Design and preparation method for variable-stiffness surface of insect-imitating foot pad with strong friction characteristic

Abstract

The invention discloses a design and preparation method of an insect-imitating foot pad variable-stiffness surface with a strong friction characteristic. The insect-imitating foot pad variable-stiffness surface structurally comprises a curved surface film, orientation micro-columns and a substrate layer, the three components are all composed of temperature-sensitive shape memory epoxy resin, and the modulus depends on the glass transition temperature of the temperature-sensitive shape memory epoxy resin. In the normal contact stage, the temperature field is set to be higher than the glass-transition temperature, the material is in a low-modulus rubber state, and the contact rigidity is low and the contact area is large due to the structural characteristics of the orientation micro-column and the curved film; before tangential loading, the material is switched to a high-modulus glassy state, the locking effect maintains interface conformal contact, the shearing effect can be effectively resisted, and the static friction force is enhanced; the structural combination of the oriented micro-columns and the curved film can adapt to the surface with large-range roughness, and the maximum static friction force of the bionic surface on smooth and rough surfaces is improved. According to the invention, the limitation of single adaptability of a traditional flexible bionic adhesion surface is broken through, and the requirement of a strong friction application scene on a surface with large-range roughness can be met.

Description

Technical Field

[0001] The present invention belongs to the technical field of bionic material design and preparation, and specifically relates to a design and preparation method for a variable stiffness surface of an insect footpad-like structure with strong friction characteristics. Background Art

[0002] For bionic adhesion materials based on the structure of insect footpads, the multi-level spherical crown-shaped friction-increasing and adhesion-reducing pads prepared by the design of a spherical contact film and a vertical cylinder array in Patent [CN 116691076 A] have high friction and low adhesion characteristics. When the curvature of the spherical contact film is 20 m -1 at 0°, the shear force on a smooth surface can reach up to about 10 N, and the normal adhesion force is within 1 N. Compared with the planar thin film-capped cylindrical fiber structure, the shear force does not show a significant attenuation as the angle increases to 3°, and at the same time, a shear force of 8-12 N can be achieved on surfaces with different roughnesses (Sq = 0.038-1.051 μm). These results indicate that the bionic friction pads have excellent high-friction, low-adhesion characteristics and good adaptability (please refer to Zhao J, Ji K, Tu C, et al. Insect-inspired design strategy for flexible attachments with strong frictional force and weak pull-off force [J]. Tribol Int, 2023, 189: 108973.). The excellent friction behavior of the bionic friction pads is attributed to their internal structure and hemispherical surface. During the attachment process, the internal structure of the bionic friction pads makes the stress distribution on the contact surface uniform, while the hemispherical surface concentrates the stress along the movement direction to resist shear. Limited by the low modulus characteristics of flexible materials, large deformations are likely to occur due to stress concentration during tangential movement, making it difficult to maintain the interface contact and adhesion state, resulting in limited ability to resist shear movement. Moreover, when the roughness of the target surface is high, due to the high elasticity of the flexible materials, conformal contact cannot be maintained during tangential movement, and the deformed contact formed with the rough peaks is easy to recover, significantly reducing the effective contact area and affecting the high-friction performance on relatively smooth surfaces.

[0003] Learning from nature, the excellent dynamic friction regulation performance required for insect movement not only comes from the static adhesion structure of the footpads, but also requires the synergistic effect of the footpads and limbs to provide directional strong friction and easy desorption during high-dynamic attachment and separation movements. The variable stiffness characteristics of biological surfaces help to improve the self-adaptability of the contact interface and the stability of the contact interface. For example, when an insect applies pressure to the footpad, the internal oriented fibers enhance the local stiffness through compressive deformation to achieve uniform distribution of the interface contact stress and increase the contact area. At the same time, when a gecko applies pressure to the footpad, the setae adapt to the surface micro-rough structure, and then the sole muscles harden and the stiffness increases, promoting the uniform distribution of the contact stress over the entire sole, inhibiting the interface contact failure caused by the expansion of interface cracks or shear action, thereby improving its adhesion and friction on different rough surfaces. Research shows that micro-columns prepared from shape memory epoxy resin can effectively enhance the adhesion of the bionic surface and have switchable adhesion characteristics. Shape memory epoxy resin can achieve the conversion between the rubber state and the glass state through temperature field regulation. When in the rubber state, it separates from the glass state. Using the variable stiffness and locking effect of the material, it affects the stress distribution at the separation interface and inhibits desorption, thereby producing an adhesion-enhancing effect. Returning to the rubber state at high temperature again can significantly reduce the adhesion and achieve the switchability of normal adhesion (see Tan D, Wang X, Liu Q, et al. Switchable Adhesion of Micropillar Adhesive on Rough Surfaces[J]. Small, 2019.). Therefore, a bionic surface coupling the biological static adhesion structure and the variable stiffness characteristics of shape memory epoxy resin is expected to achieve high friction and controllable normal adhesion characteristics on surfaces with relatively high roughness. Summary of the Invention

[0004] The object of the present invention is to solve the above problems and provide a bionic footpad variable stiffness surface with strong friction characteristics, which has a simple and convenient preparation process, low preparation cost, and can be prepared to have strong friction characteristics on surfaces with a wide range of roughness.

