Central Collapse of non-Newtonian Droplets

This study focuses on the impact dynamics of non-Newtonian droplets compared to Newtonian droplets, specifically investigating the central collapse phenomenon induced by capillary waves when non-Newtonian droplets impact a lubricated surface with low Weber numbers (We=ρU²R/γ < 10). The experimental techniques employed include laser-sheet visualization, reflection interference microscopy (RIM), and total internal reflection microscopy (TIRM), which enable the visualization and analysis of the re-entrant jet and the evolution of the air layer beneath the droplets. By studying Xanthan gum (XG) droplets, known for their shear-thinning behavior leading to an effective Ohnesorge number (Oh_eff) below the critical Ohnesorge number (Oh_cr), the excitation of capillary waves along the liquid-air interface is observed. These capillary waves generate a downward jet that contributes to the inversion of the air dimple, ultimately causing the central collapse of the droplets. In contrast, the capillary wave is dampened in viscous Newtonian droplets with a zero-shear viscosity above the critical Ohnesorge number (Oh>Oh_cr), and thus, central collapse is not observed. The experimental findings provide valuable insights into the impact dynamics of droplets with non-Newtonian properties, and a physical model is proposed to explain the formation of a squeezing film between the jet and the air dimple.