Rheology of complex fluids in micro/nanoscale passages

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Many natural or industrial fluids such as foods and beverages, pharmaceuticals, coatings, and detergents involve liquids in forms of emulsions or foams. In most cases, a dispersed liquid forms droplets in a second immiscible liquid or gas creating an interface where a thin film separates close-contact droplets. The dispersed droplets interact with each other through this thin film so it is importance to measure the physicochemical properties of these formed interfaces. Viscosity is a fundamental fluid property and is defined as the relation between stress and deformation in fluid systems. For emulsions and foams, the viscosity at the interface of liquid-liquid or liquid-gas layer can be very different from the bulk viscosity and dominates how the dispersed droplets interact with each other. That is why interfacial viscosity has been primarily linked to the long-term stability of dispersion, emulsions, and foams. Exact measurement of viscosity of interfaces is extremely challenging and is usually done by indirect methods such as addition of tracing particles that could affect the chemistry of the interfaces. Moreover, large scale measurement techniques lack the sufficient sensitivity to produce reliable measurements. This research program focuses on developing a nanofluidic platform coupled with total internal reflection microscopy (TIRM) as well as numerical models to directly measure the viscosity of thin liquid films without addition of any tracer particles. The nanofluidic system containing parallel channels with characteristic sizes as small as 10 nm will be fabricated to evaluate interfacial viscosity at length scales comparable to the thin film in real dispersion systems. Many-channel geometry can produce large amount of data to increase the statistical reliability of our measurement while maintaining the same experimental conditions for all the channels. Capillary forces will be used to induce the flow in nanochannels. Incorporating coating methods, the effects of wettability on apparent viscosity of the thin liquid film will be studied. TIRM microscopy will be employed to directly measure dynamic contact angle of liquids during capillary motion in the channels. A theoretical model will be developed to capture the capillary flow of both Newtonian and non-Newtonian liquids in channels. The general model will be incorporated in our experimental results to calculate thin film viscosity. The expected contributions from the proposed program can significantly advance our knowledge of complex liquid systems and result in developing techniques to control the stability of emulsions and dispersions according to the desired outcome. These techniques have many applications in the areas under investigation in my group, including water-in-oil emulsion stability, mixing and mass transfer in porous media during in-situ bitumen extraction, and non-aqueous bitumen extraction.