pseudoplastic flow
Pseudoplastic flow, more plainly called shear-thinning, describes a material that becomes thinner and runnier the harder you stir, shake, or push it. Wall paint is the classic example: thick and drip-free in the can, but it spreads easily under the brush and then thickens again on the wall. The more shear you apply, the lower the apparent viscosity becomes.
The mechanism is usually long, tangled polymer molecules or loosely structured particles. At rest they are randomly coiled and entangled, giving high resistance to flow. As shear rate rises they untangle and line up with the flow direction, so they slip past one another more easily and the viscosity drops. Remove the shear and they re-randomise, so the thickness returns. Unlike plastic flow, there is no yield value: even the gentlest stress produces some flow.
Pseudoplasticity is the single most useful flow type in pharmaceutics. It lets a suspension stay thick enough at rest to keep particles from settling, yet pour and swallow easily when shaken, and lets gels, lotions, and ophthalmic preparations spread smoothly while still clinging where applied. Common shear-thinning agents include methylcellulose, sodium carboxymethylcellulose, xanthan gum, and acacia.
An oral antibiotic suspension thickened with xanthan gum stays uniform on the shelf yet thins instantly when shaken, so the child receives an accurate, evenly dosed spoonful.
Shear-thinning keeps a suspension stable at rest but pourable on demand.
Pseudoplastic curves bend toward the shear-rate axis and start at the origin (no yield value); this distinguishes them from plastic flow, whose curve only begins after a yield stress.