Particles in Fluids: From the Synthesis, Properties and Applications of Micro-Emulsions, Janus Particles to Microrheolog

Colloidal particles in fluids and fluid interfaces give rise to a number of interesting phenomena which include the formation of particle-stabilized emulsions called "Pickering-Ramsden" or simply "Pickering" emulsions. These emulsions were first brought to notice by S.U. Pickering in 1907. However, it is only in the past two decades that there has been any major surge of interest in them.;Pickering emulsions are widely found in nature and industry: food, pharmaceuticals, oil industry etc. While the most common type of Pickering emulsion studied has a Newtonian dispersed phase, our interest is in emulsions with a dispersed phase that can be non-Newtonian such as one that can be subjected to a phase change under certain conditions of concentration, temperature or pressure. Further, there are Pickering emulsions stabilized by non-spherical colloidal particles. The shape and aspect ratio of these emulsifiers can give rise to interesting emulsion properties and can affect the way the emulsion droplets flocculate, pack, form microstuctures and the overall stability of the emulsions. Analyzing the rheology and flow properties of such emulsions is crucial as far as understanding their stability and breakdown are concerned. Such studies also provide insights into the designing processes involving their synthesis, transport and storage.;Particles at fluid interfaces can also be engineered to obtain smart materials such as "Janus" particles or particles which have "two faces" by virtue of differences in their surface chemistry and composition giving rise to anisotropy in properties. Among the plethora of Janus particle applications, we are interested in the synthesis of "self-propelling particles" which can be useful in drug delivery, sensing or chemotaxis.;Another interesting manifestation of colloidal particle behavior in fluids is "microrheology" wherein they can extract information about the mechanical properties of the medium they are swimming in. Complex fluids such as polymer solutions and biological materials are viscous or visco-elastic in nature and tracking the motion of colloidal particles in such fluids can give useful insights into the microscopic basis of the fluid properties and how these can influence the motion of such particles.;With these broad guides in mind, the overall aim of our research is twofold.;The first aim is to gain a better understanding of the stability and bulk viscoelastic properties of Pickering emulsions (oil-in-water or water-in-oil) including both Pickering emulsions with non-Newtonian dispersed phase and emulsions that are stabilized by non-spherical particles. We synthesize "model" oil-in-water emulsions where the oil phase is a solid at room temperature. "Model" non-spherical particles (ellipsoids, rods and discs) are either purchased or synthesized from polymeric microspheres using the standard film-stretching" technique followed by the synthesis of Pickering emulsion. Time sweeps, linear and oscillatory shear tests are imposed on these emulsions and their response at different concentration, temperature and time are analyzed.;Colloidal particles are also used to obtain Janus particles. Two synthesis approaches are adopted: the "Pickering Emulsion route", where we make use of the particles' natural tendency to be at the interface to impart chemical anisotropy and the direct deposit of "Masking" agent route where one hemisphere of a spherical particle is directly coated with suitable metals using physical vapor deposition methods. These Janus particles are made self-propelling (Janus motors) when their "active" half catalyzes a chemical reaction, converting the chemical energy to mechanical energy of the motor. The catalyst can either be a metal or active biomolecules such as enzymes. The basic principle suggests that anisotropy in the catalysis of the reaction creates a local concentration gradient of "fuel" around the Janus particle resulting in "propulsion". The fuel for our system is the commonly used Hydrogen Peroxide (H 2O2) while Platinum (Pt) is used as an active metal, catalyzing the decomposition of H2O2. The enzyme used is catalase, naturally found in mammals, also decomposing H2O2. In search of an alternate fuel source we have also attempted to make enzyme-catalyzed Janus motors using Alcohol Dehydrogenase/NADH/Isobutyraldehyde as the enzyme/co-factor/fuel system.;We have used the Pt/H2O2 system to understand how solution viscosity affects the motion of Janus motors and have concluded that it is not only solution viscosity but also the kind of viscosifying agent used that uniquely influences motor motion. Our enzyme-based system presents an interesting alternative to the commonly used metal-based systems most importantly due to the biocompatibility of the former that attracts applications in biology and medicine. We have found that the enzyme-based Janus motors are not as propulsive as the metal-based systems due to a number of factors but they still constitute a system with tremendous potential if ways to improve enzyme kinetics are harnessed.