The normal force evolution of the substrate during the withdrawal process

The evolution of normal force on the smooth plate immersed in Newtonian and yield stress fluids was investigated during the dip coating. The influences of withdrawal velocity, liquid rheological property, and plate roughness were elaborated. For all fluids, the variation of force versus the withdrawing height was divided into four stages. Importantly, the control forces for coating in each stage were determined, which are in turn viscous force-capillary, viscous force-gravity, capillary-viscous force, and gravity. The critical value of normal force controlled from capillary to viscous effect during drainage was adopted to calculate the surface tension. The possibility of measuring the surface tension was verified under the negligible interference of viscous effect and film instability through a detailed analysis on the evolution of force in a series of tests. Accordingly, the applicable conditions with the low withdrawal speed, high viscosity, and rough surface were finally achieved. This work can provide a theoretical basis for the film-formation mechanism and surface tension measurement in dip coating. KEYWORDS coatings, shear, tension, viscosity, yielding
1 | INTRODUCTION Coating a layer of thin film on a solid surface is commonly encountered in industrial production and application, such as environmental protection, medicine, spinning, material protection, magnetization, [...]