To empirically validate our hypothesis, we developed a hydrophobic pseudo-hydrogel (HPH) using Ecoflex 00–30, a commercially available silicone rubber known for its hydrophobic properties. To induce a porous structure within the HPH, we employed a sacrificial template method using NaCl microparticles as the template, with details provided in Fig. S2 and the "Methods" section. The cross-sectional morphology of the pristine silicone elastomer, as shown in Fig. 2a, was non-porous, and it exhibited a surface contact angle of 100° (Fig. S3a), confirming its hydrophobic nature. Following the pore-forming treatment, the HPH displayed a porous structure, as illustrated in Fig. 2b, where the HPH was fabricated using template particles approximately 60 µm in size. It is crucial to note that the pore-forming treatment is a purely physical process, leaving the physicochemical properties of the elastomer unchanged. The surface contact angle of the HPH was 105° (Fig. S3b), indicating that the physical porosity does not compromise the hydrophobic properties of the material itself. And the increase of the contact angle was attributed to the formation of porous microstructures on the elastomer surface31. Unlike the pristine elastomer, which did not swell when immersed in water for an extended period (six days) (Fig. 2c), the HPH, despite retaining its hydrophobic nature, unexpectedly undergoes swelling when immersed in water. As shown in Fig. 2d, after 132 hours of immersion, the volume of the HPH expands over twofold compared to its original size.
The water-swelling ability of HPH prompts us to reminiscent of conventional hydrogels. In the case of conventional hydrogels, their water-swelling capability is rooted from their intrinsic hydrophilicity. These hydrogels typically consist of highly hydrophilic networks of polymer chains. Consequently, water serves as a thermodynamically compatible solvent for such hydrophilic networks, facilitating easy wetting of the dried hydrogel by permeating the hydrophilic polymeric network. Upon wetting, the hydrophilic polymer chains undergo solvation by water, creating an osmotic pressure gradient between the hydrogel and the surrounding aqueous environment. This gradient drives more water molecules from the surroundings into the hydrogel polymeric network, resulting in a macroscopic swelling phenomenon32, 33. In contrast, HPH is a purely hydrophobic material, implying a distinct swelling mechanism compared to conventional hydrogels.
Inspired by the capillary-induced swelling observed in hydrophobic Sphagnum, we speculate that capillary forces also play a pivotal role in the swelling of HPH. The swelling process of HPH can be delineated into wetting and expansion stages, similar to the swelling process of hydrogels, as schematically illustrated in Fig. 2e-g. Given the hydrophobic nature of HPH, water cannot readily penetrate its polymeric network for wetting. However, upon immersion of HPH in water, hydrostatic pressure compelled water into the micropore structure of HPH. During this process, the air in the micropore structure was replaced by water, evidenced by the experimental observation of bubbles emerging from the HPH surface when immersed in water (Fig. S4). Upon the entry of water into the pore structures under hydrostatic pressure, two menisci form between water/air and water/elastomer due to the unique geometry of the pore structures (Fig. 2e)34, 35. Owing to cohesive forces among water molecules, the meniscus of water/elastomer generates a capillary force induced by liquid-elastomer surface tension along the periphery ((θH−W1 + θH−W2)·π·RH−W/180°) of the meniscus. Its vertical component of capillary force propels the water moving into pores, and the Finner can be expressed as
$$\:{F}_{inner}=\frac{2{\theta\:}_{H-W1}\pi\:{R}_{H-W}}{{180}^{^\circ\:}{F}_{H-W}\text{cos}\theta\:}\:\left(1\right)$$
θ denotes the angle between the force exerted by the hydrophobic pore wall on water and the vertical direction. Simultaneously, a capillary force induced by liquid-air surface tension is generated in the opposite direction, expelling water from the pore structures36, and the Fouter can be expressed as
$$\:{F}_{outer}=\frac{{\theta\:}_{A-W}\pi\:{R}_{A-W}}{{180}^{^\circ\:}{F}_{A-W}\text{cos}{\theta\:}_{1}}\:\left(2\right)$$
θ1 denotes the angle between the force exerted by air on water and the vertical direction. Given the hydrophobic and low surface energy nature of the HPH surface, which strongly repels water, the liquid-elastomer surface tension37, 38 generates a significantly stronger capillary force than the liquid-air surface tension39 (\(\:{F}_{inner}>{F}_{outer})\). Consequently, a net force acting on the water droplet propels liquid movement into the pores. This capillary force continues to progress, drawing water into the pore structures of HPH and concurrently expelling the originally entrapped air (Fig. S5).
