Spongy all-in-liquid materials by in-situ formation of emulsions at oil-water interfaces

Abstract

Printing a structured network of functionalized droplets in a liquid medium enables engineering collectives of living cells for functional purposes and promises enormous applications in processes ranging from energy storage to tissue engineering. Current approaches are limited to drop-by-drop printing or face limitations in reproducing the sophisticated internal features of a structured material and its interactions with the surrounding media. Here, we report a simple approach for creating stable liquid filaments of silica nanoparticle dispersions and use them as inks to print all-in-liquid materials that consist of a network of droplets. Silica nanoparticles stabilize liquid filaments at Weber numbers two orders of magnitude smaller than previously reported in liquid-liquid systems by rapidly producing a concentrated emulsion zone at the oil-water interface. We experimentally demonstrate the printed aqueous phase is emulsified in-situ; consequently, a 3D structure is achieved with flexible walls consisting of layered emulsions. The tube-like printed features have a spongy texture resembling miniaturized versions of “tube sponges” found in the oceans. A scaling analysis based on the interplay between hydrodynamics and emulsification kinetics reveals that filaments are formed when emulsions are generated and remain at the interface during the printing period. Stabilized filaments are utilized for printing liquid-based fluidic channels.

Associative Liquid‐In‐Liquid 3D Printing Techniques for Freeform Fabrication of Soft Matter

“…Reproduced with permission. [ 33 ] Copyright 2022, Springer Nature. F) A helical spiral printed from an aqueous solution of surfactant within an oil phase composed of oleic acid and castor oil.…”

“…They then identified the conditions leading to the formation of a stable tubule regime (shown in green, Figure 4F) for their liquid printing. [ 33 ]…”

“…[32] E) Spontaneous formation of an emulsion zone at the water-oil interface composed of silica nanoparticles and swollen reverse micelles. [33] F) Self-assembly of cationic surfactants from the aqueous ink phase and cosurfactant (oleic acid) from the oil bath, forming the viscoelastic nanostructures at the water-oil interface. [8,34] G) Formation of droplet networks by lipid bilayers acquired at the interface of aqueous droplets due to their surface activity.…”

“…[58] Besides bijel systems, in another water-oil system involving the assembly of nanoparticles and surfactants, Bazzazi et al have demonstrated an associative LL3DP platform enabled by the spontaneous formation of an emulsion zone at the water-oil interface composed of silica nanoparticles and swollen reverse micelles (Figure 2E). [33] The authors have manually printed an aqueous solution of silica nanoparticles into liquid letters with sharp and rounded edges inside the micellar solution bath (Figure 3E). However, no computer-controlled printing was demonstrated, and a high concentration of surfactant micelles and nanoparticles was required for the successful formation of stable aqueous tubules in the oil bath.…”

“…[10,13,28,37,38] High interface activity in associative LL3DP drives the formation of complex interfaces often possessing high viscoelasticity. [8,12,29,31,33,42] On one hand, the construction of such complex interfaces with elastic properties obstructs the contact between two liquid phases, changing the interfacial energy and thus deterring the breakup. [69,70] On the other hand, the breakup timescale (τ) discussed for the RP instabilities (Equation ( 1)) may not be applied as a quantitative timescale in associative LL3DP and a new timescale has to be developed to account for such elastic interfaces.…”