Analysis And Numerical Simulation Of The Liquid Film Characteristics In Non-Newtonian Fluid Dip Coating Process

In recent years,the dip coating method,due to the advantages of simple process operation and various optional substrate shapes,has been gradually applied to many fields such as electronics,environmental protection,decoration and so on.With the continuous improvement of requirement for the functionality of film materials in the modern industry,stricter requirements have also been imposed on the dip coating process.In this paper,the substrate stress,liquid film thickness,free surface morphology and fluid flow in terms of the lifting speed,substrate roughness and fluid properties were studied systematically and systematically in a combination of experiment and simulation.This study could provide theoretical foundation for for optimizing the coating process.The main contents are as follows:Newtonian fluid(Glycerol)and non-Newtonian fluid(characterized by shear-thinning fluid,silicone oil and yield stress fluid,polyacrylic acid hydrogels)were used as experimental fluids,and three steel plates with different roughness were employed as the substrate.The physical and rheological properties of fluids were measured.A special fixture device was used to connect the substrate to the rotating rheometer,and the dip coating experiment was controlled automatically.The variation of normal force on the substrate with the lifting height was measured by the force sensor on the rotating rheometer.The film thickness was measured by capacitance micrometer.The results show that the dip coating process could be divided into four stages according to the change of normal force.For the second stage of the substrate rising at a constant speed,the force on the substrate was linearly proportional to the lifting height,and the slope was directly related to the fluid viscosity and the thickness of the liquid film on the substrate.Through the force analysis,the law of the normal force curve of the substrate under different conditions was studied.Both the normal force and the liquid film thickness on the substrate increase with the increase of lifting speed,substrate roughness,fluid viscosity or yield stress.Through the analysis of the normal force at third and fourth stages,a new method for measuring the surface tension of a fluid was popsoed,and the appropriate test conditions for this method were determined.The results showed that when the substrate with a roughness(Ra)of 272.4 nm was used,the test was performed in the range of 0.05-0.3 mm/s,and the error between the new method and the result measured by the surface tension meter was less than 10%.A high-speed camera was used to monitor and record the film thickness and free surface morphology under the effects of different lifting speeds and fluid viscosity during dip coating process.The particle tracer technique was used to study the fluid flow field.The polyflow software was used to simulate the fluid velocity field during the coating process,and the coupling mechanism between the film thickness and the fluid flow field was explored.The results indicate that the simulation results were in good agreement with the experimental results under the same conditions,which proved the accuracy and validation of the numerical simulation.With the increment of lifting speed and viscosity,the liquid film thickness increases,the degree of free surface depression aggrandizes,and the curvature of dynamic meniscus area decreases.The augment of lifting speed also led to a reduction in the area near the dynamic meniscus,and an acceleration of the internal circulation of the fluid in the tank,which was the opposite of the effect of viscosity.The surface fluid near the edge of the substrate tends flowed toward the middle of the substrate,and the lifting speed hardly altered its horizontal velocity,while was proportional to the vertical downward velocity.