Evaporation-induced foam stabilization in blended lubricants

Mitigating lubricant foaming is of primary concern to lubricant manufacturers, especially in the context of high performance machinery such as wind turbines. In this study, we explored foaming in blended lubricant base oils. Utilizing single bubble experiments coupled with interferometry, we found that blending lubricant base oils exacerbates the foamability of the resulting mixture. This increased bubble stability is a consequence of Marangoni flows driven by the differential evaporation of various components in the blended system. Interestingly, the resulting flows are also seen to cause the bubble wall thickness to ‘pulse’ (spontaneous dimpling - O. Velev et.al, JCIS 1993), resulting from the interplay between capillary forces draining the bubble and Marangoni flows entraining liquid into the bubble. As the blending of lubricant oils is important in formulation, we utilized silicone oil mixtures to further characterize the evaporation-induced bubble stabilization as a function of the Marangoni number and the dimensionless dimple formation number (C. A Miller et.al, JCIS 1997).