Anisotropic Friction of Epoxy-Bydrogel Based Surfaces with Soft-Hard Combination

Anisotropic friction caused by oriented microstructure is one of the most common causes in biological systems. Typical examples like the snake locomotion with the aid of highly ordered fiber-like microstructures in ventral body side, the wheat awns propel the seeds on and into the ground with the help of silicified hairs that cover the awns, and the tree frogs have excellent wet attachment and friction performance can freely and repeatedly climb on vertical surfaces or overhang with the help of hierarchical pillar arrays. It has been demonstrated that the biological surface of soft-hard combination exhibits the more advantages for obtaining the anisotropic friction force, especially for the combination of hard structure and soft substrate. Even though the research of anisotropic friction based on the surface microstructures has obtained significant progress, the complicated preparation method, the little-span of modulus variation, and the unobvious switching of friction force are still limited the development of this project. Therefore, the hook-like spines, as one of the most effective topographies from various plants and animals for generating anisotropic force, were employed in this study. Furthermore, take advantages of additive manufacturing with the merits of creating sophisticated, bespoke and low-cost materials/devices, the surface with hook-like spines was prepared by a digital light process (DLP) 3D printer. And the optical microscope images displayed the printed surface was covered with hook-like spines exhibiting the same orientation and morphology, which were necessary for exploring the anisotropic friction force. In addition, for mimicking the typical combination of soft-hard combination and obtaining the dynamic friction force, the low modulus hydrogel was grown in-situ on the bionic oriented microstructures. To further enhance the mechanical performances, the prepared epoxy-hydrogel samples were immersed in Fe³⁺ solution for a secondary crosslinking by ionic coordination. The results demonstrated that with the increase of secondary cross-linked time, the hydrogel had an increased modulus. Integrating with the element of ruptured strain, the immersed time of 6 h was chosen with the modulus of 3.6 MPa and the ruptured strain of 288%. Comparing with epoxy resin, whose modulus was 13.6 MPa, the enhanced hydrogel was still a soft material. Finally, the anisotropic friction force of the prepared biomimetic epoxy-hydrogel surface under different conditions were investigated. Typically, when the normal load was increased from 1 N to 5 N, the negative friction force was greater than that of the positive friction force gradually. Importantly, with the normal increasing, the corresponding anisotropic friction force exhibited the larger difference. For example, under the 5 N load, the positive friction force of the biomimetic epoxy-hydrogel surface was 1.64 N, while the opposite sliding direction was 3.44 N, which showed the largest anisotropic friction difference. In this way, we demonstrated the prepared soft-hard surface had an isotropic friction force between positive and negative sliding directions at the lower normal load, while the friction force became anisotropic with the normal load increasing (>4 N). This study had a certain theoretical significance for the biological surface with the soft-hard combination, and was expected to play a certain role in intelligent frictional control, smart actuators and microbots, and some other friction-induced/controlled devices.