Drop Encapsulated in Bubble: A New Encapsulation Structure

A new fluid encapsulation structure, which is characterized by a bubble encapsulating a drop, is reported. It is stably generated from the breakup of a liquid column inside a bubble, which is achieved via the injection of Taylor flow into liquid. A model is constructed to explain the liquid column breakup mechanism. A dimensionless control guidance, which enables the possibility to create different-scale capsules, is provided. The encapsulation stability in external flows is verified, and a method to trigger the release of the encapsulated drop is provided, which supports potential applications with great advantages such as fluid transport.

Figure 1

(a) Drop encapsulated in bubble, which is the last unknown area in the fluid encapsulation map. (b) Diagram of the experiment setup. (c) Stable production of uniform capsules (see Supplemental Material for the multimedia view [9]).

Figure 2

(a) Injection of Taylor flow into the liquid makes a liquid column penetrate a bubble; (b) the liquid column fails to break up with a small aspect ratio $L_0$; (c)–(f) at various Oh, the encapsulation structure is generated by the breakup process of a liquid column with a sufficiently large $L_0$.

Figure 3

Encapsulation events plotted in terms of the liquid column radius $R$ and radical necking time $t_n$ (circle: $\mathrm{Oh}<{\mathrm{Oh}}_C$; triangle: $\mathrm{Oh}>{\mathrm{Oh}}_C$ and $w_g=60\%$; star: $\mathrm{Oh}>{\mathrm{Oh}}_C$ and $w_g=67\%$; square: $\mathrm{Oh}>{\mathrm{Oh}}_C$ and $w_g=70\%$). The solid line indicates the model $t_n=\beta t_i$ when $\mathrm{Oh}<{\mathrm{Oh}}_C$, and the other three lines indicate the model $t_n=\alpha t_v$ when $\mathrm{Oh}>{\mathrm{Oh}}_C$.

Figure 4

Phase diagram showing the outcome of the experiments (solid circle: encapsulation; open circle: no encapsulation) in terms of $\mathrm{Oh}$—$L_0$. The thick solid line indicates our model of the critical aspect ratio: $L_{0C}=\beta$ for $\mathrm{Oh}<0.052$ and $L_{0C}=\alpha Oh$ for $\mathrm{Oh}>0.052$. The light black line is the model in Ref. [22]. The light dashed line indicates the limit $L_{0M}$ on the increase in aspect ratio. The thick dashed line indicates the location of ${\mathrm{Oh}}_C$.

Figure 5

(a) Critical encapsulation events plotted in terms of $v-d/R_B$ at different viscosities (triangle: $\mu =1.81\mathrm{mPa}\mathrm{s}$; circle: $\mu =5.08\mathrm{mPa}\mathrm{s}$; square: $\mu =19.2\mathrm{mPa}\mathrm{s}$). The solid, dashed, and dotted lines indicate the encapsulation region bounds with different $\mu$. (b) Phase diagram of the control guidance in terms of $\mathrm{Re}(R_B^2/d^2)$ to produce single-ended liquid columns with $L_{0C}<L_0<L_{0M}$ at any Oh.

Figure 6

(a) Encapsulation stability with a sudden shear flow. (b) Release of the encapsulated drop by depositing the capsule on a surface.