suspension viscosity
Viscosity is simply how thick a liquid is — how hard it resists flowing. Water is thin (low viscosity), honey is thick (high viscosity). For a ceramic slurry, viscosity is the single most-watched property, because it decides whether the slip pours cleanly into a mould, spreads evenly under a casting blade, or clogs and drags. Formally it is the ratio of the shear stress you apply to the rate of shear it produces, measured in pascal-seconds (Pa.s); water is about 0.001 Pa.s, a pourable casting slip might be 0.1 to 1 Pa.s.
What sets a suspension's viscosity is how crowded and how sticky the particles are. Add a few particles to a liquid and each one disturbs the flow around it, thickening the fluid a little: Einstein's result for a very dilute suspension is eta = eta0 times (1 + 2.5 phi), where eta0 is the liquid's own viscosity and phi is the volume fraction of solid. But ceramic slips are far from dilute, and as phi climbs the viscosity does not rise gently — it shoots up toward infinity as the particles approach maximum packing (around phi = 0.64 for random spheres), because they start to jam and touch. The Krieger-Dougherty equation captures this: eta = eta0 times (1 - phi/phi_max)^(-2.5 times phi_max). Two other things push viscosity up hard: flocculation, because clumps trap liquid inside them and act like bigger, awkward particles, and fine particle size, because more surface means more particle-particle interaction. This is exactly why a good dispersant, by un-clumping the particles, can slash viscosity so dramatically.
The practical goal is a slip that carries as much powder as possible while staying low enough in viscosity to process — high solids for a dense, low-shrinkage green body, low viscosity for clean flow and filling. Those two pull against each other, and colloidal control is how you win room between them: disperse well and you can load more solids at the same viscosity. But be careful reading a single viscosity number, because a slurry is usually not a simple (Newtonian) liquid — its viscosity changes with how fast you shear it (shear thinning) and even with time (thixotropy), so you must always ask at what shear rate a quoted viscosity was measured.
Two alumina slips are made at the same 50 vol% solids. The flocculated one, full of liquid-trapping clumps, is a stiff paste near 5 Pa.s; add the right dispersant to break the clumps and the very same solids loading pours at about 0.3 Pa.s — a fifteen-fold drop with no change in how much powder is present.
Dispersing the same solids loading can cut viscosity many-fold, because breaking flocs frees the liquid they had trapped.
A single viscosity value is meaningless for a ceramic slurry unless you state the shear rate, because most slips are non-Newtonian — they thin as you stir them faster and can change with time. Always ask at what shear rate a viscosity was measured.