Spreading and evaporation of sessile droplets

We measure the contact angle at which a drop of liquid sitting on a solid surface de-pins as a function of both the concentration of solute in the drop and the amount of time the drop has been allowed to sit on the surface. We do this by removing liquid from a drop via a small hole in the substrate. This procedure does not disturb the liquid surface. However, viewing the drop from the side only allows a view of two diametrically opposed points of the contact line which can create artifacts in the drop width data. Instead, we monitor the footprint of the drop from behind the substrate, using total internal reflection, to obtain simultaneous information about the entire contact line. We find that the initial spreading of the drop on the surface for both upright and inverted sessile drops can last for several minutes but is largely independent of the solute concentration. However, during evaporation we find that the de-pinning angle is highly dependent on concentration and on the waiting time.