A bilayer color digital image correlation method for the measurement of the topography of a liquid interface

One of the most essential characteristics of the liquid interface is the topography dominated by hydrodynamics or capillary effects. Although there have developed 3D imaging techniques for the measurement of such topography, their setups, operations and reconstructions are relatively complex and suffer from low efficiency. Here we report a bilayer color digital image correlation (BC-DIC) method for the measurement of the topography of a liquid interface. The basic principle of the new method is the refraction of beneath bilayer color speckle patterns at the liquid interface, which makes it possible to perform 3D reconstruction through 2D measurement of the virtual displacement field of the speckle patterns. The equations for BC-DIC in different situations are deduced and discussed in detailed. Validation experiments are carried out to reconstruct the topography of triangular prism, planoconvex, complex transparent surface and sloshing water surface. The results show that BC-DIC is feasible and accurate for measuring the topography of transparent objects, including liquid interface. This new method paves the way for investigating surface and interface phenomena, such as capillarity and wetting issues, hydrodynamics on liquid interface, etc.