Collaborative Research: The Physical Biology of Leaves in Wind and Waves

Mathematical, numerical and physical modeling will be applied to reveal the morphological and mechanical adaptations of broad leaves that allow them to survive extreme fluid environments. For example, the principal investigators will determine how the shape and structure of tulip poplar leaves enhance cooling on nearly-stagnant hot summer days while also reducing drag in tropical storm or even hurricane force winds.

The models and tools developed in this project will also be applied to determine under what conditions the waving motion of seagrass augments waste removal and enhances photosynthesis. The scientific results of this project will inform the selection of plants that can survive extreme environmental conditions, including low levels of CO2, high temperatures, or strong wind and wave forces.

The significance of the proposed also work extends beyond gaining insight into mechanical adaptation of plants in the natural world. The physical principals discovered could drive innovations in the engineering design of flexible structures such as sails, flags, and cables. Furthermore, the computational tools developed in this project will find immediate application in other systems where exchange occurs across flexible structures in air and water, including gas exchange in the lungs, odor capture and pheromone release in a variety of animals, nutrient uptake in the gut, and heat loss in appendages.

Flexible plants, fungi, and sessile animals are thought to reconfigure in strong wind and floodwaters to reduce the drag acting upon them. In fast flows, for example, leaves roll up into cone shapes that reduce flutter and drag when compared to paper cutouts of similar shape and flexibility. In light breezes and currents, leaf flutter can be beneficial to heat dissipation and gas exchange.

It is not clear how the shape and mechanical structure of broad leaves results in different passive movements across this range of flows. The specific goals of this project are to determine the mechanisms by which 1) single leaves flutter in low winds and flows and roll up into drag reducing shapes in strong flows, 2) leaf flutter enhances heat dissipation and photosynthesis in light winds and flows, and 3) some leaves, such as the touch-me-not, actively reconfigure by changes in turgor pressure initiated by electrical signaling.

A combination of numerical simulations and laboratory experiments with real and artificial leaves will be used to quantify both passive and active movements as well as the concentrations of gases and heat. The fluid-structure interaction problem will be solved using the immersed boundary and inviscid vortex sheet methods. A new immersed boundary-style method for modeling the leaf as a source or sink of gases or heat will be developed.

Hyperelastic material models will be developed and implemented in the immersed boundary framework to determine how strain-softening or strain-hardening elasticity affects leaf performance.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.