Nanofluid induced droplet pinch-off in the jetting regime of liquid-liquid flow

The paper reports visualization-driven experiments of liquid-liquid co-flow in a millimetric T-junction geometry, using dilute silica-based nanofluids, deionised water, and surfactant (sodium dodecyl sulphate) solution as the aqueous phase and toluene as the organic phase. The results reveal three distinct characteristics of nanofluid induced droplet pinch-off at high throughputs which are not exhibited during droplet pinch-off by water and surfactant solution, viz - (i) under the same flow conditions, nanofluids uniquely produce stable monodisperse droplet/plugs in the jetting regime via an "exit pinch-off" mode while in absence of nanoparticles in the aqueous phase, polydispersity results due to random pinch off, (ii) a distinct "widening jetting" pathway leads to inverted dispersed regime (nanofluid droplets in toluene) that is not exhibited in absence of nanoparticles; and (iii) the neck-thinning dynamics transition from a classical inertio-capillary power law (∼ 2/3) for water/surfactant solution to an exponential trend for nanofluids. The droplet formation regimes are presented as a phase diagram in non-dimensional coordinates, decided based on the dominating forces governing pinch-off under different flow conditions. Although these jetting types have been reported earlier, the formation of organic and aqueous phase droplets in response to the different jetting mechanisms is a hitherto unreported phenomenon. The study is of importance as stabilizing jetting, preserving monodispersity at high throughput, remains a practical and conceptual bottleneck. Further, the qualitative regime phenomenology described in the paper can be valuable for both mesoscale droplet generation and understanding of interfacial physics.