Abstract
The laminar nature of microfluidic flows is most elegantly demonstrated via the confluence of two fluids forming two stable parallel flows within a single channel meeting at a highly stable interface. However, maintenance of laminar conditions can become complicated when there is a large viscosity contrast between the neighbouring flows leading to unique instability patterns along their interface. Here, we study the dynamics of high viscosity contrast confluent flows – specifically a core flow made of highly viscous glycerol confined by sheath flows made of water within a microfluidic flow focusing system. Our experiments indicate the formation of tapered core structures along the middle of the channel. Increasing the sheath flow rate shortens the tapered core, and importantly induces local instability patterns along the interface of core-sheath flows. The dynamics of such tapered core structures is governed by the intensity of instability patterns and the length of the core, according to which the core structure can experience stable, disturbed, broken or oscillated regimes. We have studied the dynamics of tapered core structures under these regimes. In particular, we have analysed the amplitude and frequency of core displacements during the broken core and oscillating core regimes, which have not been investigated before.
Highlights
The laminar nature of microfluidic flows is most elegantly demonstrated via the confluence of two fluids forming two stable parallel flows within a single channel meeting at a highly stable interface
We have investigated the dynamics of highly viscous core flows surrounded by low viscosity sheath flows in a microfluidic flow focusing channel
Our experiments indicate the formation of tapered core structures under the massive shear stress induced at the interface of core-sheath flows
Summary
The laminar nature of microfluidic flows is most elegantly demonstrated via the confluence of two fluids forming two stable parallel flows within a single channel meeting at a highly stable interface. If the two fluids are miscible mixing will eventually occur due to diffusion; if the two fluids are immiscible (e.g. water/oil or air/water) the parallel flows can be sustained as stable indefinitely so long as laminar conditions are maintained The widths of these two parallel flows can be precisely determined according to the ratio of the velocity and viscosity of the two fluids. Maintenance of laminar conditions can become complicated when there is a large viscosity contrast between the neighbouring flows leading to unique instability patterns[6,7,8,9], as comprehensively reviewed by Sahu et al.[10] Both experimental[11,12,13] and numerical[14] analyses have shown the existence of pearl and mushroom instability patterns when applying two miscible liquids into a circular pipe with the viscous liquid acting as the sheath (annular) flow. Core/sheath viscosity ratios is associated with rolling up thin filaments of ‘viscous thread’, leading to ‘inertial instabilities’[23]
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