External field induced forming method for stiffness gradient adhesion structure

Abstract

An external field induced forming method for a stiffness gradient adhesion structure comprises the following steps: firstly, a layer of low surface energy dielectric material is prepared on the surface of a first conductive substrate; then, a layer of polymer material mixed with dielectric nanoparticles is prepared on the surface of a second conductive substrate; then, an induced upper electrode plate is pressed on an insulating support structure, and a DC high voltage is applied, so that the electric field force exerted on the polymer and the dielectric nanoparticles overcomes the surface tension and the viscous resistance rheology, and a layer of microstructure is grown according to the most unstable wavelength; then, the voltage is continuously applied until the polymer contacts the induced upper electrode plate, wets the surface of the low surface energy dielectric material, and spreads to obtain a mushroom structure; finally, the polymer is solidified while the voltage is kept unchanged, and the induced upper electrode plate is removed, so as to obtain a micro-micro or micro-nano stiffness gradient structure; the invention reduces manufacturing cost and improves processing efficiency.

Description

Technical Field

[0001] The invention belongs to the technical field of micro-nano engineering bionic manufacturing, and in particular relates to an external field induced forming method of a stiffness gradient adhesion structure. Background Art

[0002] In nature, some animals have evolved adhesion organs with gradient material properties. For example, the toe bristles of beetles have a gradient elastic modulus from 7.2GPa at the root to 1.2MPa at the tip, which is conducive to close contact with the target surface and high stability of the structure. In addition, the opposite elastic modulus gradient distribution trend was detected in the toe pad of the tree frog: the keratinized layer on the toe surface showed a modulus of 5-15MPa, and the dense soft capillary network under the keratinized layer continued to decrease from 20kPa to 4kPa with the increase of the depth of the toe pad. This type of gradient can show excellent wear resistance while maintaining compliant contact. The above-mentioned soft-on-hard-on-hard-on-soft-on stiffness gradient structures are likely to improve the adhesion performance of bionic structures. In order to imitate this stiffness gradient structure, two materials with elastic modulus are generally required to be composited. One is a soft material, that is, a material with a small elastic modulus, which is conducive to forming a reliable interface contact; the other is a hard material, which can enhance stability and enhance the adhesion effect by inhibiting interface separation. At present, the mushroom-shaped end form has been proven to be the best geometric shape for enhancing adhesion. Compared with hemispherical, blunt or flat tips, it has better interface contact ability, thus providing a larger contact area; in addition, the mushroom-shaped form can reduce the stress around the contact surface and inhibit the generation of cracks. It can be seen that the stiffness gradient mushroom-shaped adhesion structure is a bionic structure with superior performance.

[0003] However, the material properties of the stiffness gradient and the mushroom-shaped structure have brought great difficulties to processing and manufacturing. Traditional preparation methods have many shortcomings in large-scale preparation and forming efficiency: (1) The process is complicated, such as the mold replication layered production method (Michael P. Murphy, Seok Kim, and Metin Sitti ACS Applied Materials & Interfaces 200 9.1 (4), 849-855, DOI: 10.1021 / am8002439), which requires first making a layer of columnar arrays with a smaller or larger elastic modulus, and then covering the top with other materials with a large difference in modulus; (2) The equipment is precise, requiring expensive and complex processing equipment; (3) The process parameters need to be precisely controlled, such as the hard shell molding method (Michael P. Murphy, Seok Kim, and Metin Sitti ACS Applied Materials & Interfaces, 2009.1(4), 849-855, DOI: 10.1021 / am8002439), it is necessary to precisely insert the inverse structure into the micro-nanoscale mold groove in order to form a hollow hard shell; (4) The modulus transition at the junction of the two phases is not smooth enough. For example, in the hard core molding method (Composite Microposts with High Dry Adhesion Strength, HK Minsky and Kevin T. Turner, ACS Applied Materials & Interfaces, 2017.9(21), 18322-18327, DOI: 10.1021 / acsami.7b01491), the elastic modulus at the junction of the two phases changes suddenly, which will cause stress concentration, thereby reducing the durability and strength of the structure. Summary of the invention

[0004] In order to overcome the shortcomings of the prior art, the object of the present invention is to provide an external field induced forming method of a stiffness gradient adhesion structure, which can realize a stiffness gradient structure with micro-nano or micro-micro scale.

