dilatant flow

Dilatant flow, or shear-thickening, is the opposite of pseudoplasticity: the material grows thicker and more resistant the faster you try to move it. The famous kitchen demonstration is a thick paste of cornstarch in water, which you can stir slowly with ease but which seizes up and feels almost solid if you strike or stir it suddenly.

The mechanism arises in concentrated suspensions of small, deflocculated particles. At rest and at low shear the liquid just fills the gaps between closely packed particles and lubricates their movement. When sheared rapidly the particles are forced to ride up over one another, the packing opens up (dilates), and there is no longer enough liquid to fill the enlarged voids, so internal friction rises sharply and the viscosity climbs.

Dilatancy is generally an unwanted nuisance in pharmaceutics, not a designed feature. It typically appears in dense suspensions containing a high percentage of small, deflocculated solid particles, where it can cause real manufacturing problems: a mixture that handles fine at low speed may solidify, overload a mixer motor, or jam high-speed processing equipment. Formulators usually try to avoid it by adjusting solids content or particle state.