Filling kinetics of liquids in nanochannels as narrow as 27 nm by capillary force

Abstract

We report the filling kinetics of different liquids in nanofabricated capillaries with rectangular cross-section by capillary force. Three sets of channels with different geometry were employed for the experiments. The smallest dimension of the channel cross-section was respectively 27, 50, and 73 nm. Ethanol, isopropanol, water and binary mixtures of ethanol and water spontaneously filled nanochannels with inner walls exposing silanol groups. For all the liquids the position of the moving liquid meniscus was observed to be proportional to the square root of time, which is in accordance with the classical Washburn kinetics. The velocity of the meniscus decreased both with the dimension of the channel and the ratio between the surface tension and the viscosity. In the case of water, air-bubbles were spontaneously trapped as channels were filled. For a binary mixture of 40% ethanol and water, no trapping of air was observed anymore. The filling rate was higher than expected, which also corresponds to the dynamic contact angle for the mixture being lower than that of pure ethanol. Nanochannels and porous materials share many physicochemical properties, e.g., the comparable pores size and extremely high surface to volume ratio. These similarities suggest that our nanochannels could be used as an idealized model to study mass transport mechanisms in systems where surface phenomena dominate.

Capillarity-driven dynamics of water–alcohol mixtures in nanofluidic channels

“…This observation is striking since one might expect that specific interactions of ethanol and water molecules with the SiO x channel walls would affect the dynamics of fluid flow in channels with a thickness of merely 20-50 water molecules. Yet, like in earlier studies of significantly wider nanochannels (from 27 to 73 nm; Han et al 2006) surface effects do not seem to play a major role. Han et al had reported reasonably consistent filling dynamics of 40% ethanol-water mixtures in a 27 nm deep and 900 nm wide nanochannel compared to pure water and ethanol.…”

“…The general finding of all those experiments is that the filling dynamics of typically pure simple fluids follows the classical Lucas-Washburn law (Lucas 1918;Washburn 1921), in which the meniscus advances proportional to the square root of time. However, in most instances, it was found that the prefactor in that algebraic law was reduced compared to the theoretical expectations, which was attributed to various possible effects including an electrokinetic counter flow (Tas et al 2004) (later disputed by others; Mortensen and Kristensen 2008), the presence of bubbles (Thamdrup et al 2007), elastic deformations of the channel wall, and an increased dynamic contact angle (Han et al 2006). One of the most important problems is that most experiments do not allow for distinguishing between confinement effects arising from a variation of the macroscopic materials properties (viscosity, surface tension) and retarding effects on the moving meniscus, e.g., due to wall roughness, pinning, and slip.…”

“…To this end, several groups investigated the spontaneous filling of hydrophilic nanochannels driven by capillarity (Tas and Haneveld et al 2004;Han and Mondin et al 2006;Thamdrup et al 2007;van Delft and Eijkel et al 2007;Haneveld et al 2008;Mortensen and Kristensen 2008). The general finding of all those experiments is that the filling dynamics of typically pure simple fluids follows the classical Lucas-Washburn law (Lucas 1918;Washburn 1921), in which the meniscus advances proportional to the square root of time.…”

Wetting considerations in capillary rise and imbibition in closed square tubes and open rectangular cross-section channels

“…These advances are leading to studies with microfluidic (e.g. Yang et al, 2011) and nanofluidic capillaries widths of a few tens of nm (Han et al, 2006) or with depths as small as 6 nm (Oh et al, 2009). Whilst non-constant channel cross sections have been a focus of study experimentally and theoretically (Legait, 1983;Staples andShaffer, 2002, Reysatt et al, 2008;Liou et al, 2009), increased solid-liquid contact area, and hence increased capillary pull can be achieved using a range of in-channel structures.…”

Machining technologies for silicon-based nanochannels and some properties of aqueous solutions confined in these channels

“…A possible explanation for the enhanced concentration of Alexa is the adsorption of Alexa at the liquid-air interface. In the traditional picture, the interface is free of ions, however it was shown recently [40] that larger polarizable ions may have a propensity for the interface. The liquid pockets in our experiment are the results of transport through the channel corners, which involves a large surface-to-volume ratio and therefore can contribute the concentration of species that are adsorbed at the interfaces.…”