Visualization and tracking of spontaneous liquid-liquid slug flow in microchannels

Spontaneous liquid-liquid slug (bislug) [1] flow in microchannels has been observed for both circular and square cross-sections. Flow is induced via an imbalance in interfacial tensions and curvatures between the two gas-liquid surfaces and the liquid-liquid surface. Liquid A (red) has a higher surface tension than liquid B, the resulting bislug flow occurs in the direction of the higher surface tension. Interface (racking was performed on each of the images to extract the position and time ot the slug as it traversed the microchannel. The bar graph shows the average bislug velocities for prewet and non-prewet microchannels of circular and square cross-sections. Bislug velocity in the prewet square microchannel is four times the velocity on the non-prewet square microchannel. The prewet square microchannel has a velocity more than an order of magnitude greater than that found in either the prewet and non-prewet circular microchannels. For the prewet, square microchannels, the Capillary number Is greater than that predicted by the laminar flow equation for LA/R> 5, where LA/R is the length of the fluid A slug and R is the channel radius. The actual Capillary number is much lower than the theoretical Capillary number for the prewet circular microchannels regardless of the bislug aspect ratio, LA/R, due todissipation from the menisci motion. Corning wicking reduces the viscous dissipation allowing the bislug to flow faster. When L./R <5, hydrodynamic interaction between menisci increases dissipation so that the Ca is less than predicted. © 2007 by ASME.