# Do superhydrophobic/hydrophobic materials generally have a lot of friction or only a little friction

Also, are there any exceptions that you know of?

• Welcome to Chemistry.SE. Would you share a little bit more about what you ope to learn? For example, do you want a low-friction or high-friction superhydrophobic surface? – Ben Norris Dec 17 '13 at 13:48
• I want to find a high-friction superhydrophobic or preferably icephobic surface. – Thomas Dec 18 '13 at 14:34

However, if you have a droplet moving, instead of a full liquid stream, then there is a counter-acting effect which is called contact angle hysteresis, which allows a droplet to sustain a difference in contact angle at the front and back of the droplet. This causes a force counter-acting motion of the order of the surface tension of the liquid $\gamma$ and the difference between the contact angles $\Delta \theta$: $F=\gamma l \Delta \theta$, where $l$ is a lengthscale of the order of the droplet radius. This force basically acts as a static friction on the droplet.
The value of $\Delta \theta$ is a complex function of the amount of surface roughness, being 0 for a atomically smooth surface but also approaching 0 for roughness with a small typical lengthscale. Therefore it is hard to make a general statement whether superhydrophobic surfaces will have a lot or a little friction. It all depends on the details of the surface roughness.