Structural analysis of ellipsoidal colloid clusters formed under depletion interactions

Abstract

The behavior of colloidal particles in fluid is an important subject of interest in many industries, such as cosmetics, food, paints and electronics. Especially, the phase behaviors of the particles with mutually attractive interactions have been researched thoroughly. Various phases, including free particles, fluid of clusters, gels and glasses, have been observed based on multiple factors, such as the volume fraction, particle interaction and particle size. While the particle shape is also known to significantly affect the colloid behavior, most of the studies have been limited to spherical particles both in experiments and simulations. In this study, both experimental and theoretical treatment of anisotropic particles will be probed. Experimentally, method of synthesizing of monodisperse, prolate ellipsoids will be discussed. Theoretically, the phase behavior and response to shear of prolate ellipsoids interacting under depletion forces will be examined. The difficulty in experimental studies of anisotropic particles is in obtaining monodisperse particles with well-defined interaction behavior. Naturally existing anisotropic particles such as laponite or calcium carbonate are often polydisperse in size. In this study, we examine the mechanical deformation method for the fabrication of prolate ellipsoidal particles. The effect of extensional strain, heat and polymer film on both polystyrene particles and polymethylmethacrylate (PMMA) particles will be discussed. Theoretically, the difficulty in dealing with anisotropic particle results from lack of analytical definition of the particle interaction. While equations, such as the Gay-Berne equation, exist for treating particle anisotropy, they lack in the theoretic validation and matching with actual experimental situations. Therefore, prolate ellipsoidal particles interacting under depletion forces are described in this study through Derjaguin approximation extended to arbitrary convex surface. While the Derjaguin approximation is widely used for the description of spherical particle interaction, its applicability is not limited to spheres. By extending its use to the surfaces of ellipsoids, the interaction of ellipsoids depending on their configurations are calculated. Using this method, the phase behavior and the response to shear of ellipsoidal particles are studied through Brownian dynamics method. The results show the effect of volume fraction, interaction potential and applied shear rate on the ellipsoidal particle system.