[0005] To solve the above technical problems, the technical solution of the present invention is: a bionic footpad variable stiffness surface with strong friction characteristics, the structure includes a curved film, oriented micro-columns and a base layer; all three are composed of a temperature-sensitive shape memory epoxy resin, and its modulus depends on its glass transition temperature. In the normal contact stage, the temperature field is set higher than the glass transition temperature, and the material is in a low-modulus rubber state. The structural characteristics of the oriented micro-columns and the curved film result in low contact stiffness and large contact area; before tangential loading, the material switches to a high-modulus glass state, and the locking effect maintains the conformal contact at the interface, which can effectively resist shear action and enhance the static friction force; the structural combination of the oriented micro-columns and the curved film can adapt to surfaces with a wide range of roughness and improve the maximum static friction force of the bionic surface on smooth and rough surfaces.

[0006] Further, the curved film is a finger pulp-shaped spherical arc surface; the oriented micro-columns connect the curved film and the base layer; the curved film and the oriented micro-columns are formed in a step-by-step manner; the base layer is used to transfer heat upward;

[0007] Further, the thickness of the curved film is 0.3 mm and the curvature is 20 m -1 ; the oriented micro-columns are a cylindrical array with an inclination angle, and the height of the cylindrical array gradually decreases from the center of the bionic surface outward; the inclination angle of the cylinders of the oriented micro-columns is 65°, the diameter is 0.5 mm, the center height is 1.5 mm, and the center distance between adjacent inclined cylinders is 1.7 mm, arranged in a staggered pattern.

[0008] Further, when the temperature field of the thermosensitive shape memory epoxy resin is set above the glass transition temperature (Tg), the shape memory epoxy resin will turn into a rubber state with a low modulus (as low as several MPa), while when the temperature is below Tg, the shape memory epoxy resin will turn into a glass state with a high modulus (as high as several GPa); when the temperature is above Tg, the epoxy resin deforms under an external load, and when the temperature drops below its glass transition temperature, it changes from a low-modulus rubber state to a high-modulus glass state, and the deformation is locked at the same time; therefore, the shape memory epoxy resin produces a locking effect after experiencing the process of variable stiffness.

[0009] Further, the structural combination of the oriented micro-columns and the curved film can adapt to surfaces with a wide range of roughness, ranging from a smooth surface to a surface with a relatively high roughness (Sq = 0.073 - 48.102 μm), and even a surface with a certain inclination angle (0° - 3°).

[0010] Further, the glass transition temperature (Tg) of the thermosensitive shape memory epoxy resin is adjustable and is controlled by the curing ratio of epoxy resin E44 and curing agent D230; the curved film and the oriented micro-columns are composed of epoxy resins with different glass transition temperatures due to heat transfer problems, and the mass ratios of epoxy resin E44 and D230 are 1:0.65 and 1:0.4 respectively; the curing mass ratio of the base layer and the oriented micro-columns is the same, and the mass ratio of E44 and D230 is 1:0.4; the glass transition temperature of the curved film is 44.77 °C, and the glass transition temperatures of the oriented micro-columns and the base layer are 74.98 °C.

[0011] Furthermore, the curved film and oriented microcolumns are composed of epoxy resins with different Tg, which exist in three states, namely R2G (rubber state to glass state), rubber state and glass state, and the temperature of a certain structure cannot be controlled alone. Therefore, when the temperature field is set higher than 85°C, the oriented microcolumns and the curved film are both in a low modulus rubber state, when the temperature is between 74.98°C and 44.77°C, the oriented microcolumns and the curved film are respectively in a high modulus glass state and a low modulus rubber state, and when the temperature is lower than 44.77°C, the oriented microcolumns and the curved film are both in a glass state. Due to the structural combination of oriented microcolumns and curved films with different Tg on the surface of the simulated insect foot pad, a combination of oriented microcolumns and curved films in different states can be achieved during normal separation or tangential friction.

