Decompressing Emulsion Droplets Favors Coalescence

The destabilization process of an emulsion under flow is investigated in a microfluidic device. The experimental approach enables us to generate a periodic train of droplet pairs, and thus to isolate and analyze the basic step of the destabilization, namely, the coalescence of two droplets which collide. We demonstrate a counterintuitive phenomenon: coalescence occurs during the separation phase and not during the impact. Separation induces the formation of two facing nipples in the contact area that hastens the connection of the interfaces prior to fusion. Moreover, droplet pairs initially stabilized by surfactants can be destabilized by forcing the separation. Finally, we note that the fusion mechanism is responsible for a cascade of coalescence events in a compact system of droplets where the separation is driven by surface tension.

Figure 1

Microfluidic device dedicated to the investigation of the coalescence of emulsion droplets.

Figure 2

(a) Evolution of the distance $D$ between the two centers of mass of the droplets until they coalesce as a function of the distance $x$ along the flow: ● without forcing (b) and ○ with a converging channel ($L_1=60\mu \mathrm{m}$, $L_2=200\mu \mathrm{m}$) (c). The scale bar in (b) and (c) is $50\mu \mathrm{m}$. (d) Close view of the trajectory $D(x)$ without forcing.

Figure 3

Time sequence showing the destabilization of a droplet pair passing through a symmetrical coalescence chamber: collision, relaxation, separation, and fusion. The channel width expands from $36\mu \mathrm{m}$ to $72\mu \mathrm{m}$. From top to bottom, the time in milliseconds is 0, 3, 5, 7, 7.1, and 10.

Figure 4

(a) Correlation between the location of coalescence $x_c$ and the parameter $K$ when droplet pairs are passing through a converging channel ($L_1=60\mu \mathrm{m}$, $L_2=200\mu \mathrm{m}$) for two sets of flow rate conditions (units are $\mu \mathrm{l}/\mathrm{h}$: ● $q_o=400$, $q_w=100$ and ■ $q_o=200$, $q_w=50$ ). (b) Definition of the parameter $K$ that reflects the initial configuration of the pair ($K=D_{\mathrm{eff}}/2R$ and reduces to $W{D}_0/4R^2$ by mass conservation).

Figure 5

Formation of two facing nipples in the contact area prior to coalescence induced by separating the droplets. The scale bar is $20\mu \mathrm{m}$.

Figure 6

Time sequence of the cascade of coalescence initiated in a compact train of droplets. The time step between two frames is 1 ms; the main channel width is $65\mu \mathrm{m}$.