Deformation upon impact of a concentrated suspension drop

We study the impact between a plate and a drop of non-colloidal solid particles suspended in a Newtonian liquid, with a specific attention to the case when the particle volume fraction, $ϕ$, is close to - or even exceeds - the critical volume fraction, $ϕ_c$, at which the steady effective viscosity of the suspension diverges. We use a specific concentration protocol together with an accurate determination of $ϕ$ for each drop and we measure the deformation $β$ for different liquid viscosities, impact velocities and particle sizes. At low volume fractions, $β$ is found to follow closely an effective Newtonian behavior, which we determine by documenting the low deformation limit for a highly viscous Newtonian drop and characterizing the effective shear viscosity of our suspensions. By contrast, whereas the effective Newtonian approach predicts that $β$ vanishes at $ϕ_c$, a finite deformation is observed for $ϕ>ϕ_c$. This finite deformation remains controlled by the suspending liquid viscosity and increases with increasing particle size, which suggests that the dilatancy of the particle phase is a key factor of the dissipation process close to and above $ϕ_c$.

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