  Understanding about both the range and the strength of the effective force
between two hydrophobic surfaces suspended in water is important in many areas
of natural science but unfortunately has remained imperfect. Even the
experimental observations have not been explained quantitatively. Here we find
by varying distance (d) between two hydrophobic walls in computer simulations
of water that the force exhibits a bi-exponential distance dependence. The long
range part of the force can be fitted to an exponential force law with
correlation length of 2 nm while the short range part displays a correlation
length of only 0.5 nm. The crossover from shorter range to longer range force
law is rather sharp. We show that the distance dependence of the tetrahedrality
order parameter provides a reliable marker of the force law, and exhibits
similar distance dependence.
