Drop rebound dynamics of complex fluids

High-molecular-weight polymer drop impact plays a crucial role in various industrial applications, particularly in controlling drop deposition behavior in agricultural pesticide spraying. Numerous studies have shown that high-molecular-weight polymers can reduce drop rebound effects, though the exact mechanism remains unclear. Herein, we hypothesize that the molecular weight (Mw) of the polymer is not sufficient to prevent drop rebounds; rather, the rebound suppression also depends on the absorption of polymer molecules at the liquid-solid interface, governed by the polymer molecular charge and its diffusivity towards the surface. In this study, four types of polymers—sodium alginate, SA (Mw: 0.012-0.04 MDa), polyacrylamide, PAM (Mw: 5-6 MDa), Guar Gum, GG (Mw 1-2 MDa), and Tamarind, TAM (Mw: 0.25-0.65 MDa )—were used to investigate drop rebound dynamics on a Parafilm surface. These polymers varied in molecular weight and zeta potential, with GG and TAM being neutral, and PAM and SA being negatively charged. We also used various polymer concentrations (0.1 – 5000 ppm), drop sizes (600 – 700 μm), and impact heights (50 – 200 mm). Our results show that at the same Weber number (We = 20), the GG and TAM suppressed the drop rebound on a Parafilm surface, whereas PAM and SA did not. We also found that GG and TAM suppressed the drop rebound before reaching the critical overlap concentration (c^*), whereas SA and PAM exhibited the drop rebound behavior after reaching the c^* . Since the Parafilm surface is negatively charged, these observations indicate that when the charge between the polymer and the surface is unfavorable, the drop will rebound due to electrostatic repulsion between the polymer particles and the surface. Conversely, when the charges are favorable, the polymer drop will stick to the surface even in the dilute regimes.