Amusing lines from the literature...

[meanfreepath]

From Onoda, G. Y., "Direct Observation of Two-Dimensional, Dynamical Clustering and Ordering with Colloids," Phys. Rev. Lett. 55, 226 (1985):

It appeared that the possibility of obtaining reversible clustering phenomena at the air-water interface should be possible if the right interparticle bonding conditions are found. I, in fact, have succeeded in finding such a condition.