Interaction, micromechanics, and applications of colloidal particles at fluid interfaces

The goal of this dissertation is to study both the interactions between colloids and micromechanics of colloidal aggregates to comprehensively understand the colloidal particle behavior at 2D fluid-fluid interfaces. This will ultimately lead to the ability to control the material properties and find new pathways for novel materials.We begin by validating laser tweezer measurements at the oil-water interface. We calculate the optical trapping forces exerted by a single laser beam focused on a dielectric sphere located at a 2D oil-water interface. The calculated lateral trapping forces, based on the geometrical optics approximation (GOA), agree with experimental measurements of the trapping force. Importantly, the calculations verify that the radiation force exerted on particles perpendicular to the interface is not sufficient to induce capillary interactions between particle pairs, which could be mistaken for particle-particle interactions. We further find that the trapping forces depend on the three-phase contact angle of the particle at the interface.Using this laser tweezer setup, we study the interaction potential between two particles confined at a pure oil-water interface, and examine how the pair interaction potential relates to bulk properties of colloidal suspensions, such as the suspension radial distribution function, RDF. We use optical laser tweezers to directly measure the interaction force for many particle pairs. We find the sample preparation protocols (i.e., washing the particles) affects the potential the interaction force of unwashed particles is significantly lower than that of washed particles. This also means that unwashed samples include some attractive pairs, while in washed particles all pairs exhibit a strong, long-range repulsion. The repulsion shows a good agreement with the reported force scaling Frep &sim r -4 in the latter case.Next, we examine the tailored interactions between two colloidal particles confined at oil-water interfaces by introducing additives in each fluid or by changing particle shape (i.e., doublet colloids). The fluid conditions are manipulated by the addition of a monovalent salt, anionic, or nonionic surfactants. In the presence of 0.25 M NaCl or 0.25 M NaCl/0.1 mM SDS in the aqueous sub-phase, or 25 muM SPAN 80 in the decane super-phase, the power law scaling for the long-range repulsive force (Frep &sim r-4) between two spherical particles vanishes and a long-range attractive interaction becomes significant. Moreover, we measure the interaction force between doublet particles at a pure decane-water interface using active and passive methods. The interactions are purely attractive at long-range. Interestingly, the interaction is anisotropic and consistent with capillary quadrupole interactions (Fcap &sim r -5) which account for the observed aggregated microstructure.We induce colloidal aggregates at oil-water interfaces by introducing either 0.25 M NaCl/0.1 mM SDS in the sub-phase or 25 muM SPAN 80 in the super-phase. Such additives alter the interactions between colloids by changing the Debye screening length and wetting properties. Changes in the interaction lead to difference in the macrostructures at bulk interfaces. Aggregates formed when NaCl/SDS is added to the sub-phase are more rigid than those formed when SPAN 80 is added. This is consistent with the more open microstructure in NaCl/SDS. Meanwhile, the weak bond rigidity in SPAN 80 allows the individual particles to rearrange in the microstructure (i.e., breakup, formation, rolling, and sliding), resulting in dense microstructure.Finally, we develop new methods for fabricating materials and controlling their properties. First, we assemble colloidal particles on a solid substrate via capillary forces mediated by the presence of a nearby oil-water interface. We demonstrate that the resulting crystals are soft and anneal defects rapidly, making this a facile and possibly scalable method of directed colloidal assembly. We analytically calculate the lateral capillary interaction caused by overlapping the deformed interfaces above the colloids on the solid substrate, which drives such two-dimensional (2D) crystals. We solve the augmented Young-Laplace equation to predict the interface profiles, followed by calculating the deformed surface area and the free energy change. (Abstract shortened by UMI.)...