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VISCOSITY. Definitions. Fluid : a material that flows when subject to a force (liquids and gases) Solids : may be deformed by a force, but do not flow Semi-solids : exhibit characteristics of both solids and liquids

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definitions
Definitions
  • Fluid: a material that flows

when subject to a force

(liquids and gases)

  • Solids: may be deformed

by a force, but do not flow

  • Semi-solids: exhibit characteristics of both solids and liquids
  • Viscosity: property by which fluids offer resistance to objects moving through them
slide3

Liquids

Solids

Semi-solids

slide4
Suspensions: mixtures in which solid matter is suspended in a fluid medium
  • Emulsions: mixtures in which small liquid droplets are dispersed in another liquid
  • Glasses: amorphous materials which behave as solids due to high viscosity
slide5

Emulsion

Suspension

viscosity7
Viscosity
  • Viscosity is the resistance that a fluid offers to flow when subject to a shear stress

Isaac Newton: “the resistance which arises from the lack of slipperiness of the parts of a fluid, other things being equal, is proportional to the velocity with which the parts of the liquid are separated from each other”

a closer look

Force

A Closer Look
  • In an ideal liquid there would be no viscosity, as there would be no forces acting between layers of a fluid
slide9
In reality, fluid layers interact through “frictional” forces. Each layer of fluid will drag the next layer to some extent
describing viscosity the model
Describing Viscosity:The Model

(Fastest moving layer of fluid)

(fluid is still at lower surface)

Velocity gradient at y1: g =

(shear rate)

.

du

dy

slide12

Shear Stress: t = F/A

Shear stress is the force delivered parallel

to an area of fluid surface

Force

Area

A

newton s law
Newton’s Law
  • Shear stress and shear rate are related by the proportionality constant h (known as the viscosity)
laminar versus turbulent flow
Laminar Versus Turbulent Flow
  • Newtons description applies to laminar flow fluids.
  • Laminar Flow: “streamline” flow. Fluid moves in a straight line with the applied force. Layers slide by with no swirls.
  • Turbulent Flow: disorderly flow. Small packets of fluid moving in all directions and all angles to normal line of flow.
reynolds number
Reynolds Number

The Reynolds number describes

the degree of turbulence

Laminar Flow: [Re] < 2000

Turbulent Flow: [Re] > 3000

Turbulent fluids have viscosity, but we usually measure viscosity in laminar flow

molecular basis of viscosity
Molecular Basis of Viscosity
  • Viscosity represents the internal friction between molecules in a fluid. The origin is net transfer of momentum between layers of fluid moving at different velocities in parallel flow by the mechanism of molecular collisions.
slide18
MOMENTUM = p = mv

Newton’s Second Law

slide19
A “layer” of fluid subject to a force has m=ADxr, velocity v1, momentum=mv1
  • Adjacent layers are not subject to the force, yet they move with a speed v<v1
  • Thus, momentum is “transferred” to other layers of fluid

Dx

Area A

v1

v2

viscosity in foods
Viscosity in Foods
  • Determines mouth feel, body of fluid foods, perceived thickness, and ease of swallowing
  • Influences pourability, how well batters stay on
  • Determines pumping needs of juice, milk, and other fluid foods
slide25

Viscosity and Temperature

  • For liquids, temperature decreases viscosity
  • The way in which it varies depends on the fluid
  • It is critical to know the temperature at which viscosity is measured
  • Arrhenius model often applies
slide27
Ea is the “activation energy”, that is, the energy barrier to flow
  • As a molecule moves it will experience minimum and maximum intermolecular forces with its neighbors. The difference represents the activation energy
slide28

Ea

Energy

Direction of Flow

slide29
Williams-Landel-Ferry Model: applies to very viscous systems
  • where C1 and C2 are constants, Tg is the glass transition temperature, and hg is the viscosity at Tg (~1012 Pa.s).
transfer processes
Transfer Processes
  • Shear stress is considered as momentum flux
viscosity units
Viscosity Units
  • Viscosity has the dimensions of ML-1T-1
  • In units
    • CGS: poise or centipoise (cp)
    • SI : Pa.s = 1000 m Pa.s
    • kinematic viscosity in centistokes (cSt)
  • Conversions
    • 1000 m Pa.s = 1 Pa.s = 1Ns/m2
    • 100 cP = 0.1 Pa.s = 100 m Pa.s =1 dyne.s/cm2
types of viscosity
Types of Viscosity
  • Dynamic or absolute viscosity
slide34

Other descriptions of viscous forces

  • Fluidity: the inverse of dynamic viscosity.

f = 1/h

  • Kinematic Viscosity: absolute viscosity divided by the density of the fluid:

u = h/r

Kinematic viscosity is measured by capillary viscometers

slide36
Often used for polymer solutions. When a polymer is added to a solvent, there is a noticeable increase in the viscosity.
  • If h is the viscosity of the solution, and ho the viscosity of only the solvent, then the following definitions apply: