An unexpected flow regime found via selective withdrawal

This paper explores various flow regimes that can be easily achieved through selective withdrawal. By varying the distance between the pipette and the interface, we observed that increasing the distance leads to an increase in droplet frequency and a decrease in droplet diameter. The study also highlights the transition to a different regime characterized by a corrugated liquid column, which we refer to as a Brâncuși column due to its striking resemblance to Constantin Brâncuși’s sculpture, The Infinity Column. This phenomenon occurs when the distance between the pipette and the interface is reduced, acting as an upper boundary for droplet formation. When the viscous-capillary time scale, which governs droplet breakup, matches the convective time scale of the continuous phase, the transition to a corrugated fluid column takes place. Interestingly, the characteristic wavelength of these columns remains constant regardless of the distance between the pipette and the interface but increases with the extraction flow rate. The study also explores the selective withdrawal of water droplets standing at the interface between two immiscible liquids. Complete extraction or partial coalescence with the heavier liquid are observed during the extraction process of the droplet. The present exploratory study may prove beneficial for various droplet production processes which involve selective withdrawal as a basic method.