What is Capillary action?

Answer

When a pipette tip is placed at or below the liquid surface, a small volume of the liquid may get pulled up into the tip without any external pressure being applied. This is because the adhesion of some liquids is greater than the cohesion, meaning the liquid binds to other surfaces more strongly than its own inter-molecular attraction. As a result of the capillary action, the aspiration volume is greater than expected.

This is especially true in low-volume tips (e.g. 50 μL) as the bore hole is narrower. In narrow tubes, the surface area of the liquid in contact with the tube is relatively greater compared to the volume of the liquid. This means there's more area for the adhesive forces between the liquid and the tube to act, which results in a greater capillary rise.

The degree of capillary action is determined by the liquid's adhesive/cohesive properties and other factors like vapour pressure and the lab environment. A greater source volume will result in less capillary action. Increasing the aspiration volume and then returning the excess to source will also help to mitigate against the volume increase. 

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