dilatants

Dilatants are materials that increase in volume when subjected to shear. The term is used for two main classes: dilatant fluids, also called shear-thickening fluids, and dilatant granular materials such as certain sands and gravels that dilate under shear. In both cases, the response involves a change in microstructure driven by particle contacts and rearrangement.

Dilatant fluids: In suspensions, viscosity rises as shear rate increases, often abruptly. The mechanism is thought to involve transient networks or hydroclusters of suspended particles that resist flow and become more pronounced at higher shear. The phenomenon is typically reversible when shear is reduced. Common demonstrations include cornstarch in water, sometimes called oobleck.

Dilatant soils: In dense granular soils, shear causes particles to rearrange into a looser configuration, requiring additional volume to move. This positive dilatancy influences shear strength, drainage, and pore pressure during rapid loading. Dilatant behavior is more pronounced in sands and gravels and is central to concepts such as the dilatancy angle and critical-state soil mechanics.

Applications and implications: Dilatant fluids are engineered for protective gear and impact mitigation because their viscosity increases with sudden forces. In geotechnical engineering, dilatancy affects foundations, slopes, and earthworks, particularly under dynamic or rapid loading where temporary strength changes and pore-pressure responses occur.