We experimentally investigated the splashing of dense suspension droplets impacting a solid sur-

face, 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 overcome 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.

1. •Martin H. Klein Schaarsberg, Ivo R. Peters, Nachi Stern, Kevin Dodge,Wendy Zhang, and Heinrich M. Jaeger, “From splashing to bouncing: the influence of viscosity on the impact of suspension droplets on a solid surface”, preprint
   

The experimental part of the work was started by Martin Klein Schaarsberg a visiting student to our lab from Detlef Lohse’s Physics of Fluids group at the University of Twente. The simulations were performed in close collaboration with Nachi Stern and Kevin Dodge from Wendy Zhang’s group.

The state diagram above delineates splashing and non-splashing regimes as function of two dimensionless control parameters. The horizontal dotted line is the previous result we obtained for inviscid solvents, such as water. As the solvent viscosity increases, the Stokes number St becomes smaller. Increasing impact velocity of the droplet increases both the particle-based Weber number We_p and St (that’s why the “error” bars are slanted; these bars indicate the width of the observed transition region).

Bouncing occurs at very small Stokes numbers and high Weber numbers.

The movie below shows a droplet of glass spheres in glycerol impacting a glass slide and bouncing off: