Peclet Number Dependence of Mass Transfer in Microscale Segmented Gas–Liquid
Flow
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Abstract
A detailed
understanding of the scaling behavior associated with
the fluid flow and the transport of gas molecules from a train of
elongated gas plugs into neighboring liquid segments is of great importance
for a broad range of microscale applications. The indirect dependence
of the parameters affecting the <i>Capillary</i> and <i>Peclet</i> numbers and thereby scaling behavior (i.e., the velocity
and length of the gas plugs, and the length of the liquid segments)
on the directly adjustable experimental inputs (i.e., flow rate or
pressure of each phase) has hindered the systematic investigation
of scaling behavior in microscale gas–liquid flows. Here, we
take advantage of an image-based feedback strategy that allows us
to directly impose Capillary and Peclet numbers. We custom fabricated
a long, straight microchannel (width 300 μm, length-to-width
ratio 700) in a gas impermeable silicon–glass substrate. We
automatically determined the length reduction of initially uniformly
sized gas plugs at different positions along the microchannel and
elucidated the gas concentration within adjacent liquid segments.
In accordance with penetration theory, we analytically estimated the
gas–liquid mass transfer time to scale with the Peclet number, <i>Pe</i>, to the power of −0.5. The experimentally measured
scaling exponent −0.55 ± 0.5 for carbon dioxide dissolution
in methanol and ethanol at <i>Pe</i> = 2060–16500
compared favorably with the analytical prediction and provides a guideline
for predicting physical transport for a wide range microscale gas–liquid
flow processes