Nanofluids, 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.

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Le comportement d’un fluide confiné dans un milieu poreux peu perméable (micro- and méso-pores) a été étudié en ce qui concerne ses propriétés de diffusion de masse, de conductivité thermique et de thermodiffusion. Pour ce faire des simulations de dynamique moléculaire hors équilibre ont été réalisées sur des mélanges binaires modèles placés dans des conditions thermodynamiques diverses, confinés dans des milieux poreux de géométrie lamellaire de différentes natures (lisse ou atomique, plus ou moins adsorbant) en utilisant l’ensemble __//_ et l’ensemble grand canonique. Les résultats ont montré que les effets du milieu poreux sur les propriétés de transport sont d’autant plus marqués que lataille de pore est petite, que l’adsorption est forte et que la température est basse. Les résultats ont permis d’évaluer quantitativement ces effets. Il a aussi été montré que la rugosité des murs a un impact très important sur le coefficient de diffusion de masse et non négligeable sur celui de thermodiffusion. / The aim of this work was to study how a fluid confined in a low permeability porous medium (micro- and meso-porous) behaves concerning its properties of mass diffusion, thermal conductivity and thermal diffusion. For this purpose, non-equilibrium molecular dynamics simulations have been performed on simple binary mixtures placed in various thermodynamic conditions, confined in a porous medium of lamellar geometry of different types (structure-less or atomistic, more or less adsorbent) in __//_ and grand canonical ensembles. The results show that the effects of porous medium on transport properties are more pronounced when the pore size is small, the adsorption is strong and the temperature is low. The results allowed to evaluate these effects quantitatively. In addition, it has been found that the wall roughness has a major impact on the mass diffusion coefficient and a non negligible one on the thermal diffusion coefficient.