Blebbing dynamics in an oil-water-surfactant system through the generation and destruction of a gel-like structure

Interfacial dynamical blebbing of an oil droplet on an aqueous solution is reported. The oil droplet and the aqueous solution contain a fatty acid and a cationic surfactant, respectively. When the oil droplet was placed on an aqueous surface, the oil-water interface formed blebs, spherical extrusions on the oil-water interface and circular ones on the edge of the oil droplet. With increase of the concentration of the surfactant and/or the fatty acid, the generated blebs become smaller. Based on the experimental observations together with theoretical considerations, we conclude that the bleb formation is induced through generation and destruction of a gel-like intermediate phase.

Figure 1

Snapshots of droplet deformation, where $C_{\mathrm{s}}=10\mathrm{mM}$ and $C_{\mathrm{p}}=5\mathrm{mM}$. A $500\text{−}\mathrm{\mu }\mathrm{l}$ oil droplet was added to $2000\mathrm{\mu }\mathrm{l}$ of aqueous solution on a plastic plate. The arrow indicates a bleb that is formed (0–5) and collapses (6,7). The snapshots were taken every $1\mathrm{s}$. The scale bar corresponds to $10\mathrm{mm}$. See also the movie [5].

Figure 2

Spatiotemporal plot of droplet deformation at different $C_{\mathrm{p}}$: (a) 1, (b) 5, and (c) $10\mathrm{mM}$. A $500\text{−}\mathrm{\mu }\mathrm{l}$ oil droplet was added to $2000\mathrm{\mu }\mathrm{l}$ of the aqueous solution $(C_{\mathrm{s}}=10\mathrm{mM})$. The position of the edge of the oil droplet was measured in cylindrical coordinates $r(t,\theta )$ every $0.1\mathrm{s}$. In the right diagram, $t=0$ corresponds to ca. $1\min$ after the addition of the oil droplet to the surface of the aqueous solution. Note the bleb size decreases as $C_{\mathrm{p}}$ increases.

Figure 3

(a) Geometrical configuration of the theoretical model. It is assumed that the droplet exhibits a small deformation on its periphery. (b) Mechanism of bleb formation. The oil droplet, with a radius ${\rho }^{\left(1\right)}$, is covered with the gel whose steady-state thickness is $h_0$. From $-\theta$ to $\theta$, the gel thickness is perturbed to be slightly thinner than $h_0$. If $\theta <{\theta }^{*}$, the perturbation is stabilized, whereas for $\theta >{\theta }^{*}$ the bleb forms until the gel disappears. After the disappearance of the gel, the bleb shrinks quickly due to the increased interfacial energy between the oil and the water.

Figure 4

Snapshots of droplet deformation, where $C_{\mathrm{s}}=1\mathrm{mM}$ and $C_{\mathrm{p}}=10\mathrm{mM}$. The scale bar corresponds to $20\mathrm{mm}$. See also the movie [5].