Surface Tension

When a bug walks on water, it is supported by the surface tension of the water. It is a bit like walking on a chain suspended between two posts. Each link is pulled on by its neighbor under the tension created by the weight resting on it. If the weight increases the tension beyond the ability of the proverbial "weekest" link to hold onto its neighbors, the chain breaks.

Links

The links on the surface of the water are water molecules. They link to their neighboring water molecules all around except above where the air is. Water bug legs have waxy substances on them that keep the water molecules from "linking" to them. So water doesn't stick to them and can't be absorbed. Only the bugs weight influences the water.

Each "link" in the surface of water holds onto its neighbors by sharing its two electrons. The energy required to break that bond is about 0.00667 ozin per square inch at 68 degrees Farinheit. This is a measure of water's surface tension, t.

The tension mounts!

How do we know if a water bug will be supported by this surface tension? The weight of the water bug must be matched by an equal upward force provided by the water surface. Let's consider the energy exerted on the water by the bug when it landed there. Assume the undisturbed surface was perfectly flat and calm. When the bug landed, the weight of the bug caused it to push down on the water surface with its feet. The surface fell away slightly under each foot like rubber until it became "tight" enough to support the bug's feet.

In summary, the bug pushed the water surface down some height, h, using an amount of energy equal to its weight, W, times that height.

If the surface area dimpled by each foot is A, then the amount of energy absorbed by the water surface is Wh/6A since the bug has 6 legs. If this amount is greater than 0.00667 ozin, the surface tension of water at 68 degrees, the surface will break and the bug will get wet!

We can write this condition as:

Wh/6A < t implies the bug will be supported.

For a bug weighing W = 0.009 oz, about the size of a big fly, and dimples that are about a hundreth of a square inch in area, A = 0.01,

h < 6*0.01*0.00667/0.009 = 0.0445 inches

That means each dimple caused by each foot in the water can be about a twentieth of an inch deep before the bug would fall through the surface. For a water bug, that's pretty deep!

Really Walking

We really haven't said anything about how bugs walk on water at all. We just showed a bit about how it is possible for them to stay above the water. The fact that water bugs have waxy feet and legs also means they have very little friction since water won't stick to them. Walking on land requires friction! Walking on water requires creating depressions in the dimples that can push water under the surface so the bug can move forward. Its like using the surface molecules of the water to make a temporary paddle to push with.

Better Boats

Does a water bug then inspire a means of building faster boats? Yes, and no. "Yes" because it tells us it is possible to stay on the surface of the water without getting wet. Low weight and waxy "legs" are the key points. Staying on the surface of the water means no pressure drag from the water around the boat so friction is much lower. "No" because of the following calculation for a W = 1 oz boat with contact (dimple) area of 6 in x 2.5 in = 15 in2 = A:

Wh/A = h/15 < 0.00667 ozin/in2 implies

h < 0.1 inches means that the "dimple" in the water surface can be almost a tenth of an inch deep, which seems possible.

But if we calculate the amount of water displaced, we find 1 ounce of water over 15 square inches piles up to a depth H of,

W/HA = 0.577 oz/in3 implies

H = W/0.577A = 1/(0.577*15) = 0.116 in.

H, the depth of displacement, is just a bit greater than h, so the one ounce boat probably wouldn't be supported on the water by surface tension alone. A two ounce boat would sink twice as far and so can definitely NOT be supported by surface tension.