Nucleation kinetics and virtual melting in shear-induced structural transitions

Large shear deformations can induce structural changes within crystals, yet the microscopic kinetics underlying these transformations are difficult for experimental observation and theoretical understanding. Here, we drive shear-induced structural transitions from square (square) lattices to triangular (triangle) lattices in thin-film colloidal crystals and directly observe the accompanying kinetics with single-particle resolution inside the bulk crystal. When the oscillatory shear strain amplitude 0.1 <=gamma(m)<0.4, triangle-lattice nuclei are surrounded by a liquid layer throughout their growth due to localized shear strain at the interface. Such virtual melting at crystalline interface has been predicted in theory and simulation, but have not been observed in experiment. The mean liquid layer thickness is proportional to the shear which can be explained by the Lindemann melting criterion. This provides an alternative explanation on virtual melting.