[0012] The present invention also discloses a method for preparing an insect foot pad-imitation variable-rigidity surface with strong friction characteristics, comprising the following steps:

[0013] S1. According to the curvature of the bionic sample surface, a steel ball with the corresponding curvature is selected as the inverted mold surface, and a molding mold of a curved membrane assembly negative mold is obtained by 3D printing technology. The silicone rubber solution mixed with components A and B is vacuum degassed for 3 minutes, and after pouring, it is degassed again for 3 minutes and cured at room temperature for 6 hours to obtain a smooth curved membrane molding assembly negative mold;

[0014] S2, spray a uniform layer of release agent on the surface of the positioning frame and the mold base and let it dry, then place the curved membrane forming assembly female mold on the positioning frame assembly and assemble it with the base mold, then pour the silicone rubber solution and degas for 3 minutes, and after curing, obtain the curved membrane forming mold imitating the surface of the insect foot pad;

[0015] S3, according to the design of the sample oriented micro-column size parameters, 3D printing to obtain the oriented micro-column positive mold, spraying the release agent, and then pouring the silicone rubber solution to degas and solidify, and then demolding to obtain the oriented micro-column negative mold;

[0016] S4, pouring the mixture of epoxy resin E44 and curing agent D230 into the oriented microcolumn negative mold, scraping off the excess epoxy resin mixture after degassing for 10 minutes, and pre-curing at 80°C for 1 hour, then assembling it with the curved membrane forming mold in step S2, pouring the mixture of epoxy resin E44 and curing agent D230, degassing for 30 minutes, pre-curing at 50°C for 2 hours, then curing in an oven at 100°C for 1 hour, curing in an oven at 130°C for 1 hour and then demolding;

[0017] S5. Scrape a layer of epoxy resin mixture with the same curing mass ratio as the oriented microcolumns on the pre-cured epoxy resin base layer with the same glass transition temperature as the oriented microcolumns, place the combined structure of the curved film and oriented microcolumns obtained after demolding in step S4 and cure it at 110°C for 1 hour to finally obtain a sample imitating the surface of an insect foot pad.

[0018] Furthermore, the volume ratio of component A to component B in S1 is 1:1; the mass ratio of epoxy resin E44 to curing agent D230 poured into the orientation micro-column female mold in S4 is 1:0.4, and the mass ratio of the epoxy resin E44 and curing agent D230 mixture poured into the curved film forming mold is 1:0.65.

[0019] The beneficial effects of the present invention are as follows:

[0020] 1. The present invention provides an insect-inspired footpad variable stiffness surface with strong friction characteristics and a preparation method thereof, which can actively adjust its own structural stiffness and surface contact state to adapt to different roughness surfaces, and is of great significance for intelligent bionic surfaces with controllable normal adhesion, anti-shearing ability and stiffness.

[0021] 2. The insect-inspired footpad variable stiffness surface of the present invention can actively adjust its own structural stiffness and surface contact state. Compared with the static bionic friction pad made of insect footpad-inspired flexible material (PDMS) which is only applicable to relatively smooth surfaces, it can have high tangential friction and controllable normal adhesion force on a large range of roughness surfaces (Sq = 0.073 - 48.102 μm), and can be applied to rough surfaces with a certain inclination angle (0° - 3°).

[0022] 3. The present invention learns from nature. The excellent dynamic friction regulation performance required for insect movement not only comes from the static adhesion structure of the footpad, but also needs to utilize the synergistic effect between the footpad and the limb to provide directional strong friction and easy desorption during high-dynamic attachment and separation movements. The variable stiffness characteristics of the biological surface help to improve the self-adaptability and contact interface stability of the contact interface. For example, when an insect applies pressure to the footpad, the internal oriented fibers enhance the local stiffness through compressive deformation to achieve uniform distribution of the interface contact stress and increase the contact area. At the same time, when a gecko applies pressure to the footpad, the setae adapt to the surface micro-rough structure, and then the sole muscle hardens and the stiffness increases, promoting the uniform distribution of the contact stress over the entire sole, inhibiting the interface contact failure caused by interface crack propagation or shear action, thereby improving its adhesion and friction on different rough surfaces. Inspired by this, the present invention proposes an insect-inspired footpad variable stiffness surface with strong friction characteristics on a large range of roughness surfaces and a preparation method thereof, which has high tangential friction and controllable normal adhesion characteristics on smooth and a large range of roughness surfaces based on the synergistic effect of orientation micro-columns and curved films.

[0023] 4. The variable-stiffness surface of the insect-inspired footpad with strong friction characteristics of the present invention is prepared from a temperature-sensitive shape memory epoxy resin. In the normal contact stage, the temperature field is set above the glass transition temperature, and the material is in a low-modulus rubber state. The structural characteristics of the oriented micro-columns and the curved film result in a low contact stiffness and a large contact area. Before tangential loading, after cooling, the material switches to a high-modulus glass state, and the locking effect maintains the conformal contact at the interface. The uniform distribution of the interface stress can effectively inhibit the propagation of interface cracks, improve the normal adhesion force, and at the same time can effectively resist the shear action and enhance the static friction force. The structural combination of the oriented micro-columns and the curved film can adapt to surfaces with a wide range of roughness, and improve the maximum static friction force of the bionic surface on smooth and rough surfaces. In addition, the present invention can achieve normal adhesion grasping and tangential grasping, and can achieve flexible detachment through temperature field regulation. At the same time, it breaks through the limitation of the single surface adaptability of traditional flexible bionic materials, and can meet the requirements of strong friction and switchable adhesion application scenarios on surfaces with a wide range of roughness. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 FIG. is a schematic diagram of the structural composition of the variable-stiffness surface of the insect-inspired footpad with strong friction characteristics of the present invention;

[0025] Figure 2 FIG. is a flowchart for preparing the variable-stiffness surface of the insect-inspired footpad of the present invention;

[0026] Figure 3 FIG. is a schematic diagram of the adhesion and friction principles of the variable-stiffness surface of the insect-inspired footpad of the present invention;

[0027] Figure 4 FIG. is a graph of the tangential friction test results of the variable-stiffness surface of the insect-inspired footpad on a rough plane (Sq = 13.426 μm) under different normal loads of the present invention.

