An Experimental Investigation of Droplet Detachment in High-Speed Microchannel Air Flow

This paper experimentally investigates the mechanism of water droplet detachment in a confined microchannel under highly inertial air flow. Experimental observations show that as the Reynolds number in the channel is increased, the droplet transitions from a nearly spherical droplet to a high aspect ratio slug. Scaling arguments are then made to address this behavior in an order of magnitude sense. These results show that although shear stress at the droplet-air interface may contribute to droplet elongation, the major mechanism of droplet detachment appears to be the pressure drop across the droplet. This pressure drop is a result of a highly inertial air flow being squeezed through a narrow gap between the droplet and the adjacent microchannel wall. This pressure difference creates a force imbalance across the droplet that overcomes the surface tension force pinning the droplet at the injection site, thereby leading to droplet detachment. A formulation of a nondimensional number is then presented that relates the pressure force acting across the droplet to the surface tension force at the droplet wall interface, which indicates that detachment occurs when this number is of order 1.