Stable and Unstable Marangoni Flows in Drying Droplets Measured by Particle Image Velocimetry

We use particle image velocimetry to measure the flow fields in a small, 2 mm-radius sessile droplets of water, ethanol, octanol, and water/ethanol mixtures. We observe regular axisymmtric circulatory flows in water, ethanol, and octanol droplets, and irregular circulatory flows in water/ethanol mixtures above an ethanol concentration of 0.2%. Both the speed and direction of circulatory flow vary widely depending on the fluid, with an apparent thermal Marangoni flow driving strong circulation from the edge of the droplet towards the top in the case of ethanol and octanol droplets, and a reverse circulatory flow from the top to the edge of the droplet in the case water and water/ethanol mixtures with low ethanol concentration. The flow velocities vary enormously, from only 5 microns/sec for a drying water droplet to 1 mm/sec for ethanol/water mixtures. The sources of these varations in flow direction and speed will be discussed, including the effects of the contact angle, and of thermal and solutal Marangoni stresses.