steric stabilization
/ STAIR-ic /
The second way to keep particles apart is not electrical at all — you wrap each one in a fuzzy coat of dangling polymer chains, like giving every particle a fur collar. When two coated particles drift close, their soft polymer layers touch and resist being squashed together, holding the hard cores at arm's length so they never reach the range where van der Waals attraction takes over. This is steric stabilization — stabilization by sheer bulk and crowding of adsorbed molecules rather than by charge.
Why does the coat push back? Two reasons, both rooted in the polymer chains being happier free than crushed. As two coats overlap, that region suddenly holds too many chain segments, which is thermodynamically unfavourable in a good solvent (an osmotic pressure sucks solvent in to push the particles apart), and the chains also lose wiggle room, so their entropy drops and they resist the crowding. For this to work you need the chains firmly anchored (so they cannot just be shoved aside), dense enough to cover the surface, thick enough to hold cores beyond the attraction's reach (typically several nanometres), and dissolved in a solvent they like. Get any of those wrong — too little polymer, or a poor solvent that makes the chains collapse — and the coats can even bridge two particles together, causing flocculation instead. Often the dispersant does both jobs at once: a charged polymer like sodium polyacrylate anchors to the surface, sticks out charged loops, and stabilizes by charge and bulk together — electrosteric stabilization.
Steric stabilization is the workhorse where electrostatics fails. In organic solvents (tape casting in alcohols, ceramic injection moulding in wax and polymer) there is almost no surface charge to exploit, so polymer coats are the only practical route. It is also far more forgiving of salt and of high solids loading, because it does not rely on a fragile double layer that salt can crush — this is why concentrated, high-quality casting slips so often use polymeric or electrosteric dispersants. The trade-off is that the same polymer must later be burned cleanly out of the green body, and too much of it can leave residue or pores after firing.
A tape-casting slip of barium titanate in a toluene-ethanol mix has essentially no usable surface charge, so it is stabilized sterically with an adsorbed fish-oil or polymer dispersant whose chains reach out several nanometres; those soft layers keep the powder dispersed all the way up to 40 vol% solids, where a purely electrostatic slip would have failed long before.
Polymer coats hold particles apart by bulk, so steric stabilization survives salt, solvents and high solids where charge alone cannot.
Too little polymer, or chains that reach across to two particles at once, produces bridging flocculation — the opposite of what you wanted. Steric stabilization needs full, dense surface coverage in a good solvent to actually stabilize.