Meeting Abstract

52.4  Monday, Jan. 5  Hygromorphs REYSSAT, E*; MAHADEVAN, L; Harvard University; Harvard University ereyssat@seas.harvard.edu

Pine cones open and close in response to environmental humidity fluctuations, as can be checked with pine cones picked up from the backyard. This is just one example of how plant organs and tissues from leaves to flowers respond passively to humidity. This behavior arises from the bilayer structure of the scales that are made of water-responsive cells that can swell or shrink anisotropically. A century of work in the botanical community on these structures has catalogued examples and described the structural and ultrastructural aspects. However there is to date, no dynamical description of the opening and closing events. We complement this by studying the dynamics of these processes at the microscopic cellular scale and at the macroscopic level of individual scales and whole pine cones ranging in size from a few mm to 30 cm. The dynamics of opening and closing shows a marked hysteresis that arises from the difference in the physical processes underlying the wetting and drying of soft porous structures. We quantify this in terms of a theoretical model coupling fluid transport to mechanics and geometry that explains our observations. Inspired by these natural hygromorphs, we also built a few simple biomimetic devices with bilayered structures made of polymer and paper and analyze their response to humidity fluctuations. We show that the large geometrical amplification provided by slender bodies enables several interesting applications ranging from sensors to oscillators and even artificial flowers that have a controllable blooming and wilting response.