= Solution
<DLVO theory> combines the attractive <Van der Waals force> with repulsion between overlapping electrical double layers. A charged colloid attracts counterions; their electrostatic and osmotic response screens its field over the <Debye length>. Adding salt generally shortens this range and can lower the repulsive aggregation barrier, allowing particles to reach the strongly bound primary minimum. A shallow secondary minimum can instead give weak reversible association. Whether Brownian encounters cause rapid aggregation depends on barrier heights relative to $k_BT$, not just on the sign of the force at one separation.
Adsorbed or grafted polymer layers can provide <steric stabilization>: overlap costs chain <entropy> and increases the local <osmotic pressure>. Nonadsorbing smaller particles or polymers can create a <depletion force>, because bringing large surfaces together increases the volume accessible to the smaller species. Polymer bridging can be attractive when one chain binds more than one particle. A useful comparison experiment varies salt concentration while monitoring scattered light or aggregation, then repeats with a sterically protected suspension. Charge, solvent quality, polymer coverage and particle size determine which interaction dominates; none is a universal constant of all colloids.
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