A flexible skin material with switchable wettability for trans-medium vehicles (2025)

2.1. Conceptual design of trans-medium vehicles

To clarify the application scenario and design basis of the flexible skin material, the conceptual design of a trans-medium vehicle is presented. Typically, the trans-medium vehicle operates across multiple environments, including aerial flight, underwater cruising, and water entry/exit, as depicted in . It can be launched from land-based, airborne, or shipboard platforms, rapidly reaching the combat area through high-speed aerial cruising. Upon approaching enemy defense zones, it descends to skim over the sea or dive into the water (i.e. water entry) to evade detection or interception by enemy defense systems. It can also exit the water (i.e. water exit) and regain aerial maneuverability if required during underwater cruising. Once the trans-medium vehicle is near enemy ships or submarines, it launches an attack on the underwater components of the target. In general, the motion of the trans-medium vehicle is confined within the vertical plane most of the time.

Due to enemy interceptions or the differences in launch platforms and mission objectives, the initial motion states of water entry can vary significantly. For instance, when launched from the deck of a warship, the trans-medium vehicle may enter the water at speeds ranging from 10 m/s to 20 m/s with a small inlined angle (see ). In this scenario, its trajectory stability can be compromised due to initial pitch moments caused by an asymmetrical cavity, potentially leading it to overturn or ricochet off water. Conversely, when launched from an airborne platform, the trans-medium vehicle may enter the water almost vertically at speeds ranging from 60 m/s to 100 m/s (see ), where the large impact loads can cause structural damage. In situations requiring evasive maneuvers within enemy defense zones, it might need to enter the water at higher speeds with a large angle of attack (see ), further compromising structural safety and trajectory stability. Additionally, to adapt to specific water depths or attack targets, the initial motion states of water entry can also be diverse.

Based on these considerations, the aero/hydrodynamic design of the trans-medium vehicle can be determined. To accommodate aerial flight, underwater cruising, and water entry/exit, the overall configuration integrates design elements from the bodies of missiles and the tails of torpedoes, as shown in ). The wings, essential for providing lift in the air, are deployed during aerial cruising and retracted before water entry to reduce impact loads and redundant lift during underwater cruising. To achieve load reduction and trajectory stabilization as needed across various motion states, the head morphs its upper and lower sides independently to vary slenderness and sharpness. Additionally, the surface wettability of the entire body, including the head, is designed to be switchable and independently controlled on the upper and lower sides of the body. This design can increase flexibility in configurations and surface wettabilities, enabling adaptation to various initial motion states during water entry. In this paper, the trans-medium vehicle measures 7.5 m in length and 0.75 m in diameter, with a span of 4.5 m when the wings are deployed, as shown in ). The head, extending 1.5 m from the front, is where the flexible skin material with switchable wettability is applied.

For simplification in subsequent analysis, four feature lines, including an upper feature line, a lower feature line, and two side feature lines, are utilized to parameterize the head configuration, as shown in . The upper and lower feature lines, located in the vertical plane (also the symmetry plane of the trans-medium vehicle), independently represent the primary features of the upper and lower shapes, respectively. Meanwhile, the side feature lines are symmetric about the vertical plane and located in the plane that contains the central axis of the trans-medium vehicle. Both the upper feature line and the lower feature line consist of a curved section and a straight-line section, as illustrated in ). Here, the radius R₀ and length L₀ of the head are 0.375 m and 1.5 m, respectively. The curve section is described by a bicubic spline, as specified in EquationEquation 1(1) $\mathit{y}=R_0\left[\frac{6k_1\mathit{x}}{k_0{L}_0}{\left(\frac{x}{k_0{L}_0}-1\right)}^4+\frac{k_2}{2}{\left(\frac{x}{k_0{L}_0}\right)}^2{\left(\frac{x}{k_0{L}_0}-1\right)}^3+1\right.{\left.-\left(\frac{4\mathit{x}}{k_0{L}_0}+1\right){\left(\frac{x}{k_0{L}_0}-1\right)}^4\right]}^{\frac{1}{3}}$(1) , where k₀ (ranging from 0 to 1) denotes the length proportion of the curved section, k₁ (ranging from 0 to 1) represents the curvature gradient of its front part, and k₂ (ranging from 0 to 15) represents the curvature gradient of its rear part [Citation47]. The y-coordinate of the side feature line at each x-coordinate is calculated as the average of the corresponding y-coordinate of the upper and lower feature lines. Consequently, the slenderness and the sharpness of the head configuration are completely determined by the six parameters that control the upper feature line and the lower feature line.

