Structure of dilatant fluids.
Image adapted from source [3]
Dilatancy is a property that exists primarily in colloidal dispersions. A colloidal dispersion is where one substance is microscopically dispersed evenly throughout another.
When no stress is being applied, the small repulsive forces between the particles are enough to allow them to flow past each other as a liquid. However, when a stress is applied, these small interparticular forces are overwhelmed and they clump together, forming hydroclusters (shown in red on diagram). The hydroclusters make it more difficult for the particles to flow around each other, thus increasing the viscosity[3][7]. In some dilatant fluids enough stress can cause solid-like behaviour, as essentially one large hydrocluster is formed and none of the particles can flow around one another.
When the stress is relieved the interparticular forces cause the hydroclusters to dissipate as the particles diffuse from the high concentrations in the hydroclusters to the low, until an equilibrium is formed once again. This allows the dilatant material to revert back to the fluid state. However, it should be noted that some shear thickening fluids do thicken irreversibly.
Effect of rapidity of applied force
Another property of most shear thickening fluids is that they increase more in viscosity when a stress is rapidly applied than when the same stress is applied more slowly. This is because when a stress is slowly applied the particles have time to flow around the force and remain relatively fluid. When a stress is rapidly applied, the particles do not have time to flow into convenient positions, and they thus clump together as described above, causing the increase in viscosity or hardening[6].
Now click here to see how dilatants are used in body armour.
When no stress is being applied, the small repulsive forces between the particles are enough to allow them to flow past each other as a liquid. However, when a stress is applied, these small interparticular forces are overwhelmed and they clump together, forming hydroclusters (shown in red on diagram). The hydroclusters make it more difficult for the particles to flow around each other, thus increasing the viscosity[3][7]. In some dilatant fluids enough stress can cause solid-like behaviour, as essentially one large hydrocluster is formed and none of the particles can flow around one another.
When the stress is relieved the interparticular forces cause the hydroclusters to dissipate as the particles diffuse from the high concentrations in the hydroclusters to the low, until an equilibrium is formed once again. This allows the dilatant material to revert back to the fluid state. However, it should be noted that some shear thickening fluids do thicken irreversibly.
Effect of rapidity of applied force
Another property of most shear thickening fluids is that they increase more in viscosity when a stress is rapidly applied than when the same stress is applied more slowly. This is because when a stress is slowly applied the particles have time to flow around the force and remain relatively fluid. When a stress is rapidly applied, the particles do not have time to flow into convenient positions, and they thus clump together as described above, causing the increase in viscosity or hardening[6].
Now click here to see how dilatants are used in body armour.