Device for experimental characterization of the 4D flow inside an evaporating sessile water droplet.

We describe an experimental system based on optical microscopy, permitting the analysis of the four dimensional structure of the flow inside evaporating sessile droplets by monitoring the motion of tracers in horizontal planes localized at different heights. Inter-plane particle identification is accomplished via 3D tracking algorithms. The multiple plane observation is achieved using a piezoelectric device to make the microscope objective oscillate vertically, while a high-speed camera captures images. The droplet evaporation process lasts several minutes and greatly accelerates as the fluid advances toward complete evaporation. In order to capture the dynamics of the whole process, two cameras with the same optical output but different temporal resolution are used sequentially. Using image processing algorithms, we obtain the full trajectories of multiple tracers, velocities of particles on the free surface of the droplets, and velocity fields. The information available may be used to understand the geometry of the sedimentation pattern.