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>Why grass is always greener – megadroplets

5 Oct

>As everyone who enjoys picnics on grass…
I always wondered why it is always so wet.

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Fig1. Even hours after sunrise under the scorching sun grass retains droplets. Photo: Brussels Park.

However, other plants spices are not usually as wet:
examples: clover, sun-shy ivy plants. So how come grass manages to remain
wetr even at midday? And how does it manage to attract droplets of dew to the tips?

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Fig2. The grass leaf has the ability to concentrate
water against the gravity force.

One explanation might be in the microstructure of the grass leaf:

The characteristic fusiform and V-shape channel of the grass
leaf acts like is in fact a half-funnel. At the micro level, the ability of funnels to propel micro
droplets against gravity is a consequence of capillary force law and has been demonstrated here.

The V-shape is an even more powerful force as reported here. Giant Tropical trees use conducts with embedded star prism structures so that capillary force can pump water hundreds of meters. A mathematical description of how a V-shaped wedge enhances capillary force can be found here.

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Fig3. V-shaped grass leaf.

Hence the Grass leaf uses booth: V-shape and fusiform funnel effect to collect and transport small droplet into larger droplets that concentrate on the tips. Why? Droplets from Dew form mostly at sunrise. The amount of water harvested from dew has been reported here be highly dependent on atmospheric changes such as wind and humidity. And here the ability to make larger droplets counts because the larger the droplet the greater its average life expectancy against evaporation. Additionaly, larger droplets are easier to fall to the ground and thus protect against heat and evaporation.

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Fig4. A mega droplet captured in a 4-way intersection of leaves.
A megadroplet life expectancy increases with size.

Summary:
Grass V-shape wedge form functions as a directional collector of dew droplets. The ability to capture dew into large droplets megadroplets prevents droplet loss by evaporation. The grass mesh plays a significant role in merging droplets.

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