What is the function of drilling fluid in rotary drilling in site investigation?

Drilling fluid in rotary serves two main purposes:

(i) Facilitate the rotation of drilling tube during rotary drilling;

(ii) Act as a cooling agent to cool down heat generated during drilling operation.

Traditionally, water is normally employed as drilling fluid. However, it suffers from the following drawbacks:

(i) It affects the stability of nearby ground with the introduction of water into the borehole (borehole for soil; drillhole for rock);

(ii) It affects the quality of sample by changing the water content of soil samples collected from the borehole/drillhole.

Substitutes are available in market to replace water as drilling fluid (e.g. white foam).

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.