Antifoam Hinders Air Release in Lubricating Oil

Aeration in lubricating oil occurs when small air bubbles are generated and entrapped in the liquid bulk phase. If left unaddressed, this unavoidable phenomenon may cause degradation of the lubricant performance or even damage to the lubricated machinery. Silicon-based antifoam additives are commonly added to lubricating oil to destabilize foams, but we have observed that they also hinder the release of air from the aerated lubricating oil. Mechanistic understanding of how antifoam additives stabilize air bubbles in the bulk lubricating oil is limited. In this study, we tracked individual bubbles freely rising in a column of lubricating oil with systematically increased antifoam additive concentration. The concentration-dependent retardation of bubble rising velocity in lubricating oil demonstrates a unique time-based evolution compared to the bubble rising velocity in an aqueous system. Insights have been obtained into the underlying mechanisms of stabilized bubble dispersions in lubricating oil containing antifoam additives.