Surfactant effects on the coalescence of a drop in a Hele-Shaw cell

Abstract

In this work the coalescence of an aqueous drop with a flat aqueous-organic interface was investigated in a thin gap Hele-Shaw cell. Different concentrations of a nonionic surfactant (Span 80) dissolved in the organic phase were studied. We present experimental results on the velocity field inside a coalescing droplet in the presence of surfactants. The evolution of the neck between the drop and the interface was studied with high-speed imaging. It was found that the time evolution of the neck at the initial stages of coalescence follows a linear trend, which suggests that the local surfactant concentration at the neck region for this stage of coalescence can be considered quasiconstant in time. This neck expansion can be described by the linear law developed for pure systems when the surfactant concentration at the neck is assumed higher than in the bulk solution. In addition, velocity and vorticity fields were computed inside the coalescing droplet and the bulk homophase using a high-speed shadowgraphy technique. The significant wall effects in the Hele-Shaw cell in the transverse axis cause the two vertical velocity components towards the singularity rupture point, from the drop and from the bulk homophase, to be of the same order of magnitude. This movement together with the neck expansion creates two pairs of counteracting vortices in the drop and in the bulk phase. The neck velocity is the average of the advection velocities of the two counteracting vortex pairs on each side of the neck. The presence of the surfactant slows down the dynamics of the coalescence, affects the propagation direction of the pair of vortices in the bulk phase, and reduces their size faster compared to the system without surfactant.

Coalescence-Induced Self-Propulsion of Droplets on Superomniphobic Surfaces

“…Utilizing our superomniphobic surfaces, we have also investigated the coalescence-induced self-propulsion of droplets of water containing different concentrations of SDS, with different surface tensions (γ lv ≈ 72–40 mN m –1 for experiments and γ lv ≈ 400–10 mN m –1 for numerical simulations) but approximately constant droplet radius ( R 0 ≈ 710 μm) and viscosity (μ ≈ 1 mPa·s), so as to result in a range of Ohnesorge numbers ( Oh ≈ 0.003–0.004 for experiments and Oh ≈ 0.0009–0.009 for numerical simulations). We have limited our numerical simulations to surface tension γ lv ≥ 10 mN m –1 because there are no known liquids with surface tension γ lv < 10 mN m –1 (at room temperature and atmospheric pressure). , Recent work − has demonstrated that when two similar droplets containing the same surfactant with equal concentrations coalesce the temporal gradients in the surface tension (due to reconfiguration of the surfactant) disappear very quickly and do not influence the dynamic coalescence process significantly. Consequently, in our experiments with two nearly identical droplets with the same concentration of SDS, we do not anticipate a significant influence of the surface tension gradients on the coalescence process and the coalescence-induced jumping velocity V j .…”

Transient freezing of water between two parallel plates: A combined experimental and modelling study

“…A thicker ice layer close to the wall would obstruct the field of view of the camera and deteriorate the quality of the optical measurements. The dimensions of the channel and copper plate are chosen so that the liquid flow and the heat transfer through the solidified layer followed a quasi-two-dimensional assumption, similarly to a Hele-Shaw cell [37,38] and any boundary effects along the wall-normal (z) direction can be neglected as the measurements are taken in the middle of the channel (see bottom Fig. 1(b)).…”

Optical measurements in evolving dispersed pipe flows

“…Chinaud et al (2016) showed that two more counteracting vortices form in the bulk coalescing phase. However, the spatial resolution in the current experiments is not high enough to capture these vortices.…”