WHEN particles of colloidal sizes aggregate “chain formation” is frequently observed. An extreme example is the flocculation of magnet particles1, where stearic mechanisms are undoubtedly involved, but a wide range of spherical particles including both liquid sols and aerosols also contain unexpectedly long strings of particles (for example, refs. 2–5). Recent attempts to investigate the geometrical structure of flocs by simulation on a digital computer also had the unexpected result of generating chains of particles6. Because the simulation model was based on a purely random aggregation process no directional mechanism was allowed and the important factor governing chain formation seemed to be the collision sequence leading to the formation of the aggregate. Chain formation was much more extensive, for the aggregate was formed by the addition of clusters of particles rather than of individual units.