Geometry-driven jets underlie dispersal of plants and fungi by raindrops

Plants and fungi have independently evolved a wide range of reproductive strategies, including the use of wind, insects, fruit-eating animals, and rain to disperse their reproductive units. In some cases, convergent evolution has led to the emergence of similar mechanisms in unrelated organisms. A striking example is the development of specialized cup-shaped structures called splash-cups that store reproductive units. Upon raindrop impact, these structures enable the ejection of the units away from the parent organism, facilitating local colonization.

While the role of rain in plant dispersal has been first observed by Brodie [1], and later examined through the lens of fluid mechanics by Amador et al. [2], the underlying mechanisms governing dispersal remain poorly understood. Most existing research on droplet impact has focused on flat surfaces [3], with only limited attention given to complex concave geometries like splash-cups [2, 4]. Dispersal by rain therefore remains understudied despite its biological significance.

We investigate how splash-cup structures enable the rain-driven dispersal of reproductive units in diverse organisms. When impacted by raindrops, these structures can redirect the incoming water to form fluid jets that eject the reproductive units away from the parent organism. We conducted drop impact experiments on 3D-printed biomimetic cups. Our results reveal two distinct dynamical regimes and their boundaries within the morphological parameter space, as well as two jetting behaviors that we associate with dispersal strategies optimized for different classes of organisms. We thus provide a mechanistic understanding of dispersal by rain and introduce a class of drop impact problems of fundamental interest.

[1] H. J. Brodie, “THE SPLASH-CUP DISPERSAI. MECHANISM IN PLANTS’”.

[2] G. J. Amador et. al, “Splash-cup plants accelerate raindrops to disperse seeds,” J. R. Soc. Interface, vol. 10, no. 79, p. 20120880, Feb. 2013.

[3] C. Josserand and S. T. Thoroddsen, “Drop Impact on a Solid Surface,” Annu. Rev. Fluid Mech., vol. 48, no. 1, pp. 365–391, Jan. 2016.

[4] M. O. Hassett et. al, “Splash and grab: Biomechanics of peridiole ejection and function of the funicular cord in bird’s nest fungi,” Fungal Biol., vol. 117, no. 10, pp. 708–714, Oct. 2013.