Statica en dynamica van deeltjesbevochtiging: vorm is essentieel

This work focusses on the dynamic and static events occurring for ellipsoidal particles during and after adsorption to a liquid-liquid interface. Because the ellipsoidal shape has already proven its use in the area of interface stabilization and shows promise in drug delivery due to its unique interaction with membranes, it is crucial that the adsorption process of such particles is investigated in detail. In terms of dynamics, when an ellipsoid adsorbs to a fluid interface, theoretical models with a different level of complexity propose intriguing pathways, but are qualitatively and quantitatively quite different, with no experimental verification tools available. After adsorption, the ellipsoidal particles assume a horizontal position when there are no steric obstructions. The non-spherical shape causes interface undulations which in turn induce capillary interactions that change when the particle's wetting characteristics change. Therefore, it is important to acquire accurate knowledge about this relation between particle shape and wetting properties. Furthermore, there are indications of an aging contact line around particles adsorbed at an interface. Because a changing contact angle might change the microstructure of high-interface substances, such as a transition between a Pickering-Ramsden emulsion and a bi-continuous interfacially jammed emulsion gel, a more thorough investigation of the contact line's behavior over time is needed. To obtain experimental proof of the modeled adsorption trajectories, we visualize these trajectories using high-speed confocal laser scanning microscopy. With this technique, fluorescently labelled ellipsoids are tracked in time by combining two- and three-dimensional imaging methods. The trajectories resulting from these experiments show a good qualitative agreement with the available models, but only the more complex of the two models considered here, is accurate on a quantitative scale. As long as viscosity of both phases is kept constant within a relatively limited range, the trajectory does not seem to depend on the absolute value of phase viscosities, considering both the time scale and the particle orientation. Whereas one is unable to determine the role of hydrodynamics directly from these constant viscosity experiments, the application of a large viscosity gradient, clearly shows that they need to be taken into account when predicting trajectories from calculations. The capillary number of the adsorption process for the cases with constant viscosities also indicates that hydrodynamics are indeed indispensable. To measure the three-phase contact angle of ellipsoids, a technique called freeze-fracture shadow-casting cryogenic scanning electron microscopy is used, in which the immersion depth of a particle can be derived from the shape of its shadow cast behind it. A systematical increase of the aspect ratio of the ellipsoids showed a gradual decrease of the contact angle. This linear decrease allows to simplify the general Young-Laplace equation for ellipsoids to a linearized version. Using this linearization, the relation between shape and contact angle provides information about the value of the effective line tension, a term containing both the effects of an increased contact line length reflected by the actual line tension and the possible surface energy change when particles are becoming more stretched. The existence of contact line pinning is inherently linked to a particle's out-of-plane rotation at the interface, with a pinned particle showing no rotation at all. By photo-bleaching a dark spot on a fluorescent spherical particle, we can track its rotational diffusion with confocal laser scanning microscopy. Both interfaces with a high and a low surface coverage seem to pin the particle, inhibiting all rotation within the experimental time window both out-of-plane and in-plane. Sudden jumps at high surface coverage, indicate that when sufficient energy is provided the contact line may jump towards a new position closer to its true equilibrium position. Roughness seems to play an important role in this matter for it causes interface undulations, capillary trapping a particle's position and orientation in a colloidal crystal and increasing its hydrodynamical radius. The latter greatly reduces in-plane rotation on the single particle level.nrpages: 153status: publishe