(490g) Experimental Two-Phase Fluid Displacement in Microchannels with Pure Viscoelastic Fluids

Fluid displacement, defined as the removal of fluid by another fluid, is a common phenomenon in activities all around us. Examples ranging from the flow of water down a drain to inoculations of vaccines all require the knowledge of the behaviour of the liquid being displaced by another liquid. In industrial processes, emerging technologies such as microreactors, lab-on-chips and tertiary enhanced oil recovery (EOR) are getting more attention from their additional advantages of better efficiencies and lower costs, with the common theme of being in the milli to micro-meter scale. In their majority, the fluids involved in industrially relevant displacement are viscoelastic with complex rheological behaviour. Thus, it is important to better understand the fundamentals of the fluid displacement flow in microchannels involving viscoelastic liquids.

The displacement of a liquid by another fluid has been studied extensively during the last decades. As shown by Saffman and Taylor (1958) for porous media and Hele-Shaw cells, when a viscous liquid is pushed by a less viscous one, the interface between them tends to become unstable. This would result in branching of the leading interfaces during the displacement process, known as viscous fingering effect. This observation can be extended to displacement in microfluidic channels, where many experimental studies have been performed for Newtonian flows (Foroughi et al., 2012; Lu et al., 2020). In contrast to Hele-Shaw cells, only a single viscous finger is observed during displacement inside a microchannel, with a film of the initial stagnant fluid remaining around it (Figure 1).

For complex non-Newtonian flows, Gan et al. (2007) highlighted that viscoelastic flow instabilities can promote a more effective mixing than in viscous-viscous or viscous-inertial flow cases. However, the impact of the viscoelastic properties of complex fluids on the space-time scales of these instabilities is still poorly understood.

In the present study, hydrodynamic behaviours of a Newtonian organic liquid and a pure viscoelastic aqueous liquid (Boger fluid) were investigated during displacement. The initial stagnant fluid was a blend of silicone oils while the displacing phase comprised of a mixture of polyethylene glycol (PEG) and polyethylene oxide (PEO). For reference, experiments were also carried out with a Newtonian aqueous liquid of just PEG alone. Density and viscosity ratios were kept the same by varying the blends of silicone oil in the displaced phase.

Shadowgraphy technique involving high-speed imaging on a diffused light source background were employed to observe the displacement behaviour (Figure 2). Experiments were performed in a circular quartz microchannel with an internal diameter of 200 µm. The aqueous phase was supplied by high precision syringe pump at flowrates between 0.03 to 0.3 ml/min. To capture details such as film thickness and interfacial instabilities, images were taken with a 12-bit high-speed camera with up to 1280×800 pixels resolution equipped with a microscopic lens (50×).

The results at the initial viscous finger front showed a slightly thinner trapped film when displacement was performed with the viscoelastic liquid, compared to the Newtonian cases (Figure 3a). In addition, the time it takes for the interface to become unstable after the viscous finger front was shorter by up to 50% for the non-Newtonian fluid compared to the Newtonian one, with the difference diminishing at higher flowrates (Figure 3b). This could be attributed to the additional elastic force present in the viscoelastic liquid, which changes the interaction between existing forces (such as interfacial and shear) at the interface of the two liquids.

Furthermore, the shapes of the instabilities were categorised, with two main types of instability observed. An axisymmetric instability resembling sausage waves or bamboo waves (Figure 4a) and asymmetric instabilities which resemble corkscrew waves (Figure 4b). The findings support the linear stability analysis study by Bai et. al. (1991). Next, the instabilities were further quantified using 1-dimensional wavelet transformation analysis. Frequencies of the instability were identified with three distinct modes throughout the displacement phenomena (Figure 5).

Finally, to understand better the linkage between velocity fields and local flow phenomena, measurements are carried out with the inclusion of microparticles in both phases, allowing the flow fields to be tracked via 2D micro-Particle Shadow Velocimetry (μPSV).

Reference

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Foroughi H, Abbasi A, Das KS, Kawaji M (2012) Immiscible displacement of oil by water in a microchannel: Asymmetric flow behavior and nonlinear stability analysis of core-annular flow. Physical Review E 85(2):026309

Gan HY, Lam YC, Nguyen NT, Tam KC, Yang C (2007) Efficient mixing of viscoelastic fluids in a

microchannel at low Reynolds number. Microfluidics and Nanofluidics 3(1):101-108

Lu Y, Kovalchuk NM, Che Z, Simmons MJ (2020) Interfacial instabilities due to immiscible fluid displacement in circular and non-circular microchannels. Experimental Thermal and Fluid Science 113:110045

Saffman PG, Taylor GI (1958) The penetration of a fluid into a porous medium or hele-shaw cell containing a more viscous liquid. Proceedings of the Royal Society of London Series A Mathematical & Physical Sciences 245(1242):312-329