Oleoplaning droplets on lubricated surfaces

Recently, there has been much interest in using lubricated surfaces to achieve extreme liquid repellency: A foreign droplet immiscible with the underlying lubricant layer was shown to slide off at a small tilt angle <5°. This behaviour was hypothesized to arise from a thin lubricant overlayer film sandwiched between the droplet and solid substrate, but this has not been observed experimentally. Here, using thin-film interference, we are able to visualize the intercalated film under both static and dynamic conditions. We further demonstrate that for a moving droplet, the film thickness follows the Landau-Levich-Derjaguin law. The droplet is therefore oleoplaning - akin to tyres hydroplaning on a wet road - with minimal dissipative force and no contact line pinning. The techniques and insights presented in this study will inform future work on the fundamentals of wetting for lubricated surfaces and enable their rational design.