(1) $\mathit{y}=R_0\left[\frac{6k_1\mathit{x}}{k_0{L}_0}{\left(\frac{x}{k_0{L}_0}-1\right)}^4+\frac{k_2}{2}{\left(\frac{x}{k_0{L}_0}\right)}^2{\left(\frac{x}{k_0{L}_0}-1\right)}^3+1\right.{\left.-\left(\frac{4\mathit{x}}{k_0{L}_0}+1\right){\left(\frac{x}{k_0{L}_0}-1\right)}^4\right]}^{\frac{1}{3}}$(1)

Based on the conceptual design of the trans-medium vehicle outlined above, the application scenario for the flexible skin material is clearly defined. The remainder of this section will evaluate the detailed design requirements of the flexible skin material through numerical simulation of water entry, considering only the body of the trans-medium vehicle for simplicity.

2.2. Requirements for load-bearing capacity

The load-bearing capacity requirements are primarily considered in the early stage of high-speed vertical water entry, where the most severe impact loads and structural stresses typically occur [Citation48]. As previously mentioned, since the head configuration is the primary factor that affects impact loads and structural stresses in this situation, the trans-medium vehicle morphs its head to mitigate these loads and stresses during water entry, resulting in, however, variable load-bearing requirements. To simplify the design process without sacrificing conservativeness, the structural stresses associated with the optimal head configuration, which minimizes the loads and stresses, are selected as the basis for the analysis of design requirements. This subsection preliminarily explores the relationship between the head configuration and water entry dynamics, followed by an optimization process to determine the optimal head configuration and the corresponding load-bearing capacity requirements. Since an asymmetrical head configuration does not contribute to the reduction of loads and stresses, the upper feature line and the lower feature line are assumed to be identical in this subsection. Consequently, the head configuration can be simplified to a body of revolution, thereby reducing the number of control parameters from six to three.

Before proceeding, it is necessary to introduce the FSI analysis model of water entry, which is used for simulating the water entry process and evaluating impact loads and structural stresses. As shown in ), the 5 m × 5 m × 15 m calculation model consists of water, air, and trans-medium vehicle, where the depth of the water is 10 m and the height of the air is 5 m. The fluid, including the water and the air, is described by the Navier–Stokes equations in Arbitrary Lagrangian-Eulerian form. Meanwhile, the trans-medium vehicle is described by solid control equations in Lagrangian form, coupled with the fluid through a penalty function. In view of the symmetry of the calculation model, a 1/4 model is used to replace the original for computational efficiency. Correspondingly, the nodes on the symmetry plane are constrained to suppress normal displacement, and other boundaries of the fluid domain are set to non-reflective to avoid interference.

To explore and verify the effects of head configuration on water entry FSI dynamics characteristics, three typical head configurations (conical, elliptical, and flat) are generated and presented in ), with control parameters listed in . At the initial speed of 60 m/s, the water entry process of the trans-medium vehicle is simulated, as shown in ). The flat head produces the greatest deceleration peak, which is attributed to its largest contact area. A slender head with a smaller contact area, such as the conical head, generally experiences minimum deceleration. However, this does not necessarily lead to smaller impact loads and structural stresses compared to the elliptical head, due to differences in dynamic pressure and load-bearing modes. Given the complex relationship between the head configuration and water entry FSI dynamics characteristics, the optimal head configuration and its structural stress must be determined through global optimization.

