2.1 Emulsification and LS

Like graphene, graphene oxide (GO) nanosheets are essentially one-atom thick but can be as wide as tens of micrometers (Figure S1, Supporting Information).^[32-34] Recently, it was shown^[35] that GO nanosheets in water (W-GO, see Table 1 for abbreviations) instantaneously migrate and assemble at the oil/water (O/W) interface due to their tendency to spread over high-energy interfaces, to minimize the Helmholtz free energy.^{[32, 36]} This propensity stems from the dual nature (hydrophobic and hydrophilic) of GO; the hydrophobic nature of GO nanosheets originates from the basal plane, i.e., carbon rings. Meanwhile, GO hydrophilicity is imparted by the functional groups, e.g., hydroxyl, carboxylic, and epoxy, that exits on the surface and edges of the GO.^{[35, 37, 38]} While the basal plane of GO is hydrophobic, segregating the GO to the interface replaces the exposure of the basal plane on both sides from water to one where one side is exposed to the hydrophobic oil, a far more favorable interaction.^[35] The GO interfacial network structure acts as a barrier, reducing the interfacial surface energy (Figure S2a,b, Supporting Information) between the aqueous GO suspension and oil, and thus improves the mechanical integrity of the O/W interface (Figure S2c, Supporting Information).^{[35, 39, 40]}

The high elasticity of the O/W interface in the presence of GO nanosheets (Figure S2c, Supporting Information), even at extremely low concentrations (e.g., 0.04 vol%), makes the GO assemblies exceptional candidates for applications where stabilization of the O/W interface is essential. For instance, a recent work showed that interfacial elasticity plays a decisive role in emulsion stabilization.^[41] Thus, here, we achieved ultrastable oil-in-water emulsions at extremely low concentrations of the GO nanosheets by low energy (hand-shaking) emulsification (see Figure S2a,d, Supporting Information, corresponding to the digital and optical microscope images, respectively, of a 50:50 oil-in-water emulsion in the presence of 0.3 vol% GO). The fabricated emulsions were stable over the course of months.

Interestingly, following the same low energy emulsification process, upon the incorporation of 1 mg mL⁻¹ PSS-[3-(2-aminoethyl)amino]propyl-heptaisobutyl substituted POSS (POSS-NH₂) into the silicone oil (O-POSS), a phase inversion occurred, leading to the formation of water-in-oil emulsions. Figures S2f and S3a (Supporting Information) show the 50:50 oil-in-W-GO emulsions in the presence of POSS. Figure S3b (Supporting Information) compares the interfacial properties of O/W-GO 0.04 vol% with and without POSS. With POSS dissolved in the silicone oil against W-GO 0.04 vol%, the equilibrium interfacial tension (IFT), the free energy penalty of exposing one immiscible liquid to another, was found to be ≈ 6 mN m⁻¹, which is considerably lower than that of the pure silicone oil and W-GO 0.04 vol% (≈ 35 mN m⁻¹). This confirms that POSS molecules display activity at the O/W interfaces, where they assemble in high number density. Moreover, a solid interfacial skin, wrapping the aqueous phase, is observable during contraction of a pendent droplet of W-GO 0.04 vol% in the O-POSS phase (Figure S3b, Supporting Information). The wrinkles on the droplet's surface upon decreasing the volume confirm the synergistic effect of the nanosheets and POSS at the interface, leading to the formation of a mechanically robust interfacial skin with high binding energy that can withstand the compressive force. The solid skin can hold the droplets in highly nonequilibrium shapes, even with stable sharp edges (Figures S2e and S3d, Supporting Information), leading to the formation of structured emulsions (i.e., nonspherical dispersed phase). The interfacial film formation has been previously investigated by various means including grazing incidence X-ray scattering, atomic force microscopy, and X-ray photon correlation spectroscopy.^[42-45]

Herein, the interfacial skin formation can be attributed to strong interactions between rigid POSS molecules and GO nanosheets (i.e., hydrogen bonding between the amino groups of POSS and superficial functional groups of GO). Moreover, the existence of POSS changes the charge at the interface from being negative to positive.^[39] Thus, interfacial jamming induced by electrostatic interactions between nanomaterials and oppositely charged surfactants, e.g., ammonium-carboxylate ion pairing, can be another reason for the strong film formation of negatively charged GO nanosheets at the POSS incorporated interfaces (i.e., interfacial complexation).^[46] The electrostatic interactions between nanoparticles and surfactants at interfaces have been studied in details in a recent study by Luo et al.^[47] What is more, the rigid structure of POSS molecules, cubic inorganic silica-cage core with side chains, significantly contributes to the mechanical strength of the GO/POSS interfacial film. Further discussion on the performance of GO with and without POSS at the O/W interface using different techniques (IFT and interfacial rheology) can be found in the Supporting Information.

