Evaporation of water droplets on polymer surfaces

The evaporation of water droplets on polymer surfaces was investigated by using a digital image analysis technique. There were three distinct stages in the water evaporation process: a constant contact area mode, a constant contact angle mode, and a mixed mode that is independent of both the initial quantity of water droplets and the hydrophobic properties of the polymer surfaces. The physical factors influencing the first and second transitions in the evaporation process were found to be the attainment of the receding angle on the polymer surfaces and the Marangoni instability in the evaporating water droplets, which result from the concentration gradient of contaminants. This study also provides qualitative information about the microfluid flows inside the evaporating water droplets and the morphology of drying stains on polymer surfaces. The contaminants were found to be concentrated at the perimeter of the stains, in agreement with the observed outward microfluid flow in the mixed mode of the evaporation process.