In this paper, the optimization of head configuration is conducted using a multi-island genetic algorithm with the support of a radial basis function surrogate model [Citation49]. The structural stress is regarded as the primary optimization objective, and speed loss is also included in the objective function with equal weight to ensure comprehensive water entry performance. To conservatively evaluate the load-bearing capacity of the optimized head configuration, an initial water entry speed of 100 m/s is considered here instead. Given the thickness of head skins is limited to 5 mm, which is for manufacturing and application considerations, the control parameters of head configuration are optimized to k₀ = 1.00, k₁ = 0.33, k₂ = 14.15, as shown in ). In this case, the maximum impact load during water entry is 1422 MPa, and the maximum structural stress is 25 MPa. Assuming a maximum allowable strain of 3%, a minimum elastic modulus of 825 MPa is required to safely withstand the impact loads.

2.3. Requirements for deformability and surface wettability

The essence of analyzing requirements for deformability and surface wettability is to identify which head configuration and surface wettability effectively contribute to load reduction and trajectory stabilization. In scenarios necessitating load reduction, particularly during high-speed vertical water entry, only the head configuration is pertinent, as discussed in Section 2.2, while the surface wettability is not further considered due to its negligible influence. This subsection will focus on scenarios of trajectory stabilization, where both factors are relevant.

As previously mentioned, configuration and surface wettability affect trajectory stability by generating a compensating moment, which arises from the difference in lateral forces acting on either side of the trans-medium vehicle. Therefore, the effectiveness of these factors in stabilizing the trajectory can thus be assessed by examining the lateral forces on each side of the trans-medium vehicle, which is fundamentally a hydrodynamic problem. In this paper, the early stage of vertical water entry is selected as a representative scenario for exploring the relationship between these factors and lateral forces. For analytical simplicity, it is assumed that the flow evolution and pressure distribution on each side of the trans-medium vehicle are independent. This ensures the generalizability and validity of the analysis for each side of the trans-medium vehicle, regardless of the configurations and surface wettabilities applied to the other side. Consequently, the trans-medium vehicle can be treated as symmetrical, allowing this subsection to focus exclusively on a single side of the vehicle.

Conventionally, researches on water entry hydrodynamics problems involving surface wettabilities can be conducted via numerical simulation or experimentation. However, experiments are often expensive and time-consuming, while simulations considering wettability are complex due to the required six-degree-of-freedom (6DOF) dynamic solutions and dynamic mesh operations. To reduce analysis costs, a two-dimensional simplified numerical model, which can quickly simulate the cavity and forces at the early stage of the water entry, is used to evaluate the effectiveness of specific configurations and surface wettability for the trans-medium vehicle. In addition, a verification experiment is conducted to validate the accuracy of the model. Accordingly, the trans-medium vehicle is scaled to 0.1 m in length with a diameter of 0.01 m to fit the experimental environment in this subsection.

The simplified numerical model for hydrodynamics analysis, inspired by the principle of relative motion, sets the water and the air in motion while keeping the trans-medium vehicle static, thereby eliminating the need for 6DOF dynamic solutions and mesh updates, as shown in . Due to symmetry, only one side of the trans-medium is considered, with the central surface designated as a symmetry boundary. The flow is driven upward by pressure differences between the pressure-inlet and pressure-outlet, described by the Eulerian-form Navier–Stokes equations with the Volume of Fluid model. To simulate surface wettabilities, the continuous surface force model is introduced, and the contact angle on the wall of the trans-medium vehicle is set. Since only the early stage of the water entry is considered, the changes in the motion states of the trans-medium are minimal during the whole simulation, which means the discrepancy between this simplified model and the traditional numerical model involving 6DOF motion can be ignored.

