Enhanced Coalescence in Driven Foams

Abstract

External driving leads to the emergence of unique phenomena and properties in soft matter systems. We show that driving quasi-2D foams by mechanical vibration results in significant bubble coalescence, which is enhanced by the continuous phase yield stress. The competition between coarsening and coalescence can be modulated through vibration amplitude and foam liquid fraction, which can be used to create unusual structural motifs. The combined effect of coarsening and coalescence is captured through a statistical model that quantitatively describes the time evolution of the number of bubbles.

Figures (6)

• Figure 1: Snapshots of the system's structure once the foam becomes 2D (time $t_0$) and after 5 hours for vibrated ($f$ = 53 Hz, $A$ = 0.26 mm) and non-vibrated foamed emulsions with $\phi=\{65,80\}$$\%$. The emulsion with oil fraction $\phi=80$$\%$ has a yield stress of $\tau_c\sim 16.5$ Pa, while none is measured at $\phi=65$$\%$. The edge of each photograph is 70 mm.
• Figure 2: Rescaled number of bubbles as a function of time for different shaking amplitudes in foamed emulsion with $\varepsilon=10$$\%$ and $\phi=80$$\%$.
• Figure 3: a) Rescaled number of bubbles as a function of time for different liquid fractions in foamed emulsion with $\phi=80$$\%$ vibrated at amplitude $A=0.26$ mm. System's structure at the final time $t_{\rm f}$ of the experiment for b) $\varepsilon=10$$\%$ ($t_{\rm f}\sim 11$ h) and c) $\varepsilon=20$$\%$ ($t_{\rm f}\sim 36$ h).
• Figure 4: Panels a, b and c: derivative of the rescaled number of bubbles as a function of the number of bubbles for three different shaking amplitudes, $\varepsilon=10\%$ and $\phi=80\%$. The data $\dot{n}(t)$ (blue squares) are obtained by differentiating a high-order polynomial fit of the $n(t)$ curves. We provide a comparison between experimental data and the model prediction obtained by fitting $\dot{n}(t)$ with Eq. \ref{['eq::modII']}. We also highlight the resulting contribution of coarsening and coalescence in the model. Panels d, e, f, g, h and i: coarsening rate $b$ and the product $a\sigma$ obtained by fitting Eq. \ref{['eq::modII']} over different experimental curves obtained varying $A$, $\varepsilon$ and $\phi$. Fixed control parameters are $\varepsilon=10\%$ in panels d, e, h, i; $\phi=80$$\%$ in d, e, f, g; $A=0.26$ in f, g and i.
• Figure 5: Rescaled number of bubbles as a function of time for different liquid fractions in aqueous foams ($\phi=0$$\%$) vibrated at amplitude $A=0.26$ mm.