Abstract
Water-butanol and water-hexane flows were visualized in ultra-shallow straight and serpentine microchannels with a cross-junction. At the inlet cross-junction, three major flow patterns including tubing/threading, dripping and jetting were mapped using the aqueous Capillary number versus the organic Weber number. Correspondingly, in the main microchannel, annular flow, slug flow and droplet flow were mapped using combined dimensionless numbers (Weber number times Ohnesorge number) of both phases. The flow pattern transitions were explained based on a force analysis, considering the phase flow rates, junction angle between the side feeding channels and the central feeding channel as well as aspect ratios. Compared to the straight microchannel, the dripping regime at the inlet junction and the slug flow occupy larger zones in serpentine microchannels because the centrifugal force tends to break up the organic annular core into slugs and droplets over the bends.
Highlights
IntroductionConcerning process intensification and scale miniaturization, micro-structured devices (e.g., microscale heat exchangers, microreactors, micromixers) are widely used because they possess merits such as high heat and mass transport rates, enhanced mixing, fast reaction, energy and raw material savings, and easiness of numbering up [1]
Concerning process intensification and scale miniaturization, micro-structured devices are widely used because they possess merits such as high heat and mass transport rates, enhanced mixing, fast reaction, energy and raw material savings, and easiness of numbering up [1]
A cross-junction, as a hydrodynamic focusing geometry, consists of three microchannels that intersect at a certain angle, with one phase from two side inlets and with the other from the central inlet [11, 13]
Summary
Concerning process intensification and scale miniaturization, micro-structured devices (e.g., microscale heat exchangers, microreactors, micromixers) are widely used because they possess merits such as high heat and mass transport rates, enhanced mixing, fast reaction, energy and raw material savings, and easiness of numbering up [1]. A cross-junction, as a hydrodynamic focusing geometry, consists of three microchannels that intersect at a certain angle, with one phase from two side inlets and with the other from the central inlet [11, 13]. Cubaud and Mason [15] observed tubing, threading, displacement, dripping and jetting flow regimes for various immiscible fluid pairs in a cross-junction of straight microchannels with square cross sections, covering more than an order of magnitude of difference in viscosity ratios and interfacial tensions. Fu et al [17] identified viscous displacement, tubing, dripping and jetting in a microfluidic cross-junction for silicone oilwater flow. Wu et al [11] mapped flow patterns at cross-junctions and studied slug hydrodynamics for five liquid-liquid systems flowing in three square glass microchannels of channel depths from 200 μm to 600 μm
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