= Solution
<Flocculation> indicates that the attraction/kinetic balance has changed: <electrolyte> may screen a double-layer barrier; <pH> may reduce surface charge; multivalent ions can promote aggregation; added <polymer> can bridge particles or cause <depletion force>; <poor solvent> conditions can collapse stabilizing layers. Rapid evaporation concentrates both particles and salts and can force contacts faster than they anneal. Aggregates then sediment much faster because their effective buoyant size has grown.
Check <ionic strength>, <pH>, surface charge and the <particle size distribution> before and during assembly. Dialyse or reduce salt where appropriate, restore surface charge, add a nonbridging <steric stabilization> with a compatible <solvent>, and slow the <concentration> step. Compare untreated and deliberately salt-screened controls. Restoring repulsion after irreversible aggregation may not redisperse the particles, so prevention is preferable to assuming all settled material can be recovered.
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