la5b02183_si_001.pdf (1.06 MB)
Science of Water Leaks: Validated Theory for Moisture Flow in Microchannels and Nanochannels
journal contribution
posted on 2015-10-27, 00:00 authored by Wenwen Lei, Nicole Fong, Yongbai Yin, Martin Svehla, David R. McKenzieWater is ubiquitous; the science
of its transport in micro- and
nanochannels has applications in electronics, medicine, filtration,
packaging, and earth and planetary science. Validated theory for water
vapor and two-phase water flows is a “missing link”;
completing it enables us to define and quantify flow in a set of four
standard leak configurations with dimensions from the nanoscale to
the microscale. Here we report the first measurements of water vapor
flow rates through four silica microchannels as a function of humidity,
including under conditions when air is present as a background gas.
An important finding is that the tangential momentum accommodation
coefficient (TMAC) is strongly modified by surface layers of adsorbed
water molecules, in agreement with previous work on the TMAC for nitrogen
molecules impacting a silica surface in the presence of moisture.
We measure enhanced flow rates for two-phase flows in silica microchannels
driven by capillary filling. For the measurement of flows in nanochannels
we use heavy water mass spectrometry. We construct the theory for
the flow rates of the dominant modes of water transport through each
of the four standard configurations and benchmark it against our new
measurements in silica and against previously reported measurements
for nanochannels in carbon nanotubes, carbon nanopipes, and porous
alumina. The findings show that all behavior can be described by the
four standard leak configurations and that measurements of leak behavior
made using other molecules, such as helium, are not reliable. Single-phase
water vapor flow is overestimated by a helium measurement, while two-phase
flows are greatly underestimated for channels larger than 100 nm or
for all channels when boundary slip applies, to an extent that depends
on the slip length for the liquid-phase flows.