Evaporation-induced cavitation in 2-D multisection nanochannels
Cavitation is the formation of vapor bubbles in a liquid that is a consequence of tensions acting on the liquid. It is of great interest to lots of different scientific fields such as fluid mechanics, acoustics, hydraulic engineering and biology. Although widely studied in macroscale and microscale confined liquids, heterogeneous cavitation at the nanoscale has only been experimentally observed recently in 2-D nanofluidic channels during an evaporation process, where vapor bubbles form and expand inside the nanochannels instead of menisci receding along the channels. Such evaporation-induced cavitation shows a strong correlation with the nanochannel cross-section non-uniformity and exhibited lots of interesting phenomena, including fast evaporation rate and self-controlled bubble dynamics. In this work, we further investigated this new cavitation phenomenon using a series of specially designed 2-D multi-section nanochannels. Each of these channels includes two or three sections of nanochannel with heights of 25 and/or 35 nm and the same width of 3 μm. A modified sacrificial layer etching method was developed to fabricate these nanochannel devices. Water evaporation processes in these channels were recorded using a high-speed camera mounted on an inverted microscope. We observed that cavitation only occurred in multi-section nanochannels with a “Low to High” channel design. In such nanochannels, when menisci receded to the “Low to High” step, bubbles occurred in the higher channel section and started expansion until they occupied the whole section. We explored the origin of these cavitation phenomena and discovered that that initial bubbles were formed during a snap-off process, where meniscus curvature difference induced reverse liquid flows cause air trapping right at the step. The following bubble expansion is a result of evaporation-induced negative pressure (up to -58 bars) as water inside the nanochannels is in a metastable state. We also analyzed water evaporation rates (bubble growth rates) in these nanochannels in the presence of cavitation. While most evaporation rates can be explained by classic vapor diffusion theories or the kinetic limit of evaporation, water evaporation rates in nanochannels with a Low-High-Low design in the presence of cavitation were as high as 630 μm/s, which is even much higher than the kinetic limit of evaporation and cannot be explained by any current theories. This study further expands our understanding of cavitation and provides new insights and explanations for phase-change phenomena at the nanoscale, including cavitation in plants and quick drying process in nanoporous media. The discovered ultra-high evaporation rates in the Low-High-Low nanochannels also offer a new solution to address thermal management needs for next generation electronic devices.
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