Evaporation in microfluidic systems : From radially evolving capillary structures to phyllotaxic spirals

Abstract : Capillarity is a common phenomenon encountered in Nature. In the context of the drying of porous media with pore size in the micrometer-millimeter size range, capillary effects play a dominant role in controlling the phases (liquid or vapor) distribution in the pore space as drying occurs. The basic idea of the present work is to study the drying of pure, wetting fluids in micro-fabricated, quasi-2D, model porous media (hereafter called micromodels). We present results obtained for different micromodel geometries. Typically, the micromodels used consist of arrangements of cylinders sandwiched between a top and bottom plate. Phases distribution and evaporation rates in such micromodels can easily be measured by direct visualizations and subsequent image processing.By tuning the cylinders pattern, one can first obtain micromodels for which the drying rate is almost constant, from the beginning of the drying experiment to the total evaporation of the liquid initially filling the system. Typically, this situation is obtained when the pores size decreases from the micromodel center to the periphery (the micromodels are axisymmetric). On the contrary, when the pores size increases from the center to the periphery, invasion of a stable drying front is observed, resulting in a much longer total drying time.We also designed another type of micromodel where the cylinders are arranged in a Fibonacci spiral pattern, a design inspired by phyllotaxic structure. In such systems, thick liquid films develop along the spirals during drying and play a key role in the drying kinetics. This situation is reminiscent of that already studied by Chauvet in capillary tubes with square cross-sections. However, it is more complex because of the porous nature of the micromodel (whereas a single capillary tube, as studied by Chauvet, can be viewed as a unique pore), and because of the much more complex liquid films shapes. For such systems, we present some experimental results on the liquid films effects on the drying kinetics, together with theoretical prediction, based on a visco-capillary drying model. Such a modelling requires the use of the Surface Evolver software to model the film shape, coupled with DNS simulations of the Stokes flow within the liquid films to compute the viscous resistance to the evaporation-induced flow.Finally, as a last part of this thesis, several evaporation experiments performed on deformable micromodels are presented. This preliminary work aims at reaching a situation where elasto-capillary effects modify the pore space geometry during evaporation. This, as seen above, should in turn alter the phase distribution during evaporation and the drying kinetics.
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Chen Chen. Evaporation in microfluidic systems : From radially evolving capillary structures to phyllotaxic spirals. Construction durable. INSA de Toulouse, 2016. English. ⟨NNT : 2016ISAT0020⟩. ⟨tel-01553833⟩

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