Roumpea, Evangelia Panagiota; (2019) Formation and flow of droplets in complex fluids in microfluidic channels. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In this thesis, the flow dynamics and characteristics of liquid-liquid, organic-aqueous flows in small channels were studied for cases when the aqueous phase is a complex fluid. In particular, polymeric non-Newtonian solutions and solutions with surfactants were used to investigate the effects of two different physical properties i.e. shear-thinning viscosity and interfacial tension, on two-phase flows. The organic Newtonian phase was a silicone oil, with 0.0046 Pa s viscosity. For the studies with non-Newtonian fluids, two aqueous glycerol solutions containing xanthan gum (1000 and 2000 ppm) were used. The two immiscible phases were brought together in a glass T-junction microchannel (with 200 μm inner diameter), and the effects of flowrate and viscosity on the plug flow characteristics and velocity fields were examined. Experiments were carried out both at the inlet and in the main part of the microchannel. For the studies with the surfactant solutions two ionic surfactants, C12TAB (50 mM) and C16TAB (5 mM) were added in the water-glycerol solution separately. In this case, a flow-focusing channel inlet was used (with cross-junction equal to 190 μm x 195 μm). For all experiments, an innovative micro-Particle Image Velocimetry (µPIV) technique was developed as part of this project. In this technique, two laser wavelengths illuminated two different particle types, each seeded in one of the phases of the two-phase mixture, in order to capture the shape of the interface and to measure the velocity fields in both phases simultaneously. // From the studies with the non-Newtonian aqueous phases, two different flow patterns were identified, namely plug and parallel flow, depending on the competition among interfacial, inertia and viscous forces. The plug flow was further investigated because it offers enhanced transport rates and has many applications in (bio) chemical analysis and synthesis. It was found that in the squeezing regime the plug formation at the T-junction could be divided in three discrete stages: expansion, necking, and pinch-off. The experimental results revealed that the organic plugs formed in the non-Newtonian solutions had larger tip (bullet-shaped profile) and neck curvatures and smaller neck widths compared to the Newtonian ones. In addition, an increase in xanthan gum concentration shifted the break-up location of the neck of the forming plug further downstream inside the channel and resulted in higher continuous phase velocities. Based on the velocity profiles and the geometric characteristics of the forming plugs, the forces acting on them were estimated. It was found that drag forces gradually increased and overcame the surface tension ones in the beginning of the pinch-off stage changing the curvature of the neck from convex to concave. // The hydrodynamic characteristics of the plug flow of the organic phase in the continuous non-Newtonian aqueous solutions were further studied in the main channel, downstream from the T-junction inlet. With increasing polymer concentration, longer plugs were produced with increased film thickness around them due to the increase in visocity. From the shear rate 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. The velocity profiles obtained from the PIV measurements showed that the addition of xanthan gum resulted in higher plug velocity. Within the non-Newtonian slug, the velocity profiles were found to be flat in the middle of the channel as expected for a power law fluid, while the circulation times were increased with the concentration of xanthan gum. // When the surfactants were employed in the aqueous phase, three distinct drop formation stages were also identified, namely expansion, necking and pinch-off for all solutions studied. The surfactant-laden solutions produced smaller drops with larger tip curvature than the surfactant free system. The forces acting on the forming drop were estimated and showed that the drag force overcame the interfacial tension force at the transition between the expansion and necking stages. During this transition, the curvature of the neck of the forming drop changed from convex to concave while the thinning rate of the neck increased. The velocity profiles showed that during the expansion stage, an internal circualtion appeared at the centre of the forming drop which gradually weakened as the droplet moved into the main channel. This circulation was less prominent in the surfactant-laden system which was attributed to the accumulation of surfactants at the drop tip. The averaged velocity fields revealed that the addition of surfactants increased the local velocity difference between the two phases compared to the surfactant-free case, for the same phase flowrates.

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