Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2022 / Paper 2 / Section D / 25 / d / i / C

Secondary-minimum flocculation traps particles at a larger separation in a shallow attractive well. The primary barrier can remain intact, so particles form loose aggregates without reaching molecular contact. Such flocs are often reversible on dilution or a suitable change in ionic strength. If the secondary well is very weak compared with $k_{B}T$, the dispersion remains largely unassociated; if screening removes the primary barrier, the particles can proceed to the much deeper primary minimum. “Aggregation” and “flocculation” have variable usage, so the physical distinction here is explicitly shallow, separated association versus near-contact coagulation.