The creation of intricate, porous materials with controlled architectures is pivotal for a range of advanced applications, from biomedical scaffolds to catalytic converters. Inspired by prior research on emulsion stabilization, this study details an innovative two-step emulsification process leveraging the synergistic interaction of hydrophilic nanoparticles and a zwitterionic surfactant to generate stable nanodroplets. These nanodroplets serve as versatile 3d Printing Templates, enabling the fabrication of nanoporous structures with hierarchical porosity.
Our methodology employs phosphatidylcholine (PC), a zwitterionic surfactant, and silica nanoparticles, as a model system to demonstrate this process (Fig. 1a). Initially, a coarse emulsion of corn oil in water is stabilized by PC, resulting in micron-sized droplets. Subsequently, silica nanoparticles are introduced into the aqueous phase, and the mixture undergoes ultrasonication. This high-energy sonication further reduces the oil droplet size to the nanoscale. During this phase, excess PC molecules in the oil phase rapidly adsorb at the oil-water interface, lowering interfacial tension and facilitating droplet break-up. This approach effectively overcomes the limitations of slow particle diffusion and adsorption at liquid interfaces, crucial for achieving smaller, stable droplet sizes.
Cryo-scanning electron microscopy (cryo-SEM) reveals a dense layer of silica nanoparticles adsorbed around the oil nanodroplets, confirming the emulsion microstructure (Fig. 1b). This dense nanoparticle layer imparts viscoelastic properties to the interface, preventing nanodroplet coarsening and coalescence over extended periods, as validated by interfacial rheology measurements (Fig. S1a). While the primary droplets exhibit predominantly negative electric charges at the processing pH, electrostatic forces alone cannot account for the adsorption of negatively charged nanoparticles. Non-electrostatic forces and potential interactions involving the positive charges of the zwitterionic surfactant at the oil-water interface are likely contributing factors (Fig. S1b). In contrast, formulations using only particles or PC fail to form a viscoelastic film, leading to larger and less stable droplets post-emulsification (Fig. S2). The interfacial adsorption of particles effectively stabilizes emulsions against coalescence and Ostwald ripening, consistent with previous findings. Dynamic light scattering experiments demonstrate the remarkable long-term stability of these nano-sized droplets, maintained for up to three months due to the interfacial viscoelastic film (Fig. 1c).
Figure 1: (a) Schematic illustration of nanoemulsion formation using phosphatidylcholine (PC) and silica nanoparticles. (b) Cryo-SEM images revealing silica nanoparticle layer around nanodroplets. Inset: Silica particle monolayer detached during preparation. Scale bars: 200 nm. (c) Droplet size distribution over time, demonstrating nanoemulsion stability.
These robust nanodroplets can be concentrated via ultracentrifugation, forming a jammed template directly convertible into nanoporous structures upon drying or sintering (Fig. 2a), contingent on oil volatility. The porosity of the resulting structure is directly controlled by the oil droplet concentration in the jammed nanoemulsion, estimated to exceed 70 vol%. To broaden the applicability of this 3D printing template approach, we fabricated nanoporous structures using nanoparticles with varying surface chemistries (Fig. 2b,c). Beyond bare silica, alumina-coated silica nanoparticles were employed, demonstrating the method’s versatility while maintaining consistent particle size. The zwitterionic nature of the surfactant is key to this versatility, enabling electrostatic attraction of both positively and negatively charged hydrophilic particles during the secondary emulsification (Fig. S1a).
Figure 2: (a) Schematic of nanoporous structure preparation from nanoemulsions. (b) Zeta potential of bare and alumina-coated silica particles. (c) SEM images of porous structures from negatively (left) and positively (right) charged nanoparticles. Scale bars: 400 nm.
Indeed, zeta potential measurements confirmed that bare silica (Ludox TM50) and alumina-coated silica particles (Ludox CL-P) exhibit negative and positive net surface charges, respectively, at a processing pH of 5.5 (Fig. 2b). Stable nanoemulsions were successfully created with alumina-coated silica nanoparticles using PC. Sintering these centrifuged emulsions yielded porous structures with either bare silica or alumina-coated silica nanoparticles forming the pore walls (Fig. 2c). As an alternative to sintering, chemical consolidation methods can be used to preserve temperature-sensitive components. Furthermore, smaller 7 nm silica particles and bare alumina particles were also successfully used to produce porous structures, highlighting the method’s flexibility (Fig. S3).
The size of the nanopores is a direct reflection of the precursor nanodroplet template size. Using bare silica nanoparticles as an example, nanoemulsions with droplet sizes ranging from 150 to 1000 nm (Fig. S2) resulted in porous structures with pore sizes of 100 to 900 nm post-drying and sintering. The dense nanoparticle layer on the droplet surface often leads to closed nanopores after processing. However, open pores can be achieved by slightly destabilizing the emulsions, resulting in partially particle-covered droplet surfaces. Replacing corn oil with decane as the dispersed phase can induce such controlled destabilization, enabling the generation of open porosity (Fig. S4). This ability to tune the porosity – open or closed – allows for tailoring the structure to meet specific application requirements.
Beyond nanoporous structures, these nanoemulsions can be combined with micron-sized sacrificial templates to create porous architectures with two levels of hierarchy. We explored two types of sacrificial templates for micron-sized pores: oil droplets and polymer particles. The choice of template influences the consolidation step and the final porous structure. Oil droplet templates allow for consolidation through simple evaporation and chemical cross-linking, while polymer particle templates typically require sintering to avoid lengthy polymer dissolution. Furthermore, oil droplets are susceptible to deformation during drying and consolidation, unlike the rigid nature of polymer particle templates.
