Nanoscience #6 Why Don’t Water Striders Get Wet?

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Nature’s ingenious design gave these water striders the power to skip effortlessly across lakes and ponds.

Hey Steemians,

I am back with one more article from the world of nano-science. Have you ever tried to walk on water or saw someone walking on water? You must be wondering whether it is possible to do that, the answer is yes!! You can do it!! You just need to modify your body to one of these.

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Water striders are among the aquatic creature which are fascinating to watch. You can find them in almost all the places – in lakes, creeks, urban ponds even mud puddles. Here is the video link for the water strider movement in water YouTube.

Water striders or the water skater/skimmer, they resemble with spider but are insects from the family Gerridae (Order- Heteroptera) of about 350 different species. They are not more than 5mm in size and are dark colored, slender body are often seen to skate in groups.

This eight-legged creature move across the water to hunt and to escape. But how does it skate? It must be pushing itself forward and the water backwards, but what is doing the pushing for movement?

How do they do it?

Nanoscience has the answer!! If you go deep into the micro to nanoscale, you will find the legs are covered by large number of oriented tiny hairs (microsetae) with the fine grooves, which is the main reason for this water resistance.

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If you see in the images, the legs do not pierce the water surface up to the depth of 4.38±0.02 mm. The maximum force exerted on a single leg is about 152 dynes which is almost 15 times of the total body weight of the insect. The volume of water that was ejected is roughly 300 times that of leg itself, suggesting the enormous water repellent surface (1). (Making the strider legs superhydrophobic)

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Microscopic images of strider’s microsetae revealed the size to be less than 3 micrometer in diameter whereas, a human hair is of 80-100 micrometer. The fine grooves of well-ordered microsetae are measured in nanometers, even smaller than billionth of a meter.

Air gets trapped in the air cushion formed by these nano grooves and microsetae, which prevents the legs from getting wet (2).

Scientists have made a hydrophobic leg from a smooth quartz fibre similar to the strider's leg just to compare with the original one. The surface of the artificial legs were modified by self-assembling monolayer of low-surface-energy heptadecafluorodecyltrimethoxysilane (FAS-17), making the conract angle of 109° between water droplet and the flat surface. The artificial legs were supported by water with maximum force of 19.05 dynes, this can led the artificial strider to withstand but not glide or swim rapidly on the surface.

This model gave an idea about the force exerted by the strider's leg could be due to a 'superhydrophobic' effect. And to enhance the hydrophobicity of these artificial legs, researchers coated them with wax, making it more hydrophobic. The contact angle the cuticle wax secreted on a strider's leg and water is about 105°, which is not enough to account for its marked water repellence.

So, the main reason behind their water repellent legs are the unique hierarchical micro and nanostructuring on the leg surface.