Pattern formation in drying of particle-laden sessile drops of polymer solutions on solid substrates

The drying of particle-laden sessile drops of water or polymer solutions was investigated experimentally. The contact angle of water on the solid surface was 45°. Particles used in the experiment were polystyrene particles of 1 and 6 μm and hollow glass spheres of 9–13 μm. As the polymers polyethylene oxide of 200 and 900 kDa and xanthan gum were used. Depending on particle size and fluid viscosity, the drying pattern varied and there was a competition of the inward movement due to the capillary and/or buoyant force acting on the particles trapped at the liquid interface and the outward movement due to the Deegan flow. The typical ‘coffee ring’ of colloidal particles was not always observed. Either inward or outward motion was observed depending on low shear viscosity. Elasticity does not appear to change the Deegan flow qualitatively. However, elasticity appears to change the pinning characteristics of the contact line.