Entropic torque
  Quantitative predictions are presented of a depletion-induced torque and force acting on a single colloidal hard rod immersed in a solvent of hard spheres close to a planar hard wall. This torque and force, which are entirely of entropic origin, may play an important role for the key-lock principle, where a biological macromolecule (the key) is only functional in a particular orientation with respect to a cavity (the lock). 
R. Roth, R. van Roij, D. Andrienko, K. R. Mecke, and S. Dietrich