Abstract
The three-dimensional two-phase flow dynamics inside a microfluidic device of complex geometry is simulated using a parallel, hybrid front-tracking/level-set solver. The numerical framework employed circumvents numerous meshing issues normally associated with constructing complex geometries within typical computational fluid dynamics packages. The device considered in the present work is constructed via a module that defines solid objects by means of a static distance function. The construction combines primitive objects, such as a cylinder, a plane, and a torus, for instance, using simple geometrical operations. The numerical solutions predicted encompass dripping and jetting, and transitions in flow patterns are observed featuring the formation of drops, ‘pancakes’, plugs, and jets, over a wide range of flow rate ratios. We demonstrate the fact that vortex formation accompanies the development of certain flow patterns, and elucidate its role in their underlying mechanisms. Experimental visualisation with a high-speed imaging are also carried out. The numerical predictions are in excellent agreement with the experimental data.
Highlights
Two-phase flow in microchannels is of central importance to applications in chemical, medical, and pharmaceutical processes such as inkjet printing, DNA chips, lab-on-a-chip technology, micro-propulsion, and microfluidics (Andersson and Berg 2003; Kuswandi et al 2007; Martinez et al 2010; Squires and Quake 2005; Mark et al 2010)
These fluid combinations, when driven through the junction, have four generic interface shapes for the dispersed phase at the exit branch: (1) spherical drops with a diameter smaller than the cross-junction height (1 90 μm ), (2) ‘pancakes’ resembling a flattened sphere with radius between 190 μm and 390 μm, (3) plugs which have an elongated
The isolated droplets and stable jets are predicted by the modelling, but for brevity we have omitted them and concentrated upon the more commonly occurring pancake and plug droplets
Summary
Two-phase flow in microchannels is of central importance to applications in chemical, medical, and pharmaceutical processes such as inkjet printing, DNA chips, lab-on-a-chip technology, micro-propulsion, and microfluidics (Andersson and Berg 2003; Kuswandi et al 2007; Martinez et al 2010; Squires and Quake 2005; Mark et al 2010). The front-tracking technique (Unverdi and Tryggvason 1992), and the variants developed by Shin and Juric (2009a, b) and Shin et al (2017, 2018), exhibit no numerical instabilities, and parasitic currents This approach is ideally suited to multiphase flow simulation, in the case of surface tension-dominated flows, and it is employed to study the physics of breakup, the influence of the flow-focussing at junctions in microfluidics devices, which are potentially key, as shown in previous experimental work. Chinaud et al (2015) have developed a new technique for flow visualisation, termed “complementary micro Particle Image Velocimetry ( PIV)”, which allows velocity fields in both phases to be imaged These experiments highlight the apparent existence of an intriguing vortex formation during the squeezing regime; we utilise the results of our simulation technique to detect, and quantify numerically, the role of this vortex in the breakup mechanism.
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