Given the complexity of water entry problems considering surface wettability, a set of experiments is conducted to verify the accuracy of the numerical model. In this paper, the experimental system consists of a tempered glass tank, a high-speed camera, a light source, a model launch system, and a control and sampling system, as shown in . The tempered glass tank measures 1.5 m in length, 0.8 m in width, and 0.8 m in height, illuminated by the light source behind. Here, the light source comprises two LED arrays, each equipped with a softbox. The high-speed camera is positioned in front of the tank, with resolution, frame rate, and exposure time set to 1024 × 1024, 6400 fps, and 1/10000 sec, respectively. During a test, the model is accelerated by the model launch system on a rail until released and entered into the water, while the high-speed camera records and uploads the images to a computer, all controlled by the control and sampling system.

A scaled trans-medium vehicle with the optimal head configuration obtained in Section 2.2 is used for the verification experiment. As shown in , three aluminum experiment models are fabricated, with surface wettability reaching superhydrophilic (Water Contact Angle, WCA = 0°), neutral (WCA = 90°), and superhydrophobic (WCA = 150°), respectively, by means of surface treatment. The cavities formed during their vertical water entry at initial speeds of 3.1 m/s are compared with corresponding results from the simplified numerical model, as presented in . It can be seen that the superhydrophilic model produces no cavity, the neutral model produces a shallow cavity, and the superhydrophobic model detaches the interface line earlier, producing a larger cavity. These results align well with the numerical results, demonstrating the effectiveness of the simplified numerical model.

Utilizing the simplified numerical model, the requirements for deformability and surface wettability are analyzed through comparing the lateral forces experienced by trans-medium vehicles with a typical head configuration and surface wettability during the early stages of the water entry. Based on the three head configurations presented for water entry dynamics analysis in Section 2.2, three additional typical head configurations, which differ in curvature while taking into account the requirements of load reduction by setting k₀ to 1 (equivalent to the optimal configuration), are proposed for trajectory stabilization analysis, as listed in and graphically presented in . For clarity, these configurations are denoted as Config A, Config B, and Config C, respectively. For each head configuration, three types of surface wettabilities are considered, including superhydrophilic (WCA = 0°), neutral (WCA = 90°), and superhydrophobic (WCA = 180°).

When the trans-medium vehicle with these configurations and surface wettabilities enters the water at speeds of 1 m/s, 3 m/s, 5 m/s, and 10 m/s, the lateral force on its single side is monitored and presented in . It is evident that both the head configuration and surface wettability influence the lateral force. Shortly after contacting the water, before the side of the trans-medium vehicle is immersed and wettability effects are established, the lateral force is primarily determined by the head configuration. However, as the trans-medium vehicle continues to descend, the impact of wettability on lateral forces increases over time. In addition, the specific manifestation of this effect varies with the initial motion states. For example, Config C produces relatively small lateral forces across a wide range of initial speeds, but when utilized along with a superhydrophobic surface, it generates larger lateral forces, particularly at lower speeds. Config A consistently results in large and positive lateral forces, with wettability significantly affecting the lateral force at lower speeds. When Config B is employed, the magnitude of the lateral forces generally falls between those of Config A and C, and it also shows similar relationships with the surface wettability.

To exploit the variable lateral forces and achieve flexible compensation of pitch moments, all of these configurations and surface wettabilities can be used to adjust the lateral force differences between the two sides of the trans-medium vehicle. Incorporating the optimal configuration presented in Section 2.2, the maximum area among these configurations is 1.6 m², while the minimum is 1.4 m², corresponding to the original size of the trans-medium vehicle. Therefore, the deformability requirement of the flexible skin material is set to 15% to enable morphing between these configurations. Regarding surface wettability, the flexible skin material must be capable of switching between superhydrophilic and superhydrophobic states. The requirements for deformability and surface wettability are thereby determined, which also demonstrates the significance of flexible skin with switchable wettability.

