Adhesion strategies for climbing robots typically prioritize maximizing adhesive force for secure gripping, leading to high friction and requiring multiple adhesive pads with intermittent detachment and reattachment, resulting in a discrete "walking" movement. This approach presents shortcomings including complex control systems, heavy transmission structures, energy waste, and reduced adhesive force during movement. Snails, however, employ a different strategy: sliding suction. They maintain a high suction force while sliding, a phenomenon seemingly contradictory to typical suction cup behavior. This study investigates the role of mucus in reducing friction and enhancing suction in snails, aiming to replicate this mechanism using readily available water as an artificial lubricant. The central challenge involves designing materials and structures to efficiently introduce the liquid film at the suction interface while maintaining effective suction.

Existing research focuses on enhancing suction adhesion, often involving lifting the suction pad to minimize friction, which results in leakage and high energy consumption from centrifugal pumps. Other studies explored using water as a vortex medium, although leakage remained an issue. A gel-enhanced snail-like robot showed climbing capabilities, but adhesion was limited by the gel's stickiness, and inverted adhesion was not achieved. Previous attempts using silicon oil as a lubricant did not achieve autonomous long-distance sliding in dry environments and resulted in substrate contamination. This work directly addresses these limitations by proposing a water-enhanced sliding suction mechanism that combines effective suction with low friction.

The research explores the principle of sliding suction by examining the role of friction at the suction interface. Snails use mucus to reduce friction and enhance suction; water serves as an analogous lubricant in this study. Hydrophilic silicone, fabricated by adding poly(dimethylsiloxane-b-ethylene oxide) (PBP) to plain silicone, is used to create a suction cup bottom pad that promotes water self-spreading via capillary action. The coefficient of friction (CoF) was measured under various conditions (dry, wetted by water, wetted by detergent solution) for hydrophobic and hydrophilic silicone pads. A sliding suction cup with a top PU reinforcement layer and a PBP-silicone pad was designed and tested. Experiments evaluated sliding suction under different conditions (dry, immersed in water, immersed in detergent solution) with a linear stage applying tangential force. The influence of perpendicular pulling force and substrate materials on sliding friction was investigated. A sliding suction robot (SSR) was designed, incorporating a water secreting system using super absorbent (SA) foam and silicone tubes to deliver water to the suction interface. The SSR's physical model was developed, considering factors like static and kinetic friction forces, water secretion rate, and the influence of gravity. Experiments included rotation and translation tests on a tilted PMMA sheet at various angles (0°, 45°, 90°, 135°, 180°), as well as payload sliding tests with a 1kg mass. Real-world applications were demonstrated through climbing tests on glass and painted metal walls, and user-controlled weight transportation with obstacle avoidance.

The hydrophilic silicone with 2% PBP demonstrated a significant friction reduction (up to 94%) compared to hydrophobic silicone when wetted with water. The water-enhanced sliding suction cup maintained suction while sliding under various conditions, with minimal friction force compared to a dry state. The kinetic friction force increased slightly with increasing perpendicular pulling force, indicating the high load-carrying capacity. The SSR successfully achieved rotation (53°/s) and translation (19 mm/s) at various tilt angles, including upside down. The SSR demonstrated high payload capability, carrying a 1kg mass (10 times its own weight). The water film left behind evaporated quickly, leaving no residue. The SSR demonstrated successful untethered climbing on both glass and metal walls, and user-controlled weight transportation with obstacle avoidance. Movement deviations were observed due to factors like tyre slip and insufficient wetting, but the SSR remained stable. Gravity and robot weight influenced translational movement, with upward climbing being more challenging than downward sliding. The maximum theoretical payload capacity was calculated to be over 460N (480 times its own weight), while the measured maximum static suction force was 50.3N (52 times its own weight). The maximum power consumption was 1.7W, with zero power consumption during static adhesion.

The findings demonstrate the feasibility and advantages of the water-enhanced soft sliding suction mechanism. The high payload capacity, low energy consumption, and continuous sliding motion represent significant advancements over traditional climbing robot designs. The ability to achieve stable adhesion and controlled movement on various surfaces, including upside down, highlights the potential for versatile applications. The successful real-world demonstrations further validate the practicality of this approach. The slight movement deviations observed highlight opportunities for future improvements through closed-loop control and sensor integration.

This research presents a novel water-enhanced soft sliding suction mechanism for climbing robots, inspired by snail locomotion. The resulting robot demonstrates high payload capacity, low energy consumption, and stable operation on various surfaces. Future work should focus on improving the robot's performance on rough surfaces, addressing the limitations related to step changes in surface topography, and incorporating sensors for autonomous control. This technology holds significant promise for diverse robotic applications.

The current SSR design is optimized for relatively smooth surfaces. Rougher surfaces can lead to leakage and increased friction, requiring improvements like a small vacuum pump for leakage compensation. The SSR currently cannot traverse step-like changes in surface height. Future iterations may incorporate multiple suction cups or other mechanisms for negotiating such obstacles. The observed movement deviations highlight the need for advanced sensor integration and closed-loop control for precise and autonomous operation.