Fabien Chauvet

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.

Threshold of Bénard-Marangoni instability in drying liquid films

We here show how evaporation/condensation processes lead to efficient heat spreading along a liquid/gas interface, thereby damping thermal fluctuations and hindering thermocapillary flows. This mechanism acts as an effective thermal conductivity of the gas phase, which is shown to diverge when the latter is made of pure vapor. Our simple (fitting-parameter–free) theory nicely agrees with measurements of critical conditions for Benard-Marangoni instability in drying liquid films. Heat spreading is also shown to strongly affect wavelength selection in the nonlinear regime. In addition to providing a quantitative framework for analyzing transitions between complex evaporation-driven patterns, this also opens new perspectives for better controlling deposition techniques based on drying.

Study of Evaporation in Capillary Tubes by Infrared Thermography and Ombroscopy Techniques

We study the evaporation of a volatile liquid in a capillary tube of square internal cross section (side length = 1mm ) by combining an infrared thermography technique with ombroscopy visualizations. This makes it possible to obtain the temperature profile along the outer surface of the capillary together with the evolution of the bulk meniscus position within the tube. Evaporation in such a tube is significantly faster than in tubes of circular section owing to the liquid films developing along the tube internal corners under the effect of capillary forces as the bulk meniscus recedes inside the tube. When the tube is held horizontal the temperature minimum stays at the tube entrance and the evaporation rate reaches a stationary value. This is the signature of the liquid films, which transport the liquid up to the tube entrance, where the mass transfer is important. When the tube is vertical and open at the top, the temperature minimum stays at the entrance (top) for a while. In contrast with the horizontal case, however, the position of the temperature minimum changes when the bulk meniscus has sufficiently receded inside the tube. The rate of evaporation then decreases significantly. The different behaviour between the vertical case and the horizontal one is explained by the thinning of the corner films in the vertical tube entrance region under the conjugated effects of gravity and viscous forces. When the tube is held horizontal, the capillary effects are dominant and the film thickness remains essentially constant in the tube entrance region.© 2008 ASME