Abstract
Nanoscale liquid transport governs the behaviour of a wide range of nanofluidic systems, yet remains poorly characterized and understood due to the enormous hydraulic resistance associated with the nanoconfinement and the resulting minuscule flow rates in such systems. To overcome this problem, here we present a new measurement technique based on capillary flow and a novel hybrid nanochannel design and use it to measure water transport through single 2-D hydrophilic silica nanochannels with heights down to 7 nm. Our results show that silica nanochannels exhibit increased mass flow resistance compared to the classical hydrodynamics prediction. This difference increases with decreasing channel height and reaches 45% in the case of 7 nm nanochannels. This resistance increase is attributed to the formation of a 7-angstrom-thick stagnant hydration layer on the hydrophilic surfaces. By avoiding use of any pressure and flow sensors or any theoretical estimations the hybrid nanochannel scheme enables facile and precise flow measurement through single nanochannels, nanotubes, or nanoporous media and opens the prospect for accurate characterization of both hydrophilic and hydrophobic nanofluidic systems.
Highlights
Understanding liquid transport through nanoscale confinements is critical in a variety of practical applications, including energy conversion/storage[1,2], water desalination[1,3], phase-change thermal management[4], biological and chemical separations[5], and lab-on-a chip devices[6]
In this method–which is mainly applicable to hydrophilic channels–the driving pressure in the nano-conduits is not experimentally measured, but calculated based on classical theories[28,29,30,31,32,33,34,35,36] with bulk liquid properties or molecular simulations[18], which can be quite different from the actual values, resulting in inaccurate calculation of the actual hydraulic resistance
Given the limitations of the current measurement techniques, it is necessary to develop a technique for liquid flow measurement in single nano-conduits[37,38] which can be applied to both hydrophobic and hydrophilic conduits without using any theoretical estimations
Summary
The hybrid nanochannel design consists of a test channel (the channel under investigation) seamlessly connected to a reference channel with a different but known mass flow resistance (Fig. 1). The location of the meniscus in the reference channel during the second capillary filling process x(t) is recorded, which is expected to be described by the following equation (Supplementary information, section I): x2 + 2L⁎ x = 2At η (3). By designing a long test channel and choosing a high frame rate, the hybrid nanochannel scheme is able to detect very large values of η (> 104) with a small error (Fig. 1d). This scheme is adequate for the study of enhanced liquid transport in carbon nanotubes and graphene nanochannels where a wide range of flow enhancements have been reported[9,18,19,20,21]. Position of the meniscus as a function of time was extracted from the recorded frames using a MATLAB image processing code
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