Fluid flow in an evaporating droplet on a solid surface*

We experimentally study internal fluid flow in evaporating droplets on a solid surface. Optical and fluorescent microscopy were used for the measurements. Water and isopropanol droplets of 0.2 microliter, glass and polydimethylsiloxane (PDMS) substrates, particle of 1.1 micrometer diameter, and 0.001% (v/v) particle concentration were considered. For a droplet of water on a glass surface, classical radially outward flow was seen in experiments. The droplet of water on PDMS shows two vortices in opposite directions without axisymmtericity in the flow. A droplet of isopropanol on PDMS shows Marangoni loops, as reported earlier in the literature. We also quantify the flow field using the two-dimensional Particle Image Velocimetry (PIV) technique. We also carry out theoretical analyses and compare the prediction with measurements. Fundamental insights into the internal convection will help to design application such as inkjet printing, bioassays, particle sorting, etc.