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First Thermal and Fluids Engineering Summer Conference

ISSN: 2379-1748
ISBN: 978-1-56700-430-4

MOTION OF BUBBLES AND PARTICLES IN A DRYING PAINT LAYER DUE TO SURFACE-TENSION-DRIVEN FLOWS

DOI: 10.1615/TFESC1.mnp.012579
pages 1573-1582

Nazli Saranjam
Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Ontario, Canada

Sanjeev Chandra
Center for Advanced Coating Technologies, Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Ontario, Canada, M5S 3G8

Javad Mostaghimi
Center for Advanced Coating Technologies, Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Ontario, Canada, M5S 3G8


KEY WORDS: Mass transfer and drying, evaporation, two-phase flow, surface-tension-driven instability

Abstract

Paint films with thicknesses ranging from 150 to 1000 µm were applied on glass substrates using a paint applicator with a knife edge that created a layer of uniform thickness. A model paint was developed for the experiments with known solvent and resin compositions and the samples were cured in a natural convection oven at temperatures varying from 100-200°C. Paint weight loss during curing was measured and photographs of the paint surface were taken during curing. Photographs of the surface of drying paint films showed regular undulations appearing on the surface. This is believed to be induced by surface-tension-gradients that occur as soon as the curing/drying process begins. Tests were done with either bubbles or tracer particles, 10-300 µm in diameter, introduced in the paint films. The motion of the particles or bubbles was photographed and particle image velocimetry (PIV) software used to determine the trajectory and velocity of particles. An analytical solution of the mass-diffusion equation was used to model solvent evaporation from the paint film surface and to calculate the mass diffusivity of the solvent. The model allowed calculation of the concentration gradient and therefore surface tension variations in the paint films. Particles and bubbles were observed to move towards each other in the paint films, forming self- assembling agglomerations in the dried paint films.

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