A Water Acquisition System Inspired by the Extreme Desert Plant Tillandsia
Abstract
Animals and plants have evolved intriguing adaptations to absorb water from hard to access water sources such as fog and dew. Some of the most striking adaptations include the hydrophobic islands of tenebrionids beetles, the spines of some Opuntia cacti, and the absorbing trichomes of bromeliads. Inspired by these fascinating structures, researchers have designed new biomimetic devices with enhanced water harvesting properties. This project was inspired by the ability of Tillandsia plants to capture water from sporadic fog events within the hyper-arid Atacama Desert. TillandsiaÕs unique fog-capturing ability emerges from the so-called trichomes that cover the surface of the leaf. The trichomes are made of a thick cellulose-rich layer sitting atop a narrow impermeable tube penetrating the leaf. The challenge faced by these plants can be summarized as follow: the cellulosic layer must be highly permeable to allow liquid water uptake during short fog exposures while, at the same time, preventing evaporative water losses when exposed to high temperature and low relative humidity. Objectives - We took a classical biomimetic approach whereby the principles extracted from an in-depth biophysical analysis of the Tillandsia trichome were used to design cellulose membranes endowed with the same asymmetric water transport properties. The project was articulated along two broad scientific objectives. First, we studied the Tillandsia trichome to establish which structural features are critical for asymmetric transport. Second, we characterized the ability of a composite membrane made of bacterial cellulose to serve the functions of the trichome. Results - Our experimental analyses demonstrated that the design of the Tillandsia trichome forces fog water to move along three successive structural elements before entering the leaf: the thick shield walls, the lumen of the dome cell, and the semipermeable membrane of the outermost foot cell. When exposed to fog water, the highly hygroscopic shield walls absorb the droplets by capillarity, allowing a continuous path of liquid water from the surface of the trichome to the cytoplasm of the foot cells; the flow of water being driven inward by an osmotic gradient. When the fog disappears and the effective osmotic potential of the environment falls below the osmotic potential of the foot cells, the dome cell cavitates thus breaking the column of liquid water. The main resistance to evaporation comes from the extreme reduction of the evaporative surface by the cavitation of the dome cell and the accrued resistance of the shield wall at low relative humidity. The composite membrane of the trichome can achieve a 5800-fold asymmetry in water conductance. Based on those observations, we designed a biomimetic device with a composite membrane made of a thick layer of bacterial cellulose above a semi-permeable membrane. These were laid over a chamber loaded with an osmotic solution (sucrose or salt solutions). The water transport properties of the biomimetic device were as follow. The resistance to the absorption of liquid water was set by the semi-permeable membrane while the thickness of the cellulose layer had virtually no effect on the absorption resistance. In contrast, the resistance to the outward flow of water vapor increased linearly with the thickness of the layer of bacterial cellulose. Moreover, the resistance of the cellulose layer also increased when the system was exposed to lower relative humidity. These features are the key responses also identified in the trichome of Tillandsia. The device equipped with a bacterial cellulose membrane of 0.25mm had a water transport asymmetry of 149. Extrapolating to a 10mm bacterial cellulose membrane, the asymmetry would be nearly 6000 times the asymmetry factor of the semi-permeable membrane alone.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Mar 11, 2019
- Source ID
- W911NF1610434
Entities
People
- Jacques Dumais
Organizations
- Army Contracting Command
- United States Army