Migration Mechanisms Of Liquid Lubricants And Regulation Strategies

Lubricant migration is one of the most significant factors which can result in the failures of space devices.Here,migration is a phenomenon that lubricant can freely expand on a contact surface without external forces.In consideration of the fact that lubricants of mechanical components in spacecrafts are always of lower dosage and can not be replenished,once migration occurs,it will seriously affect the lubrication condition and cause potential lubrication failure of these devices.Therefore,it is essential to study the characteristics of lubricant migration and explore various regulation strategies to obstruct the migration to ensure the long operating life of mechanical components.In this study,we fouce on the migration induced by thermal gradients.The influence of properties of liquid lubricants,additives,surface structures,functional fluids and other factors on migration behavior are investigated.Firstly,the characteristics of lubricant migration on a smooth surface and the corresponding lubrication properties are discussed,and the effects of lubricant additives on the migration behavior are studied.Then the migration theory basing on Navier-Stokes equations is established.After that,the migration performance on structured surfaces and functional fluid of ferrofluids are investigated,and various regulation strategies are proposed.The main conclusions drawn from this study are as follows:(1)In the aspect of basic migration performances of the lubricants: Under the effect of thermal gradients,the lubricant will migrate from the high temperature zone to the low temperature zone.Adding polydimethylsiloxane or oleic acid to the lubricant will reduce its surface tension coefficient or increase its adsorption ability respectively,which can both slow down the migration velocity of the lubricant.Beside that,increasing the viscosity or decreasing the dosage of the lubricant,or decreasing the thermal gradient can also decrease the migration velocity.All of the above mentioned factors give a guidance of regulation strategies on migration.(2)In the aspect of migration theory: Basing on the Navier-Stokes equations,an analytical approach using the thin-film lubrication equation with a temperature-dependent viscosity is proposed for the prediction of migration velocity of droplets on a horizontal surface and an inclined surface.The numerical results show good agreements with the trends of experimental data.Under a given thermal gradient,there exists a critical inclination angle under which the droplet migration is halted,and the critical inclination angle can be readily predicted using the expressions derived in this study.(3)In the aspect of anti-migration surface structures design: Utilisation of grinding scars angled against the unidirectional thermal gradient is an effective way of preventing the migration.Microgrooves perpendicular to the unidirectional thermal gradient effectively obstructs the migration,and increasing the groove depth could obtain a better obstruction effect.Microdimples can impede the migration induced by the omnidirectional thermal gradients,and increasing the depth or the area density of microdimples amplifies the resistance to migration.(4)In the aspect of functional fluids: Utilization of ferrofluids as a lubricant demonstrates a strong anti-migration performance under an external magnetic field,and ferrofluids with higher saturation magnetization can hinder the migration behavior more effectively.A magnetic switch is designed to restrict the migration and re-concentrate the migrated ferrofluids.All the conclusions drawn from this study provide a wide range of references for the migration regulation strategies.