[0005] In order to achieve the above object, the technical route adopted by the present invention is:

[0006] An external field induced forming method for a stiffness gradient adhesion structure comprises the following steps:

[0007] The first step is to prepare the induction upper electrode: a layer of low surface energy dielectric material 2 with a thickness of micrometer or nanometer level is prepared on the surface of the first conductive substrate 1;

[0008] The second step is the selection and treatment of the substrate: using the second conductive substrate 4 as the substrate, preparing a layer of polymer material 3 mixed with dielectric nanoparticles with a thickness of micrometers or nanometers on the surface thereof;

[0009] The third step is electric field induced rheological forming of the soft-hard composite structure: applying pressure to press the induced upper electrode plate onto the insulating support structure 5, applying a DC high voltage, so that the electric field force on the polymer and the dielectric nanoparticles overcomes the surface tension and viscous resistance to flow, and grows a layer of microstructure according to the most unstable wavelength; wherein, by controlling the matching of the relative dielectric constants of the nanoparticles and the polymer, a micro-nanostructure with different distributions of nanoparticles inside is obtained; thereafter, the voltage is continuously applied until the polymer contacts the induced upper electrode plate, wets on the surface of the low surface energy dielectric material 2, and spreads to obtain a mushroom structure;

[0010] The fourth step is the curing and demolding of the polymer material: the polymer is cured while maintaining the voltage unchanged, and the induced upper electrode is removed to obtain a micro-micro or micro-nano stiffness gradient structure.

[0011] In the third step, by controlling the matching of the relative dielectric constants of the nanoparticles and the polymer, a micro-nano structure with different distributions of internal nanoparticles is obtained. When the dielectric constant of the nanoparticles is greater than the dielectric constant of the polymer, the nanoparticles will be concentrated on the upper side of the micro-nano structure to form a hard-upper and soft-lower structure; otherwise, a soft-upper and hard-lower structure will be formed.

[0012] The present invention proposes an external field induced forming method for a stiffness gradient adhesion structure, which has a simple process route, does not require expensive processing equipment and complex process control, reduces manufacturing costs, and improves processing efficiency; the micro-micro or micro-nano two-level structure prepared by the method can be widely used in belt conveyors, manipulators, micro suction cups, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a schematic structural diagram of the induced upper electrode plate of the present invention.

[0014] Figure 2 The schematic diagram is a structural diagram of preparing a layer of polymer material on a substrate according to the present invention.

[0015] Figure 3 This is a schematic diagram of the present invention using external pressure to press the induction template onto the substrate through the dielectric support structure.

[0016] Figure 4 Schematic diagram of the rheological mechanism of the micro-nanostructure induced by electric field in the present invention.

[0017] Figure 5 It is a schematic diagram of the formation of mushroom structure of the polymer of the present invention after effective wetting is formed upon contact with the upper electrode plate.

[0018] Figure 6 The schematic diagram of the upper soft and lower hard structure obtained by removing the upper electrode plate after curing of the present invention.

[0019] Figure 7 It is a schematic diagram of the upper hard and lower soft structure obtained by removing the upper electrode plate after curing of the present invention. DETAILED DESCRIPTION

[0020] The present invention is described in detail below with reference to the accompanying drawings.

[0021] An external field induced forming method for a stiffness gradient adhesion structure comprises the following steps:

[0022] The first step is to prepare the upper electrode plate: a layer of low surface energy dielectric material 2 with a thickness of micrometer or nanometer is prepared on the surface of the first conductive substrate 1 by spin coating process, such as Teflon solution, and then a part of the dielectric layer is removed from the surface of the first conductive substrate 1 to lead out the conductive tape, such as Figure 1 As shown;

[0023] The second step is the selection and treatment of the substrate: a second conductive substrate 4 is used as the substrate, and a layer of polymer material 3 mixed with dielectric nanoparticles with a thickness of micrometers or nanometers is prepared on its surface by spin coating process, such as Figure 2 As shown;

[0024] The third step is to induce the soft-hard composite structure into rheological shape by electric field: Figure 3 As shown, applying pressure will press the induced upper plate onto the insulating support structure 5, such as a PI film; applying a high DC voltage will cause the electric field force on the polymer and dielectric nanoparticles to overcome the surface tension and viscous resistance rheology, and grow a layer of microstructure according to the most unstable wavelength; wherein, by controlling the relative dielectric constant matching of the nanoparticles and the polymer, a micro-nanostructure with different distributions of internal nanoparticles can be obtained; when the dielectric constant of the nanoparticles is greater than the dielectric constant of the polymer, the nanoparticles will be concentrated on the upper side of the formed structure to form a hard-upper and soft-lower structure; otherwise, a soft-upper and hard-lower structure will be formed; thereafter, the voltage is continued to be applied until the polymer contacts the induced upper plate, wets on the surface of the low surface energy dielectric material 2, and spreads to obtain a mushroom structure, such as Figure 4 , Figure 5 As shown;

[0025] The fourth step is the curing and demolding of the polymer material: the polymer is cured while maintaining the voltage unchanged. After the structure is completely cured, the induced upper electrode is removed to obtain a micro-micro or micro-nano stiffness gradient structure.

