Influence of surfactant kinetics on rapid interface creation via microjet impact on liquid pools

TL;DR

This work addresses how surfactant adsorption kinetics influence dynamic surface tension during rapid interface creation by submillimeter jet impact. It combines controlled jet impact experiments with SDS (moderate adsorption) and Surfynol 465 (ultrafast adsorption) to quantify cavity depth $d(t)$ and retraction under varying jet speeds, interpreting results through dimensionless groups such as $We$, $Oh$, $Ma$, and $Pe_s$. A simple overdamped harmonic oscillator model for cavity retraction yields a damping ratio $\zeta$ that tracks the effective dynamic surface tension $\sigma_{\text{ef}}$, and the observed ratios of $\zeta^{-2} d_{\text{max}}^{-3}$ between surfactants match the measured $\sigma_{\text{ef}}$-ratio, validating the link between kinetics and mechanics. The main finding is that ultrafast adsorption can deepen and prolong cavities, while SDS behaves similarly to pure water at these rapid timescales, highlighting a kinetic limit in dynamic surface-tension reduction with potential implications for fast interfacial processes in cooling, emulsification, and needle-free injection technologies. These insights provide a diagnostic framework for selecting surfactants in applications requiring rapid interface formation.

Abstract

We experimentally investigate the influence of surfactant adsorption kinetics on cavity dynamics during the rapid formation of interfaces. For this purpose, we use a submillimeter jet impacting onto a surfactant-laden liquid pool much larger than the jet dimensions. Cavity retraction and closure occur on a submillisecond timescale, posing a stringent test of the ability of surfactants to reduce surface tension dynamically. Our experiments reveal the difference between the effects of sodium dodecyl sulfate (SDS), a surfactant with moderately fast adsorption kinetics, and Surfynol 465, a surfactant with ultrafast adsorption kinetics. For SDS, the collapse pathway is nearly indistinguishable from that of pure water, suggesting negligible dynamic surface tension reduction. In contrast, Surfynol allows the emergence of deeper cavities that persist longer in the liquid pool. The harmonic oscillator model accurately captures the cavity retraction in the deep seal regime. The fitted values of the damping ratios are consistent with the dynamic surface tensions.

Figures (8)

• Figure 1: Cavity closure modes: shallow seal (a), deep seal (b), and surface seal (c). The images were taken with pure water for a jet radius $r_j=$ 30.9 $\mu$m (a), 36.4 $\mu$m (b), and 38.5 $\mu$m (c) and velocity $v_j=$ 14.9 m/s (a), 19.9 m/s (b), and 29.3 m/s (c), respectively. The arrows indicate the pinch-off location.
• Figure 2: Sketch of the fluid configuration. A submillimeter jet (red) impacts on a centimeter liquid pool (blue), creating a cavity filled with air (cyan).
• Figure 3: Surface tension $\sigma$ as a function of the surfactant volumetric concentration $c$ for SDS and Surfynol.
• Figure 4: Schematic representation of the experimental setup: (A) high-speed camera, (B) optical lenses, (C) laser, (D) liquid bath, (E) microfluidic chip, (F) syringe, (G) light source, (H) translation stage. The inset shows a typical snapshot sequence of the thermocavitation and jet generation process.
• Figure 5: (a) Images of the cavity evolution for $r_j\approx$ 39 $\mu$m, $v_j \approx 20$ m/s, and the three liquids analyzed in this work (top: water, middle: SDS, bottom: Surfynol). The images correspond to the instants indicated in the graphs. (b) Mean cavity penetration depth $d(t)$. The shaded gray area represents the uncertainty in the estimate of the mean.