[0028] Figure 5 FIG. is a graph of the tangential friction and normal adhesion test results of the variable-stiffness surface of the insect-inspired footpad and planes with different roughnesses under a 2N preload of the present invention;

[0029] Figure 6 FIG. is a graph of the tangential friction and normal adhesion test results of the variable-stiffness surface of the insect-inspired footpad on a smooth surface and a rough (Sq = 13.426 μm) surface with an inclination angle of 0° - 3° under a 2N preload of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] The present invention will be further described below with reference to the accompanying drawings and specific embodiments:

[0031] As Figures 1 to 6As shown, a variable-stiffness surface of an insect-inspired footpad with strong friction characteristics provided by the present invention has a structure including a curved film, oriented micro-columns, and a base layer; all three are made of a thermosensitive shape memory epoxy resin, and its modulus depends on its glass transition temperature. In the normal contact stage, the temperature field is set above the glass transition temperature, and the material is in a low-modulus rubber state. The structural characteristics of the oriented micro-columns and the curved film result in a low contact stiffness and a large contact area. Before tangential loading, the material switches to a high-modulus glass state, and the locking effect maintains the conformal contact at the interface, effectively resisting shear forces and enhancing the static friction. The structural combination of the oriented micro-columns and the curved film can adapt to surfaces with a wide range of roughness, improving the maximum static friction of the bionic surface on smooth and rough surfaces.

[0032] The curved film is a finger pulp-shaped spherical arc surface; the oriented micro-columns connect the curved film and the base layer; the curved film and the oriented micro-columns are formed in a step-by-step manner; the base layer is used to transfer heat upward;

[0033] The thickness of the curved film is 0.3 mm, and the curvature is 20 m -1 The oriented micro-columns are a cylindrical array with an inclined angle, and the height of the cylindrical array gradually decreases from the center of the bionic surface to the outside. The inclined angle of the cylinders of the oriented micro-columns is 65°, the diameter is 0.5 mm, the center height is 1.5 mm, and the center distance between adjacent inclined cylinders is 1.7 mm, arranged in a staggered pattern.

[0034] When the temperature field of the thermosensitive shape memory epoxy resin is set above the glass transition temperature (Tg), the shape memory epoxy resin will change into a rubber state with a low modulus (as low as several MPa), while when the temperature is below Tg, the shape memory epoxy resin will change into a glass state with a high modulus (as high as several GPa); when the temperature is above Tg, the epoxy resin deforms under an external load, and when the temperature drops below its glass transition temperature, it changes from a low-modulus rubber state to a high-modulus glass state, and the deformation is locked at the same time. Therefore, the shape memory epoxy resin produces a locking effect after undergoing the process of variable stiffness.

[0035] The structural combination of the oriented micro-columns and the curved film can adapt to surfaces with a wide range of roughness, ranging from smooth surfaces to surfaces with relatively high roughness (Sq = 0.073 - 48.102 μm), and even surfaces with a certain inclination angle (0° - 3°).

[0036] The glass transition temperature (Tg) of the thermosensitive shape memory epoxy resin is adjustable and controlled by the curing ratio of epoxy resin E44 and curing agent D230; the curved film and oriented microcolumns require epoxy resins with different glass transition temperatures due to heat transfer problems, and the mass ratios of epoxy resins E44 and D230 are 1:0.65 and 1:0.4 respectively; the curing mass ratio of the base layer and the oriented microcolumns is the same, and the mass ratio of E44 and D230 is 1:0.4; the glass transition temperature of the curved film is 44.77°C, and the glass transition temperature of the oriented microcolumns and the base layer is 74.98°C.

[0037] The curved film and oriented microcolumns are composed of epoxy resins with different Tg, which exist in three states: R2G (rubber state to glass state), rubber state and glass state. At the same time, the temperature of a certain structure cannot be controlled alone. Therefore, when the temperature field is set higher than 85°C, the oriented microcolumns and the curved film are both in the low modulus rubber state. When the temperature is between 74.98°C and 44.77°C, the oriented microcolumns and the curved film are in the high modulus glass state and low modulus rubber state, respectively. When the temperature is lower than 44.77°C, the oriented microcolumns and the curved film are both in the glass state. Due to the structural combination of oriented microcolumns and curved films with different Tg on the surface of the insect foot pad, the combination of oriented microcolumns and curved films in different states can be achieved during normal separation or tangential friction.

