Van der Waals' attraction between anisometric particles

Abstract

Calculations based on additivity of the London-van der Waals' dispersion forces show that for particles of colloidal dimensions the total attractive energy is of the same order of magnitude as thermal energies when the mean diameter (v1/3, where v is the volume) is of the order of magnitude of the particle separation, regardless of whether the particles are ball-, rod-, or plate-shaped. At smaller separations the order of attractive energies is plates > rectangular rods > cylinders > spheres. At separations such that V 10 kT the attraction between spheres varies nearly as r−1, but for rods and cylinders it varies approximately r−2 and for platelets r−3.

Likewise, (for rods) parallel orientation is greatly favored over perpendicular orientation at small separations, and (for rectangular rods) orientation with the largest faces opposite each other is favored. These differences diminish as the particle separation increases but remain important so long as the van der Waals' attraction itself is of the order of kT or larger.