arXiv subjects

S. Rodríguez-Aparicio

Publications and source records attributed to S. Rodríguez-Aparicio.

5 recordsLinked to original sources

arXiv2026

Effect of surfactant kinetics on the wetting following the drop impact onto rough surfaces

We experimentally analyze the effect of a surfactant on wetting following drop impact on rough surfaces, paying special attention to the role of dynamic surface tension. To this end, we compare the results obtained with Triton X-100, SDS, and Surfynol 465. For concentrations below the critical micelle concentration $c_{\textin{cmc}}$, the evolution of the coverage area is nearly identical for all three surfactants, suggesting that the surfactant concentration is too low to significantly influence droplet spreading. In contrast, pronounced differences emerge due to the distinct dynamic surface tensions of the surfactants at $c/c_{\textin{cmc}}=2$. The evolution of the coverage area during spreading is nearly the same for pure water droplets and those containing Surfynol 465, indicating that surfactant depletion is negligible during the rapid spreading stage. As the Weber number increases, droplet spreading becomes progressively less sensitive to surface tension, thereby reducing the influence of surfactant adsorption kinetics. Nevertheless, Surfynol 465 produces larger coverage areas than Triton X-100 and SDS. The final coverage area is governed by the quasi-static recession of the triple contact line, which is controlled by the receding contact angle. Surfynol 465 consistently yields substantially larger final coverage areas across the range of surface roughness considered in this study.

physics.flu-dyn↗

arXiv2026

Interface pinch-off in the presence of a soluble surfactant

We study numerically and experimentally the breakup of a pendant droplet loaded with a soluble surfactant. We consider the limit in which surfactant sorption is limited only by diffusion. Surfactant transfer toward the interface is enhanced by convection. As a consequence, diffusion does not constitute a significant barrier over most of the breakup, and surfactant sorption maintains the surface tension practically constant across the interface. Diffusion hinders the surfactant sorption only very close to the interface pinch-off. The droplet shape in the diffusion-limited model deviates significantly from that in the insoluble case over most of the breakup. In the insoluble case, the droplet shape is affected by surfactant depletion, which leads to a local increase in surface tension and Marangoni stress. The dynamics of a millimeter-sized droplet loaded with Surfynol 465 agree remarkably well with predictions from the diffusion-limited model, without any parameter fitting, down to pinching times of the order of $10-20$ $μ$s. Sodium dodecyl sulfate (SDS) produces essentially the same effects as those for Surfynol 465. Therefore, both Surfynol 465 and SDS maintain a practically constant surface tension throughout most of the droplet breakup. Slow-kinetics surfactants, such as Triton X-100, differ significantly from Surfynol 465 and SDS. The most evident effect of the surfactant adsorption energy barrier is the shortening of the filament that bridges the upper meniscus and the detached lower drop. Comparing the filament length to that of a clean interface with the same surface tension allows one to evaluate the rate of surfactant adsorption.

physics.flu-dyn↗

arXiv2025

Critical bubble bursting in real water. Effect of surface-active contaminants

We study the bursting of a bubble on a liquid free surface under critical conditions, i.e., those leading to the minimum (maximum) size (velocity) of the first-emitted jet droplet. Our experiments show that a tiny amount of surfactant considerably increases (decreases) the droplet radius (velocity). The volume of the first-emitted droplet increases by a factor of 20 for a concentration that produces an insignificant reduction in the bubble surface tension. The total liquid volume ejected by the bubble increases with the surfactant concentration. Surfactant accumulates at the bubble base due to cavity bottom shrinkage and surfactant convection. The resulting reduction in surface tension narrows the region of free surface reversal. Despite this effect, the size of the emitted droplet increases due to the Marangoni stress acting on the jet surface. Marangoni stress slows down the interface of the liquid jet, delaying the detachment of the droplet. More liquid flows into the droplet, increasing the mass and energy transfer to the resulting spray. A significant increase in the droplet size is also observed with a weak surfactant. This indicates that natural water contamination can substantially alter the bursting of bubbles under critical conditions. Our results may explain the size of the particles emitted by bubble bursting in seawater.

physics.flu-dyn↗

arXiv2025

Stability analysis of the flow in a coflowing device

We analyze the stability of the coflow configuration. The experiments and the global stability analysis show that the emitted jet always destabilizes before the tapering conical meniscus. This implies that the parameter conditions at which polydisperse dripping arises cannot be determined from the linear stability analysis of the steady jetting mode. Transient simulations show that the linear superposition of decaying eigenmodes triggered by an initial perturbation can lead to the jet breakup. The breakup process significantly depends on the initial perturbation. These results question the validity of the linear stability analysis as applied to the coflowing and other similar configurations.

physics.flu-dyn↗

arXiv2025

A novel microfluidic method to produce monodisperse micrometer bubbles

We present a novel microfluidic method to produce quasi-monodisperse bubbles with diameters from tens to very few microns. A gaseous rivulet flows over the shallow groove printed on a T-junction exit channel. The triple contact line delimiting the rivulet is pinned to the groove edges. The rivulet breaks up into bubbles much smaller than the exit channel. When operating under adequate conditions, the flow transitions toward a singular mode where the rivulet remains quasi-static and emits bubbles smaller than the groove width. This allows the production of bubbles with diameters in the 3-5 $μ$m range, which is preferable for relevant therapeutical applications.

physics.flu-dyn↗