Rheological Dilatancy
Non-Newtonian fluid behavior wherein apparent dynamic viscosity increases as the applied shear rate rises defines the dilatant response of concentrated suspensions. Under the mechanism of shear thickening, densely packed solid particles suspended within liquid continuous phases transition from structured, layered flow regimes into chaotic, jamming networks under abrupt mechanical stress. Pigmented coating formulations used in the paper industry, containing high concentrations of calcium carbonate, clay, and latex binders, encounter this flow anomaly when forced through restricted geometry gaps.
When metering blades or coating rolls apply extreme shear strain, particle hydrodynamic lubrication fails, forcing mineral solids to make direct frictional contact and expand their hydrodynamic volume. Apparent viscosity surges instantaneously, inducing high hydraulic pressures that disrupt coating application equipment and mark the passing substrate.
Blade Metering Disruption
Metering blades operating on high-speed coater heads generate local shear rates exceeding one million reciprocal seconds. Encountering shear thickening at these velocities generates immense hydraulic repulsion against the metering element, deflecting the blade tip and causing coating weight variations, blade bleeding, and scratch marks along the moving web. Pigment slurries must be engineered through dispersant additions and optimized particle size distributions to suppress dilatant transitions across all machine operational speeds.
Formulation Boundaries
Dilatant behavior ceases immediately once applied shear rates fall below critical threshold levels, allowing formulations to return to lower baseline viscosities. Shear thickening differs strictly from thixotropic breakdown or shear-thinning pseudoplasticity, presenting operational challenges primarily in ultra-high-solids coating regimes where free liquid volumes become insufficient to separate suspended solids.
