Measurement of Lamella Thickness Evolution during Droplet Spreading on a Dry Surface and Insights into Energy Distribution during the Process

The present study deals with the evolution of a liquid film formed by droplet impact onto a dry flat surface at high Weber and Reynolds numbers. A Chromatic Confocal Sensor is employed to measure the temporal evolution of the lamella thickness of the impacting droplet, and droplet spread factors are measured using shadow imaging. These measurements were then used to identify appropriate theoretical representations for different phases of the evolving droplet height/lamella thickness. The evolution of the lamella thickness is a critical input for estimating the energy dissipation in the boundary layer during lamella spread. Existing energy budgeting approaches consider a constant boundary layer thickness to estimate the boundary layer dissipation and predict the maximum spread of the droplet. The present study instead proposes the use of a transient boundary layer thickness along with a suitable velocity scale. Furthermore, the significance of considering early-stage energy dissipation in energy budgeting is demonstrated, and an approach for integrating both early-stage and wall boundary layer dissipations into an energy-based model is presented. A differential form of the energy balance equation is proposed for high Weber and Reynolds number impacts and is used to capture the time variation of the spreading diameter. The temporal evolution of the droplet spread factors obtained using this model is in good agreement with our measurements.