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Uncover rock-climbing fish's secret of balancing tight adhesion and fast sliding for bioinspired robots - PubMed

  • ️Sun Jan 01 2023

Uncover rock-climbing fish's secret of balancing tight adhesion and fast sliding for bioinspired robots

Wenjun Tan et al. Natl Sci Rev. 2023.

Abstract

The underlying principle of the unique dynamic adaptive adhesion capability of a rock-climbing fish (Beaufortia kweichowensis) that can resist a pull-off force of 1000 times its weight while achieving simultaneous fast sliding (7.83 body lengths per second (BL/S)) remains a mystery in the literature. This adhesion-sliding ability has long been sought for underwater robots. However, strong surface adhesion and fast sliding appear to contradict each other due to the need for high surface contact stress. The skillfully balanced mechanism of the tight surface adhesion and fast sliding of the rock-climbing fish is disclosed in this work. The Stefan force (0.1 mN/mm2) generated by micro-setae on pectoral fins and ventral fins leads to a 70 N/m2 adhesion force by conforming the overall body of the fish to a surface to form a sealing chamber. The pull-off force is neutralized simultaneously due to the negative pressure caused by the volumetric change of the chamber. The rock-climbing fish's micro-setae hydrodynamic interaction and sealing suction cup work cohesively to contribute to low friction and high pull-off-force resistance and can therefore slide rapidly while clinging to the surface. Inspired by this unique mechanism, an underwater robot is developed with incorporated structures that mimic the functionality of the rock-climbing fish via a micro-setae array attached to a soft self-adaptive chamber, a setup which demonstrates superiority over conventional structures in terms of balancing tight underwater adhesion and fast sliding.

Keywords: Stefan adhesion; bioinspired; crawling robots; rock-climbing fish; underwater adhesion; underwater robots.

© The Author(s) 2023. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd.

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Figures

Figure 1.
Figure 1.

Adhesion and crawling ability, and mechanism of the rock-climbing fish and biomimetic prototype design. (A) Among the adhering species, the rock-climbing fish can generate great adhesion force with amazing crawling ability underwater. (B) The setae array on the edge region of the suction cup conforms to the substrate and forms a sealing chamber through the Stefan force; the inner cavity of the suction cup presents a pressure change due to the volume change when the pull-off force is imposed. (C) The mechanism for generating negative pressure adhesion: when the pull-off force occurs, the micro-setae at the edge of the suction cup are kept in contact with the substrate by force ‘FA-setae’ due to hydrodynamic interactions, and the suction cup keeps sealed during the pull-off process. However, the suction cup deforms to increase its volume and decrease its inner pressure, generating negative pressure adhesion to resist the pull-off force throughout the process. (D) A bionic underwater crawling/climbing fish named Climbot was designed based on the adhesion mechanism of the fish, and can crawl at a maximum speed of 3.7 BL/S and has an adhesion force of 25.67 ± 2.81 N.

Figure 2.
Figure 2.

The adhering-sliding synergistic process is shown by the contact zone and propelling flow field image during starting and stopping. (A) The starting movement of the rock-climbing fish. (i) T = 0 ms, initial state, the fish stay static; (ii) T = 350 ms, start-up is realized via cooperation between adhering and swinging the caudal fin. (iii) The flow field after the caudal fin swings at T = 350 ms, containing three swings. (B) The stopping movement of a rock-climbing fish. (i) T = 0 ms, initial state, the fish is about to brake during sliding. (ii) T = 215 ms, stopping is realized by increasing adhesion via enlarging the suction cup area and the forepart of the suction cup first adhering to the substrate, followed by the posterior half. (iii) The flow field around the caudal fin at T = 215 ms.

Figure 3.
Figure 3.

Characterization of the morphology of the rock-climbing-fish suction cup, and its adhesion characteristic. (A) Bottom view of the fish suction cup. (B and C) Setae morphology. (D) Microscope view of setae adhesion measurement with atomic force microscope (AFM). (E) Force-displacement curve of setae adhesion measurement with AFM shows that there is no jump in the trace curve, indicating that there is no attraction force such as van der Waals force or electrostatic force, only adhesion force. (F) Variation of adhesion with approach velocities of AFM tip-less cantilever; the adhesion force increases with the AFM probe speed. The retrace curve shows the adhesion force from setae. The trace curve shows the adhesion force from a flat surface (petri dish). (G) The friction of fish under different pulling forces. The pulling forces are provided by the normal load of weights.

Figure 4.
Figure 4.

Simulation of setae hydrodynamic adhesion. (A) Schematic for the topographical features and the adhesion testing process. (B) The analytic solution of the force-displacement curve is obtained from Equation S5 in the online supplementary data. (C) The analytic solution of adhesion vs. separating speed obtained from Equation S5. (D) The simulation result of adhesion between setae and a moving plate. When the plate is just separating from the four setae, and a negative pressure area is generated (pressure reaches −1.63 kPa), this negative pressure causes the adhesion force. (E) The plate lifts to 5 μm, and the pressure goes to −0.2 Pa.

Figure 5.
Figure 5.

Adhesion ability of Climbot, and outdoor experiments. (A) Biomimetic setae. The length of the biomimetic setae is 15 μm, and the cylinder setae diameter is 12 μm. (B) The force-displacement curve of setae and the flat substrate; the biomimetic setae show a better adhesion performance than the flat base. (C) The adhesion force and work contrast the flat base and setae structure; the adhesion force of the setae structure is more stable than the flat base, and the adhesion force of the biomimetic setae is 42.7%. The adhesion work of the biomimetic setae is 11.9 times stronger than that of the flat base. (D) The friction comparison between a bionic underwater crawling robot with a setae structure, and one without a setae structure, with a different load, shows that the former has lower friction. (E) An adhesion comparison between a bionic underwater crawling robot with a setae structure and one without a setae structure shows that the former has higher adhesion. (F) Climbot can slide on the bottom lateral surfaces of the tank. (G) Climbot attached tightly to a model ship while the vessel was sailing on a river. (H) Climbot was directed to move on the bottom surface of a model ship (Mann−Whitney-Wilcoxon test, *** P < 0.001).

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