with a large specific surface area featuring both hydrophilic and hydrophobic characters. [4] The hydrophobic nature of GO nanosheets originates from the basal plane, i.e., carbon rings, while hydrophilicity is imparted to GO by the surface and edge functional groups, e.g., hydroxyl, carboxylic, and epoxy groups. Due to this dual nature, GO nanosheets self-assemble at oil/water (O/W) interfaces, forming nanometer-thick barriers separating water and oil. Thus, by tuning the carbon to oxygen ratio, GO can be assembled at liquid-liquid interfaces to generate hierarchical structures with defined functionalities. [5,6] The promise of GO for numerous applications has increased interest in its interfacial behavior.The assembly of GO at liquid-liquid interfaces has been investigated primarily by dynamic interfacial tension (IFT). Previously, we [7] showed that GO assembles at the O/W interfaces, forming, predominantly a tessellated, nanosheet barrier reducing the interfacial surface energy between the liquids. The low bending modulus of GO enables the assemblies to conform to the curvature of the interface. Similarly, Kim et al. [1] investigated the activity of GO nanosheets at air-liquid, liquid-liquid, and liquid-solid interfaces, showing that, despite the stable dispersion of GO in water, GO segregates to the interfaces to reduce the interfacial tension. Imperiali et al. [8] reported on GO film formation at air/W interfaces, performing compression/expansion experiments in a Langmuir trough. They found that GO assembles at the surface and, upon compression, maitains a single layer thickness, resisting overlap due to attractive lateral forces. However, the influence of GO on the viscoelastic properties of the O/W interfaces has not been thoroughly investigated, which is critical for applications, including emulsification, enhanced oil recovery (EOR), and all-liquid 2D and 3D printing. We recently demonstrated, for example, the importance of the interfacial rheology of O/W on the stabilization of Pickering emulsions. [9] It was shown that the interfacial rheology plays a decisive role in emulsion formation, [9,10] controlling the emulsion morphology and stability. [11,12] In all-liquid 3D printing, reducing the interfacial tension to retard Plateau Rayleigh (PR) instabilities and stabilize the interface [13] is essential. Several nanomaterials have been recently proposed for sculpting liquids. [14] For instance, the printability Tailoring the oil/water (O/W) interface is a prerequisite for structuring these two immiscible liquids into prescribed architectures, i.e., liquid-in-liquid printing, which is an emerging area in material science. Here, assemblies of graphene oxide (GO) at O/W and air/W interfaces are characterized using a wide range of interfacial rheological techniques. It is shown that the GO nanosheets assemble at the interface, even at extremely low concentrations as low as 0.04 vol%, significantly increasing the elasticity at O/W or air/W interfaces. This is attributed to the combined hydrophobic and hydrop...

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.

This paper reports the experimental results of a water droplet spreading on a glass substrate submerged in an oil phase. The radius of the wetted area grows exponentially over time forming two distinct regimes. The early time dynamics of wetting is characterized with the time exponent of 1, referred to as the viscous regime, which is ultimately transitioned to the Tanner's regime with the time exponent of 0.1. It is revealed that an increase in the ambient phase viscosity over three decades considerably slows down the rate of three-phase contact line movement. A scaling law is developed where the three-phase contact line velocity is a function of both spreading radius and mean viscosity, close to the geometric mean of the droplet and ambient fluids' viscosities. Using the proposed scaling and mean viscosity, all plots of spreading radius for different viscosity ratios collapse to a master curve. Furthermore, several cases with multiple rupture and spreading points, i.e., wetting in a nonideal system, are considered. The growth of an equivalent wetting radius in a multiple point spreading system is predicted by the developed scaling law.

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