Page 30 - Fluid mechanics, heat transfer, and mass transfer

NEWTONIAN AND NON-NEWTONIAN FLUIDS 7

as paints and printing inks of high concentrations, . What are thixotropic fluids?
gum arabic in water, and solutions of certain & Thixotropic fluids are time-dependent fluids for
surfactants. which structural rearrangements occur during defor-
& Most shear thickening fluids tend to show shear mation at a rate too slow to maintain equilibrium
thinning behavior at very low shear rates. However, configurations. As a result, shear stress changes with
at high shear rates, the suspension expands (dilates), duration of shear. These show decreasing shear stress
such that the amount of liquid present cannot over- with time at constant shear rate. In other words,
come the frictional forces between the particles and apparent viscosity decreases with time under shear
the consequent increase in apparent viscosity, m a . conditions. After shear ceases, apparent viscosity
. What are power law fluids? returns to its original value, with the time for recovery
varying for different fluids. Increased agitation nor-
& Power law fluids are fluids with non-Newtonian
mally decreases apparent viscosity, but this depends
behavior, exhibiting anonlinear relationship between
on duration of agitation, viscosity of the fluid, and
the shear stress (t) and shear rate (l).
rate of motion before agitation.
. What is a Bingham plastic?
& Examples of thixotropic fluids include mayonnaise,
& Non-Newtonian fluids for which a finite stress is
margarine, honey, shaving cream, clay suspensions
required before continuous deformation takes place
used as drilling muds, ketchup, gelatin solutions,
are called yield stress materials, which are also
some polymer solutions and food materials, and
called viscoplastic materials. These fluids will not
some paints and inks.
flow when only a small shear stress is applied.
. What are rheopectic fluids?
The shear stress must exceed a critical value, known
as the yield stress t 0 , for the fluid to flow. For & For rheopectic fluids, shear stress increases with time
example, when a tube of toothpaste is opened, certain at constant shear rate, their behavior being opposite
amount of force is needed to be applied before the to thixotropy.
toothpaste will start flowing. In other words, visco- & Rheopectic behavior has been observed in bentonite
plastic fluids behave like solids when the applied clay suspensions, gypsum suspensions, certain sols
shear stress is less than the yield stress. Once it such as vanadium pentoxide, and some polyester
exceeds the yield stress, the viscoplastic fluid will solutions.
flow just like a fluid. . What are rheomalectic fluids?
& Bingham plastics are a special class of viscoplastic & In rheomalectic fluids, viscosity decreases with time
fluids that exhibit a linear behavior of shear stress under shear conditions but does not recover, with the
against shear rate. fluid structure being irreversibly destroyed.
& Bingham plastics have definite yield value that must . What are viscoelastic fluids?
be exceeded before flow starts. After flow starts, & Viscoelastic fluids exhibit both viscous and elastic
viscosity decreases with increase in agitation.
behavior. They behave like elastic rubber-like solids
& Examples of viscoplastic fluids are drilling mud,
and as viscous liquids.
toothpaste, paper pulp, greases, soap, nuclear fuel & They exhibit elastic recovery from deformation when
slurries, mayonnaise, margarine, chocolate mixtures,
stress is removed.
blood, suspensions of chalk, grain, and thoria, and
& Viscoelastic fluids exhibit tendencies such as climb-
sewage sludge.
ing up a rotating shaft, swelling when extruded out of
& Bingham plastic materials are the simplest of the
a dye, and the like. This phenomenon is called
yield stress materials. Highly concentrated suspen-
Weissenberg effect. Polymeric liquids, flour dough,
sions of fine solid particles frequently exhibit Bing-
egg white, and bitumen are examples of this class.
ham plastic behavior. Slope of the line shown in
& Relaxation time, the time required for elastic effects
Figure 1.1 for a Bingham plastic is called infinite
to decay, is a property of these fluids.
shear viscosity.
& Viscoelastic effects may be important with
& The equation for a Bingham plastic material is
sudden changes in rates of deformation, as during
given as
flow start-up or stoppage, rapidly oscillating flows,
t ¼ t y þ m g: ð1:9Þ and during expansions and contractions involving
1
accelerations.
& Figure 1.1 illustrates shear rate–shear stress behavior & In many fully developed flows, viscoelastic fluids
of Newtonian and non-Newtonian fluids. behave as purely viscous fluids.