Nanofluid induced continuous production of monodispersed plugs during biphasic liquid flow in meso-scale

The study proposes a novel technique for continuous generation of monodispersed droplets during biphasic flow in mesoscale. The technique uses aqueous nanofluids to pinch-off toluene droplets during flow in a 2.38 mm diameter glass conduit. Visualisation studies over a flow rate range of 2–80 mL/min show a significant increase in plug flow range compared to water-toluene flow. The range is even higher than that reported in literature for water-toluene flow in microchannels. This is a favorable outcome as plug flow enhances rate of transport processes. Additionally, we observe an enhanced range of monodispersity (polydispersity index ≤ 1.01) compared to water-toluene flow under the same conditions. The values for critical aqueous capillary number and organic Weber number for monodispersity (Ca_aq* and We_org*) ≈ 0.024 and 7 for nanofluids and 0.01 and 2 for water. We further note the formation of inverted dispersed flow at low aqueous and moderate organic flow (We_org ≤ 8). Comparison with the flow pattern map for surfactant-toluene flow under identical flow conditions shows that the observations cannot be rationalised solely from changes in phase physical properties. We postulate interface jamming to play a significant role. A simplistic analysis shows that availability of nanoparticles exceeding 400 % of that required to cover the surface created during droplet formation ensures monodispersity under jetting at high phase flow rates. Likewise, inverted droplet flow appears when the availability of nanoparticles exceeds 100 % of that required for interface coverage and We_org ≤ 8. For higher Weber numbers, inertial forces dominate, resulting in thread flow.