2.4. Summary of design requirements

In this paper, the fundamental design principle for the flexible skin material includes both load-bearing capacity and deformability. Specifically, the material should be sufficiently strong and stiff to withstand impact loads during water entry, while also maintaining flexibility to accommodate large in-plane deformations. Furthermore, to enable switchable surface wettability, a smart, responsive coating, which can maintain effective wettability performance during the deformation of the substrate, must be applied to the surface of the flexible skin material. In conclusion, the overall design objective of the flexible skin material is to simultaneously achieve load-bearing capacity, deformability, and switchable surface wettability, as illustrated in .

According to the analysis above, considering that the strain of the flexible skin material is limited to 3% under the impact loads during water entry, a minimum ultimate strength of 25.0 MPa and an elastic modulus of 835 MPa are required. To satisfy the variation in the configuration and wettability, the deformation rate should reach at least 15%, and the surface wettability should be able to switch between superhydrophilic and superhydrophobic states, corresponding to WCA ≤ 5° and WCA ≥ 150°, respectively. Moreover, the thickness of the material should not exceed 5 mm to ensure feasibility in manufacturing and practical application.

3.1. Design and fabrication of flexible skin materials

The substrate of the flexible skin material must possess adequate load-bearing capacity, high deformability, and compatibility with surface coatings. Shape memory polymer (SMP), a variable stiffness material with shape memory effect, exhibits stiffness responsive to external stimuli and allows for reversible, large deformations. As temperature rises, SMP transitions from a stiff to a flexible state, thereby facilitating deformation. Conversely, lowering the temperature increases its stiffness, fixing the deformed shape and enabling resistance to impact loads [Citation38]. Moreover, SMP can recover its original shape through the shape memory effect, even after significant deformation. With further advantages such as smoothness, impermeability, and ease of surface treatment, SMP emerges as a promising candidate for the substrate of the flexible skin material. However, due to brittleness in the glassy state and susceptibility to tearing at high temperatures [Citation37], pure SMP is limited in its ability to withstand impact loads in water entry scenarios.

In this paper, to enhance the load-bearing capacity of pure SMP, S-shaped shape memory alloy (SMA) wires are embedded within the SMP matrix, which draws inspiration from the concepts of SMP composite (SMPC). SMA is characterized by high tensile strain and high recovery rates upon deformation (also known as superelasticity) [Citation50], aligning well with the design requirements of the flexible skin material. By integrating SMP with S-shaped SMA wires into a single-layer SMP-SMA laminate, both the strength and the elastic modulus of the SMP are improved for load-bearing applications, without significantly increasing the burden on the morphing actuator. Furthermore, a double-layer configuration, consisting of SMP and two layers of SMA wires arranged orthogonally, can further enhance the mechanical performance of the SMP in all directions, as shown in . With reinforcement from S-shaped SMA wires, SMP can thus be employed as the substrate of the flexible skin material.

The substrate of the flexible skin material, comprising SMA and SMP, is fabricated according to the procedure illustrated in . First, the mold is cleaned with ethanol and coated with a release agent. The S-shaped SMA, with a radius of 5 mm and a thickness of 0.5 mm, is neatly positioned in the mold utilizing the shifted method. Here, the mold is made of silica gel, which can facilitate the release of the epoxy resin and preserve the integrity of the embedded materials. Subsequently, epoxy resin E-44 is mixed with polyether amine D230, the curing agent, at a mass ratio of 2:1. The mixture is thoroughly stirred with glass rods to ensure homogeneity. To eliminate air bubbles generated during mixing, which can impair the mechanical performance of the resin, vacuum degassing is performed on the premixed solution. The degassed resin is then slowly poured into the silicone mold, followed by an additional vacuuming step to remove any remaining bubbles. The resin is cured in an oven at 100°C for 1 hour, then at 130°C for another hour. After demolding while still warm, the substrate of the flexible skin material is obtained.

