A cutting fluid must reach into the small gap at the chip–tool and tool–work interfaces to cool and lubricate. To do that it has to wet the metal, i.e. spread out into a thin film rather than gather into rounded droplets. Wetting is governed by surface tension: a high surface tension pulls the liquid inward to minimise its surface area, producing beads with a large contact angle, whereas a low surface tension lets the liquid flatten out and creep into fine crevices.

Plain water has an unusually high surface tension (about 72 mN/m at room temperature), one of the highest of common liquids because of strong hydrogen bonding between its molecules. As a result water beads up on a machined surface and does not penetrate the cutting zone well, so its cooling and lubricating action is limited even though its heat capacity is excellent.

• To fix this, industrial water-based coolants add surfactants (wetting agents) and emulsifiers. These molecules migrate to the liquid surface and lower its surface tension, so the fluid spreads, wets the tool and reaches the interface.

The choice naming low surface tension is exactly the opposite of the truth — a low surface tension would promote spreading, not prevent it, so it cannot be the reason water fails to wet. High buoyancy force is irrelevant to how a thin film spreads over a solid surface. High volatility would concern how quickly the fluid evaporates, not whether it wets the metal; water is in fact only moderately volatile. Hence the correct reason is water's high surface tension.