Electric field induced self-assembly of mesoscale structured materials and smart fluids

This dissertation aims to study the forces that drive self-assembly in binary mixtures of particles suspended in liquids and on fluid-liquid interfaces when they are subjected to a uniform electric or magnetic field. Three fluid-particle systems are investigated experimentally and theoretically : (i) Suspensions of dielectric particles in dielectric liquids; (ii) Suspensions of ferromagnetic and diamagnetic particles in ferrofluids; and (iii) Dielectric particles on dielectric fluid-liquid interfaces. The results of these studies are then used to estimate the parameter values needed to assemble materials with desired mesoscale microstructures. The first fluid-particle system studied is an electrorheological (ER) fluid formed using a mixture of positively and negatively polarizable particles. An important property of ER fluids is that their rheological properties can be modified on demand, within a few milliseconds, by applying an external electric field. Then, after the field is switched off, they go back to their original state. However, if only positively or negatively polarizable particles are used, the distribution of particles will fragment into chains and columns. Experimental results show that if a suitable mixture of positively and negatively polarized particles is used for making the ER fluid, the particle chains come closer, and the volume they occupy decreases. These results agree with the direct numerical simulations (DNS) based on the Maxwell Stress Tensor (MST) and point dipole methods. The application of the electric field results in the formation of a closely packed three-dimensionally connected structure. The influence of varying the electric field intensity, particle size, polarizabilities, and number ratio are characterized in terms of the extent of connected pattern formation which is obtained numerically and the experimentally measured yield stress. The yield stress for an ER fluid formed using a particle mixture is larger than that for an ER fluid containing only one type of particles and is maximum for a critical volume fraction. The second problem studied is the magnetorheological fluids (MR) formed using mixtures of micron-sized iron and glass particles in a liquid. The rheological behavior of MR and ER fluids is similar. For example, when an external magnetic field is applied to a MR fluid, the particles are magnetized and rearrange relative to one another, which modifies its rheological properties almost instantly. Also, when only one type of particles is used to prepare MR fluids, i.e., either positively or negatively magnetized particles, the particle distribution becomes fragmented into chains and columns. If a suitable mixture of positively and negatively magnetized particles is used, individual particle chains of one type attract the other type, creating a band with no gaps. This results in the formation of a closely packed connected structure. The MR fluids’ yield stress behavior is experimentally investigated, formed by suspending mixtures of ferromagnetic and diamagnetic particles in ferrofluids (FF), which show that the yield stress is maximum when the volume fraction of ferromagnetic particles is around sixty percent. The rheological response of MR fluids depends on parameters such as the particles’ concentration, magnetic susceptibilities of the suspending liquid, and the applied magnetic field intensity. The third problem investigated is that of making UV-cured thin films with embedded monolayers of gold particles on their surfaces. This is achieved by self-assembly of gold nanoparticles on a UV curable liquid’s surface by applying an electric field normal to the surface. The substrates are then used for Surface-Enhanced Raman Scattering (SERS) applications. The experimental results show that the substrates’ performance depends on the particle concentration and the inter-particle distance. The laboratory-built substrates are found to be more efficient than the commercial SERS substrates