[0038] The present invention also discloses a method for preparing a variable-rigidity surface imitating an insect foot pad with strong friction characteristics over a wide range of roughness, comprising the following steps:

[0039] S1. According to the curvature of the bionic sample surface, a steel ball with the corresponding curvature is selected as the inverted mold surface, and a molding mold of a curved membrane assembly negative mold is obtained by 3D printing technology. The silicone rubber solution mixed with components A and B is vacuum degassed for 3 minutes, and after pouring, it is degassed again for 3 minutes and cured at room temperature for 6 hours to obtain a smooth curved membrane molding assembly negative mold;

[0040] In step S1, the volume ratio of components A and B is 1: 1. The design of the curvature size of the bionic sample surface in this embodiment refers to the reference insect smooth foot pad prototype, which is a combination of a hemispherical outer skin and internal oriented fibers. The hemispherical shape can adapt to various surfaces and obtain strong friction and low adhesion properties.

[0041] S2, spray a uniform layer of release agent on the surface of the positioning frame and the mold base and let it dry, then place the curved membrane forming assembly female mold on the positioning frame assembly and assemble it with the base mold, then pour the silicone rubber solution and degas for 3 minutes, and after curing, obtain the curved membrane forming mold imitating the surface of the insect foot pad;

[0042] S3. Design according to the size parameters of the sample-oriented micro-columns, and 3D print to obtain the oriented micro-column male mold. Spray the release agent, let it stand, then pour the silicone rubber solution, degas it, and cure it. After demolding, obtain the oriented micro-column female mold;

[0043] S4. Pour the mixture of epoxy resin E44 and curing agent D230 into the oriented micro-column female mold. After degassing for 10 minutes, scrape off the excess epoxy resin mixture and pre-cure it at 80 °C for 1 hour. Then assemble it with the curved film forming mold in step S2, pour the mixture of epoxy resin E44 and curing agent D230, degas for 30 minutes, pre-cure at 50 °C for 2 hours, then cure it in an oven at 100 °C for 1 hour and then in an oven at 130 °C for 1 hour, and finally demold;

[0044] In step S4, the mass ratio of epoxy resin E44 to curing agent D230 poured into the oriented micro-column female mold is 1:0.4, and the mass ratio of the mixture of epoxy resin E44 and curing agent D230 poured into the curved film forming mold is 1:0.65.

[0045] S5. Scrape a layer of epoxy resin mixture with the same curing mass ratio as the oriented micro-columns on the pre-cured epoxy resin base layer with the same glass transition temperature as the oriented micro-columns. Place the combined structure with the curved film and oriented micro-columns obtained after demolding in step S4, and cure it at 110 °C for 1 hour. Finally, obtain the sample imitating the surface of the insect footpad.

[0046] The combination of the variable stiffness surface of the bionic insect footpad with different Tg of the oriented micro-columns and the curved film can achieve the combination of different state oriented micro-columns and the curved film during normal separation or tangential friction. The oriented micro-columns and the curved film of the variable stiffness surface of the bionic insect footpad contact the surface in the high-temperature low-modulus rubber state, and separate in the low-temperature high-modulus glass state, experiencing a variable stiffness process, named F R2G _M R2G group. And when separating between 74.98 °C and 44.77 °C, only the oriented micro-columns experience a variable stiffness process, named F R2G _M R group. When contacting and then performing normal separation or tangential friction in the high-temperature state, it is named F R _M R group, and when contacting and performing friction tests in the low-temperature state, it is named F G _M G group.

[0047] The present invention also provides a friction and adhesion test method for the variable stiffness surface of the bionic insect footpad with strong friction characteristics on a large-range roughness surface, including the following steps:

[0048] S11. First, paste the smooth or rough counter surface onto the specimen fixture of the UMT friction and wear testing machine, and fix the sample in the lower fixture with heating and cooling functions.

[0049] In this step, UMT refers to the UMT Tribolab friction and wear testing machine of Bruker Corporation.

[0050] S12. Then, turn on the heating switch of the lower fixture and heat it until the sample substrate temperature reaches T max .

[0051] In step S12, the sample substrate temperature T max is 90 °C. When T max = 90 °C, the bionic surface is in a low-modulus rubber state, showing low stiffness. The F R2G _M R2G group, F R2G _M R group, and F R _M R group need to go through this heating process, while the glassy F G _M G group does not go through the heating process.

[0052] S13. After that, move the counter surface of the upper specimen of the UMT to 1 - 2 mm above the sample surface. After the program starts, the counter surface moves downward at a certain speed and continues to load until the preset load F n , and hold the load for time t. During the holding time, turn on the cooling switch to cool the variable-stiffness bionic sample group to temperature T min ; the rubbery F R _M R group always maintains the temperature at T max and does not need to be cooled.

