Microstructure, Morphology, and Lifetime of Armored Bubbles Exposed to Surfactants

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Original entry: Pratomo Putra (Tom) Alimsijah, APPHY 226, Spring 2009

Microstructure, Morphology, and Lifetime of Armored Bubbles Exposed to Surfactants

Anand Bala Subramaniam, Cecile Mejean, Manouk Abkarian, and Howard A. Stone, Langmuir, Published on Web 04/05/2006


Soft Matter Keywords

Jamming, long-range, attraction, surfactant, shell, cmc, surface, tension


Abstract

The authors reported the behavior of particle-stabilized bubbles when exposed to various concentrations of surfactants. Starting with non-spherical jammed bubbles, surfactant (Triton X-100) is added causing the bubbles to dissolve. The manners the bubbles dissolve depend on the concentrations of the surfactant. For low surfactant concentration, armored bubbles remains nonspherical while dissolving while for concentrations above or close to cmc, initially nonspherical bubbles transform to spheres before dissolving. The authors then proposed explanations for these occurrences.


Soft Matter Discussion

In the experiment, they noticed how the buoyant armored bubbles rise to the top of the water droplet and deforms at the bulk air-water interface producing a flat facet as shown below.


Fig. 1: Experimental setup (a) and a drop sample containing the armored bubbles deforming at the interface (b).


Because of the jamming properties of the particles, the bubbles did not coalesce with the bulk air. In this case, the particles form a bridge between the air phase in the bubble and the atmosphere with a thin layer of water in between. Without the particle jamming effect, the gas will rapidly diffuse out of the bubble into the atmosphere through the thin water film.


Without surfactants, the bubble takes on a nonspherical shape by losing some gas and then remaining stable without further volume changes for at least 2 days. The buckling and nonspherical shapes suggest that the armored bubbles’ interface take on a solidlike behavior. Ordinary bubbles with isotropic surface tension will revert to spherical shapes.


When surfactant (Triton X-100) at a concentration of c=0.66mM (cmc ~ 0.2mM) is added, the nonspherical bubble transforms to spheres and ejects excess particles from the interface. This continues until it disappears completely. Armored bubbles with an initial radius if 20um took approximately 100s to dissolve under this condition.


At a concentration below the cmc (0.066mM), different morphology was observed. The bubble remains nonspherical throughout the whole ordeal but starts ejecting particles while losing volume. However, the particles are still immobile on the interface, compared to the earlier case (c=0.66mM) where the particles on the interface resume Brownian motion. The lifetime for the bubbles (20um) under this condition (c=0.066mM) is highly variable from 1190-1340s.


They propose that the difference in bubble lifetime, microstructure, and morphology are correlated with the surfactant concentration. These differences are shown in the figure below:


Fig. 2


For c>c(critical,2) – the earlier case – the surfactant promotes the wetting of the particles by adsorbing onto the particles and the air-water interface. This thus lead to the unjamming of the shell. However, unexpectedly, the particles mostly remain in a shell-like configuration (did not diffuse away) for the duration of the bubble lifetime. The authors believe that this is caused by the attraction between the particles and surfactant-covered air-water interface.


As expected, the free air-water interface in this case is not effective in preventing the dissolution of the bubble. The disparity of time between a normal bubble dissolving and shelled bubble dissolving is caused by the gas impermeable particles closeness to the surface and reducing the gas flux out of the dissolving bubble.


For c<c(critical,1), no destabilization occurred. For c(critical,1)<c<c(critical,2), the author tried to propose an explanation for the longer destabilization time scale compared to c>(critical,2). One of the explanations that the authors considered was the slower diffusion rate of the surfactants under this condition. However, the authors realize that this does not account for the microstructural and morphological difference between c(critical,1)<c<c(critical,2) and c>(critical,2). After doing another experiment, the author realizes that the particles desorbed only when the bubbles decreased in size when c(critical,1)<c<c(critical,2). This shows that the process of gas dissolution is necessary for the ejection of particles from the bubble interface when c(critical,1)<c<c(critical,2).


The shell is actually providing an active resistance to the dissolution of the bubble. The deflation of the bubble increases the stresses on the shell, halting the dissolution. The addition of surfactant stresses these particles (which were originally in a stable configuration) and particles are slowly ejected to relieve this stress.


Further studies can be done to determine the validity of the apparent long-range attraction between the bubble and the particles and why the particles remained in shell-like configurations even though they are unjammed.