Our group has shown reversible and switchable adhesion of liquid Gallium (Ga) droplets on smooth, rough, wet, and dry surfaces for the first time [ ]. Phase-change ability and low melting point (30℃) of Ga enable conformal adhesion at the warm liquid state, strong adhesion in the cooled solid state, and easy detachment after going back to the liquid state above 30℃.
Although substrates play an important role upon crystallization of supercooled liquid Ga, the influence of surface temperature and thermal property of substrate materials have remained elusive. Using seeding method, the crystallization of supercooled phase-change Ga on substrates with different thermal conductivities is investigated. In liquid state, the mechanical deformation of the highly bendable native oxide skin on the surface of Ga droplet leads to the formation of wrinkles when brought into contact with a rigid substrate [ ]. The applied compressive stress at the interface is relieved through the native Ga2O3 sheet, transitioning from a circular to a radial wrinkled state. The circular wrinkles enhance fracture strength by trapping cracks that propagate along the radial direction.
During crystallization, the interfacial temperature appeared to be critical on the crystallization kinetics, which dictates the thermo-mechanical stresses between the substrate and the crystallized Ga. The spatiotemporal evolution of the crystallization dynamics was visualized on transparent glass, indium tin oxide coated glass, and zinc selenide substrates [ ]. At an elevated surface temperature, close to the melting point of Ga, an extended single-crystal growth of Ga on dielectric substrates was realized due to layering effect and annealing. The adhesive strength at the interfaces depends on the thermal conductivity and the initial surface temperature of the substrates.
We demonstrate the potential use of Ga adhesive by performing pick-and-place operations that can be implemented in broad adhesive applications including transfer printing, reconfigurable and climbing robots, electronic packaging and biomedicine. This insight can be applicable to other liquid metals for industrial applications, and sheds more light on phase-change crystallization.