Antifoam hindrance of air release in lubricating oils

Air entrainment in lubricating oils occurs when small air bubbles are trapped in the bulk, which may degrade performance if left unaddressed. Silicon-based antifoam additives suppress foams, but can hinder air release from the bulk. It is possible that antifoam droplets irreversibly attach to rising bubble surfaces, developing rear stagnant caps that immobilize the bubble surfaces, leading to the hindered air release.

Individual bubbles are generated in lubricating oil columns and tracked while rising under the condition of Re < 1, with systematically increasing antifoam concentration. High speed imaging and image analysis provide the local and instantaneous velocity profiles of the rising bubbles. Measurements in aqueous surfactant solutions are performed for comparison.

Antifoam-induced deceleration of individual bubbles in lubricating oils is first demonstrated, addressing the longstanding challenge of understanding the hindered air release. Unlike the surfactant-induced deceleration dominated by surfactant adsorption and Marangoni stress, antifoam-induced deceleration exhibits a distinct asymptotic tendency in velocity profiles. A novel two-stage collision-attachment model was established based on antifoam attachment and progressive growth of rear stagnant caps, capturing the asymptotic deceleration tendency observed in experiments. The model yields a characteristic decay time scale that decreases with antifoam concentration, directly quantifying the air release hindrance induced by antifoam additives.