Liquid–Liquid Encapsulation: Penetration vs. Trapping at a Liquid Interfacial Layer

Abstract

Encapsulation protects vulnerable cores in an aggressive environment and imparts desirable functionalities to the overall encapsulated cargo, including control of mechanical properties, release kinetics, and targeted delivery. Liquid–liquid encapsulation to create such capsules, where a liquid layer (shell) is used to wrap another liquid (core), is an attractive value proposition for ultrafast encapsulation (∼100 ms). Here, we demonstrate a robust framework for stable liquid–liquid encapsulation. Wrapping is achieved by simple impingement of a target core (in liquid form) on top of an interfacial layer of another shell-forming liquid floating on a host liquid bath. Poly(dimethylsiloxane) (PDMS) is chosen as the shell-forming liquid due to its biocompatibility, physicochemical stability, heat curability, and acceptability as both a drug excipient and food additive. Depending on the kinetic energy of the impinging core droplet, encapsulation is accomplished by either of the two pathwaysnecking-driven complete interfacial penetration and subsequent generation of encapsulated droplets inside the host bath or trapping inside the interfacial layer. Combining thermodynamic argument with experimental demonstration, we show that the interfacially trapped state, which results in a low kinetic energy of impact, is also an encapsulated state where the core droplet is wholly enclosed inside the floating interfacial layer. Therefore, despite being impact-driven, our method remains kinetic energy independent and minimally restrictive. We describe the underlying interfacial evolution behind encapsulation and experimentally identify a nondimensional regime of occurrence for the two pathways mentioned above. Successful encapsulation by either path offers efficient long-term protection of the encased cores in aggressive surroundings (e.g., protection of honey/maple syrup inside a water bath despite their miscibility). We enable the generation of multifunctional compound droplets via interfacial trapping, where multiple core droplets with different compositions are encapsulated within the same wrapping shell. Further, we demonstrate the practical utility of the interfacially trapped state by showing successful heat-curing of the shell and subsequent extraction of the capsule. The cured capsules are sufficiently robust and remain stable under normal handling.

Manipulation of Ferrofluid‐Wrapped Drops: Translation, Coalescence, and Release

“…Owing to the sufficient kinetic energy, the droplet pierces through the interfacial layer, where it overcomes the interfacial barrier offered by the ferrofluid‐water interface and results in the formation of the encapsulated droplet as reported by Misra et al. [ 65,66 ] In both situations, the interface undergoes a similar evolution process, as evident from the high‐speed time‐stamped images of Figure 2a,b, respectively. The color images of the final encapsulated droplets are shown in Figure 2a (rightmost) and Figure 2b (rightmost).…”

“…The encapsulation of the core liquid (laser oil) inside the oil‐based ferrofluid is achieved by using the robust impact‐driven liquid‐liquid encapsulation technique developed by our group. [ 65,66 ] In those works, Misra et al. [ 65,66 ] exploited the thermodynamically favorable tendency of a liquid‐triplet to demonstrate impact‐driven stable and ultrafast encapsulation by impinging a target core analyte on the interfacial layer of a shell‐forming liquid floating on the pool of another host liquid.…”

“…[ 65,66 ] In those works, Misra et al. [ 65,66 ] exploited the thermodynamically favorable tendency of a liquid‐triplet to demonstrate impact‐driven stable and ultrafast encapsulation by impinging a target core analyte on the interfacial layer of a shell‐forming liquid floating on the pool of another host liquid. This technique is further utilized by Yin et al.…”

“…We first enclose the EG droplet inside a carrier droplet of laser oil using a Y‐junction flow arrangement and then encapsulate the laser oil – EG compound droplet with a ferrofluid shell layer using the same impact‐driven liquid‐liquid encapsulation technique. [ 65,66 ] Subsequently, we use the controlled motion of a permanent magnet to manipulate the compound cargo and demonstrate on‐demand coalescence, allowing controlled mixing of the inner EG core without perishing in the surrounding aqueous medium. Finally, we show the release of the EG core inside the water bath using the magnetic field.…”

Bioinspired Scalable Lubricated Bicontinuous Porous Composites with Self-Recoverability and Exceptional Outdoor Durability

“…Oncoming droplets of different surface tensions can attain different equilibrium thermodynamic states − (cloaked vs non-cloaked) upon contact with the surface lubricant layer of LuBiCs. The lubricant cloaks a droplet of high surface tension test liquid (e.g., water) , as the combined interfacial tension of the lubricant–air interface and the lubricant–water interface is still lower than the water–air interface, making the cloaked state a thermodynamically favorable one.…”