It is important to note that, despite water entering the HPH during the wetting process, it merely replaces the volume of air within the HPH, without expanding the overall volume of the material. To elucidate this dynamic behavior of the wetting process, we plotted the volume change of HPH against time (t), as depicted in Fig. 2h. During the initial 30 minutes of being immersed, no significant changes in volume were observed. Concurrently, we monitored the mass change of the HPH, revealing a continuous increase. This weight increment indicates that the denser water is replacing the air inside the HPH. After approximately 30 minutes of immersion, the volume of the HPH begins to steadily increase, marking the commencement of the expansion stage. Due to capillary forces, continuous imbibition is sustained, leading to the high stretchability of the elastomer and resulting in macroscopic volume expansion (Fig. 2g). On one hand, the elastomer's expansion increases the radius of the pores, leading to a decrease in capillary force. On the other hand, the elastomer's expansion generates an increasing elastic force that impedes water imbibition. Ultimately, a balance is achieved between the capillary force and the elastic force, resulting in the equilibrium expansion of HPH. As depicted in the volume curve (Fig. 2i), the swelling of HPH reaches an equilibrium state after 144 hours of immersion. The swelling behavior of HPH is also evident in the microscopic pore structure changes. In Fig. S6, a thin film of HPH (~ 78 µm) immersed in water is observed under a microscope, clearly revealing the pore structure. Approximately 10 minutes later, the pore sizes on the HPH surface progressively increase with the prolonged immersion time, eventually reaching equilibrium (Fig. S7). The swelled HPH was also investigated by Environmental Scanning Electron Microscope (ESEM). As shown in the cross-sectional ESEM images of swelled HPH (Fig. S8), the pore structure of HPH after swelling is demonstrated, showing that its pore size ranges from 80 to 200 µm. This result matches the swelling rate of macroscopic HPH, confirming the unity between microscopic and macroscopic observations.
It should be noted here that, at the critical transition point between the wetting and expansion stages, a small portion of air remains sealed by the water inside the HPH (Fig. 2f), playing an important role in the subsequent expansion process. If all the air within the HPH were evacuated, leaving only the water/elastomer interface, the capillary force induced by surface tension around the pore periphery would be nullified, resulting in the absence of a driving force to draw water in. However, we consistently observe an increase in both volume and weight of the HPH. This sustained imbibition should be attributed to the trapped air, which generates asymmetric capillary forces (Fig. 2g). By analyzing the volume and weight curves, we calculated that the density of the HPH at the transition point was 0.927 g/cm³, an intermediate value between the fully dried and fully wetted states. This suggests that at the transition point, about 11.69% of the pore volume is occupied by air (Fig. S9), supporting the hypothesis of entrapped air. To futher demonstrate the importance of residual air in the pore for HPH swelling, we performed vertical water absorption and swelling experiments on long strips of HPH. The HPH strip was positioned perpendicular to the surface of red ink, with their lower ends immersed in the ink (Fig. S10). Unlike immersing HPH in water, where air is easily trapped in the hydrophobic matrix, this setup allows water to be transported upward from the bottom of the HPH, wetting the pores layer by layer. This process expels most of the internal air from the HPH, leaving only a small portion to provide the driving force for the subsequent expansion process. As a result, the swelling ratio of HPH strips in the vertical swelling experiment is significantly lower than the one fully immersed in water. The observed swelling ratio in this setup is only 130%, demonstrating the crucial role of residual air in achieving full swelling.
As discussed earlier, the water-swelling process of HPH is linked to capillary forces induced by liquid-elastomer surface tension along the periphery of the pore meniscus, which is also associated with the radius of the pores. Thus, by adjusting the pore size within HPH, the water-swelling ability of HPH becomes tunable. To explore the relationship between pore structure and elastomer swelling, the tunable pore sizes in the HPH were achieved by manipulating the size of the soluble template NaCl particles. The internal pore structures of HPH with different pore sizes are illustrated in Fig. S11. Observations reveal that smaller pore sizes correspond to a faster water absorption rate of HPH and a larger swelling size (Fig. 3a, and Fig. 3b). Additionally, there exists a threshold pore size determining whether swelling will occur in HPH. Obvious swelling is observed only when the pore size is less than 152 µm. Conversely, when the pore size exceeds 152 µm, the pore structure in HPH is wetted but does not exhibit obvious expansion. This could be attributed to the fact that, when the pore size exceeds 152 µm, the capillary forces generated by surface tension are insufficient to overcome the elastic force of the elastomer and induce expansion.