We demonstrated two pathways to achieve hierarchical porosity using polycaprolactone (PCL) particles or decane droplets as micron-sized sacrificial templates (Fig. 3). Route 1 utilizes oil droplets as a secondary soft template, involving an additional emulsification step to create larger oil droplets within the pre-formed nanoemulsions (Fig. 3a). Confocal microscopy confirmed nanodroplet adsorption onto the larger oil droplets (Fig. S5), crucial for preventing phase separation during centrifugation. The size of these larger droplets can be adjusted from 15 to 250 µm by controlling emulsification intensity and duration. Simple drying of the multiscale emulsion at 25 °C yields a porous structure with both nano and microscale pores (Fig. 3b). To consolidate and strengthen this structure at room temperature, chitosan was incorporated into the emulsion’s continuous phase and cross-linked with glutaraldehyde (Fig. S6). This room-temperature consolidation is particularly advantageous for bio-scaffold fabrication, preventing thermal degradation of organic components. Pore size distribution analysis reveals two distinct pore families (Fig. 3c,d). The smaller pore size distribution aligns with the original nanoemulsion droplet size, indicating minimal distortion during drying. However, larger pore size distribution is broader than the templating droplets, likely due to shrinkage and capillary forces during liquid removal. Despite the small nanopores within macropore walls (Fig. 3b), open porosity as high as 90% was achieved in the sintered structure. Increased open porosity with larger macropore windows can be achieved by reducing emulsion stability through lower stabilizer concentrations.
Figure 3: (a) Route 1 schematic: hierarchical porous materials from nanoemulsions and microdroplets. (b) SEM image of porous structure after drying. (c,d) Size distributions of µm-templates and nm-droplets (c) before and (d) after drying.
Route 2 employs polymer particles as sacrificial templates for microscale pores, offering enhanced templating fidelity (Fig. 4). Using monodisperse 100 µm PCL particles, fabricated via microfluidics, results in µm-sized pores with narrow size distribution after sintering (Fig. 4a). Polymer microparticles are directly added to jammed nanoemulsions post-centrifugation. Heat treatment at 850 °C removes liquid phases and polymer particles, creating a two-level hierarchical porous structure with well-defined pore sizes. SEM imaging confirms a strong correlation between pore and sacrificial template size distribution (Fig. 4b). The monodispersity of particle templates translates into pores with a polydispersity index below 0.03 (Fig. 4c,d), contrasting with the higher polydispersity (0.68) from large droplet templates (Fig. 3d). Microfluidic emulsification allows for monodisperse particles across a 5 µm to 500 µm range, making polymer microtemplates combined with nanoemulsions a powerful approach for precisely designing material pores across a broad size range.
Figure 4: (a) Route 2 schematic: hierarchical porous materials from nanoemulsions and PCL particles. (b) SEM image of porous structure after sintering. (c,d) Size distributions of µm-templates and nm-droplets in (c) wet state and (d) after sintering at 850 °C.
3D printing these dual templating soft materials expands pore size design to the millimeter scale, providing digital control to shape porous architectures into complex 3D geometries (Fig. 5). Direct ink writing allows for tailoring filament spacing, controlling macroscale pore openings. Water-based inks of jammed nanoemulsions loaded with PCL microparticles were printed into a chiral 3D geometry, challenging to achieve via conventional methods (Fig. 5a,c). Micro-computed tomography reveals the intricate geometry (Fig. 5b). This helicoidal geometry features an internal channel with two-level hierarchical porosity in its walls, stemming from droplet and microparticle templates after sintering at 850 °C (Fig. 5d). Archimedes measurements show the chiral structure walls have 85% open and 6% closed porosity. Grid-like cellular geometries are also achievable by modifying ink rheology to prevent filament sagging.
Figure 5: (a,b) 3D printed helicoidal structure using nanoemulsion ink with PCL particles in wet state (a) and micro-CT reconstruction (b). Scale bars: 1 cm. (c) Hierarchy levels in printed structure. (d) Photos and SEM images of sintered 3D printed structure. Scale bars: 0.5 cm, 0.5 cm, 100 µm, 200 nm (left to right), 100 nm (inset). (e) Pore size range of additive manufacturing technology.
The tortuous internal channels and multiscale porous walls of the helicoidal geometry are promising for catalytic applications. Adjusting ink formulation for open porosity would optimize it for catalytic supports at high temperatures. Alternatively, similar 3D printed structures could be used in thermal management, with coarser pores for fluid permeability and nanoemulsions containing non-volatile oil for thermal storage via phase changes. Furthermore, these hierarchical porous structures may offer improved mechanical efficiency compared to single-scale porosity materials.
While various 3D printing techniques exist, the ability to create porous materials with features across nano-, micro-, and macroscales in a single, rapid process is a unique advantage of this technology (Fig. 5e). This is enabled by combining printing spatial control with the self-assembly of ink building blocks. The achieved porosity and pore size ranges surpass capabilities of previous porous material processing methods. This nanoemulsion-based 3D printing template approach opens new avenues for designing and manufacturing advanced materials with tailored hierarchical porosity for diverse applications.
