| Selective withdrawal occurs when a thin spout of fluid A is viscously entrained by a convergent flow of a second, immiscible fluid B. The associated selective withdrawal transition is the process by which the interface between the two fluids undergoes a topological transition from a bounded surface with no entrainment to an unbounded surface with a spout of fluid A piercing the bulk of fluid B. This transition which occurs due to changes in the imposed flow of fluid B, is well studied for the case of a planar interface between the two fluids. This paper examines the transition when the interface consists of a curved droplet of water (fluid A) protruding from a capillary within a volume of oil (fluid B). A qualitatively new, non-stationary state is observed in which the interface oscillates and intermittently ejects small volumes of water. As a control parameter is varied, both the timescale of this ejection, Tspit, and the amplitude of oscillation, A, decrease by several orders of magnitude consistent with a power law: Tspit ∝ A3/2. Within this scaling, mode locking onto various externally influenced frequencies and complex multiperiodic oscillations are observed. |
