Hygroscopic Plant Structures
Plants build thin, smart structures that change their shape depending on amount of moisture in the air (hygroscopic motion). A typical hygroscopic structure is based on the combination of rigid fibrous cells within spongy tissue- when humidity drops, the spongy tissue emits water, while the fibers retain their length. As a result, the structure shrinks only perpendicular to the fibers. Similarly, when humidity increases, the tissue absorbs water, and the structure swells perpendicular to the fibers. Seed pods and pinecones consist of layers in which the fibers are organized in different directions. When the humidity changes, each layer contracts / swells with a different intensity and direction, causing the structure to bend or twist. This is how pinecones and seed pods open in dry air.
How to create a 3D shape?
The mathematical tool that characterizes three-dimensional surfaces is the curvature field, which describes the rate and direction of bending of the surface at each point. The product of the principal curvatures is the Gaussian curvature, K, which distinguishes between dome-like regions where K>0, flat or cylindrical regions where K=0, and saddle-like regions, where K<0. The great mathematician Carl Friedrich Gauss showed that a change of K requires a non-uniform planar growth or contraction. Inverting the Gaussian relation makes it clear that if a flat surface "grows" non-uniformly, it can turn into a dome, a saddle, or any three-dimensional shape we want!
A seed pod uses a different principle: uneven growth across the thickness. While it is young and moist, the two sides of the pod are flat and fit together, thus protecting the developing seeds. Each pod is constructed of two fibrous layers that shrink in perpendicular directions as they dry. The result is a curvature that causes the valves of the pod to twist in opposite directions. This is how the pod opens and “shoots” the seeds into the distance.
This is how the lily opens: young petals with a positive Gaussian curvature form a "container". Later, the edges of the petals grow faster than the inside, turning the Gaussian curvature into negative (saddle) and causing the flower to develop a beautiful "ruffle"– a structure typical of surfaces with negative curvature.
Unevenness across the surface of the sheet dictates Gaussian curvature, while unevenness between the layers of the sheet prescribes local bending. Since the beginning of research on self-morphing about 20 years ago, methods were developed for the use of various materials such as gels and nematic elastomers. Recently, there has been an attempt to implement these principles using familiar materials from the field of construction and design.
Self-Morphing With Artificial Materials
When we want to apply the principles of self-morphing to artificial materials, we need to find materials that react geometrically (swell, contract, elongate) to an external trigger, such as heat, moisture, light, and more. Then, we need to develop methods for creating heterogeneity in thematerial, which will result in a variable geometric response either in the direction of "growth" or in its intensity.
The Restless Awn
Spiralling Erodium awn activated by humidity variations, causing the twisting/untwisting of the awn’s cells
