Visualization of surface deformations during thin film drying using a Digital-Image-Correlation method

In this paper, an optical on-line measurement technique for the topography of thin polymer films during the drying process is presented. Based on the refraction of light at the interface between two transparent fluids (cf. free-surface synthetic schlieren method), this technique allows the indirect measurement of the surface slope and thus the time-resolved topography of liquid films from the displacement field of the refracted image of a random pattern below the film using a Digital-Image-Correlation algorithm. A comparison of the topography reconstructed (calculated) from the last picture and a profilometer measurement of the dry film showed that the numerical reconstruction based on linearization of the 2-D nonlinear equation can follow the time-dependent deformation of the drying polymeric film. Thus this measurement technique is a powerful and practical tool for the theoretical understanding of fluid motions and the developing surface topographies during the drying process of polymer solutions.