the van der Waals attraction
/ van der VAHLS /
Every two lumps of matter feel a faint, universal pull toward each other, even when they carry no net charge. It comes from the electrons inside them, which are never perfectly still: at any instant one atom's electron cloud is a little lopsided, making a fleeting tiny dipole, and that dipole tugs the neighbouring atom's cloud into line, so the two flicker in step and attract. Sum that flicker over the trillions of atoms in two ceramic particles and you get the van der Waals attraction — the reason fine powders clump, dust sticks to a wall, and a suspension left alone tends to gum together.
For two particles the pull grows sharply as they get close. For two equal spheres of radius R, separated by a small gap D, the attractive energy is roughly V_A = -(A times R) / (12 times D), where A is the Hamaker constant — a material property, typically around 10^-20 to 10^-19 joule for oxides across water. The minus sign says attraction; the 1/D says that as the gap shrinks toward atomic contact the pull rockets up toward (in this idealised form) minus infinity. That deep well at contact is the primary minimum: once two particles touch there, ordinary stirring will not pull them apart. Crucially, van der Waals attraction is always on and cannot be switched off — the medium between the particles only weakens it (water cuts it below what it would be in air, but never to zero).
This single ever-present attraction is the villain the whole field is organised against. Left unopposed it flocculates a slurry into lumps, traps air, and ruins the uniformity of the green body. You cannot remove it, so instead you must overpower it with a repulsion — either electrostatic (like-charging the particles) or steric (coating them with polymer). DLVO theory is precisely the bookkeeping of van der Waals attraction versus electrostatic repulsion, and every dispersant you add is, at heart, a way to keep particles far enough apart that this attraction stays weak.
Two 0.5 micron alumina particles almost touching (a 1 nm gap) with a Hamaker constant near 5 x 10^-20 J feel an attractive energy of order tens of times the thermal energy kT — far too strong for Brownian jiggling to shake them loose. That is why an untreated fine powder always wants to aggregate.
The pull is small far away but overwhelming at contact, so particles that drift close tend to stick for good.
A common misconception is that van der Waals forces are negligibly weak. Between single molecules they are; summed over a whole particle at short range they dominate, which is exactly why a dispersion must actively fight them rather than ignore them.