Liquid-liquid encapsulation with a constrained interfacial layer

Liquid-liquid encapsulation enables rapid wrapping of liquid core droplets using liquid interfacial layer(s) floating on a host liquid bath, but is conventionally limited to shell-forming liquids lighter than the host bath and suffers from uncontrolled lateral spreading of the interfacial layer. We hypothesize that introducing a simple hydrophobic loop at the air-host liquid interface will (i) anchor denser interfacial liquids by interfacial pinning, circumventing the density constraint, and (ii) confine lateral spread, allowing thicker films to form from the same volume of the interfacial layer and improving process control.

Encapsulation experiments were performed using two hydrophobic loops placed one at a time at the surface of deionized water or surfactant-laden aqueous baths. Shell-forming liquids included silicone oils and dibutyl phthalate. Top and side view imaging characterized lens formation with and without the loop. Core droplets were released from controlled heights to vary the impact Weber number, and dynamics were captured using high-speed imaging. A regime map was constructed for three cases: no loop and two loops with different dimensions. Minimum and maximum effective volumes were identified for each loop.

The hydrophobic loop enabled stable confinement of denser interfacial liquids and produced visibly thicker lenses. The trapping-to-penetration transition boundary shifted upward with confinement, more strongly for smaller loops. Each loop exhibited a finite operable volume window. The thicker confined layers also reduced air entrapment by prolonging the residence time of the core droplet. Despite differences in geometry, the two loops showed similar transition Weber numbers at overflow, suggesting geometric similarity at the overflow limit. Overall, the loop enhances shell-liquid utilization, reduces air inclusion, and extends the versatility of liquid-liquid encapsulation.