[0053] In step S13, cooling to temperature T min , specifically, cooling from 90 °C to 30 °C or 63.5 °C, corresponding to the F R2G _M R2G group and F R2G _M R group respectively.

[0054] S14. Finally, unload normally at a speed of 0.1 mm / s or apply a tangential displacement of 1.5 mm to obtain the adhesion force-time curve and the friction force-time variation curve, and finally obtain the maximum normal adhesion force and the maximum static friction force;

[0055] In the adhesion and friction tests, for the rough counterface with an inclination angle of 0°, the resin surfaces imprinted with 80#, 800#, 1000#, and 3000# sandpapers were selected, and for the smooth counterface, the resin surface imprinted with smooth glass was selected. The roughness Sq of the resin surfaces imprinted with 80#, 800#, 1000#, 3000# sandpapers and smooth glass was 48.102 μm, 31.29 μm, 13.426 μm, 7.49 μm, and 0.073 μm respectively. Additionally, rough surfaces with inclination angles of 1°, 2°, and 3° (Sq = 13.426 μm) were also imprinted; the normal loads were selected as 1 N, 2 N, 3 N, and 4 N.

[0056] As Figure 3 (a) and the state Ⅱ in (b), conformal contact is achieved at the contact surface in the high-temperature and low-stiffness state; when cooled to room temperature without removing the load, it becomes in the high-stiffness state, and through the variable stiffness process, the propagation of interface cracks can be inhibited and shear deformation can be resisted, realizing the function of picking up objects or playing a role in increasing friction, as Figure 3 (a) and the state Ⅲ in (b). When reheated to the high-temperature state, the lock-in effect disappears after changing from high stiffness to low stiffness. Due to the shape memory effect, when a normal upward load is applied, cracks propagate from the edge to achieve tearing at the edge of the contact interface, enabling the object to fall off from the surface of the insect-inspired footpad, corresponding to Figure 3 (a)'s state Ⅳ; meanwhile, when a tangential load is applied, it is difficult to resist the tangential load and large deformation is likely to occur, thus enabling the switch from the high-friction state to the low-friction state, corresponding to Figure 3 (b)'s state Ⅳ.

[0057] The tangential friction and normal adhesion tests of the variable stiffness surface of the insect-inspired footpad of the present invention are as follows:

[0058] For the variable stiffness surface of the insect-inspired footpad in the glassy state (F G _M G group), rubbery state (F R _M R group), and the transition from the rubbery state to the glassy state (including F R2G _M R group and F R2G _M R2G group), different magnitudes of preloads were applied to conduct tangential friction tests on the rough surface (Sq = 13.426 μm), and the test results are as Figure 4 shown. It can be seen from the test results that the variable stiffness F R2G _M R2G group obtained the maximum tangential friction force on the rough plane compared with the other three groups, and the friction force continued to increase significantly with the increase of the load.

[0059] For the variable stiffness surface of the insect-inspired footpad in the glassy state (F G _MG group), rubber state (F R _M R group) and the transition from rubber state to glass state (including F R2G _M R group and F R2G _M R2G group), a 2N preload is applied to conduct tangential friction tests and normal adhesion tests on surfaces with different roughnesses, and the test results are as Figure 5 shown. Figure 5 (a) in it is the result of the tangential friction test with a 2N preload on surfaces with different roughnesses, and (b) is the result of the normal adhesion test with a 2N preload on surfaces with different roughnesses. It can be seen from the test results that the variable stiffness F R2G _M R2G group obtains the maximum static friction force and normal adhesion force on surfaces with different roughnesses (Sq = 0.073 - 48.102 μm) compared with the other three groups, and the most excellent static friction force and normal adhesion force can be obtained when on a smooth surface (Sq = 13.426 μm). Although the maximum static friction force and the maximum normal adhesion force will decrease with the increase of roughness, it still has a significant friction-increasing and adhesion-increasing effect compared with the other three groups.

[0060] For the variable stiffness surface of the insect-inspired foot pad in the glass state (F G _M G group), rubber state (F R _M R group) and the transition from rubber state to glass state (including F R2G _M R group and F R2G _M R2G group), a 2N preload is applied to conduct tangential friction tests and normal adhesion tests on rough surfaces (Sq = 13.426 μm) with different inclinations (0° - 3°), and the test results are as Figure 6 shown. Figure 6 (a) in it is the result of the tangential friction test with a 2N preload on surfaces with different roughnesses, and (b) is the result of the normal adhesion test with a 2N preload on surfaces with different roughnesses. It can be seen from the test results that the variable stiffness F R2G _M R2G group obtains the maximum static friction force and normal adhesion force on rough surfaces with different inclinations compared with the other three groups, and as the inclination increases, the maximum static friction force does not immediately decrease, but only starts to decrease when the inclination increases to 3°. Although the maximum normal adhesion force decreases with the increase of the inclination, the decrease rate is slow, and it still has a significant normal adhesion-increasing effect compared with the other three groups.

