Diffusion in Multi Component Mixtures

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Thermal gradients are ubiquitous in human and industrial activities, and can induce a wide range of thermal transport phenomena, mass separation in multicomponent mixtures (thermodiffusion), or solute motion in solutions (thermophoresis). These phenomena are of practical interest in defining the composition of oil reservoirs, the replication and accumulation of short DNA fragments, or the design of microscale and nanoscale thermophoretic devices for the manipulation of small volumes of liquids or solutes in scales ranging from nano to micrometer. In subsurface hydrocarbon reservoirs, the temperature varies vertically and sometimes horizontally (i.e. there may be non-negligible horizontal gradients). Over time, some hydrocarbon components segregate towards the cold side and others towards the hot side of the reservoir due to thermodiffusion. Thermal segregation will also initiate molecular diffusion driven by concentration gradients and natural convection. As a result, it is crucial to determine the fluid compositional variation in the oil reservoir. Current models that are used in oil reservoir simulators are based on Fick’s law, which cannot account for these compositional interactions and the non-ideal behaviour of the species. The response of a suspension or a fluid mixture to a temperature gradient depends in a subtle and essentially unknown way on the molecular interactions. Understanding how these molecular interactions determine thermal transport is essential to advance the development of devices and processes of technological and industrial interest. An accurate molecular multicomponent diffusion model is needed to be able to predict this mechanism. We propose to measure experimentally these diffusion coefficients and introduce for the first time a molecular dynamics model combined with irreversible thermodynamic theory. Comparison between experimental and numerical results will help us develop an accurate molecular diffusion model. Such experimental measurements have been performed on board the International Space Station but sofar had a limited success in ground laboratory. This is due to the presence of convection, which creates mixing and therefore destroys the separation process. With a high cost and limited access to space experiment, an innovative experimental design to measure these diffusion coefficients in our laboratory for different average mixture temperature is proposed. This experimental approach requires a precise sample design to overcome the gravity effect (i.e. convection which leads to mixing). Succeeding in measuring the diffusion coefficients in laboratory will lead to an accurate molecular multicomponent diffusion model, which is in great needs in oil reservoir simulation.