Two-phase pressure-driven flow at the entrance to a channel

When a droplet attached to a syringe is aspirated into a comparable or small diameter tube by applying suction pressure, with wetting fluid present in the tube and the surroundings, we observe the formation of a thin film separating the droplet fluid from the tube. Similar results have been observed in micropipette aspiration experiments. We characterize this two-phase entry flow by reporting the entry length versus time for varying aspiration pressures and viscosity ratios of the droplet to the surrounding fluid. We also observe pinch-off in the tube and investigate its occurrence for different values of the applied pressure and viscosity ratio. We characterize the power law for the pinch-off radius at different time scales for various viscosity ratios, including viscosity ratios close to 1.