Measuring interactions between colloidal (nano)particles

Colloidal dispersions, which are liquids with finely dispersed particles of between 1 nanometer and 10 micrometer in size, are ubiquitous in nature and in man-made applications. The stability of colloidal particles and the processes underlying their assembly into functional materials are governed by the interaction forces between the particles, and knowledge of these interaction forces is thus crucial to understand and predict their properties. In this thesis we explore methods to measure these interaction forces experimentally, with a focus on measurements of nanoparticles because their interactions are particularly difficult to predict using theory. We first review the different techniques which have been used to perform such measurements and their relevance to nanoparticles, and subsequently study two conceptually different methods by which interaction forces may be extracted from real-space microscopy data. This is done firstly through the analysis of equilibrium distribution functions which may be obtained from a single snap-shot of the particle positions. We demonstrate such measurements for a number of different (nano)particle systems using different 3D optical and electron micron microscopy techniques. Finally, we demonstrate using simulations and experiments that for micron-sized colloids interaction forces may be extracted without relying on equilibrium conditions through measurement of the particle dynamics using high-speed 2D and 3D optical microscopy, and extend this method to allow for measurement of particles with anisotropic (dipolar) interaction forces out of equilibrium.