the yield stress
Some slurries behave like a solid until you push hard enough, then flow like a liquid. Toothpaste sits in a firm bead on the brush and does not run, yet squeeze the tube and it flows out smoothly; mayonnaise holds a peak and stays put until stirred. The minimum stress you must apply before such a material will flow at all is its yield stress. Below it the slurry holds its shape like a soft solid; above it, it flows.
The yield stress comes from a continuous network the particles have built across the whole slurry — a weak, space-filling scaffold of flocculated particles linked by van der Waals attraction. That network can bear a small load elastically, like a fragile lattice, but once the stress exceeds what the inter-particle bonds can hold, the network breaks and the material starts to flow. A common model is the Bingham plastic: tau = tau_y + eta_pl times (shear rate), where tau_y is the yield stress and eta_pl the plastic viscosity once it is flowing. So a slurry with yield stress is not just thick — it is fundamentally in-between solid and liquid, and how big the yield stress is tells you directly how strongly flocculated the particles are: a well-dispersed slip has essentially no yield stress, while a flocculated one can have a large one.
Yield stress is a double-edged property you deliberately design for. You want a large yield stress when a body must hold its own shape: it stops heavy particles from settling out (they cannot sink because the network holds them), lets a screen-printed line or an extruded rod keep its form the instant it leaves the die, and is the whole basis of gel casting and of pastes for ceramic 3D printing, where the printed shape must not slump before it sets. But you want a low or zero yield stress when a slip must pour and fill a mould completely and drain cleanly, as in slip casting. So formulators flocculate on purpose to raise yield stress, or disperse to kill it, depending on the forming route. The honest caveat: yield stress is famously hard to measure unambiguously — different instruments and methods give different numbers — so treat a quoted value as method-dependent, not absolute.
A gel-casting slurry is formulated with a deliberate yield stress so that after it is poured into a mould and the gel sets, coarse and fine particles alike are held in place and cannot segregate by settling. The same yield stress lets a paste-extruded ceramic filament for 3D printing keep its round cross-section the moment it leaves the nozzle instead of flattening into a puddle.
A yield stress makes a slurry hold its shape until pushed — essential for suspending particles and for shape-holding forming.
Yield stress is not a single hard number: it depends on the measurement method and time scale, and a very slow flow can occur even below the apparent yield stress. Treat it as a practical, method-dependent handle on flocculation, not an exact material constant.