DLVO theory
/ dee-el-vee-OH /
DLVO theory is the simple, powerful idea that whether two particles stick or stay apart is decided by adding up just two forces as a function of how far apart they are: the van der Waals attraction, which always pulls them together, and the electrostatic repulsion from their overlapping double layers, which pushes them apart. Named for its four originators (Derjaguin and Landau, Verwey and Overbeek), it is the bookkeeping of that tug-of-war. Draw the total interaction energy against separation and the shape of that curve tells you the fate of the whole suspension.
The two contributions behave very differently with distance. The attraction falls off gently, roughly as 1/D. The repulsion falls off steeply, decaying exponentially with the Debye length of the double layer. Add them and a characteristic curve appears: at very short range attraction wins, giving a deep primary minimum (particles stuck fast in contact); at intermediate range repulsion can win, raising an energy barrier that particles must climb to reach that minimum; and sometimes at longer range there is a shallow secondary minimum where particles loosely associate without truly touching. Stability comes down to the barrier height, measured in units of the thermal energy kT. If the barrier is tall — say more than about 15 to 20 kT — Brownian collisions almost never carry particles over it, so the suspension stays dispersed for a long time. If the barrier is low or gone, every collision sticks and the slurry flocculates fast.
DLVO explains, in one framework, most of the levers a ceramist pulls. Lower the pH toward strong charge and you raise the barrier and disperse; move to the isoelectric point and the repulsion vanishes, the barrier drops to zero, and the slip flocculates. Add salt and you compress the double layer, shrink the barrier, and coagulate — the theory even predicts a critical coagulation concentration of salt. Be honest about its limits, though: classical DLVO ignores steric (polymer) forces, solvation and hydration layers, surface roughness, and ion-specific effects, so real slurries stabilized by polymers or in concentrated, non-ideal conditions often need an extended DLVO or a separate steric treatment. It is the right first picture, not the last word.
For an alumina slurry at pH 4, DLVO predicts a repulsion barrier of tens of kT — collisions bounce off and the slip stays fluid for days. Nudge the pH up to about 9 (the isoelectric point) and the barrier collapses to zero; now every particle that touches sticks, and within minutes the same slurry sets into a lumpy gel.
The height of the DLVO energy barrier, in units of kT, decides whether a slurry stays dispersed or flocculates.
Classical DLVO counts only van der Waals plus electrostatic forces. It cannot describe steric stabilization by polymers, hydration forces, or ion-specific effects, so a polymer-dispersed or concentrated ceramic slurry usually needs an extended (steric) treatment on top.