To further elucidate the intriguing pore size-related water-swelling phenomena of HPH, detailed principal calculations have been conducted to unveil the underlying mechanism, as shown in Fig. 3c, According to Gor et al.40, in porous media with regularly spaced pores, the engineering strain of a single thick-walled cylindrical unit cell can approximate the overall volumetric strain. To account for pore interaction, analysis is conducted on seven adjacent thick-walled cylindrical unit cells. In a single thick-walled cylindrical unit cell, pore walls endure internal pressure \(\:{p}_{i}\), while the effect of neighboring pores on the central cell is akin to an external boundary pressure \(\:{p}_{o}\). The water-swelling of HPH will provide the unbalanced force contributed by the surface tension near the liquid-solid interface, which will provide a driven pressure \(\:{p}_{c}\) to break the balance between the internal pressure and external pressure. As Fig. 3d shows, before HPH wetting, \(\:{p}_{i}={p}_{o}\), and the HPH will not swell. When we place the HPH in the water, this balance will be broken, and \(\:{P}_{i}+{P}_{c}\gg\:{P}_{o}\). When pressurized liquid is injected into porous media with dual porosity, it undergoes two typical stages sequentially. Firstly, the macroscopic pores are rapidly filled, compressing the liquid inside the pores and exerting pressure on the pore walls, thereby stiffening the structure. Secondly, the pressurized liquid in the macroscopic pores gradually permeates into the microscopic pores within the framework under the driving force of pressure gradients, leading to complex changes in the overall macroscopic elastic behavior of the structure. Compared to the second stage, the first stage is typically completed in a very short time. Therefore, the focus here is mainly on the evolution of the macroscopic equivalent properties of porous media during the liquid infiltration process in the second stage. The HPH will swell very fast, thus, the pore pressure in porous media during the swelling will be\(\:\:{P}_{f}=({P}_{i}+{P}_{c})-{P}_{o}\). In porous media containing fluids, there exist complex coupling effects between the solid framework and the pore fluid. The fluid pressure acting on the pore walls has a significant influence on their macroscopic mechanical behavior. A thorough understanding of the equivalent mechanical properties of fluid-containing porous media is crucial for promoting their practical engineering applications. Previous research has addressed this issue to some extent41. However, a systematic understanding is still lacking. Therefore, here, we analyze this problem based on a micromechanical model. Firstly, a theoretical model predicting the effective modulus of dry porous media will be established, followed by an analysis of the influence of pore pressure. The pore load modulus in porous media can be expressed as
$$\:{M}_{pd}=\frac{E\{\left[1+\left(1+\nu\:\right)k\right]-[1-(1+\nu\:)(1-2\nu\:)k\left]\xi\:\right\}}{\left(1+\nu\:\right)\{\left(1-2\nu\:\right)\left[1-\alpha\:\left(1+\left(1+v\right)k\right)\right]-[\alpha\:\left(1-\left(1+\nu\:\right)\left(1-2\nu\:\right)k\right)\xi\:-1\left]\right\}\xi\:}\:\left(3\right)$$
In Fig. 3e, it can be observed that for nanoporous specimens with the same porosity, as the pore size decreases, the influence of surface effects becomes increasingly apparent. Conversely, for specimens with the same pore size, the higher the porosity, the more pronounced the surface effects. Additionally, soft surfaces lead to a decrease in the poroelastic modulus with decreasing pore radius. This means that the higher porous HPH will more fully and more easily swell with water. Thus, the equivalent volume modulus of porous media can be defined by the formula,
$$\:{K}_{pd}=\frac{E(1-\xi\:)}{2[\left(1-\nu\:\right)+(1+\nu\:\left)\xi\:\right](1-\alpha\:\xi\:)}\:\left(4\right)$$
And the equivalent Young’s modulus of porous media can be defined by the formula,
$$\:{E}_{pd}=\frac{E{\left(1-\xi\:\right)}^{3}}{\left(1-\alpha\:\xi\:\right)}\text{exp}\left\{\frac{3\alpha\:\xi\:\left[2+\left(1+\alpha\:\right)\xi\:\right]}{2\left[2-\left(2-\alpha\:\right)\xi\:\right]}\right\}\:\left(5\right)$$
As shown in Fig. 3f, the equivalent Young’s modulus will decrease very fast due to the change of porosity. So the HPH strain under the pore pressure of swelling water can be expressed as \(\:\epsilon\:={P}_{f}/{M}_{pd}\).
Since HPH exhibits similar water-swelling ability to conventional hydrogels, it functions similarly and has similar potential applications. Conventional hydrogels have been extensively studied as self-morphing materials due to their water-triggered shape transformation phenomena. The development of self-morphing materials is inspired by the widespread shape-transformation phenomena observed in plants42, generating growing interest in various fields such as energy harvesting, metamaterials, soft robotics, sensors, and multifunctional bioscaffolds. In the following section, we explore the potential applications of HPH as programmable water-responsive self-morphing materials. Typically, the shape transformation of hydrogels arises from inhomogeneous swelling behaviors within the material. Traditional shape-morphing hydrogels achieve this by incorporating diverse components with different swelling behaviors in response to specific stimuli. However, introducing diverse components may encounter intrinsic limitations, such as unstable interfaces and time-consuming fabrication. Therefore, it is of great significance to develop monocomponent hydrogel that enable precisely programmable deformations43. Our HPH materials can take the advantages of the microstructure programmed swelling ability to realize monocomponent self-morphing materials (Fig. S12).