[0026] The size of the stiffness gradient adhesion structure achieved by the above method is closely related to the magnitude of the applied voltage U and the height of the dielectric support structure 5:

[0027] Influence of voltage: As the voltage increases, the distribution density of nanoparticles inside the structure increases, the contact wettability improves, and the brim of the mushroom-shaped structure increases;

[0028] Influence of the height of the dielectric support structure 5: As the height of the dielectric support structure 5 increases, the depth-to-width ratio of the formed column increases, the overall modulus gradient increases accordingly, and the corresponding adhesion performance increases accordingly.

[0029] A certain pressure P is applied to make the induction upper electrode plate contact the substrate through the dielectric support structure 5. The polymer material before curing has fluidity. When a suitable electric field is applied, the polymer begins to flow under the action of the electric field. Before the polymer contacts the induction upper electrode plate, the polymer material grows according to the most unstable wavelength to obtain a microstructure, such as Figure 4 As shown; when the polymer contacts the induced upper plate, the polymer material spreads out under the effect of interfacial wetting of the low surface energy dielectric material 2 to obtain a mushroom brim structure, as shown Figure 5 shown.

[0030] Due to the different dielectric constants of dielectric nanoparticles and fluid polymers, the growth rates and distribution conditions of the two substances under the action of the electric field are also different; the larger the dielectric constant, the faster the growth rate; under the same growth time conditions, the two substances will have different gradient distribution states; when the dielectric constant of dielectric nanoparticles is smaller than the dielectric constant of polymers, dielectric nanoparticles will generally be concentrated in the lower half of the mushroom structure, forming a mushroom structure with a soft upper and hard lower stiffness gradient distribution, such as Figure 6 As shown; when the dielectric constant of the dielectric nanoparticles is greater than the dielectric constant of the polymer, the dielectric nanoparticles will generally be concentrated in the upper part of the mushroom structure, forming a mushroom structure with a hard upper part and a soft lower part and a gradient stiffness distribution, as shown Figure 7 shown.

[0031] Finally, the polymer is solidified and the induction upper electrode is removed to obtain a micro-micro or micro-nano stiffness gradient adhesion structure. The adhesion structure prepared by this method overcomes the equipment and material limitations of conventional processes and can obtain a large-area stiffness gradient structure.

[0032] The present invention overcomes the limitations of the complex process and expensive processing equipment in the traditional preparation method, and can achieve one-time molding of polymer materials to obtain a micro-micro or micro-nano stiffness gradient structure.

Claims

1. A method for external field induced forming of a stiffness gradient adhesion structure, characterized in that: The following steps are involved: The first step is to prepare the induction upper electrode: a layer of low surface energy dielectric material (2) with a thickness of micrometer or nanometer level is prepared on the surface of the first conductive substrate (1); The second step is the selection and treatment of the substrate: using the second conductive substrate (4) as the substrate, and preparing a layer of polymer material (3) mixed with dielectric nanoparticles with a thickness of micrometers or nanometers on the surface thereof; The third step is to induce the rheological formation of the soft-hard composite structure by electric field: applying pressure to press the induction upper electrode plate onto the insulating support structure (5), applying a DC high voltage, so that the electric field force on the polymer and dielectric nanoparticles overcomes the surface tension and viscous resistance to flow, and grows a layer of microstructure according to the most unstable wavelength; then, continue to apply voltage until the polymer contacts the induction upper electrode plate, wets the surface of the low surface energy dielectric material (2), and spreads to obtain a mushroom structure; The fourth step is the curing and demolding of the polymer material: the polymer is cured while maintaining the voltage unchanged, and the induced upper electrode is removed, thereby obtaining a micro-micro or micro-nano stiffness gradient structure; In the third step, by controlling the matching of the relative dielectric constants of the nanoparticles and the polymer, a micro-nano structure with different distributions of nanoparticles inside is obtained. When the dielectric constant of the nanoparticles is greater than the dielectric constant of the polymer, the nanoparticles will be concentrated on the upper side of the micro-nano structure to form a structure with a hard top and a soft bottom; otherwise, a structure with a soft top and a hard bottom is formed.

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