The mentioned characteristic of the O-POSS/W-GO interfaces, i.e., rapid interfacial skin formation, prompted us to develop a facile method for devising new types of soft material with controlled morphology. In this method, named liquids streaming (LS), with the aid of a 3D printer, or a simple pump, aqueous suspensions of GO can be jetted into an O-POSS phase, see Videos S1 and S2 (Supporting Information). In other words, the water phase is extruded out of a nozzle, at a controlled flow rate, into the oil phase. The rapid co-assembly of the GO nanosheets and POSS prevents Plateau-Rayleigh instabilities,^[48] i.e., the break-up of the jetted aqueous phase into drops due to minimization of the surface energy, leading to the formation of aqueous tubules wrapped by the continuous GO/POSS interfacial skin (Figure 1a,b), which are stable over the course of months (Figure 1c). It is worth mentioning that the tubules’ formation and their mechanical integrity stem solely from the rapid GO/POSS interfacial skin formation, rather than the bulk rheology of the suspensions, which is the primary origin of the typical filament formation in direct ink writing/printing of hydrogels.^[49-52] Figure 1d and Video S3 (Supporting Information) clearly show the flowability of the GO suspensions at four different concentrations. Following a recent work by Russell and co-workers^[] and for simplicity, we named these structured liquids as “sculpted emulsions.” We obtained this type of two-phase system even at extremely low concentrations, as low as 0.04 vol% (see Figure 1e and Video S2, Supporting Information). Thus, the co-assembly of GO nanosheets and POSS at the interface of O/W creates a mechanically-rigid, solidified interfacial layer encasing the aqueous liquid (see Video S4, Supporting Information).

The formation of the tubules significantly relies on the pressure of the injection. Figure 1f schematically shows that at low or high enough injection pressures, droplet formation takes place rather than tubule formation (see Videos S5, S6, S7, S8, and S9, showing the injection at 50, 70, 90, 110, and 150psi, respectively). The transition from a continuous jet to a stream of droplets relies on the viscosity µ, density ρ, surface tension γ, and the jet diameter d, which can be estimated by Ohnesorge number Oh = µ/√(ργd). When 1 < Oh or Oh < 0.1, instability sets in, and the liquid jet breaks apart into droplets. See Figure S5 (Supporting Information) and the corresponding discussion for further information.

Indeed, the LS technique provides a versatile platform for designing two-phase soft materials with continuous domains. More importantly, the domain size, combination, and macroscale design are highly tunable. For instance, by manipulating the flow rate, needle size, and the printing head's speed, the tubules’ size can be readily controlled (see, for instance, Figure 1i; Figures S6 and S7, Supporting Information). Moreover, by virtue of the precise control of this platform, new classes of soft materials can be formed, namely multilayer (Figure 1g) and interpenetrated (Figure 1h) tubules. The latter were fabricated by simultaneous injection of different aqueous suspensions (0.04 and 0.3 vol% GO) through separated nozzles into the O-POSS. A myriad of application prospects emerges given the possibility of loading these GO suspensions with different materials (e.g., magnetic particles,^[35] metal–organic frameworks,^[54] etc.) and injecting them through different needles simultaneously. They can also expand the range of materials bestowed with multi-functionally, via the LS process. All in all, the proposed customizable LS technique opens new avenues to design novel uniform, multifunctional two-phase soft materials featuring different orders or shapes. We postulate that these novel types of soft materials can greatly expand the application of multiphase systems.

2.2 Aerogel Formation

The emulsions in the previous section were formed by injection of the aqueous phase into a low-viscous silicone oil. In this section, in order to fabricate the aerogels, the silicone oil was replaced with a volatile nonpolar liquid. This decision roots from our observation that the LS technique worked well with different volatile nonpolar phases, including toluene and hexane. Herein, for brevity, we report on the interfacial skin formation results of LS in hexane (Figure S8, Supporting Information), noting that a similar phenomenon was observed in other cases. Figures S8 and S9 (Supporting Information) and the corresponding discussion in Supporting Information present the interfacial characterization of hexane/W-GO with and without POSS. As delineated in the former section, the same procedure was used here for the fabrication of the sculpted emulsions, i.e., injection of the GO suspensions through a needle inside the hexane-POSS phase. Afterward, the system was dried utilizing one-step freeze-drying, noting that prior to such drying, the samples were kept in a freezer overnight.

