Droplet Migration on Conical Glass Fibers

Cacti have developed an efficient system to harvest water: needle-like conical spines on which fog condenses into droplets which then proceed to migrate towards the base. The mechanism responsible for this phenomenon is the spines’ conical shape. Motivated to understand this at a fundamental level, we explore self-driven motion of droplets on fibers by designing a simple experiment using conical glass fibers. The main goal is to understand precisely how the geometry of the conical structure affects the motion of the droplets. Using a simple theoretical model, we determine the droplet velocity as a function of geometrical parameters and find this to be in good agreement with our experimental data.