Coalescence of surface bubbles: The crucial role of motion-induced dynamic adsorption layer.

The formation of motion-induced dynamic adsorption layers of surfactants at the surface of rising bubbles is a widely accepted phenomenon. Although their existence and formation kinetics have been theoretically postulated and confirmed in many experimental reports, the investigations primarily remain qualitative in nature. In this paper we present results that, to the best of our knowledge, provide a first quantitative proof of the influence of the dynamic adsorption layer on drainage dynamics of a single foam film formed under dynamic conditions. This is achieved by measuring the drainage dynamics of single foam films, formed by air bubbles of millimetric size colliding against the interface between n-octanol solutions and air. This was repeated for a total of five different surfactant concentrations and two different liquid column heights. All three steps preceding foam film rupture, namely the rising, bouncing and drainage steps, were sequentially examined. In particular, the morphology of the single film formed during the drainage step was analyzed considering the rising and bouncing history of the bubble. It was found that, depending on the motion-induced state of adsorption layer at the bubble surface during the rising and the bouncing steps, single foam film drainage dynamics can be spectacularly different. Using Direct Numerical Simulations (DNS), it was revealed that surfactant redistribution can occur at the bubble surface as a result of the bouncing dynamics (approach-bounce cycles), strongly affecting the interfacial mobility, and leading to slower rates of foam film drainage. Since the bouncing amplitude directly depends on the rising velocity, which correlates in turn with the adsorption layer of surfactants at the bubble surface during the rising step, it is demonstrated that the lifetime of surface bubbles should intimately be related to the history of their formation.

Measuring mutual diffusion coefficients in aqueous binary mixtures with unidimensional drying cells.

Chemical diffusion is a mass transport process caused by thermally generated motions of species. In a binary mixture, the diffusion of one species in one direction involves the diffusion of another species in the opposite direction, which corresponds to a single mutual diffusion coefficient. Here, we report a simple and general method to measure such coefficients in binary liquid mixtures, using the PNIPAM/water system as a study case. Experimentally, we show how a simple unidirectional drying cell coupled with a spatially-resolved characterization method such as Raman microscopy can yield concentration gradients developing in between two boundaries of known and constant chemical potential. Acquiring such gradients over time leads to a time-set that is shown to collapse to a single master curve using a change of variable. Such a scaling law offers a self-checking frame for solving analytically the diffusion-advection equation. As a result, we show that a simple analytical formula relates the measured concentration gradient with the concentration-dependent mutual diffusion coefficient. In the PNIPAM/water system, the mutual diffusion coefficient sharply decreases at low water content. Our work thus highlights the importance of considering the concentration-dependence of the mutual diffusion coefficient in complex aqueous solutions and provides a method to measure it.

Assessing suspension and infectivity times of virus-loaded aerosols involved in airborne transmission.

Airborne transmission occurs through droplet-mediated transport of viruses following the expulsion of an aerosol by an infected host. Transmission efficiency results from the interplay between virus survival in the drying droplet and droplet suspension time in the air, controlled by the coupling between water evaporation and droplet sedimentation. Furthermore, droplets are made of a respiratory fluid and thus, display a complex composition consisting of water and nonvolatile solutes. Here, we quantify the impact of this complex composition on the different phenomena underlying transmission. Solutes lead to a nonideal thermodynamic behavior, which sets an equilibrium droplet size that is independent of relative humidity. In contrast, solutes do not significantly hinder transport due to their low initial concentration. Realistic suspension times are computed and increase with increasing relative humidity or decreasing temperature. By uncoupling drying and suspended stages, we observe that enveloped viruses may remain infectious for hours in dried droplets. However, their infectivity decreases with increasing relative humidity or temperature after dozens of minutes. Examining expelled droplet size distributions in the light of these results leads to distinguishing two aerosols. Most droplets measure between 0 and 40 µm and compose an aerosol that remains suspended for hours. Its transmission efficiency is controlled by infectivity, which decreases with increasing humidity and temperature. Larger droplets form an aerosol that only remains suspended for minutes but corresponds to a much larger volume and thus, viral load. Its transmission efficiency is controlled by droplet suspension time, which decreases with increasing humidity and decreasing temperature.

Lifetime of Surface Bubbles in Surfactant Solutions.

Despite the prevalence of surface bubbles in many natural phenomena and engineering applications, the effect of surfactants on their surface residence time is not clear. Numerous experimental studies and theoretical models exist but a clear understanding of the film drainage phenomena is still lacking. In particular, theoretical work predicting the drainage rate of the thin film between a bubble and the free surface in the presence and absence of surfactants usually makes use of the lubrication theory. On the other hand, in numerous natural situations and experimental works, the bubble approaches the free surface from a certain distance and forms a thin film at a later stage. This article attempts to bridge these two approaches. In particular, in this article, we review these works and compare them to our direct numerical simulations where we study the coupled influence of bubble deformation and surfactants on the rising and drainage process of a bubble beneath a free surface. In the present study, the level-set method is used to capture the air-liquid interfaces, and the transport equation of surfactants is solved in an Eulerian framework. The axisymmetric simulations capture the bubble acceleration, deformation, and rest (or drainage) phases from nondeformable to deformable bubbles, as measured by the Bond number (Bo), and from surfactant-free to surfactant-coated bubbles, as measured by the Langmuir number (La). The results show that the distance h between the bubble and the free surface decays exponentially for surfactant-free interfaces (La = 0), and this decay is faster for nondeformable bubbles (Bo ≪ 1) than for deformable ones (Bo ≫ 1). The presence of surfactants (La > 0) slows the decay of h, exponentially for large bubbles (Bo ≫ 1) and algebraically for small ones (Bo ≪ 1). For Bo ≈ 1, the lifetime is the longest and is associated with the (Marangoni) elasticity of the interfaces.