Three-Dimensional Sag Tracking in Falling Liquid Films

Coating defects often arise during application in the flash stage, which constitutes the ∼10 min interval immediately following film application when the solvent evaporates. Understanding the transient rheology and kinematics of a coating system is necessary to avoid defects such as sag, which results in undesirable appearance. A new technique named variable angle inspection microscopy (VAIM) aimed at measuring these phenomena was developed and is summarized herein. The essence of this new, non-invasive, rheological technique is the measurement of a flow field in response to a known gravitational stress. VAIM was used to measure the flow profile through a volume of a liquid thin film at an arbitrary orientation. Flow kinematics of the falling thin film was inferred from particle tracking measurements. Initial benchmarking measurements in the absence of drying tracked the velocity of silica probe particles in ∼140 μm thick films of known viscosity, much greater than water, at incline angles of 5° and 10°. Probe particles were tracked through the entire thickness of the film and at speeds as high as ∼100 μm/s. The sag flow field was well resolved in ∼10 μm thick cross sections, and in general the VAIM measurements were highly reproducible. Complementary profilometer measurements of film thinning were utilized to predict sag velocities with a known model. The model predictions showed good agreement with measurements, which validated the effectiveness of this new method in relating material properties and flow kinematics.