  Recently, there has been much interest in using lubricated flat and
nano-/micro-structured surfaces to achieve extreme liquid-repellency: any
foreign droplet immiscible with the underlying lubricant layer was shown to
slide off at a small tilt angle $<$ 5$^{\circ}$. This behavior 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
confocal optical interferometry, we are able to visualize the intercalated film
under both static and dynamic conditions. We further demonstrate that the
lubricant flow entrained by droplet motion can transform a partially dewetted
film into a continuous layer, by generating a sufficient hydrodynamic force to
lift the droplet over the solid substrate. The droplet is therefore
oleoplaning, akin to tires hydroplaning on a wet road, with minimal dissipative
force (down to 0.1 $\mu$N for 1 $\mu$l droplet when measured using a cantilever
force sensor) 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.

Fluid Dynamics