colloidal rods, where the viscous forces are of equal importance to the elastic forces. By tracking a sedimenting sphere with combined fluorescence confocal microscopy and polarization microscopy we find that the Stokes drag for motion along the director is two times larger than for motion perpendicular to the director. This brings the unique viscoelastic properties of a colloidal liquid crystal into focus.

Rigid sphere transport through a colloidal gas–liquid interface

In this paper we report on the gravity-driven transport of rigid spheres of various sizes through the fluid-fluid interface of a demixed colloid-polymer mixture. Three consecutive stages can be distinguished: (i) the sphere approaches the interface by sedimenting through the polymer-rich phase, (ii) it is subsequently transported to the colloid-rich phase and (iii) it moves away from the interface. The spheres are covered by a thin wetting film of the colloid- rich phase, to which they are eventually transported. The ultralow interfacial tension in these phase-separating mixtures results in very small capillary forces so that the process takes place in the low Reynolds regime. Moreover, it enables the investigation of the role of capillary waves in the process. Depending on the Bond number, the ratio between gravitational force and capillary force acting on the sphere, different transport configurations are observed. At low Bond numbers, the drainage transport configuration, with a dominant capillary force, is encountered. At high Bond numbers, spheres are transported through the tailing configuration, with a dominant gravitational force. By varying the sphere diameter, we observe both transport configurations as well as a crossover regime in a single experimental system.

Persistence in practice

We present a scheme for accurately calculating the probabilities of persistence on sequences of n heights above a level h from the measured n+2 points of the height?height correlation function of a fluctuating interface. The calculated persistence probabilities compare very well with the measured persistence probabilities of a fluctuating phase-separated colloidal interface for the whole experimental range.

Residence and waiting times of Brownian interface fluctuations

We report on the residence times of capillary waves above a given height h and on the typical waiting time in between such fluctuations. The measurements were made on phase-separated colloid-polymer systems by laser scanning confocal microscopy. Due to the Brownian character of the process, the stochastics vary with the chosen measurement interval Δt. In experiments, the discrete scanning times are a practical cutoff and we are able to measure the waiting time as a function of this cutoff. The measurement interval dependence of the observed waiting and residence times turns out to be solely determined by the time-dependent height-height correlation function g(t). We find excellent agreement with the theory presented here along with the experiments.

Statistics of fluctuating colloidal fluid-fluid interfaces

Fluctuations of the interface between coexisting colloidal fluid phases have been measured with confocal microscopy. Due to a very low surface tension, the thermal motions of the interface are so slow that a record can be made of the positions of the interface. The theory of the interfacial height fluctuations is developed. For a host of correlation functions, the experimental data are compared with the theoretical expressions. The agreement between theory and experiment is remarkably good.