The characteristic length scales in direct mode of drop formation from the edges of spinning discs with different surface wetting characteristics

Abstract

The characteristic length-scales responsible for primary and secondary drop size in direct droplet mode are probed using discs of different surface wettability through exposure controlled high resolution static imaging. The primary drop formation from the edge of a hydrophobic disc is found to be mechanistically different from its hydrophilic counterparts. The disc wetting factor (w) influences primary drop size only at low Weber number (We). A single mean primary/secondary drop size-We relationship represents discs of different surface characteristics. It points to insignificance of surface wettability. For hydrophilic discs the mean primary drop size and the mean incipient bulge size vary as We-0.47 and We-0.5 respectively, which hints at incipient bulge size dependence of primary drop size. The two capillary wave-based length scales tested as characteristic length scales for secondary drops show high size variability, which is a distinctive feature of secondary drops..