From splashing to bouncing: the influence of viscosity on the impact of suspension droplets on a solid surface

TL;DR

This study experimentally explores how viscosity influences the splashing and bouncing behavior of suspension droplets impacting solid surfaces, revealing a phase diagram and conditions under which droplets bounce or deform.

Contribution

It introduces a combined phase diagram based on particle Weber and Stokes numbers to predict splashing and bouncing phenomena in viscous suspension impacts.

Findings

Splashing occurs when particle kinetic energy exceeds surface tension confinement.

Increasing solvent viscosity suppresses splashing and can cause droplets to bounce.

At very low Stokes numbers, droplets deform plastically and bounce back.

Abstract

We experimentally investigated the splashing of dense suspension droplets impacting a solid surface, extending prior work to the regime where the viscosity of the suspending liquid becomes a significant parameter. The overall behavior can be described by a combination of two trends. The first one is that the splashing becomes favored when the kinetic energy of individual particles at the surface of a droplet overcomes the confinement produced by surface tension. This is expressed by a particle-based Weber number $We_p$. The second is that splashing is suppressed by increasing the viscosity of the solvent. This is expressed by the Stokes number $St$, which influences the effective coefficient of restitution of colliding particles. We developed a phase diagram where the splashing onset is delineated as a function of both $We_p$ and $St$. A surprising result occurs at very small Stokes number, where not only splashing is suppressed but also plastic deformation of the droplet. This leads to a situation where droplets can bounce back after impact, an observation we are able to reproduce using discrete particle numerical simulations that take into account viscous interaction between particles and elastic energy.