Development of hydrophobic textured surfaces for low friction applications

The object here is to produce hydrophobic textured surfaces having controllable frictional properties for lubricated systems. This will be achieved by knowing that the presence of sliding on the surfaces makes improve the lubrication, this fluid slip appears only when the surfaces are hydrophobic [1]. Finally, the surfaces are hydrophobic thanks to their low surface energy or their texturing.

The choice of material was based on two conditions. On the one hand, it must have a low surface energy and therefore a contact angle θ greater than 100°. On the other hand, it must have good frictional resistance in order to preserve patterns during tribological tests. Based on these two criteria, Polytetrafluoroethylene (PTFE) has been whose contact angle is equal to 110° and a Young’s modulus between 600 and 1000MPa.

In order to have a successful texture, we must have surfaces that answer to the Cassie-Baxter regime (whose equation is Cos θ_CB = rf cos θ + f – 1, where r is the roughness factor and f is the wet area fraction [2]), where the fluid remains suspended at the vertices of the texture. Several geometric shapes have been studied : paraboloid studs, square holes and studs, cylindrical holes, striations … Best results have been theoretically obtained with paraboloid studs, cylindrical holes and square holes. Patterns were produced with FIB. SEM observations showed that texturing of PTFE with FIB is possible. A crush test will be realised once the textured surfaces are ready. We can thus calculate the sliding length of the lubricant.

   

(a) paraboloic                                 (b) squared                                 (c) cylindral holes

[1] Large Slip of Aqueous Liquid Flow over a Nanoengineered Superhydrophobic Surface, Chang-Hwan Choi et Chang-Jin Kim, Mechanical and Aerospace Engineering Department, University of California, Los Angeles.

[2] Optimizing Super-Hydrophobic Surfaces : Criteria for Comparison of Surface Topographies, Eyal Bittoun et Abraham Marmur, Departement of Chemical Engineering, Technion-Israel Institute of Technology.