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Numerical simulation of interfacial flows in micropores Ahmadlouydarab, Majid


Recent technological developments in microfluidics and fuel cells have given special significance to interfacial dynamics in small pores. Using a diffuse-interface model and a finite-element code, I have simulated three associated problems numerically: gas-liquid flow regimes in micropores; relative permeability for two-phase flow through a model porous medium; and dynamics of sessile drops under the simultaneous action of a wettability gradient and an external flow. For two-phase flows in corrugated microchannels driven by a pressure drop, a number of flow regimes were observed: gas flow, blockage, liquid flow, bubble-slug flow, droplet flow, annular flow and annular-droplet flow. Some of the regimes are known from prior studies in macroscopic pipes, but the others are new and specific to the micropores. Then a map of flow regimes has been constructed in the plane of the liquid saturation and the imposed pressure drop. The transitions among certain flow regimes show significant hysteresis, largely owing to the pinning of the interface at sharp corners in the flow conduit. As an extension of the above study, I computed the relative permeability of a model porous media made of corrugated tubes, using an averaging scheme over a pore-size-distribution of a real porous medium. I discovered that the flow rates vary nonlinearly with the pressure gradient, and that the extended Darcy's law does not hold in general. In the third project, I found that for each prescribed wetting gradient, there is a narrow range for the cross flow within which a stationary drop can be achieved. The drop motion exhibits strong hysteresis, i.e. sensitivity to initial conditions and forcing history. Two drops merge or separate depending on the competition between wettability and external flow. In general, the wettability gradient favors catch-up and coalescence while the external flow favors separation. These numerical simulations have demonstrated that novel interfacial dynamics can be produced in micropores where capillary forces and contact line dynamics play more important roles than in larger spatial dimensions. The numerical results may serve as guidelines to future experiments and technological development in microfluidics and lab-on-chip devices.

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