[0061] The present invention utilizes the variable stiffness characteristics and locking effect of shape memory polymers during the transition between the rubber state and the glass state to design and prepare a variable stiffness surface of an insect-inspired footpad with strong friction characteristics on a large range of rough surfaces, which has the following characteristics:

[0062] 1. Excellent shear resistance: The variable stiffness surface of the insect-inspired footpad has excellent shear resistance on a large range of rough surfaces (Sq = 0.073 - 48.102 μm). Compared with the flexible insect-inspired footpad surface, it has stronger shear resistance on smooth surfaces, providing a new method for friction research and the design of friction-increasing variable stiffness bionic surfaces, and can better meet the grasping requirements of flexible grippers.

[0063] 2. Ability to actively regulate adhesion and friction: After the variable stiffness surface of the insect-inspired footpad undergoes the transition between the rubber state and the glass state, it has high normal adhesion force and high shear resistance, and can be used in normal adhesion grasping and tangential grasping scenarios; when the temperature field is heated above Tg again, the variable stiffness surface of the insect-inspired footpad switches to low adhesion and low friction characteristics, enabling flexible release of objects.

[0064] 3. High shear resistance on inclined rough surfaces: When the variable stiffness surface of the insect-inspired footpad contacts rough surfaces with different inclination angles, the curved film can easily achieve a larger conformal contact area compared with planar adhesion bionic materials. The structural characteristics of the oriented micro-columns and the curved film result in low contact stiffness, large contact area, and further increase in the conformal contact area as the load increases. Subsequently, cooling transforms from the low modulus rubber state to the high modulus glass state to lock the shape, achieving the shear resistance effect.

[0065] 4. Can adapt to rough surfaces with different inclination angles: Within a certain inclination angle range (0° - 3°), as the inclination angle increases, the variable stiffness surface of the insect-inspired footpad can still maintain excellent shear resistance and normal adhesion ability.

[0066] 5. Scalability of structural parameters: The structural parameters of the oriented micro-columns and the curved film can be reduced or increased proportionally and as needed, and have the same normal adhesion, high shear resistance effect, and surface adaptability. The adaptation range of the surface inclination degree and roughness also decreases or increases accordingly. Multiple characteristic structures of the variable stiffness surface of the insect-inspired footpad can be combined and arranged, and laid on a large area surface to meet the grasping or climbing function requirements under special working conditions.

[0067] 6. Can be recycled and reused multiple times: The shape memory epoxy resin maintains a shape recovery rate of over 99% after dozens of high-low temperature cycles, and this bionic surface can be reused.

[0068] Those of ordinary skill in the art will realize that the embodiments described herein are provided to assist the reader in understanding the principles of the present invention, and it should be understood that the scope of protection of the present invention is not limited to such specific statements and embodiments. Those of ordinary skill in the art can make various other specific deformations and combinations that do not depart from the essence of the present invention based on the technical revelations disclosed in the present invention, and these deformations and combinations are still within the scope of protection of the present invention.

Claims

1. A variable-rigidity surface imitating an insect foot pad with strong friction characteristics, characterized in that: The structure includes a curved membrane, oriented microcolumns and a base layer; all three are composed of a temperature-sensitive shape memory epoxy resin, and its modulus depends on its glass transition temperature; in the normal contact stage, the temperature field is set higher than the glass transition temperature, and the material is in a low modulus rubber state. The structural characteristics of the oriented microcolumns and the curved membrane result in low contact stiffness and a large contact area; before tangential loading, the material switches to a high modulus glass state, and the locking effect maintains conformal contact at the interface, which can effectively resist shear and enhance static friction; the structural combination of the oriented microcolumns and the curved membrane can adapt to surfaces with a wide range of roughness and enhance the maximum static friction of the bionic surface on smooth and rough surfaces.

2. The insect foot pad-imitation variable-rigidity surface with strong friction characteristics according to claim 1, characterized in that: The curved film is a finger-shaped spherical arc surface; the oriented micro-columns connect the curved film and the base layer; the curved film and the oriented micro-columns are formed in steps; and the base layer is used to transfer heat upward.

3. The insect foot pad-imitation variable-rigidity surface with strong friction characteristics according to claim 1, characterized in that: The thickness of the curved surface is 0.3 mm and the curvature is 20 mm. -1 ; The oriented microcolumns are cylindrical arrays with an inclination angle, and the height of the cylindrical array gradually decreases from the center of the bionic surface to the outside; the cylindrical inclination angle of the oriented microcolumns is 65°, the diameter is 0.5mm, the center height is 1.5mm, the center distance between adjacent inclined cylinders is 1.7mm, and they are staggered.

