There should be enough water to fill the voids, so the particles of sand rub against each other, dramatically increasing the apparent viscosity as the shear rate increases. Other materials that exhibit this type of behavior are concentrated (especially deflocculated) aqueous suspensions of china clay, titanium oxide, corn flour, and wet cement aggregates. Viscoplastic fluids are those that appear to show a "yield stress." That is, a certain amount of shear stress must be applied to the material before it can begin to flow, or deform. Whereas in fact it seems that if flow is measured over a wide enough range of shear rates, no yield stress really does exist,5 the concept of a yield stress remains very convenient, because a number of materials do closely approximate to this type of behavior.
If the material is being sheared but at less than the yield shear stress (Ryield), then the material will deform "elastically" and flow as a rigid body and not like a fluid. The classic example of this type of
behavior is the behavior of toothpaste, which flows "en masse" through the nozzle of the container tube when the tube is squeezed (shear at the nozzle wall being much higher than in the bulk of the paste), but stays firmly rod shaped on the toothbrush until at some higher shear rate it deforms when brushed onto one's teeth. Actually, this is an example of a Bingham plastic, which, once the shear stress exceeds the yield stress for a limited range of shear rates, will then flow with shear stress directly proportioned to shear rate. To be more precise, we should really say that such fluids flow with any further shear stress (R), over and above the yield stress, in a way that is directly proportioned to shear rate. This is somewhat similar to Newtonian behavior except that the yield shear stress (Ryield) must be reached before there can be any flow. After that yield stress has been reached and over a limited range of
If the material is being sheared but at less than the yield shear stress (Ryield), then the material will deform "elastically" and flow as a rigid body and not like a fluid. The classic example of this type of
behavior is the behavior of toothpaste, which flows "en masse" through the nozzle of the container tube when the tube is squeezed (shear at the nozzle wall being much higher than in the bulk of the paste), but stays firmly rod shaped on the toothbrush until at some higher shear rate it deforms when brushed onto one's teeth. Actually, this is an example of a Bingham plastic, which, once the shear stress exceeds the yield stress for a limited range of shear rates, will then flow with shear stress directly proportioned to shear rate. To be more precise, we should really say that such fluids flow with any further shear stress (R), over and above the yield stress, in a way that is directly proportioned to shear rate. This is somewhat similar to Newtonian behavior except that the yield shear stress (Ryield) must be reached before there can be any flow. After that yield stress has been reached and over a limited range of