Figure 2a and Figure S10 (Supporting Information) show the digital images of the resultant structured aerogel. As can be seen, the aerogels feature a unique worm-like structure. The aerogels’ domain size can be tuned by controlling the tubules diameter during the LS process (for instance, compare Figure 2a and Figure S10, Supporting Information). Figure 2b shows the SEM images of the worm-like aerogels. Clearly, a layer of GO encased a highly porous structure, primary porosity (see the arrow in Figure 2b). The porosity of the inner structure of the tubules is inherited from the ice crystal templates in the freezing process and can be controlled by the concentration of the GO in the precursor suspension (see the SEM images in Figure S11, Supporting Information). For instance, a porous structure is observable in the aerogel tubules (Figure 2b), derived from freeze-drying of hexane-POSS/W-GO 0.3 vol%. While the tubules of the aerogel derived from hexane-POSS/W-GO 0.1 vol% are mostly hollow (see Figure S12, Supporting Information), indicating that at low concentrations, the majority of the nanosheets migrate to the O/W interface.

The unique entangled morphology of the tubules (following worm-like contours) creates voids or a secondary macro-scale porosity (see the areas indicated with red lines, Figure 2b), forming an appealing morphology for applications where a large surface area is required, e.g., for gas sorption. The size and volume of the larger pores are dictated by the diameter of the tubules; these pores add more structural flexibility to the solid constructs fabricated by the LS technique.

The secondary pores are occupied by the oil phase in the emulsions. Figure 2c, for instance, shows that for a 3mL GO suspension, the volume increased to more than 5 mL in the case of its sculpted emulsion counterpart (Figure 2d). The green phase in the confocal images of the sculpted emulsions shown in Figure 2e also indicates the inclusion of the secondary pores by the oil phase. This increase in the volume by the secondary porosity significantly reduces the density of the resultant worm-like aerogels. These aerogels are among the lightest reported and is due to the fact that precursor constituents were low-concentration GO suspensions (0.04–0.3 vol%). As shown in Figure 2f, few cotton fibers or a plastic marker can hold the worm-like aerogels by electrostatic charges. The density of the structured aerogels was estimated to be in the range of ≈0.5 to ≈4 mg cm⁻³ (depending on the concentration of GO in the precursor emulsion and the diameter of the tubules), placing our worm-like aerogels in the category of ultra-flyweight aerogels (hence, our used abbreviation, WUFA, for worm-like ultra-flyweight aerogel). Although GO aerogels with lower densities (e.g., 0.16 mg cm⁻³)^[28] have been introduced in the literature, they lack any structural integrity. It will be shown that the density of our samples can be further reduced to ≈ 0.15 mg cm⁻³.

Figure S11 (Supporting Information) shows the digital and SEM images of the regular GO aerogels derived from GO suspensions at various loadings of the nanosheets. As the GO concentration decreased from 0.3 to 0.2 vol%, the freeze-dried aerogels were not able to sustain their structure, and an extensive shrinkage and deformation took place (Figure S11, Supporting Information). This can be attributed to the extremely low concentration of the nanosheets in the precursor suspensions, which means that the regular GO aerogels failed to retain the structure during freeze-drying. However, interestingly, no deformation was observed in the WUFAs, even for those obtained at the lowest concentration (Figures S12 and S13, Supporting Information). Moreover, no shrinkage was observed in WUFA-GO (0.1 vol%) (Figure S12, Supporting Information) and the shrinkage was negligible for WUFA-GO (0.04 vol%) (Figure S13, Supporting Information). Hence, our proposed method offers structural tunability for UFA fabrication, and these aerogels feature superior structural stability during freeze-drying, thanks to the strong GO interfacial assembly. In addition, it will be shown that worm-like structure bestows excellent mechanical properties to our WUFAs.

Here, to achieve a much lighter WUFA, we injected a (50:50 (V:V) hexane (pure)-in-(W-GO)) emulsion (the same as Figure S2a, Supporting Information) into the hexane-POSS, instead of injecting GO suspensions. This type of emulsion is inspired by double emulsions.^[55] Nonetheless, instead of encapsulating oil-in-water droplets, the oil droplets were trapped by continuous tubules of aqueous W-GO in the hexane-POSS phase (see Figure 3a,b). Thus, these emulsions can be considered as oil-in-water-in-oil double emulsions, named as “tubular double emulsions.” As 50 vol.% of the tubules is filled with a volatile hexane phase, the resultant aerogels are 50% lighter than their counterparts from the sculpted emulsions prepared by pure GO suspensions. Thus, the density reduced to ≈0.25- ≈2 mg cm⁻³. This approach adds more tunability to the microstructural features of the worm-like aerogels, as the porosity can also be tailored by the amount and size of the dispersed hexane phase in the aqueous tubules. Figure 3c–h illustrates the digital and SEM images of the WUFAs derived from the tubular double emulsions. These tubules are flyweight, and they can be actuated by electrostatic fields (see Figure 3g,h and Video S10, Supporting Information). Moreover, these tubules were strong enough to bear the weight of the entire structure (see Figure 3h). Structures with smart, stimuli-responsive (electrostatic, electrical, thermal, and photonic signals) features are essential in the design of advanced electronics.^[56] With a recently proposed approach,^[56] these tubules can be used as primary units in nonwoven structures, with a great capability for next-generation wearable systems.

