Abstract
The wettability of particles and the way they attach to liquid interfaces is important for many technologies where powders are mixed with liquids or used as emulsifiers. Most such powders are highly agglomerated into larger aggregates that are highly porous. The attachment of such porous particles to liquid–fluid interfaces has not been studied in detail, especially in cases where the porous particles are impregnated with another fluid phase. The overall aim of the thesis is to study the behaviour of particles at the liquid–fluid interface with an emphasis on non– spherical and porous solid particles. We study the orientation of anisotropic microparticles and measure the contact angle of smooth and porous microparticles with the gel–trapping technique (GTT) to find the wettability of microparticles adsorbed in fluid interfaces. This technique allows us to obtain micrographs by scanning electron microscopy (SEM) for particles resting on polydimethylsiloxane (PDMS), which replicates the non–polar phase and allows for measuring the particle contact angle. We show the results of the typical attachment and orientation of needle–like (aragonite), rhombohedra–like (calcite) microcrystals and ethyl cellulose micro–rods, as well as highly porous hydrophilic and hydrophobic silica microparticles at these liquid interfaces. The importance of these results is in gaining an understanding of the adsorption behaviour and demonstrating actual information on anisotropic particles which have potential applications in industrial formulations and products. We also investigate how carboxylate modified latex (CML) microparticles adsorb at liquid surfaces and the preferred type of emulsion they can stabilise depending on the particle size and the surface density of carboxylic groups. We also study, both theoretically and experimentally, the effect of salt in the aqueous phase on the contact angle of such microparticles. The main finding is that the wettability of CML microparticles is governed by the carboxylic group density on the particle surface rather than their ionisation. We demonstrate that the type of Pickering emulsions is governed by the wettability of microparticles at the oil–water interface. We study the effect of the initial impregnation of porous particles with polar or non–polar phases on their attachment at liquid interfaces both theoretically and experimentally. Model supra–particles have been prepared by using building blocks of smaller colloid particles packed in a spherical aggregate. The particles were produced by drying latex particle suspensions of various particle volume fractions and concentrations, followed by partial fusion of the particles achieved by thermal annealing. We have studied the particle surface morphology and porosity and showed how the annealing temperature, the initial particle volume fraction of sulphate latex suspension and the evaporation temperature can be used to control the supra–particle final structure and porosity. Furthermore, we have investigated the link between the wettability of the porous supra–particle building blocks, i.e. sulphate latex particles, and the macroscopic (apparent) contact angle of the porous supra–particle when attached to liquid surfaces. The contact angles of porous supra–particles infused with water at the oil–water interface were found to be much bigger than those at the air–water interface as expected. We also show how the type of liquid filling the pores of the supra–particle affects its macroscopic contact angle at the oil–water interface.