RESEARCH PROJECT : IS IT POSSIBLE TO WALK ON WATER ?

Insects walking on water

 

 

 

We all remember when we were young, those little walks near to a pond. We used to be fascinated by some insects that were on the surface of the water.

 

→ But how do they do this? Which mechanisms allow them to walk on water?

 

      1) The Gerridae

 

The Gerridae is an insect Hemiptera which belongs to the family of bugs also known as « water striders ; water bugs ; pond skaters ; water skippers or also jesus bugs (because of their legs in “x”)

 

 

 

 

 

 

 

 

 

                                   https://www.cals.ncsu.edu/course/ent425/spotID/Heteroptera/gerrid.html

 

               a)  Characteristics :

 

Gerrids are very small (between 15 and 17 mm), and their weight is very light (2g on average).

As all Hexapoda, Gerrids have six legs :

Two small legs situated near their head, and 4 others situated on both sides of their body like an « X ».

Some water striders can have wings, whereas other species of Gerridae do not.

The Gerrids’ hairs situated on their legs can secrete an hydrophobic substance which repels water.

 

               b)  What allows their mobility ?

 

Their mobility is due to two different phenomenons :

 

•  The Gerridae hairs are very hydrophobic and repels water, preventing the Gerridae from penetrating the water.

 

• The second notion is the creation of the topographic depression. Thanks to the surface tension, topographic depression drives a suppression which allows to  carry the insect. If Gerrids were heavier, this surface tension would not be enough to assure their lift.

 

      2) Surface Tension, How Does It Work ?

 

 

               Definition :

 

Surface tension is the name given to a strength that exists at every interface level between two different media ( between a solid or a liquid and a gas). It explains many phenomenons such as the fact that a needle can float at the water surface or the fact that Gerrids are able to walk on water.

 

 

               Physics knowledge:

 

In a liquid , there are many molecules that exercise strengths :

• they exercise attraction strengths (in this case, the molecules are attracted to one another).

• they exercise repulsion strengths (in this case, the molecules  repulse one another).

 

Besides, when they are at the surface, the part that is between the liquid and the air is not attracted by other molecules anymore.

 

The graph underneath illustrates that the molecule “M1” is at the interface between the liquid and the air. The part in contact with the interface does not undergo the attraction of the other one : the molecule is only attracted inside the liquid.

 

 

 

 

 

 

 

 

 

http://chalumo29.free.fr/160b.htm

 

 

In the case where the interface is bendable, there is a difference between the atmospheric pressure and the liquid pressure.

 

 

               For example:

 

In a test tube, if the interface obtained is bendable upward, it means that the water pressure is higher than the atmospheric pressure just as the graph underneath shows::

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

      3) Realisation :

 

Because we are creative, we created some kind of “water spider” by using copper strings so as to create topographic depression.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

As we can notice, at the interface between our spider legs and water, topographic depression appeared because of the surface tension.