4. The insect foot pad-imitation variable stiffness surface with strong friction characteristics according to claim 1, characterized in that: When the temperature field of the thermosensitive shape memory epoxy resin is set higher than the glass transition temperature (Tg), the shape memory epoxy resin will change into a rubber state with a lower modulus (as low as several MPa), and when the temperature is lower than Tg, the shape memory epoxy resin will change into a glass state with a higher modulus (as high as several GPa); when the temperature is higher than Tg, the epoxy resin will deform under the action of an external load, and when the temperature drops below its glass transition temperature, it will change from a low modulus rubber state to a high modulus glass state, and the deformation will be locked. Therefore, the shape memory epoxy resin produces a locking effect after undergoing the process of changing stiffness.

5. The insect foot pad-imitation variable-rigidity surface with strong friction characteristics according to claim 1, characterized in that: The structural combination of the oriented microcolumns and the curved film can adapt to a wide range of surfaces with roughness, ranging from smooth surfaces to relatively high roughness surfaces (Sq=0.073-48.102 μm), and even surfaces with a certain inclination angle (0°-3°).

6. The insect foot pad-imitation variable-rigidity surface with strong friction characteristics according to claim 1, characterized in that: The glass transition temperature (Tg) of the temperature-sensitive shape memory epoxy resin is adjustable and is controlled by the curing ratio of epoxy resin E44 and curing agent D230; the curved film and oriented microcolumns require epoxy resins with different glass transition temperatures due to heat transfer problems, and the mass ratios of epoxy resins E44 and D230 are 1:0.65 and 1:0.4 respectively; the curing mass ratio of the base layer and the oriented microcolumns is the same, and the mass ratio of E44 and D230 is 1:0.4; the glass transition temperature of the curved film is 44.77°C, and the glass transition temperature of the oriented microcolumns and the base layer is 74.98°C.

7. The insect foot pad-imitation variable-rigidity surface with strong friction characteristics according to claim 1, characterized in that: The curved film and oriented microcolumns are composed of epoxy resins with different Tg, which exist in three states, namely R2G (rubber state to glass state), rubber state and glass state, and the temperature of a certain structure cannot be controlled alone; therefore, when the temperature field is set higher than 85°C, the oriented microcolumns and the curved film are both in the low modulus rubber state, when the temperature is between 74.98°C and 44.77°C, the oriented microcolumns and the curved film are respectively in the high modulus glass state and the low modulus rubber state, and when the temperature is lower than 44.77°C, the oriented microcolumns and the curved film are both in the glass state. Due to the structural combination of oriented microcolumns and curved films with different Tg on the surface of the simulated insect foot pad, the combination of oriented microcolumns and curved films in different states can be realized during normal separation or tangential friction.

8. The insect foot pad-imitation variable-rigidity surface with strong friction characteristics according to any one of claims 1 to 7, characterized in that: The method for preparing the variable-rigidity surface imitating insect foot pads with strong friction characteristics comprises the following steps: S1. According to the curvature of the bionic sample surface, a steel ball with the corresponding curvature is selected as the inverted mold surface, and a molding mold of a curved membrane assembly negative mold is obtained by 3D printing technology. The silicone rubber solution mixed with components A and B is vacuum degassed for 3 minutes, and after pouring, it is degassed again for 3 minutes and cured at room temperature for 6 hours to obtain a smooth curved membrane molding assembly negative mold; S2, spray a uniform layer of release agent on the surface of the positioning frame and the mold base and let it dry, then place the curved membrane forming assembly female mold on the positioning frame assembly and assemble it with the base mold, then pour the silicone rubber solution and degas for 3 minutes, and after curing, obtain the curved membrane forming mold imitating the surface of the insect foot pad; S3, according to the design of the sample oriented micro-column size parameters, 3D printing to obtain the oriented micro-column positive mold, spraying the release agent, and then pouring the silicone rubber solution to degas and solidify, and then demolding to obtain the oriented micro-column negative mold; S4, pouring the mixture of epoxy resin E44 and curing agent D230 into the oriented microcolumn negative mold, scraping off the excess epoxy resin mixture after degassing for 10 minutes, and pre-curing at 80°C for 1 hour, then assembling it with the curved membrane forming mold in step S2, pouring the mixture of epoxy resin E44 and curing agent D230, degassing for 30 minutes, pre-curing at 50°C for 2 hours, then curing in an oven at 100°C for 1 hour, curing in an oven at 130°C for 1 hour and then demolding; S5. Scrape a layer of epoxy resin mixture with the same curing mass ratio as the oriented microcolumns on the pre-cured epoxy resin base layer with the same glass transition temperature as the oriented microcolumns, place the combined structure of the curved film and oriented microcolumns obtained after demolding in step S4 and cure it at 110°C for 1 hour to finally obtain a sample imitating the surface of an insect foot pad.

9. The insect foot pad-imitation variable-rigidity surface with strong friction characteristics according to claim 8, characterized in that: The volume ratio of components A and B in S1 is 1:1; the mass ratio of epoxy resin E44 and curing agent D230 cast into the oriented microcolumn female mold in S4 is 1:0.4, and the mass ratio of the mixture of epoxy resin E44 and curing agent D230 cast into the curved membrane forming mold is 1:0.65.

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