Experimental investigations of non-Newtonian/Newtonian liquid-liquid flows in microchannels

The plug flow of a non-Newtonian and a Newtonian liquid was experimentally investigated in a quartz microchannel (200-µm internal diameter). Two aqueous glycerol solutions containing xanthan gum at 1000 and 2000 ppm were the non-Newtonian fluids and 0.0046 Pa s silicone oil was the Newtonian phase forming the dispersed plugs. Two-color particle image velocimetry was used to obtain the hydrodynamic characteristics and the velocity profiles in both phases under different fluid flow rates. The experimental results revealed that the increase in xanthan gum concentration produced longer, bullet-shaped plugs, and increased the thickness of the film surrounding them. From the shear rate and viscosity profiles, it was found that the polymer solution was in the shear-thinning region while the viscosity was higher in the middle of the channel compared to the region close to the wall. Circulation times in the aqueous phase increased with the concentration of xanthan gum

Studies of plug formation in microchannel liquid-liquid flows using advanced particle image velocimetry techniques

Two complementary micro Particle Image Velocimetry (μPIV) techniques have been developed in this work to study plug formation at a microchannel inlet during the flow of two immiscible liquids. Experiments were conducted for different fluid flow rate combinations in a T-junction, where all branches had internal diameters equal to 200 μm. The dispersed phase was a water/glycerol solution and was injected from the side branch of the junction, while the continuous phase was silicon oil and was injected along the main channel axis. In the two-colour μPIV technique two laser wavelengths are used to illuminate two different tracer particles, one in each fluid, and phase averaged velocity profiles can be obtained in both phases simultaneously. In the high speed bright field μPIV technique, a backlight illuminates the test section, where the dispersed phase plug is seeded with tracer particles. This approach allows velocity profiles of the forming dispersed plugs to be followed in time. Non-dimensional plug lengths were found to vary linearly with the aqueous to organic phase flow rate ratio, in agreement with a well-known scaling correlation. The flowrate ratio also affected the velocity profiles within the forming plugs. In particular, for a ratio equal to one, a vortex appears at the tip of the plug in the early stages of plug formation. The interface curvature at the rear of the forming plug changes sign at the later stages of plug formation and accelerates the thinning of the meniscus leading to plug breakage. The spatially resolved velocity fields obtained in both phases with the two-colour PIV show that the continuous phase resists the flow of the dispersed phase into the main channel at the rear of the plug meniscus and causes the change in the interface curvature. This change of interface curvature was accompanied by an increase in vorticity inside the dispersed phase during plug formation

Effect of surfactant on emulsification in microchannels

Drop formation in a microfluidic flow-focusing device (cross-junction) was studied in absence and presence of one of two ionic surfactants. Four different flow regimes: squeezing, dripping, jetting, and threading were observed in line with existing literature. The effect of surfactant on the transition between flow regimes was shown to depend upon the value of critical micelle concentration and correlates with dynamic surface tension. Drop length in the channel increased as the ratio of flow rate of dispersed to continuous phase, φ increased. For drops smaller than the channel width, the increase was slow, proportional to φ 0.1 , yet was much faster, proportional to φ for larger drops. In contradiction to the expected stabilisation of drops by surfactant, surfactant-laden drops larger than the channel height coalesced inside the channel at a higher rate than surfactant-free drops. It is proposed that the coalescence is caused by the electrostatic attraction due to surfactant redistribution under the high shear stresses near the wall of the channel

Experimental studies on droplet formation in a flow-focusing microchannel in the presence of surfactants

The formation of an aqueous droplet in an organic continuous phase was studied experimentally inside a flow-focusing microchannel (190 μm × 195 μm: depth × width) in the presence of surfactants. A low viscosity silicone oil (0.0046 Pa s) was used as the continuous phase and a mixture of 48% w/w water and 52% w/w glycerol was the dispersed phase. Two ionic surfactants, C₁₂TAB (50 mM) and C₁₆TAB (5 mM) were added in the aqueous phase, at concentrations above the CMC values. Four regimes of drop formation were identified, namely squeezing, dripping, jetting and threading, whose boundaries changed when the surfactants were present. The drop formation process and the velocity profiles in both phases in the squeezing and dripping regimes were studied in more detail using a two-colour Particle Image Velocimetry technique. For all solutions studied, three distinct drop formation stages were identified, expansion, necking and pinch-off. The surfactant-laden solutions produced smaller drops. Considering the dynamic interfacial tension, rather than the equilibrium one, it was possible to explain differences in the drop formation between the two surfactant systems in the expansion stage. The forces acting on the forming drops were estimated and showed that the drag force overcomes the interfacial tension force at the transition between the expansion and necking stages. During this transition, the curvature of the neck changed while its thinning rate was increased. The transition from the necking to the pinch-off stage was signified by a flow reversal at the bottom part of the drop