The effect of raindrop velocity on splash cup seed dispersal

Splash cups are small plants with cup-shaped fruit bodies that use incoming water drops to disperse seeds. This dispersal mechanism appears in a variety of plant species and a variety of environments. When a water drop, such as a rain drop, hits the seed cup of the plant, the kinetic energy from the drop ejects the seeds away from the parent plant. Previous work has found seeds travel up to one meter away from the parent plant. In their natural environment, drops may come from rain or drip from overhead plants thus creating a variety of incoming velocities. In this experiment, we varied the velocity of the incoming drop in the approximate range of 2.5 m/s to 6 m/s by altering the height from which it was released into a splash cup model. These models were 5 mm in diameter, 5 mm tall, and had an opening angle of 40° above the horizontal and contained 5 plastic seed mimics. We used a high-speed camera and an overhead camera to record the drop velocity, impact location, and the final position of the seed mimics. Results show that as drop velocity increased, the seed dispersal distance increased approximately in proportion to the drop kinetic energy. Also the number of seed mimics ejected from the cup increased with incoming drop velocity. Seeds typically ejected from the cup in clumps, although as drop velocity increased, the number of seed mimics landing in a clump decreased, leading to a wider spread of seeds. At the higher drop velocities, seeds ejected from the cup at a wider range of impact locations, both towards the center of the cup and towards the edge of the cup.