Learning Forum

[akhemka]

I noticed something very strange. When I put a straw in a glass of water, the level of water inside the straw was higher than the level of water in the beaker. Why is this happening?

[Akismet]

Hi @akhemka

What you noticed is referred to as the capillary action. Capillary action (or capillarity) occurs in narrow spaces when hard boundaries are very close to each other. In these cases the liquid can flow upward against gravity. This is fundamental for trees to bring the nutrients up to the leaves. Capillarity is generated by cohesive and adhesive forces. The water is moved up in straw because of the interaction of the adhesive forces between the fluid an the solid surfaces with the surface tension of the liquid. The water is "pulled" along the wall of the straw by the adhesive force while the cohesive forces in order to maintain the original surface tension, pull the rest of the liquid in the motion direction.