  Recently, there has been much progress in the design and application of
oil-repellent superoleophobic surfaces. Polyzwitterionic brush surfaces are of
particular interest, because of their ability to repel oil under water, even in
the absence of micro-/nano-structures. The origin of this underwater
superoleophobicity is attributed to the presence of a stable water film beneath
the oil droplet, but this had not been demonstrated experimentally. Here, using
optical interferometric techniques, we show that an oil droplet effectively
hydroplanes over a water film, whose thickness is between one hundred and
hundreds of nanometres. In addition, using a custom-built Droplet Force
Apparatus, we measured the friction and adhesion forces to be in the nN range
for millimetric-sized droplets. These forces are much lower than for other
classes of well-known liquid-repellent surfaces, including the lotus-effect and
lubricant-infused surfaces, where the typical force is on the order of $\mu$N
