Functional Surface Textures Designing For Manipulating The Thermocapillary Migration Of Liquid Lubricants

Lubricant migration loss is one of the major reasons for the failures of space devices.Here,migration is a phenomenon that lubricant can freely expand on a friction surface without external forces.Considering the fact that lubricants in space devices are always of lower dosage,difficult to be replenished,and lack of a recirculation mechanism,continuous migration loss of lubricants will weaken its lubricating ability,resulting in lubrication failures.Therefore,it is essential to investigate the characteristics of lubricant migration and explore various regulation strategies to achieve durable localized lubrication under extreme special conditions such as space.To regulate the lubrication migration actively,this study focuses on the migration induced by thermal gradients and active regulation strategies based on gradient functional surface textures.Firstly,thermocapillary migration performances of different lubricants on different surfaces are tested.Then the multiscale gradient surface textures are fabricated to construct gradient functional surfaces,and the direction-guide capacity of the functional surfaces under the effect of thermal gradients are investigated.After that,the theoretical model is built to elucidate the migration regulation mechanism of the gradient functional surfaces.Finally,the designing principles of the surface texture with excellent directionguide and reconcentration capacity are proposed.The main conclusions drawn from this study are as follows:(1)Thermocapillary migration is directly correlated to the properties of solids and liquids.The solid surfaces with low surface tension can hinder thermocapillary migration effectively,and the migration velocity of polar lubricants on solid surfaces is almost zero.The dimensionless number H* is proposed to evaluate the migration capacities of the solid surfaces and lubricants quickly.(2)The geometric parameters of radial gradient microgrooves have a significant effect on the migration performance.The migration velocity on the convergent direction is higher than the divergent direction,and the migration velocity on the surface with a smaller divergence angle is higher than the surface with a larger divergence angle.Combining the lubrication approximation theory,the theoretical model basing on Navier-Stokes equations on the radially microgrooved surface is built for the prediction of migration velocity of droplets,and the numerical results agree well with the experimental data.(3)The superoleophobic surfaces with wedge shaped superoleophilic grooves have a robust direction-guide capacity.The migration distance on these surfaces is approximately two to three times higher than surfaces just featured with radial microgrooves pattern without coatings.The vertex angle has a significant effect on the migration performance,the larger vertex angle yields a shorter migration distance but a larger initial migration velocity.However,the vertex groove doesn’t have a significant effect on the migration performance.When the divergent direction is right opposite to the direction of thermal gradient,the gradient functional surface can reconcentrate the migrated droplet back to the designed regions.All the conclusions drawn from this study provide a wide range of references for the migration active regulation strategies.