Surface tension

Surface tension

Surfaces tension – The cohesive forces between liquid molecules are responsible for the phenomenon known as surface tension. The molecules at the surface do not have other like molecules on all sides of them and consequently they cohere more strongly to those directly associated with them on the surface. This forms a surface “film” which makes it more difficult to move an object through the surface than to move it when it is completely submersed.

  • “floating” – Only objects lighter than water can float. This insects is prevented from sinking because of surface tension effects. The surface of the water behaves like a membrane that supports the weight of the insect.

Don’t touch the tent!

Common tent materials are somewhat rainproof in that the surface tension of water will bridge the pores in the finely woven material. But if you touch the tent material with your finger, you break the surface tension and the rain will drip through.

  • Ask – How waterproof does a tent need to be? Rain is less likely to drip through if the surface tension is stronger.
  • Imagine – Many tents today are very lightweight and inexpensive to buy. How can surface tension be part of the rainproofing strategy for selecting tent material? The surface tension is stronger if the pores in the fabric are smaller because the fabric is heavier or more tightly woven.
  • Plan, Create – The tent manufacturer can keep costs and weight down if there is less fabric in each tent. How can this change how rainproof the tent is?
  • Improve – What are some ways the tent can be made more rainproof? There are also coating that can be used to fill in the spaces in the tent fabric.

That’s engineering

  • Surface tension is caused by the attraction between the molecules of the liquid, due to various intermolecular forces. In the bulk of the liquid each molecule is pulled equally in all directions by neighbouring liquid molecules, resulting in a net force of zero. At the surface of the liquid, the molecules are pulled inwards by other molecules deeper inside the liquid, but there are no liquid molecules on the outside to balance these forces, so the surface molecules are subject to an inward force of molecular attraction which is balanced by the resistance of the liquid to compression.

Engineering ideas

  • surface tension, molecules, cohesion, adhesion, film

Do It
Challenges for you to work on…

News, updates

Learn more…

  • Surface tension – an effect within the surface layer of a liquid that causes the layer to behave as an elastic sheet. Thus, the surface of any liquid behaves as if it was covered by a stretched membrane. Surface tension is measured in units of force per unit length.
  • Fluid Mechanics demonstrations