Lubricity of an oil in water emulsion in metal cutting: The effect of hydrophilic/lypophilic balance of emulsifiers

► Coefficient of friction in cutting of steel is shown to vary systematically with the Critical Packing Parameter (CPP) and the solubility of the emulsifier in hexadecane. ► A high CPP emulsifier generates a bilayer micelle which undergoes easy planar shear to release an oil film trapped in the interlayer plane to form a robust organic oxide tribofilm on the substrate. We believe this account for the observed low friction. ► A low CPP surfactant on the other hand generates spherical micelles which encapsulates oil in its core. To release the oil, the intermolecular bonds in the micelle need to be broken over the whole micelle volume. ► The expected friction to transmit traction may thus be high in this case. The tribofilm formed in this case is thin and consists of metallic oxides.