Ugaz, Victor M2014-12-122017-04-072014-12-122017-04-072012-122012-10-05http://hdl.handle.net/1969.1/148127Nanofluids, a new class of fluids engineered by suspending nanometer-sized particles in a host liquid, are offered as a new strategy in order to improve heat and mass transfer efficiency. My research was motivated by previous exciting studies on enhanced mass diffusion and the possibility of tailoring mass transport by direct manipulation of molecular diffusion. Therefore, a microfluidic approach capable of directly probing tracer diffusion between nanoparticle-laden fluid streams was developed. Under conditions matching previously reported studies, strong complexation interactions between the dye and nanoparticles at the interface between fluid streams was observed. When the tracer dye and surfactant were carefully chosen to minimize the collective effects of the interactions, no significant change in tracer dye diffusivity was observed in the presence of nanoparticles. Next, adapting tracer dyes for studies involving colloidal nanomaterials was explored. Addition of these charged tracers poses a myriad of challenges because of their propensity to disrupt the delicate balance among physicochemical interactions governing suspension stability. Here it was shown how important it is to select the compatible combinations of dye, nanoparticle, and stabilizing surfactant to overcome these limitations in low volume fraction (< 1 vol%) aqueous suspensions of Al2O3 nanoparticles. A microfluidic system was applied as a stability probe that unexpectedly revealed how rapid aggregation could be readily triggered in the presence of local chemical gradients. Suspension stability was also assessed in conjunction with coordinated measurements of zeta potential, steady shear viscosity and bulk thermal conductivity. These studies also guided our efforts to prepare new refrigerant formulations containing dispersed nanomaterials, including graphene nanosheets, carbon nanotubes and metal oxide and nitride. The influence of key parameters such as particle type, size and volume fraction on the suspension's thermal conductivity was investigated using a standard protocol. Our findings showed that thermal conductivity values of carbon nanotube and graphene nanosheet suspensions were higher than TiO2 nanoparticles, despite some nanoparticles with large particle sizes provided noticeable thermal conductivity enhancements. Significantly, the graphene containing suspensions uniquely matched the thermal conductivity enhancements attained in nanotube suspensions without accompanying viscosity, thus making them an attractive new coolant for demanding applications such as electronics and reactor cooling.suspension stabilityheat transfertracer dyemicrofluidicsnano refrigerantmass diffusionNanofluidThesis