Strength of self-pinning in coffee drops.

The equilibrium contact angle $\theta_e$ of a liquid drop placed on a solid surface is uniquely determined by a balance of surface tension forces according to Young's Equation, yet is rarely observed in real systems. Due to contact angle hysteresis, liquids can make contact with a surface at any angle between the receding and advancing contact angle: $\theta_R<\theta_e<\theta_A$. A particularly striking example of this phenomenon is the familiar coffee stain. For coffee $\theta_R=0$, thus as the drop evaporates the contact line remains pinned at its initial location. This results in the majority of the coffee being deposited in a characteristic ring at the drop's original boundary. We investigate how solid particles suspended in a liquid could so strongly influence contact angle hysteresis, by measuring the receding contact angle of a drop at various times during the evaporation process. For low solute concentrations, $\theta_R$ slowly decreases as the drop evaporates, but remains positive. Surprisingly, we find that increasing the solute concentration results in $\theta_R=0$ and a fully pinned contact line almost immediately after the drop is deposited.