Performance Improvement of Electro-Responsive Smart Fluid Based on Nanoparticle Characteristics

Abstract

Stimuli-responsive materials, so-called smart materials, change their structure, dimension, and interactions in response to external stimuli, such as pH, temperature, light, mechanical force, and electric or magnetic field. Stimuli-responsive materials have received tremendous worldwide attention because of their fascinating characteristic, e.g. adjustability of response based on environmental condition. In particular, electro-responsive materials, defined as the materials that exhibit controllable properties according to electric field, are considered as the most feasible candidate for practical applications due to their low power consumption, rapid reversible transition, and simple fluid mechanics. Up to date, various electro-responsive materials have been produced and adopted to electro-responsive fluid. However, few researches have addressed the dependence of electro-responsive activities on particle features. Moreover, there are still several limitations for substantive commercialization of electro-responsive fluids due to deficient performance and sedimentation problem. Consequently, it is still challenging to develop innovative electro-responsive materials that represent high efficiency. In addition, it is required to obviously clarify the effect of particle properties on electro-responsive behavior to better understand the mechanism underlying electro-responsive fluids. This dissertation describes electro-responsive characteristics based on different critical parameters, including particle geometry, dielectric property, and electrical conductivity, by fabricating inventive nanoparticles for electro-responsive fluids. First, electro-responsive characteristics depending on particle geometry is examined by varying the aspect ratio of graphene oxide (GO)-wrapped silica materials. Electrorheological (ER) effect of the GO-wrapped silica material-based fluid is improved as the aspect ratio of the particle increases, which is attributed to geometrical effect (e.g. flow resistance and mechanical stability) and superior dielectric properties. Second, the relationship between particle shell structure and electro-responsive property is investigated by synthesizing the size-controlled single-shell and double-shell SiO2/TiO2 hollow nanoparticles for ER fluids. It is revealed that outstanding electro-responsive performance of double-shell SiO2/TiO2 hollow nanoparticles (DS HNPs) is associated with the enhanced interfacial polarization. In addition, electro-responsive efficiency is observed to be enhanced with a decrease in the diameter of particles, ascribed to a large achievable polarizability determined by dielectric constant. Furthermore, remarkable anti-sedimentation property is observed, which promise a sufficient potential for practical application. Third, the correlation of electrical conductivity and electro-responsive behavior is identified by introducing few-layer molybdenum disulfide (MoS2) nanosheets to ER fluid. Electrical conductivity of MoS2 can be tunable by adjusting the annealing temperature because of its semiconductivity. It is meaningful that an in-depth study on the effect of electrical conductivity on ER effect is carried out. Moreover, to the best our knowledge, this is the first time to report the possibility of transition-metal dichalcogenides as a candidate of ER fluid. This study may provide promising approaches for performance improvement of electro-responsive smart fluids.