Parallel governing criteria for non-Newtonian droplet rebound suppression on superhydrophobic surfaces

Abstract

Non-Newtonian droplets are known to suppress rebound on superhydrophobic (SH) surfaces. Our previous work (Dhar et al., Phys Rev Fluids, 2019, vol 4 [1]) showed that the polymer concentration and impact velocity must exceed a certain threshold to initiate the onset of rebound suppression. Analogous to drag reduction or elastic instabilities, we proposed that rebound suppression occurs only when the Weissenberg number exceeds unity (i.e., Wi >1) at the onset of retraction. In this study, we explore four different types of SH surfaces to examine the universality of the previous observation (the importance of Wi >1 for rebound suppression). We observed that rebound suppression does not occur on all types of SH surfaces. The surfaces where rebound suppression was observed are categorized as ‘Type-I’, while the remaining as ‘Type-II’. The non-uniformity in the rebound suppression phenomena is explained by the role of Cassie to Wenzel transformation (CWT) or impalement. We showed that for ‘Type-I’ surfaces, when the ratio of dynamic pressure (PD) to minimum capillary pressure (PC) exceeds unity (i.e., PD/PC > 1), the rebound is suppressed. The ‘Type-II’ surfaces have characteristic spacing in the order of nanometers, which is three orders of magnitude lower than the microtextured ‘Type-I’ surfaces. Thereby, the minimum capillary pressure is significantly higher for ‘Type-II’ surfaces. The dynamic pressure could not surpass the minimum capillary pressure for the impact velocities and polymer concentrations studied for the ‘Type-II’ surfaces. As PD/PC remained < 1, for ‘Type-II’ surfaces, rebound suppression was not observed. It must be noted that for the same values of PD/PC > 1 values, water droplets didn’t show rebound suppression on ‘Type-I’ surfaces. The non-Newtonian droplets are known to slow down the retraction dynamics due to normal stress, extensional viscosity and higher adsorption at the substrate. We have demonstrated the role of extensional viscosity in this study, particularly how strain stiffening of radial filaments emanating from the droplet aids in dissipating energy, which further aids in arresting drop rebound. Therefore, the existence of two parallel mechanisms: the role of fluid elasticity (manifested through Wi > 1), and the role of CWT or impalement (manifested through PD/PC> 1), are necessary conditions for non-Newtonian drop rebound suppression on superhydrophobic surfaces.