Thinning and pinch-off of viscoelastic particulate suspensions

Dispensing liquid through dripping or spraying requires its breakup into droplets, each of which ultimately involving the pinching of a liquid neck. This situation is, for instance, found in additive manufacturing or during the dispersal of drops when one sneezes or speaks. For a Newtonian fluid, as the liquid neck becomes thinner, its thickness follows a power-law with time until the breakup in a finite-time singularity. However, a small quantity of polymer dissolved in the liquid removes this singularity: the neck stretches into a long, slender filament whose thickness decays exponentially. On the other hand, solid particles suspended in a Newtonian liquid are known to speed up the thinning process. Although these two separate mechanisms have been thoroughly studied, their combination remains widely unexplored. We investigate here the pinch-off of drops of a viscoelastic suspension, i.e., non-Brownian spherical particles dispersed in a dilute polymer solution. Our experiments reveal that particles primarily affect the initial Newtonian regime through a higher effective viscosity. In a second time, the viscoelastic regime remains dominated by the sole polymer with no effect of the particles. However, the interplay between particles and polymer chains affects the transition between the two regimes where the coil-stretch transition of the polymer is strongly affected by the particle content.