Printing spongy all-in-liquid materials


 rinting structured networks of functionalized droplets in a liquid medium enables engineering collectives of living cells for functional purposes [1, 2], bacterial ecology [3], and promises enormous applications in processes ranging from energy storage [4, 5] to drug delivery [6, 7]and tissue engineering [8]. Current approaches are limited to drop-by-drop printing [1, 2] or face limitations in reproducing the sophisticated internal features of a structured material and its interactions with the surrounding media [6, 9–11]. Here, we report on 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 microemulsion zone at the oil-water interface. We experimentally demonstrate that the printed aqueous phase is emulsified in-situ; consequently, a 3D structure is achieved with flexible walls consisting of layered microemulsions. 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 hydro-dynamics and emulsification kinetics reveals that liquid filaments are formed when emulsions are generated and remain at the interface during the printing period. We demonstrate the utilization of filaments of the nanoparticle dispersions for printing fluidic channels and propose to use them as lab-on-a-chip devices.

liquid-liquid systems by rapidly producing a concentrated microemulsion zone at the oil-water interface. We experimentally demonstrate that the printed aqueous phase is emulsified in-situ; consequently, a 3D structure is achieved with flexible walls consisting of layered microemulsions. 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 liquid filaments are formed when emulsions are generated and remain at the interface during the printing period. We demonstrate the utilization of filaments of the nanoparticle dispersions for printing fluidic channels and propose to use them as lab-on-a-chip devices.
2 Introduction 1 Recently developed liquid-in-liquid printed materials [6, [10][11][12][13][14] have many potential applications 2 in energy storage [4,5], microreactors [7], and for creating biomimetic materials [8,15]. These 3 types of materials are generated by the jamming of nanoparticles at the oil-water interface by 4 application of an electrical field [16], using molding [11,17], or direct ink writing (DIW) printing 5 techniques [10,12]. The materials lack the multiscale porosity created by emulsion-based inks 6 used in 3D printed solid structures [18][19][20]. Emulsions allow encapsulating hydrophilic and 7 hydrophobic cargoes together into the same printed texture [21,22]. Also, emulsions enable 8 interactions between the printed frame and the surrounding media, where the space within the 9 printed texture, i.e., porosity, is controlled by the droplet size distribution [18][19][20]. Although 10 emulsions are ideal colloidal dispersions for printing in air, their utilization in liquid printing 11 systems is limited to sticky (gel-like) emulsions [23]. The stability of the emulsion is the main 12 challenge that hinders their applications as inks in liquid media. Either the injected emulsion 13 phase is immediately dispersed in the surrounding liquid, or the droplets swell and coalescence. 14 Consequently, conventional emulsions are not appropriate candidates for printing in liquid 15 media. 16 We describe a new approach to print porous liquids in a liquid medium. Silica nanopar-17 ticles are incorporated into spontaneous emulsification systems of sorbitan monooleate (Span  The spontaneous formation of a bicontinuous phase at the oil-water interface and its further 23 penetration inside the bulk stabilizes liquid filaments with porous walls that can be used as 24 inks for liquid-in-liquid printing. Span 80 micellar solutions. The oil viscosity is 135 mPa.s, while the addition of Span increases 30 the oil viscosity to 257 mPa.s (Fig. S1b). For the case of DI water, single drops detach from 31 the injection needle tip and sediment through the micellar solution, as is familiar from the 32 Plateau-Rayleigh instability (Fig. 1a). The injection of a 4.0 wt.% silica dispersion generates 33 three flow regimes (Fig. 1b). These flow morphologies were characterized by a fast Fourier 34 transform analysis (Supporting information, section III) and named bead-on-a-string (BOAS), 35 column, and connected, and color coded respectively in blue, green, and red throughout the 36 manuscript ( Fig. 1 and Supporting Movies 1-4).

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The BOAS regime appears for the lowest injection speeds, where the detached drops 38 merge with the preceding drops, and is identified with the dominant wavelength of 39 1.9 mm < λ mean < 2.6 mm. Please note that the column and connected flow regimes do not 40 have a dominant wavelength (Supporting information, section III). Thus, we consider the cut-41 off wavelength, where the power spectrum starts reducing, as the characteristic wavelength in 42 column and connected flow regimes. In the column state, which occurs for intermediate injec-43 tion speeds, the injected liquid forms a stable thread-like shape without breaking into droplets, 44 where the cut-off wavelength is 0.7 mm < λ cut−off < 1.0 mm. The connected regime occurs at 45 high injection speeds, where a thread connects the neighbouring droplets, i.e., a combination 46 of column and BOAS shapes, with wavelengths of 2.5 mm < λ cut−off < 3.2 mm (Fig. 1b,c). 47 Here, our particular interest is on identifying conditions that lead to the column regime since 48 stable columns have potential applications for direct ink writing, as we demonstrate below.     the injection speed (Fig. S16). The BOAS regime appears at the longest residence times, 127 meaning that emulsions are generated in this regime and diffuse from the interface (Fig. 3a. are not formed within the time frame of the experiment (Fig. 3a. 3). Hence, the transition 131 from column to connected is defined as the emulsification time (Table S1).

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The experimentally defined diffusion times in Table S1 increase upon increasing the micellar 133 solution viscosity (Fig. 3b, left and bottom axes in blue). This response is consistent with the

Liquid column characteristics and applications 154
The stability of liquid filaments during continuous injection was shown in Fig. 1. We also test Moreover, the silica dispersion is loaded into a micro-pipette (volume 10 µl) and liquid 163 letters with sharp and rounded edges are manually printed in the micellar solution (Fig. 4b). surface area of each printed texture (S) divided by its initial surface area (S i ), is plotted in Fig.   168 4b. The printed structures remain stable up to two months without losing their sharpness 169 (Fig. S18).    Correspondence and requests for materials should be addressed to Parisa Bazazi.