Experiments in which oil, water and colloidal particles meet

Abstract

In this thesis, the results are reported of experimental studies in which oil, water and colloidal particles meet. Colloidal particles are particles that have at least one characteristic length scale in the range between a few nanometers (nm) and several micrometers (μm). Mixtures of oil and water, one finely dispersed in the other (called emulsions), are also colloidal systems. We have shown that monodisperse oil-in-water emulsions, with sub-10 μm droplet diameters, can be synthesized on a bulk scale using various methods, all based on a nucleation and growth process. These droplets were used as templates for the growth of elastic, partially permeable microcapsules that were fluorescently labelled. Because of the liquid nature of the template, the core could be easily exchanged after capsule synthesis. Specifically, we have shown that the oil core could be exchanged for a liquid surfactant that was sustained in the capsule interior after re-dispersing the particles in water. This surfactant cargo was spontaneously released from the capsules in the vicinity of an oil-water interface, because the surface active material prefers partitioning over the oil-water interface, and the oil phase, when compared to the aqueous phase. Applications in this direction are of potential interest for enhanced oil recovery because surfactant encapsulation prevents redundant surfactant loss in the oil reservoir, whereas the wetting behavior and the surface tension are favorably affected upon (controlled) release, facilitating emulsification. In addition to encapsulation studies, the as formed capsules are also of interest for self-assembly studies. The spherical capsules were converted into anisotropic, bowl-shaped particles by various buckling processes, affecting the amount of oil encapsulated in the core. When mixed with complementary (convex) shaped colloidal spheres, these (concave) bowl-shaped capsules were found to form lock-and-key composite particles in time after applying an electric field, a phenomenon generalized to both silica and polystyrene spheres. These electric-field-induced interactions were reversible and controllable on millisecond timescales but could also be made irreversible by playing with the force balance such that complex shaped colloids with mixed composition were synthesized. Moreover, by coating the flexible bowl-shaped capsules with an additional shell-layer, the configuration of the (partially) buckled capsules was preserved even upon drying and solvent exchange. This second shell growth generated particles not only of interest for self-assembly studies, because of the preserved anisotropic shape, but also for controlled encapsulation studies, because it enabled tuning of the shell permeability. Finally, this thesis describes a novel class of emulsions: non-touching Pickering emulsions. Typically surfactants or colloids are added which adsorb onto the interface and thereby prevent de-mixing of oil and water in emulsions. Because of their micron-sized dimensions, colloids become irreversibly trapped when touching both the oil and water. We examined experimentally an alternative scenario: non-touching (effectively non-wetting) particles that do not sit in the interface, but rather fully in the oil phase as a result of a force balance. Apart from the fundamental novelty, this new Pickering emulsion was reversible: the addition of salt enabled a triggered de-mixing of the two phases, which is promising for applications.