Infuence of the surface viscous stress on the pinch‑of of free surfaces loaded with nearly‑inviscid surfactants

Abstract

We analyze the breakup of a pendant water droplet loaded with SDS. The free surface minimum radius measured in the experiments is compared with that obtained from a numerical solution of the Navier–Stokes equations for diferent values of the shear and dilatational surface viscosities. This comparison shows the small but measurable efect of the surface viscous stresses for sufciently small spatiotemporal distances from the breakup point, and allows to establish upper bounds for the values of the shear and dilatational viscosities. We study numerically the distribution of Marangoni and viscous stresses over the free surface as a function of the time to the pinching, and describe how surface viscous stresses grow in the pinching region as the free surface approaches its breakup. When Marangoni and surface viscous stresses are taken into account, the surfactant is not swept away from the thread neck in the time interval analyzed. Surface viscous stresses eventually balance the driving capillary pressure in in the pinching region for small enough values of the time to pinching. Based on this result, we propose a scaling law to account for the efect of the surface viscosities on the last stage of temporal evolution of the neck radius.