By controlling the pore structural domain in HPH, programmable and localized swelling can be achieved, which then driving the controllable shape-morphing of HPH. The fabrication method is detailed in Fig. S13. The porous domain in HPH (the domain with swelling properties) serve as the active component, while the non-porous domain, unresponsive to water stimulus, serve as passive component. The perfect bonding between the active and passive components is attributed to the fact that they are both Ecofelx 00–30 (Fig. S14). Consequently, non-uniform volume changes generate internal stresses at their interfaces, inducing out-of-plane shape transformation. Initially, we investigate the self-buckling deformation of HPH by laterally arranging porous (active) and nonporous (passive) components. As illustrated in Fig. 4a, a series of concentric patterns is programmed in HPH, where in-plane heterogeneous swelling leads to modulated internal stresses, resulting in 3D deformations. Upon immersion in water, the planar concentric circle HPH evolves into Enneper's surfaces with controllable wrinkles. In Fig. 4a, we demonstrate patterned surfaces with three to six wrinkles. For more intricate in-plane patterning, a periodically patterned HPH is prepared, featuring an array of non-swellable discs embedded in the swellable HPH framework. In this configuration, each compartmentalized swellable and porous domain is surrounded by four non-porous and non-swellable discs, leading to an alternating concave-convex 3D shape (Fig. 4b).
In addition to self-buckling shape transformation, HPH can also achieve self-bending and twisting (Fig. 4c) by arranging the porous structural gradient across the thickness of materials. Figure 4c provides several examples of swelling-induced 3D HPH achieved through unidirectional or bidirectional folding. More examples of swelling-induced shape transformation in HPH, which borrow techniques and ideas from conventional self-morphing hydrogels such as kirigami and responsive mechanical buckling, are demonstrated in Fig. S15. Furthermore, a reconfigurable and assembled responsive lattice can be achieved. Additionally, as shown in Fig. 4d, we prepared a series of self-folding HPH strips and then assembled these strips into lattice by using 3D-printed dock connectors. These HPH strips can be assembled into various types of responsive lattice with different connection configurations as shown in Fig. 4d and Fig. S16. These responsive lattices serve as a common platform for designing soft mechanical metamaterials capable of negative swelling ratios. Upon swelling, the lattice shown in Fig. 4d achieves a negative swelling with area change of 29% and 25%. In contrast to previous literature, our responsive lattice composed by HPH strips is dismountable with reassembling ability, enabling the recycling of self-folding HPH units into reconfigurable metamaterials.
Finally, like conventional hydrogels, HPH can be combined with functional components to achieve emergent actuation properties and performance30. Here, by incorporating magnetic NdFeB microparticles into the monocomponent self-morphing HPH, we report hybrid materials that are actuated by a magnetic field after swelling-induced shape transformation. For conventional hydrogels, before incorporating NdFeB, the NdFeB should undergo surface passivation, such as coating with a thin layer of silica, to prevent its corrosion in the matrix of hydrogel44.In contrast, our HPH is composed of the hydrophobic silicone elastomer, which is an ideal matrix for NdFeB and has been reported in many literatures. As shown in Fig. 5a and Fig. S17a, by harnessing the self-buckling deformation of HPH, we obtained a floating robot featuring a buckled buoy and a magnetic propelling tail. The buckled buoy provides enough buoyancy force to keep the robot floating on the water surface. Under magnetic actuation by an N52 magnet (magnetic field strength of 0.5 T), the magnetic propelling tail flaps the water, propelling the floating robot forward (Fig. 5b and Movie S1). The swelling-induced deformation not only changes the shape of the magnetic robots but also alters the magnetization profiles inside the materials. Illustrated in Fig. 5c and Fig. S17b, we initially design a flat magnetic HPH film with a simple planar magnetization profile. With this simple planar magnetization profile, it is challenging to realize magnetic actuation movement for the HPH film. After swelling-induced shape transformation, the HPH film rolls into a wheel-like structure. Moreover, such self-rolling shape transformation also deforms the original simple planar magnetization profiles in the HPH film, resulting in 3D axially divergent magnetization. With this more complex 3D magnetization and the wheel-like shape, the HPH film can achieve wheel rolling movement under magnetic actuation (Fig. 5d and Movie S2). Figure 5e and Fig. S17c exhibits another example by adopting self-morphing to tune the magnetization profile to realize delicate magnetic actuated movement. Our design involves a planar cross-shaped HPH film endowed with two magnetic "legs". In the planar structure, these two "legs" cannot be used for walking. Upon swelling-induced shape transformation, these two magnetic "legs" can stand up to support the HPH body. Under magnetic actuation, the standing HPH robot can alternate its magnetic "legs" to walk forward (Fig. 5f and Movie S3).