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The upside-down water collection system of Syntrichia caninervis, Pan, Pitt, Zhang, Wu, Tao and Truscott, Nature Plants, 2016

Novelty/impact/significance:

It is unveiled that the Syntrichia caninervis (S. caninervis), a most abundant desert moss, collects and transports water by coupling relevant nano- and microscale physical structures with multiscale sources of water (nanoscale nucleation, microscale fog droplet, larger droplets on awn).

This natural strategy with high efficiency provides physics-based evidence of previously speculative function and insights in developing new water harvesting and transport devices in scarce water environments.

Scientific question:

What are the water collection and transport mechanisms of S. caninervis and other similar bryophytes?

Key of how:

The awn has an integrated water collection and transport system, harvesting the multiscale water resources as diverse in form and more in amount as possible through its correspondingly multicale structures: the nanogrooves accounts for the nanoscale water nucleation, the microgrooves are collection sites for microscopic droplet/fog (>1 µm) and the microscale barbs provide collection depots of droplet, the awn fiber conical shape transports larger droplets to base, and the awn and leaf reduce splash and collect raindrops.

Major points:

1. As one most common desert moss in the world, S. caninervis has a key feature of leaf hair (awn) which is correlated to the water collection and transportation, while the mechanisms not elucidated.

2. Without the strategies such as hydrophobic leaf surfaces, special chemical leaf surfaces and extensive root systems of other plants for arid environments, the S. caninervis has to develop other structures to adapt to the water scarcity.

3. Each moss leaf has an awn growing from its distal terminus, with the awn (10-50 µm in diameter) tapering towards its distal end.

Ruling out the functions of mechanical protection (awns are too delicate) and blocking ultraviolet sunlight (too sparse density), and considering the awns’ correlation to more than 20% of dew collection (despite a negligible volume fraction) and the indispensability to the plant growth (from a field study), it is deduced the awns participate in water collection.

4. Previous relevant studies are single water collection processes on slender bodies, lack experimental support, or mechanisms between the plant structural features and the scale of water collection/retention are not elucidated.

Here, water capture and transport mechanisms of the awn are proposed and experimentally verified/supported: (1) the nanogrooves of awn accounts for the nanoscale water nucleation, (2) the microgrooves of awn are collection sites for microscopic droplet (>1 µm)/fog, and the microscale barbs provide collection depots of droplet, (3) the awn fiber conical shape transports larger droplets to base, (4) the awn and leaf reduce splash and collect raindrops.

5. Detailed experimental observation (within ESEM) of the awn nanostructure versus the water nucleation verified the hypothesis of nanogrooves being the nucleation sites, and microgrooves being the growth/accumulation sites into droplets, which is also consistent with the nucleation theory.

6. Exposing the awn within fog field reveals that microscale droplets are captured at the microgrooves, and coalesce into larger droplets at the high-barb concentration sites.

The awn fiber conical shape transports larger droplets to base, driven by the Laplacian pressure difference rather than gravitation (droplet size is far below the capillary length for water).

7. The high-density clusters of flexible leaves and awns provides maximum retention of rain droplets (0.67 to 5 mm) by absorbing the impact energy of droplets via its mechanical flexibility and reducing splash via the conical shape-induced capillary force.

 

These observations and analyses support and lead to the integrated water collection and transport strategy that involves the nano-, micro-, to macroscale structures of the awn correspondingly responsible for the nano-, micro-, to macroscale water sources acquisition and transportation.

Here is the link of the fulltext: https://www.nature.com/articles/nplants201676

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