Detachment of a highly concentrated suspension drop

We study experimentally the detachment of a large pendant drop of non-Brownian monodisperse solid spheres in a Newtonian liquid, for high solid volume fractions ( 0.55 less than or equals phi less than or equals 0.605 0.55 ≤ ϕ ≤ 0.605 0.55≤ φ ≤ 0.605 ) compared with the critical volume fraction phi Subscript c ϕ c φ _c of the suspension. This set-up allows us to monitor both the bulk-effective deformation rate and the mean stress at the neck of the drop throughout detachment. The neck effective viscosity is found to decrease close to exponentially with the current neck radius, from a large initial value, eta Subscript i η i η _i , down to the suspending liquid one. Experiments with different drop-to-particle size ratios indicate that, above phi Subscript c ϕ c φ _c , the initial value eta Subscript i η i η _i is set by the viscosity-limited dilation of the particle phase. Observations on a transparent, doubly index-matched system reveal increasing deviation from axial symmetry and the collapse of capillary menisci at the drop interface during the exponential decay. A simplified model, based on the observed rheological evolution and the close-to-conical neck shape, captures the steeply accelerated detachment dynamics and shows how eta Subscript i η i η _i sets the breakup duration.