Dynamic interfacial tension effects in the rupture of liquid necks

Abstract

By examining the rupture of fluid necks during droplet formation of surfactant-laden liquids, we observe deviations from expected behaviour for the pinch-off of such necks. We suggest that these deviations are due to the presence of a dynamic (time-varying) interfacial tension at the minimum neck location and extract this quantity from our measurements on a variety of systems. The presence of such dynamic interfacial tension effects should change the rupture process drastically. However, our measurements show that a simple ansatz, which incorporates the temporal change of the interfacial tension, allows us to understand the dynamics of thinning. This shows that this dynamics is largely independent of the exact details of what happens far from the breakup location, pointing to the local nature of the thinning dynamics.

Stretching Polymers in Droplet-Pinch-Off Experiments

“…Using two syringe pumps, the polymer solution is injected into the round capillary at a flow rate of Q aq ¼ 50 l=hr, while the continuous phase (hexadecane) is injected into the rectangular capillary at Q oil ¼ 20 ml=hr. The capillaries are rendered hydrophobic using OTS [Trichloro(octadecyl) silane] to avoid the use of surfactants, as they change the dynamics of the breakup process [37,38]. The formation of droplets and their detachment is monitored with an inverted microscope, with X10 or X20 objectives in a bright-field configuration, and a high-speed camera working at 2000 frames per second (fps).…”

Viscous Effects on Inertial Drop Formation

“…These trends have been confirmed by experiment and simulation [7,10,11], and Eq. (1) has been proposed as the basis of a method to measure surface tension on a millisecond timescale [12,13].…”

The Dynamic Surface Tension of Water

“…Such a dynamic surface tension is well-known for surface-active agents, and, indeed, the same experiment as done here in the presence of surfactants shows that close to drop breakup, the surface tension is significantly higher than the equilibrium one, just as is observed here. 25 , 26 If there is no adsorption barrier, the characteristic time follows from the adsorption dynamics as τ ∼ Γ 2 / Dc 2 , where Γ is the equilibrium adsorption, c the bulk concentration, and D the diffusion coefficient of the surface-active species. Characteristic times are also found to be on the order of ∼1 ms in surfactant adsorption experiments.…”