(a) A liquid Gallium droplet is attached to an elastomeric PDMS post and brought in contact with a flat smooth glass substrate. (b) The thin Galliumoxide skin is conformed to the smooth glass surface when mechanical load is applied. Wrinkling instability emerges at the contact interface to relieve the applied stress and radial wrinkles evolve under increased load.
Biological system can reversibly adhere to and detach from many uneven terrain. For instance, tree-frog adhesive pad can adhere to smooth, rough, wet, and dry surfaces. Most of the synthetic adhesive systems are only suitable for smooth and dry surfaces. To overcome the challenges facing the synthetic adhesives, Gallium (Ga) liquid metal offers a promising performance due to its phase change property at low temperature (30$^{\circ}$C). Ga changes phase from liquid to solid isothermally at room temperature. The mechanism of liquid state adhesion of Ga resembles that of tree-frog pad, which is a very compliant system.
In this study, we present the mechanical behavior of the highly bendable native oxide skin on the surface of Ga droplet when compressed against a rigid flat substrate as it exhibits a fascinating wrinkling phenomenon, and its implications on the adhesion energy necessary to separate the interface. The applied compressive stress at the contact interface is relieved by the Ga2O3 sheet, transitioning from a circular into a radial wrinkled state (Figure b). The circular wrinkles enhance fracture strength by trapping cracks that propagate along the radial direction. The fundamental response explored in these studies is the out-of-plane wrinkling that occurs to alleviate compressive stresses. Wrinkling has implications in morphogenesis, interfacial-crack trapping similar to frog adhesive pad to enhance adhesion, and functional optical and electronic materials. Ga based liquid metals are attractive for a broad range of applications including flexible or stretchable sensors, robotics, microelectronics, 3D printing, and next generation nanomechanical devices. Additionally, we can predict when the oxide nanofilm ruptures and leaves residue on the surface, which may further facilitate cost effective room-temperature 2D patterning of liquid Ga for future semiconductor applications. Our findings provide the tools for the precise control of liquid metal interfaces.