Sandrine Geoffroy

Roles of gas in capillary filling of nanoslits

Control and understanding of flows inside fabricated nanochannels is rich in potential applications, but nanoscale physics of fluids remains to be clarified even for the simple case of spontaneous capillary filling. This paper reports an experimental and modelling investigation of the role of gas on the capillary filling kinetics slowdown in nanoslits (depth going from 20 nm to 400 nm) compared to Washburns prediction. First, the role of gas through the usually observed trapped bubbles during a nanoslits capillary filling is analysed thanks to experiments realized with water, ethanol and silicone oil in silicon-glass nanochannels. Bubbles are trapped only when slit depth is below a liquid-dependent threshold. This is interpreted as possible contact line pinning strength varying with wettability. Stagnant trapped bubbles lifetime is investigated for the three liquids used. Experimental results show that bubbles are first compressed because of the increasing local liquid pressure. Once the gas bubble pressure is sufficiently high, gas dissolution induces the final bubble collapse. Influence of the bubbles’ presence on the capillary filling kinetics is analysed by estimating viscous resistance induced by the bubbles using an effective medium approach (Brinkman approximation). Surprisingly, the bubbles’ presence is found to have a very minor effect on nanoslits capillary filling kinetics. Second, the transient gas pressure profile between the advancing meniscus and the channel exit is computed numerically taking into account gas compressibility. A non-negligible over-pressure ahead of the meniscus is found for nano-scale slit capillary filling. Considering the possible presence of precursor films, reducing cross-section for gas flow, leads to a capillary filling kinetics slowdown comparable to the ones measured experimentally.

Migration of finite sized particles in a laminar square channel flow from low to high Reynolds numbers

The migration of neutrally buoyant finite sized particles in a Newtonian square channel flow is investigated in the limit of very low solid volumetric concentration, within a wide range of channel Reynolds numbers Re = [0.07-120]. In situ microscope measurements of particle distributions, taken far from the channel inlet (at a distance several thousand times the channel height), revealed that particles are preferentially located near the channel walls at Re > 10 and near the channel center at Re 10). In this regime, we show that (i) the particle undergoes cross-streamline migration followed by a cross-lateral migration (parallel to the wall) in agreement with previous observations, and (ii) the stable equilibrium positions are located at the midline of the channel faces while the diagonal equilibrium positions are unstable. At low flow inertia, the first instants of the numerical simulations (carried at Re = O(1)) reveal that the cross-streamline migration of a single particle is oriented towards the channel wall, suggesting that the particle preferential positions around the channel center, observed in the experiments, are rather due to multi-body interactions.

Control of evaporation by geometry in capillary structures. From confined pillar arrays in a gap radial gradient to phyllotaxy-inspired geometry

Evaporation is a key phenomenon in the natural environment and in many technological systems involving capillary structures. Understanding the evaporation front dynamics enables the evaporation rate from microfluidic devices and porous media to be finely controlled. Of particular interest is the ability to control the position of the front through suitable design of the capillary structure. Here, we show how to design model capillary structures in microfluidic devices so as to control the drying kinetics. This is achieved by acting on the spatial organization of the constrictions that influence the invasion of the structure by the gas phase. Two types of control are demonstrated. The first is intended to control the sequence of primary invasions through the pore space, while the second aims to control the secondary liquid structures: films, bridges, etc., that can form in the region of pore space invaded by the gas phase. It is shown how the latter can be obtained from phyllotaxy-inspired geometry. Our study thus opens up a route toward the control of the evaporation kinetics by means of tailored capillary structures.

Towards The Computation Of Viscous Flow Resistance Of A Liquid Bridge

Flow within thick liquid films present owing to capillary effects in the pore space is of key importance in many multiphase flow applications in porous media, for example, drying or oil recovery processes. The viscous resistance to the flow is a key parameter for modelling fluid transport in such situations. It is well known for liquid films wetting the corners of tubes of polygonal cross-section. In this latter case, the liquid films shape is simple and can be readily obtained. The situation is much more involved when considering a realistic pore space, as in a packing of spherical particles, for example. In this case, X-ray tomography observations have shown that most of the liquid is confined around contact points between particles at intermediate liquid saturation. Nonetheless, a connectivity of all the liquid bridges throughout the particle packing can exist, allowing liquid transport across the porous medium. The ultimate goal of the present research is to provide the viscous flow resistance for such capillary liquid cluster of complex shape. As a first step in this direction, we present in this paper direct numerical simulation of the Stokes flow in liquid bridges obtained between two cylindrical pillars confined between two horizontal plates. The liquid bridge shape is obtained under conditions of hydrostatic equilibrium thanks to the Surface Evolver software. Then simulations of the viscous flow within the bridge are performed using Comsol Multiphysics ® Creeping flow solver.

A Review of Drying Theory and Modelling Approaches

This chapter presents an overview of the physics of slow quasi-isothermal drying in building materials and associated modelling techniques. The latter include the traditional continuum models as well as pore scale techniques such as direct simulations or pore network models. Overviews on some related problem such as the combined problem of evaporation, salt transport and crystallization are also given. The analysis of the current state of the art emphasizes the capillary liquid films in a real material as a major modelling issue, both within the framework of classical continuum models and pore scale models. Still more challenging is the development of drying models in the presence of salt crystallization, which represents a necessary step toward the modelling of damages induced by the salt crystallization in building materials.

Stagnation Points as Loci of Solute Concentration Extrema at the Evaporative Surface of a Random Porous Medium

AbstractEvaporation of a saline solution from a porous medium often leads to the precipitation of salt at the surface of the porous medium. It is commonly observed that the crystallized salt does not form everywhere at the porous medium surface but at some specific locations. This is interpreted at the signature of spatial variations in the salt concentration at the surface of the porous medium prior to the onset of crystallization. We explore numerically the link between the ion concentration spatial variations at the surface and porous medium heterogeneities considering strongly anisotropic short-range correlated permeability Gaussian fields corresponding to a vertical layering perpendicular to the top evaporative surface for the case of the evaporation–wicking situation. It is shown that the ion concentration extrema at the surfaces correspond to stagnation points with minima corresponding to divergent stagnation points and maxima to convergent stagnation points. Counter-intuitively, the ion concentration maxima are shown to correspond to permeability minima. However, the ion concentration absolute maximum does not necessarily always correspond to the permeability absolute minimum. More generally, the study emphasizes the key role played by the impact of heterogeneities on the velocity field induced in the medium by the evaporation process. It is also shown that the number of ion mass fraction maxima at the porous medium surface is generally much lower than the naive prediction based on the number of correlation lengths spanning the medium.