To construct a smart, responsive coating with switchable wettability that remains unaffected by the deformation of its substrate, functional molecules are grafted onto the surface of the flexible skin material. Specifically, perfluorooctanoic acid (PFOA) molecules, responsive to pH, are grafted onto hydrophobic SiO₂ nanoparticles via a silane coupling agent and subsequently sprayed onto the substrate. The low surface energy of the perfluorooctane chain preserves the intrinsic hydrophobicity of the SiO₂ nanoparticles. However, in alkaline environments, the introduced amide group can hydrolyze into carboxylic and carboxylic acid groups, rendering the SiO₂ nanoparticles hydrophilic. Conversely, under acidic conditions, dehydration and condensation reactions regenerate amide groups, restoring the hydrophobicity of the SiO₂ nanoparticles, as shown in ). Compared to the coatings developed using microarrays, those constructed by spraying grafted functional molecules exhibit a rich micro-/nanoscale morphology and an irregular hierarchical structure (see )), which allow the coating to retain its morphological features through self-splitting, thereby maintaining stable wettability during the deformation of the substrate [Citation51]. Additionally, the stability and adhesion of the coating can be further enhanced by optimizing the spraying and pre-curing processes.

The fabrication process of the switchable wettability coating is as follows: First, the substrate is polished, rinsed with anhydrous ethanol, and allowed to dry naturally. Next, 20 g of epoxy resin E-44 is mixed with 20 g of ethyl acetate (EAC) and stirred until it is fully dissolved. Subsequently, 5 g of D-230 curing agent is added to the mixture and stirred thoroughly. The epoxy resin solution is then sprayed onto the substrate and pre-cured in an oven at 80°C for 45 minutes. After that, 4.8 g of 7 nm hydrophobic vaporphase SiO₂ nanoparticles, grafted with PFOA, are initially dissolved in 72 g (80 mL) of ethyl acetate, followed by ultrasonic stirring for 20 minutes and magnetic stirring for 10 minutes. Finally, the PFOA-SiO₂ solution is sprayed onto the pre-cured substrate, which is then cured at 100°C for 1 hour and subsequently at 130°C for an additional hour. Through these procedures, the flexible skin material with switchable surface wettability is obtained, as summarized in .

3.2. Mechanical performance of flexible skin materials

As a preliminary validation, the mechanical performance of the flexible skin material is investigated by finite element simulation. To streamline the analysis, the composite laminates, consisting of SMP and SMA, are assumed to be perfectly bonded, neglecting any interface slip between the SMP and SMA layers. Under this assumption, a finite element model of a 145 × 20 × 2.1 mm flexible skin material sample is constructed, using the material parameters of SMA (NiTi) and SMP provided in . To simulate uniaxial tensile loading, an in-plane displacement of 3 mm is applied to one end of the sample along the stretching direction, while a fixed boundary condition is applied to the opposite end. The results of the finite element simulation yielded an elastic modulus of 2.89 GPa for the single-layer configuration and 2.91 GPa for the double-layer configuration.

Subsequently, four experimental samples, including two identical single-layer samples and two identical double-layer samples, are prepared for tensile testing to further evaluate the strength, stiffness, and deformability of the flexible skin material. The tensile tests are conducted using a WDW-20 microcomputer-controlled electronic testing machine, with experimental data collected by a computer-based data acquisition system. At room temperature, the stress-strain response of the flexible skin material is measured under quasi-static loading, as shown in . The average elastic modulus is calculated, yielding results of 2.40 GPa for the single-layer samples and 2.32 GPa for the double-layer samples, which closely aligns with the simulation results and satisfy the previously specified requirements. The average ultimate strength of the samples is found to be 37.8 MPa for the single-layer configuration and 31.6 MPa for the double-layer configuration, representing a significant improvement compared to the ultimate strength of 29.0 MPa measured prior to the embedding of the SMA, and fulfilling the established strength requirements. Failure analysis (see ) further reveals that the SMP fractures before the SMA, demonstrating the effectiveness of the SMA reinforcement. Moreover, the fracture cross-section of the sample is almost coincident with the extension of the SMA alloy wire, and the results from the double-layer samples indicate that the SMP is more likely to fracture near the SMA wires oriented perpendicular to the stretching direction. This explains the lower ultimate strength observed in the double-layer samples compared to the single-layer samples and underscores the importance of optimizing the bonding process between SMP and SMA materials to enhance load-bearing capacity. In addition to the above results, it is also found that when the flexible skin material is heated to 80°C and stretched, it exhibits a maximum deformation rate of 26.8%, thereby meeting the deformation requirement.

