Hydrodynamics of micro-scale surface flows induced by triangulated droplet stream impingement array

A study of surface flow hydrodynamics caused by triple stream of impinging droplets arranged in a triangular array is presented. Triple streams of mono-dispersed droplets were produced using a piezoelectric droplet generator with the ability to adjust parameters such as droplet impingement frequency, droplet diameter, droplet velocity and spacing between adjacent impinging droplet streams. A translucent Zinc Selenide (ZnSe) substrate was used for characterizing the hydrodynamic phenomena of the droplet impingement zone using a high speed imaging technique. Surface jet-like fluid flows were observed among impact craters during the high-frequency droplet impingement process. A transition from laminar-like to turbulent-like surface jet flow was observed by increasing droplet Weber number or decreasing droplet impingement spacing. A correlation based on visual observations has been postulated by taking into account the droplets' Weber number ($We)$ and non-dimensional droplet impingement spacing ($S^{\ast}$). The correlation has a mathematical form of $S^{\ast }\cdot We^{n}=K$, where $K$ is a constant. One major result from the study is the relative accuracy of the postulated model in predicting the laminar-turbulent like transition in terms of $We$ and $S^{\ast }$.