Impacts of the Surfactant Concentration on the Sedimentation Characteristics of Silica Hard-Shell Microcapsules Containing Phase Change Materials

The effects of surfactant concentration on the sedimentation characteristics of silica hard-shell microcapsules containing trimethylolethane clathrate hydrates as phase change materials have been investigated. A combination additive of a cationic surfactant system and poly vinyl alcohol (PVA) was used to inhibit sedimentation. The concentration of surfactants was varied between 0 and 6,000 ppm, while that of PVA was maintained at 2,000 ppm. The apparent shear viscosities and the apparent volume fractions of the sedimentation were measured. In each solution, the hydraulic diameter of the molecular structure in each solution was also determined. From the results of sedimentation experiments, it was found that the sedimentation was effectively reduced when the surfactant concentration was greater than 4,000 ppm. The extensional viscosity of the fluid was found to dominate the sedimentation characteristics of the systems with high concentrations. In contrast, the sedimentation velocity was observed to be comparatively high for a surfactant concentration of 2,000 ppm. In the present study, surfactant molecules were found to adsorb on the surface of the microcapsules. This leads to the disappearance of high-order structure of the combination additives, increasing counter-ion molar ratio, while the extensional viscosity decreases. Considering that the viscosity affects the flow characteristics, a surfactant concentration of 4,000 ppm was found to be optimal among the present conditions.