(400c) Foaming Performance of Anti-Foam Laden Lubricants Subject to Filtration

Non-aqueous lubricant foams are detrimental to the lubricated machinery, resulting in well documented operational and maintenance challenges. In practice, lubricant foaming is usually controlled through the use of special additives called as ‘antifoams’. The size and concentration of these additives are crucial to maintain a satisfactory foaming performance in the lubricant. Unfortunately, the unavoidable filtration of lubricants is known to cause unpredictable variations in the size and concentration of these additives, often resulting in unexpected and adverse foaming performance.

Here we study this phenomenon in detail by probing the coalescence stability of single bubbles in filtered lubricants with organo-modified siloxane (liquid) antifoams. From the bubble results we show that lubricant foam stability is directly correlated to the filtration cycles, and inversely correlated to both the filter pore size and the antifoam concentration. In addition, we will also propose a method utilizing the single bubble stability to gain an approximate understanding of the underlying antifoam size density distributions. Finally, utilizing a simple probabilistic model, we will also establish a necessary (but not sufficient) condition on the minimum antifoam concentration required in a lubricant for maintaining satisfactory foaming performance. We believe these results are valuable in guiding the design of lubricants with robust and superior foaming performance.