Elasto-hydrodynamics of non-Newtonian droplet collision with convex substrates

Abstract

In this article, we report the post-collision elasto-hydrodynamics of non-Newtonian elastic or Boger fluid droplets [polyacrylamide (PAAM) solution in water] on convex or cylindrical targets of various diameters. Both hydrophilic and superhydrophobic (SH) surfaces were studied to deduce the role of wettability. Different governing parameters, such as cylinder diameter, Weber number, and fluid elasticity (different polymer concentrations), were systematically varied to understand various hydrodynamic outcomes. In contrast to the Newtonian water droplets on hydrophilic surfaces, PAAM droplets resisted capillary breakup and exhibited formation of long lasting, slender, fluid filaments. In certain cases, these filaments showed the existence of satellite beads during stretching, which are generated through blistering or pearling instability (known as beads-on-a string). In the case of SH surfaces, PAAM droplets rebound at larger cylindrical diameters and higher Weber number compared to water. Thin transient filaments attached to the cylinder surface eventually suppress droplet rebound. Such rebound suppression is essentially a non-Newtonian feature, as water droplets on a cylindrical SH surface always exhibited rebound and fragmentation. Finally, we illustrate phase maps where the different regimes of post-impact elasto-hydrodynamics are correlated as functions of a proposed elastic Weber number (which incorporates the effects of both the Weber and the Weissenberg numbers) and the non-dimensional diameter D*. We show that distinct scaling regimes appear in the elasto-hydrodynamic behavior of the post-impact droplets of elastic fluids.