Doctor Blade Coating Technique

In doctor blade coating, the film thickness and morphology are jointly governed by the coordination of the upstream and downstream menisci.

The upstream meniscus, through the straightness of the wetting line, dominates the lateral spreading of the solution and limits the coating speed. The downstream meniscus controls the final film thickness through the balance between shear flow and capillary forces.

The Bond number (Bo) is the key criterion:

☆ When Bo < critical value (surface tension-dominated), the meniscus remains vertically stable.

☆ When Bo > critical value (gravity-dominated), the liquid becomes unstable and may detach.

Process parameters—including blade-to-substrate gap, coating speed, solution properties, and temperature—affect coating quality by influencing the Bond number and fluid dynamics.

The Bond number (Bo) is a dimensionless parameter that characterizes the relative importance of gravitational forces to surface tension. It is widely used to analyze liquid wetting and deformation behavior on solid surfaces.

In coating processes, it is used to evaluate liquid bridge stability, meniscus deformation, and coating uniformity.