Micro-Particle Tracking Velocimetry in the impingement zone formed by a micro-droplet train*

The flow field in the impingement zone of a single droplet train is studied experimentally. The near-wall velocities are measured at different heights from the impingement wall utilizing a Micro-Particle Tracking Velocimetry (μ-PTV) technique. The flow experiments are conducted using a dielectric cooling fluid (HFE 7100) seeded with fluorescent particles. Measurements are performed using double–exposed single-frame images at low frequency as required by the existence of high magnitude quasi-steady velocities in the impingement zone. The methodology with an algorithm to estimate the radial velocities at different heights is described. The observations from the micro-droplet experiments are then compared with a micro-jet stream case where the flow is at a steady state to establish the performance of the measurement technique. A μ-PTV technique is found to be appropriate for the study of this kind of flow field, as it requires a low fluorescent particle seeding loading, which will not interfere with the droplet generation, manage the unstable impingement point from one image to the next, and the quasi-steady nature of the flow.