Probing dissipation length-scale in spreading drops using granular suspensions

Granular suspensions are used to gain a better understanding of the multiscale physics of spreading drops. In a previous work, monomodal granular suspensions were shown to obey the Cox Voinov law which relates the capillary number to the dynamical contact angle, provided one uses an apparent viscosity. Contrary to the bulk viscosity of monomodal suspensions which solely depends on particle volume fraction,  the apparent wetting viscosity was shown to depend both the particle concentration and size: it is maximum for the smallest particles and decrease to almost the suspending fluid viscosity above a cut-off diameter of 100 μm, even for dense suspensions. The origin of this cut-off size is investigated and linked to the dissipation length-scale of this multiscale problem. Tuning dissipation can be achieved by using the confinement constraint of the particles by the contact line.

Ordering of the particles under confinement is observed in this region and is shown to affect the wetting dynamics reflected in the effective wetting viscosity. Interestingly, systems that are more complex, consisting of bimodal suspensions offer new insights into the wetting dynamics and provide evidence of the interplay between confinement and bulk effects during the spreading of a granular suspension.