Evaporation-induced natural convection of a liquid slug of binary mixture inside a microchannel: effect of confinement
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Both experimental and simulation studies have been carried out on internal convection of an evaporating liquid slug of aqueous NaCl solution inside a microcapillary. Effect of confinement due to the extended channel length beyond the interface of the liquid slug has been investigated by placing the liquid slug inside the microcapillary of different lengths. Micro-PIV technique has been used for measurement of velocity field inside the liquid slug. Simulation studies have been carried out using COMSOL Multiphysics software for reporting the evaporative flux distribution on the meniscus and the concentration field distribution inside the liquid slug. The combined experimental and simulation studies successfully explain the underlying flow physics. Evaporation from the liquid–air interface of the slug induces buoyancy-driven Rayleigh convection. Evaporative flux of the interface depends on the extended length of the microcapillary beyond the liquid slug. The presence of extended channel region beyond the meniscus suppresses the evaporation from the meniscus due to the absence of evaporation flux normal to the channel wall. Evaporation occurs primarily from only one meniscus when the slug is located at one end of a long channel. Evaporation occurs from both the menisci when both the menisci are directly exposed to the atmosphere. Evaporation from only one meniscus of a slug leads to one recirculation bubble inside the liquid slug, whereas evaporation from both the menisci leads to two recirculation bubbles inside the liquid slug. Liquid slug with asymmetric extended channel length beyond the liquid slug interface leads to asymmetric evaporative flux, concentration field distribution and recirculation bubble size. The extended channel length beyond an evaporating liquid slug can influence/control the performance of a digital microfluidic system/device.
KeywordsEvaporating slug Evaporating meniscus Micro-PIV Rayleigh convection
Authors acknowledge the Department of Science and Technology, Government of India, for the financial support.
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