Breakup dynamics of Newtonian fluids under extension

Abstract When a Newtonian fluid is stretched via uniaxial extension, it forms a narrow filament which thins until it breaks. Thinning is governed by mechanical forces, whereby an increase in filament length necessitates a reduction in width, and by capillary forces, whereby the rate of capillary thinning depends on the fluid’s viscosity and surface tension. The timing of filament breakup is a key parameter in understanding fluid fragmentation, droplet formation and the maximum filament length achievable, which are relevant to many natural and industrial processes, from volcanic eruptions to spray paints. Here, we perform uniaxial extension experiments on Newtonian silicone oils stretched between circular, parallel plates moving at equal and opposite velocities. We find that filament breakup always occurs in a regime dominated by capillary forces, irrespective of the plate separation rate, and that breakup time can be reliably predicted from initial fluid dimensions, plate separation rate and the ratio of surface tension to viscosity. As an illustration of use, we apply our findings to an example of a volcanic eruption, predicting the maximum length of fluid filaments that could be produced by clasts of ejected lava breaking up mid-flight.