Optimal functional surface coating considering internal flow behavior of viscous-liquids driven by vertical ultrasonic waves

Abstract

In ultrasonic vibration-assisted coatings, the verification of the surface roughness formation induced by the internal waves and the investigation of the maximum roughness position play a very important role and significantly contribute to the development of coating technology and to the comprehension of the induced internal flows. Therefore, this study aimed to verify the roughness formation process and to investigate the relationship between the liquid film thickness, the initial viscosity, and the applied frequency in order to fabricate the functional surface with the maximum hydrophobicity or hydrophilicity. A two-dimensional (2-D) numerical approach using a moving-mesh and non-Newtonian model with a multi-physics program (COMSOL) was applied to investigate the dynamic behavior, the velocity profile and the surface roughness of internal flow. Also, experiments were performed to investigate the existence and position of the maximum peak velocity using scanning electron microscopy (SEM) images and measured surface-roughness data. It was concluded that the surface roughness generated by the initial viscosity was remained despite increased viscosity when the ultrasonic vibration was applied for a long time. The developed velocity profile had a velocity gradient overshoot and the maximal internal flow velocity developed at the thickness of about 1.2-fold of the viscosity boundary layer edge. The applied frequency, thickness of the film and initial viscosity dominantly affect the position of maximum peak velocity related to the viscous boundary layer and the performance of the liquid film coating. © 2021 The Author(s)