Droplets in an axisymmetric microtube: Effects of aspect ratio and fluid interfaces

Abstract

The flow within a liquid droplet steadily translating along an axisymmetric microtube is investigated experimentally and compared with a model to predict the invariants of the flow, that is circulation, hydrodynamic impulse, and kinetic energy. The model is based on pipe Poiseuille flow and allows for variable aspect ratio, AR. The invariants are computed from the velocity fields, which are measured with micro digital particle image velocimetry. The non-dimensionalized experimental invariants show negligible dependence on the Reynolds number, within the small range investigated (∼1.5-13), which also agrees with the model. The effect of introducing a fluid interface is found to increase the observed magnitude of invariants in low-AR droplets above those in continuous flow. Also increased are the average rates at which invariants are advected across a hypothetical flux plane within the tube. The increase in these rates above continuous flow goes as AR−1. The momentum flux is similarly increased for low-AR and is mainly attributed to the converging/diverging radial velocities near the trailing/leading interfaces. The momentum flux is also compared with available synthetic jet data.