Roles of gas in capillary filling of nanoslits

Abstract

International audienceControl 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 Washburn's 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 siliconglass 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

Capillary imbibition in a square tube

“…The front coefficient k in Eq. (1) varies depending on the geometry and the roughness of the channel walls [20], and was shown consistently less than what Lucas-Washburn equation (1) predicts [21]. In all these calculations, the effect of the finger part has been ignored.…”

Early regimes of water capillary flow in slit silica nanochannels

“…In fact, during the last decade different explanations have been proposed for the observed reduction in the capillary filling speed. Moreover, Chauvet et al 38 found that slower capillary filling rates cannot be explained by enhanced viscous resistance due to nanobubbles. Persson et al 30 extended the results reported by Tas et al 31 and concluded that other effects, different from the electroviscosity, also contribute to the observed deviations in the imbibition rates.…”

“…Therefore, Tas et al 31 attributed the slower capillary rates to the electroviscous effect. Specifically, Chauvet et al 38 inferred that in sub 100 nm channels, the enhanced hydrodynamic resistance induced by the presence of nanobubbles is compensated for by the effect of the reduced volume to fill induced by the same gas nanobubbles. In the same context, Mortensen and Kristensen 37 found that the contribution of the electroviscous effect to the apparent viscosity is less than 1% and therefore concluded that the electroviscous effect is not sufficiently strong to account for the observed deviations.…”

“…Nevertheless, as pointed out by van Honschoten et al 14 the Washburn model is derived for water imbibition without gas bubbles. 38,50 Therefore, a comprehensive explanation to the slower than expected capillary rates observed in the filling of nanochannels compared to the LW equation predictions remains an open question. Specifically, Chauvet et al 38 inferred that in sub 100 nm channels, the enhanced hydrodynamic resistance induced by the presence of nanobubbles is compensated for by the effect of the reduced volume to fill induced by the same gas nanobubbles.…”

FIB Design for Nanofluidic Applications

“…15.1a), which involve several technological steps: PM conception/realization and PR deposition, baking, insulating, and developing [8]. Typically, nanofluidic channels are made by using a conventional lithography step followed by dry or wet etching to define the depth of the channel [9]. Due to the limitations inherent to conventional lithographic processes, almost all nanofluidic devices have simple geometries with only one nanometric dimension, the channel depth.…”