rheological behaviour of different fluids
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Answer:
Some fluids exhibit different apparent viscosities under different conditions.
Actually, the definition of viscosity implies that there is a fixed ratio between
shear stress and shear rate in the fluid. Most homogeneous fluids follow this relationship.
These are called "Newtonian" fluids. At a fixed temperature they have a
certain viscosity that has the same value when measured at any shear rate.
"Non-Newtonian" fluids show different viscosities at different shear rates.
Many non-Newtonian fluids are actually slurries of suspended particles in a liquid.
Paint, clay slips, and wet sand are examples of fluids that exhibit this strange
behavior. Figure 3-12 is a graph of shear stress vs. shear rate. In the case of a rotational
viscosimeter, it would be rpm vs. torque. Newtonian fluids show a straight
line through the origin with a slope equal to the viscosity. Paint should act as a low
viscosity material when being sheared by the action of a paint brush, but then it
should become more firm and stick to the wall without running after the brush
moves on. The particles and pigment in the paint are chosen to produce a product
that has a higher slope (apparent viscosity) at low shear rate than at high shear
rate. Clay slips usually should have similar characteristics if being applied as a
coating. Kaolinite clays do this, but montmorillonite clays and wet sand behave in
just the opposite way. As shown in Table 3-6, these non-Newtonian fluids are
called plastic, pseudoplastic, or dilatant fluids. Another term, thixotropic, is used
to describe fluids whose apparent viscosity changes with time as well as shear
rate.