So far, exceptional features such as large surface area, tunable mechanical properties (will be shown in Figure 4), good structural stability, and robust interface formation were obtained. Coupling these characteristics with the superior electrical properties of the graphene nanosheets broadens the range of potential applications of WUFAs, e.g., lightweight advanced electronics and electromagnetic interference (EMI) shields. We produced electrically conductive WUFAs via a double thermal/chemical reduction process explained in Supporting Information (see Figure S14, Supporting Information, and Figure 4). The samples were reduced via both thermal and chemical reduction to show the durability of the WUFAs. The collapse of the reduced graphene (rGO) aerogels upon reduction at elevated temperatures has been recently reported.^[57] Interestingly, no deformation or shrinkage was observed in the WUFAs after thermal treatment at 1100 °C (see Figure 4a and Figure S14, Supporting Information). This can be considered as a great advantage over direct ink writing of suspensions containing rheological modifiers (e.g., polymer), as they suffer from deformation and/or cracking induced by thermal stresses due to thermal expansion coefficient mismatch.^[31]

Figure 4b,d shows a 3-mg reduced worm-like aerogel, before and after holding a 22500 mg weight, demonstrating an excellent mechanical performance, which has rarely been found in other full graphene aerogels (see Figure S15, Supporting Information). Video S11 (Supporting Information) and Figure 4e confirm the compressibility of the reduced WUFA. Based on Figure 4e, the reduced WUFA can recover its initial shape after cyclic compression at deformations ranging from 20% to 80% (Figure S14c, Supporting Information, shows the shape recovery). To the best of our knowledge, this outstanding compressibility has never been reported for pure regular rGO aerogels. In fact, it is well known that rGO aerogels lack any mechanical properties and are fragile as a direct consequence of the weak interfaces formed by nanosheets, even at high concentrations.^[58] This limits practical applications of traditional rGO aerogels. Figure S15 (Supporting Information) confirms that the traditional rGO aerogels, derived from freeze-drying of W-GO 1.0 vol% suspension, do not recover their shape after compression. Zhao et al.^[20] also reported a great compressibility for millimeter-scale graphene aerogel spheres (GSs) with a core–shell structure and a density of 5.0–7.8 mg cm⁻³ prepared by a coagulation method. The extremely better compressibility of the GSs compared to regular graphene-based aerogels was attributed to the dense and continuous shell structure, preventing serious deformation damages. Hence, it can be postulated that the outstanding compressibility of the reduced WUFA stems from their unique worm-like structure (strong network of nanosheets encasing a highly porous structure). Of note, the mechanical properties of the WUFAs can be readily tuned by the inclusion of various polymers into the aerogels, allowing the fabrication of rigid to flexible structures (see Figures S16 and S17, Supporting Information). More importantly, the weight of the WUFAs decreased (≈40%, see the TGA graph in Figure S14b, Supporting Information) by removing the oxygen-containing functional groups upon reduction, resulting in a further density reduction (≈0.15–1.2 mg cm⁻³).

The reduced WUFAs were used to fabricate EMI shields and lightweight circuits. Aerogels prepared from the sculpted emulsions of rGO 0.3 vol% showed an appreciable average EMI shielding effectiveness (SE) of ≈ 29 dB over the X-band frequency range at a thickness of 5 mm (Figure 4f), meaning that it can shield 99.8% of the incident wave. This excellent SE was achieved at an extremely low solid content, leading to an extraordinary specific SE (SE/thickness/density) of ≈ 24 000 dB cm² g⁻¹ (Figure S14e, Supporting Information, compares this value with the literature). As can be seen in Figure 4g, the dominant mechanism of shielding is absorption, i.e., absorbance (A) > reflectance (R), in the probed frequency window, making our aerogels a wave absorber, a versatile candidate for medical and military applications.^[59] Indeed, due to the low density of the shields, the EM waves can penetrate the structure. Then, the penetrated waves will be trapped by the primary and secondary pores via wave scattering and multiple reflections at the abundant solid–air interfaces, leading to prolonging the trajectory and, thus, dissipation of the EM waves.^[60]

With regard to EMI shields, recently, it has been shown that the multilayer gradient assembly of conductive rGO nanosheets adds to the total EMI SE and improves the absorption of the waves.^[61] Indeed, the multilayer sculpted emulsions make the fabrication of the gradient aerogels possible (Figure S18, Supporting Information), opening new opportunities in the field of lightweight EMI shields. Moreover, inspired by liquid-in-liquid printing,^{[40, 53, 62-66]} the sculpted tubules can be structured in any desired shape (see, for instance Figure 4h and Videos S12 and S13, Supporting Information), and the resultant WUFA (Figure 4i,j) can be chemically or thermally reduced (rGO), realizing applications in ultralight conductive circuits (Figure 4k).