Dynamics of impact of polymer droplets on viscoelastic surfaces

Droplet impact on soft surfaces is important in many industrial, biological and agricultural applications. In this paper, we have analyzed the dynamics of impact of polymer droplets upon PDMS surfaces. We varied the impact velocity (0.5-2 m/s) and found that impact velocity plays a crucial role in the process. The elasticity of the substrate has also been varied to study its effect upon the droplet dynamics. We delineate the entire process into three different stages and employ force balance equations to identify the governing forces during each stage. The initial spreading is strongly inertia-controlled and the maximum diameter obeys a power-law relation with the Weber number (We0.25), irrespective of the impact velocity and the surface properties. The viscoelastic nature of the surface has a dominant influence upon the retraction of the droplets. The effect is more prominent at a higher velocity wherein the droplet retraction is completely eliminated. A damped harmonic oscillator-type analogy shows that the damping is higher on soft surfaces and at higher velocities.