AsianScientist (Jun. 4, 2015) – Every engineer or heavy machinery operator realizes the problem of using liquids for lubrication–at some point, they run dry either from spreading across the surface or from evaporation. Using liquids that are more viscous (and therefore less volatile) mitigates evaporation, but lubricant spreading tends to remain a concern.
The lack or ‘starvation’ of lubricant between sliding surfaces leads to increased wear and could possibly result in failure or damage to the surface; it is therefore crucial to maintain a certain critical amount of lubricant between the surfaces.
In a joint research project between Imperial College and National University of Singapore, published in Tribology Letters, we investigated two methods to prevent starvation. Firstly, we modified the liquid itself by including three types of additives—octadecylamine, dodecylamine and a multiply-alkylated cyclopentane (MAC)—mixed in the bulk lubricant hexadecane. These were then dropped on a silicon surface and their spreading behavior observed.
To our surprise, we found that certain concentrations of the amine additives resulted in the liquid droplets retracting inward, rather than spreading outward. Also intriguing was the behavior of the MAC-hexadecane combination, which neither spread nor retracted, but remained as a contained droplet with a significant contact angle.
The behavior of the mixtures can be attributed to the change in the interaction between the silicon surface and the additives. In the case of the amine additives, the inward retracting mixture would be classified as autophobic liquids. The MAC-hexadecane combination, however, behaved differently, suggesting that there is a surface-active component that causes MAC to form a film on the surface, thereby preventing the liquid from spreading.
Sliding contacts, however, expose the lubricant to a certain amount of dragging force. To combat this mode of starvation we modified the surface instead. The silicon surfaces were coated with an octadecyltrichlorosilane (OTS) monolayer to induce hydro- and oleophobicity. Portions of the monolayer were then removed with oxygen plasma for a selectively modified surface.
In this manner, we created surfaces that repelled the liquids (the OTS monolayer), but had a non-repelling area in the center of the surface (the bare silicon surface). Droplets of water and hexadecane were then placed in the bare areas and the silicon wafer was mounted on a rotating disc, which was then spun up till the droplets had moved from their original location. This allowed us to measure the force required to move the droplet, calculated by taking the centrifugal force that the droplet experienced due to its own mass.
While hydrophobic and oleophobic surfaces have low surface energies and are preferred for applications requiring low adhesion or friction between surfaces, we found that they do not retain the droplet at the desired location, implying that they are likely to be more prone to starvation, unless there is a countering force to prevent it.
The unmodified surfaces behaved normally, but the selectively modified surfaces retained the liquid droplets at higher speeds. The enhanced droplet retention could be attributed to the step in surface energies between the adjacent bare and modified regions, which represent an invisible ‘barrier’ the droplet needs to overcome in order to spread.
Both methods hold implications of improving lubrication–not only for larger devices but also for micro-devices, where re-application of lubricant is extremely difficult and where starvation of lubricant often results in sudden and catastrophic failure. Limiting drop area, in the modification of the liquid itself, also reduces the likelihood of evaporation, combating both factors of starvation at the same time.
The article can be found at: Leong et al. (2015) Confining Liquids On Silicon Surfaces To Lubricate MEMS.
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Copyright: Asian Scientist Magazine; Photo: Springer.
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