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

This paper reports visualization-driven experiments of liquid–liquid co-flow in a millimetric T-junction geometry, using dilute silica-based nanofluids, de-ionized water, and surfactant (sodium dodecyl sulfate) 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 the absence of nanoparticles in the aqueous phase, polydispersity results due to random pinch off; (ii) a distinct “widening jetting” pathway leads to an inverted dispersed regime (nanofluid droplets in toluene) that is not exhibited in the 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. This study is of importance as stabilizing jetting, preserving monodispersity at high throughput, remains a practical and conceptual bottleneck. Furthermore, the qualitative regime phenomenology described in the paper can be valuable for both mesoscale droplet generation and understanding of interfacial physics.