3.3. Wettability performance of flexible skin materials

To evaluate the static performance of the wettability for the PFOA-SiO₂ coating on the flexible skin material, the WCA of the surface is tested using solutions with varying pH levels, as shown in . The results indicate that the PFOA-SiO₂ coating does not achieve superhydrophobicity at pH 1. However, the WCA remains above 150° for pH levels ranging from 2 to 10, demonstrating stable superhydrophobic characteristics. When the pH level reaches 11, the sufficiently strong alkalinity causes the hydrolysis of perfluorooctane amide groups and the formation of perfluorooctane carboxyl and perfluorooctanoate groups, leading the WCA decline to a hydrophibic state. As the pH level further increases, the WCA decreases sharply, ultimately reaching a superhydrophilic state (WCA near 0°) at pH 13. Therefore, the flexible skin material can effectively transition between superhydrophilic and superhydrophobic states, with WCA fulfilling the respective requirements.

To investigate the dynamic performance of the wettability for the PFOA-SiO₂ coatings on the flexible skin material, the WCA of the surface is tested while cyclically switching the environment pH level between 7 and 13. The detailed testing procedure is as follows. First, the sample is treated with an alkaline solution at pH 13, and its WCA is measured. Next, an acidic solution at pH 7 is applied and allowed to sit for 5 minutes, followed by heating in an oven at 100°C for 60 minutes before recording WCA. This cycle was repeated eight times, and the results are shown in . In this figure, the point directly opposite the scale line of the horizontal axis represents the case of pH 7, while the point at the midpoint between adjacent scale lines corresponds to the case of pH 13. It indicates that the PFOA-SiO₂ coating maintains a reversible transition between superhydrophilic and superhydrophobic states even after seven cycles, demonstrating excellent stability during repeated cyclic switching. Thus, the surface wettability performance of the flexible skin material is further validated.

3.4. Interrelationship between deformation and surface wettability

To assess the effectiveness of the switchable wettability on the flexible skin material undergoing deformation, the effects of substrate deformation on the surface wettability are analyzed. Here, four identical flexible skin material samples with switchable wettability are fabricated and tested under deformation rates of 5%, 10%, 15%, and 20%, respectively. The detailed test procedure is as follows: First, a dynamic wettability performance test, as described in Section 3.3, is conducted for one cycle, and the WCA is recorded. Next, an external force is applied to stretch the sample to the desired states of deformation (as shown in ), followed by another cycle of dynamic wettability performance testing. Subsequently, the external force is removed, allowing the sample to recover its original shape. These three steps are repeated three times to reproduce the situation of multiple morphing cycles in the trans-medium vehicle.

The test results are shown in , where the bar aligned with the scale line of the horizontal axis represents the unloaded state, and the bar at the midpoint between adjacent scale lines corresponds to the loaded state. As can be seen, after the first stretching, the PFOA-SiO₂ coating of all the samples exhibits stable, switchable wettability, even at a deformation rate of 20%. Upon unloading, their performance remains consistent. Throughout three cycles of loading, stretching, and unloading, the WCA of the samples at pH 7 and pH 13 remains unchanged from their values before morphing. These results demonstrate that the switchable wettability of the flexible skin material remains effective during repeated cyclic deformations of the substrate.