What does interfacial tension indicate in transformer oil condition assessment?

Interfacial tension reflects how polar contaminants and oxidation byproducts have accumulated in the oil. As oil ages, oxidation produces acids and surface active compounds that reduce interfacial tension. A declining interfacial tension often correlates with sludge formation risk, reduced dielectric reliability, and overall oil degradation. It is used alongside acidity and tan delta to judge whether oil regeneration or replacement is needed. Because interfacial tension can respond to contamination and oxidation, trending values helps identify abnormal aging or ingress of contaminants.