Pinch-off dynamics of emulsion filaments before and after polymerization of the internal phase.

The capillary break-up of complex fluid filaments occurs in many scientific and industrial applications, particularly in bio-printing where both liquid and polymerized droplets exist in the fluid. The simultaneous presence of fluid and solid particles within a carrier fluid and their interactions lead to deviations in the filament break-up from the well-established capillary breakup dynamics of single-phase liquids. To examine the significance of the dispersed phase and the internal interactions between liquid droplets and solid particles, we prepare emulsions through photopolymerization and conduct experimental investigations into the pinch-off dynamics of fluid filaments, focusing on the impact of varying concentrations of liquid droplets (before polymerization) and polymerized droplets. Despite the increase in bulk viscosity due to the presence of polymerized droplets in the fluid and their aggregation, the results show that polymerization significantly reduces the length of the fluid filament before breakup, thus shortening the duration of pinch-off. We investigate two categories of complex fluids, characterized by their droplet sizes: (i) sub-micrometer droplets and (ii) droplets with an average diameter of 50 micrometers. In emulsions containing sub-micrometer droplets, the individual droplet contributions remain undetectable during capillary breakup, and the measured pinch-off dynamics predominantly reflect the bulk shear viscosity or viscoelasticity of the system. This is due to the droplet sizes falling below our imaging resolution. In contrast, emulsions with larger polymerized droplets exhibit behavior analogous to single-phase carrier fluids: once the filament's length equals the droplet diameter, the droplets are expelled. Concurrently, larger liquid droplets are deformed and elongated along the flow direction. Our study highlights the effect of mixing liquid and polymerized droplets on the capillary breakup dynamics of fluid filaments, providing insights to formulate 3D printing inks.