Session 1I: Examining Contact Line Pinning and the Coffee-Ring Effect

When a drop of coffee dries on a solid surface, it leaves a dense, ring-like deposit along the perimeter. Coffee which is initially distributed throughout the drop ends up concentrated at the edge. There already exist theoretical explanations for this effect but they rely on the contact line of the drop remaining fixed (pinning) and the contact angle decreasing while the droplet evaporates. This is at odds with the prediction of the Young equation which suggests that contact angle should remain constant while the contact line shrinks. In order to better understand when and why contact lines pin and thus when the coffee-ring effect will be observed, we will examine how much the contact angle of a drop changes during evaporation with varying evaporation speeds and solute concentrations