Ultra-High Friction and Adhesion in Hydrogel Layer Driven by Wet-to-Dry Transition Dynamics.

Hydrogels are well-known for their antifriction and lubricating properties, particularly in hydrated environments, where their water-rich polymer networks enable effective friction reduction. However, during the wet-to-dry transition, a critical phase is identified during which the microstructures of a polyacrylamide hydrogel layer undergo volumetric shrinkage, leading to extensive interfacial contact and enhanced intermolecular interactions at solid interfaces. This process leads to a sharp increase in friction and adhesion forces. Sliding friction tests show that under a 2 mN load, the shear force peaked at 115 mN, corresponding to a remarkably high friction coefficient of 57.5. By leveraging this wet-to-dry transition, strong object gripping is successfully achieved across a range of surfaces. Notably, the hydrogel layer exhibits a high adhesion strength of 3.48 MPa on glass and 3.64 MPa on Si substrates. These findings offer new insights into hydrogel active gripping technologies and provide promising implications for soft robotics and adhesive interfaces.