  In a recent Letter (Phys. Rev. Lett. vol 96, 066001 (2006), ref [1]), Choi
and Kim reported slip lengths of a few tens of microns for water on
nanoengineered superhydrophobic surfaces, on the basis of rheometry
(cone-and-plate) measurements. We show that the experimental uncertainty in the
experiment of Ref. [1], expressed in term of slip lengths, lies in the range 20
- 100 micrometers, which is precisely the order of magnitude of the reported
slip lengths. Moreover we point out a systematic bias expected on the
superhydrophobic surfaces. We thus infer that it is not possible to draw out
any conclusion concerning the existence of huge slip lengths in the system
studied by Choi and Kim.
