How does the presence of water in a splash cup affect seed dispersal distance?

Splash cups use falling raindrops as a method of seed dispersal. These plants launch seeds from millimeter-sized cups and use a raindrop's kinetic energy to send their seeds a horizontal distance up to ten times their height. Understanding the biomechanics of this dispersal method can be more broadly applied to better understanding similar phenomena such as soil erosion, the spread of pesticides, or 3D printing. While previous work has focused on cups that are initially dry, splash cups in nature are often found partially full of water. Here, we study how the presence of water in a splash cup changes patterns of seed dispersal. We released water drops above 3D-printed splash cup models that contained one seed and were either empty or half-full with water. We varied the raindrop's impact location relative to the center of the cup, as well as the steepness of the cup walls, to determine optimal values for dispersal distances. Our findings suggest that the presence of water affected dispersal distances in a complex and non-linear fashion. Furthermore, the presence of water shifts the optimal raindrop impact location towards the center of the cup.