Viscosity is a measure of a fluid's internal resistance to shear forces or flow. It describes a fluid's resistance to changes in shape when stress is applied. Viscosity depends on the molecular structure of the fluid and arises due to intermolecular friction. Newtonian fluids have a constant viscosity that does not change with shear rate, while non-Newtonian fluids have variable viscosities. Viscosity can be measured using capillary viscometers, rotational viscometers, and online viscometers. It is affected by temperature, with most liquids becoming less viscous when heated. Viscosity is an important consideration in applications like lubrication, pumping fluids, blending fuels, and coating or printing processes.

1. Viscosity

2. Viscosity
• Viscosity is the measure of the internal friction of a fluid.
• This friction becomes apparent when a layer of fluid is
made to move in relation to another layer.
• The greater the friction, the greater the amount of force
required to cause this movement, which is called shear.
• Shearing occurs whenever the fluid is physically moved
or distributed, as in pouring, spreading, spraying, mixing,
etc. Highly viscous fluids, therefore, require more force to
move than less viscous materials.

3. • Isaac Newton defined viscosity by considering the model
represented in the figure below.
• Two parallel planes of fluid of equal area A are separated
by a distance dx and are moving in the same direction at
different velocities V1 and V2.

4. • Newton assumed that the force required to maintain this
difference in speed was proportional to the difference in
speed through the liquid, or the velocity gradient. To
express this, Newton wrote:
• where Ƞ is a constant for a given material and is called its
viscosity.

5. • The velocity gradient, dv/dx , is a measure of the change
in speed at which the intermediate layers move with
respect to each other. It describes the shearing the liquid
experiences and is thus called shear rate.
• The term F/A indicates the force per unit area required to
produce the shearing action. It is referred to as shear
stress.
• So, mathematically Viscosity can be defines as

6. Types of Viscosity

7. Newtonian fluids

8. • This type of flow behavior Newton assumed for all fluids
is called Newtonian.
• It is, however, only one of several types of flow behavior
you may encounter.
• Graph A shows that the relationship between shear
stress and shear rate is a straight line.
• Graph B shows that the fluid's viscosity remains constant
as the shear rate is varied.
• Typical Newtonian fluids include water and thin motor
oils.

9. Non Newtonian fluids
• A non-Newtonian fluid is broadly defined as one for
which the relationship is not a constant.
• It means that there is non-linear relationship between
shear rate & shear stress.
• In other words, when the shear rate is varied, the shear
stress doesn't vary in the same proportion (or even
necessarily in the same direction).
• E.g. Soap Solutions & cosmetics, Food such as butter,
jam, cheese, soup, yogurt, natural substances such as
lava, gums, etc.

10. Non Newtonian fluids

11. Units of Absolute Viscosity

12. Kinematic Viscosity

13. Instruments for Measuring Viscosity

14. Principle of Viscosity Measurement

15. Capillary Viscometer

16. Capillary Viscometer

17. Rotational Viscometer
• Though gravity is available everywhere for free, it is sometimes not strong enough
as a driving force.
• For highly viscous fluids a measurement based on gravity would take far too long.
• Therefore, rotational viscometers use a motor drive. Unlike capillary viscometers,
rotational viscometers provide dynamic or shear viscosity results.
• A rotational viscometer consists of a sample-filled cup and a measuring bob that is
immersed into the sample. There are two main principles in use:
• The Couette Principle
• The Searle Principle

18. Rotational Viscometer
The Couette Principle
If the bob stands still and the drive rotates the sample cup, this is the Couette
principle (named after M. M. A. Couette, 1858 to 1943). Although this construction
avoids problems with turbulent flow, it is rarely used in commercially available
instruments. This is probably due to problems with the insulation and tightness of the
rotating sample cup.

19. Rotational Viscometer
The Searle Principle
In most industrially available viscometers the motor drives the measuring bob and the
sample cup stands still. The viscosity is proportional to the motor torque that is
required for turning the measuring bob against the fluid’s viscous forces. This is called
the Searle principle (named after G. F. C. Searle, 1864 to 1954).
When employing the Searle principle, the bob's rotational speed in low-viscosity
samples should not be too high. Otherwise turbulent flow could occur due to
centrifugal forces or the effects of inertia.
The motor turns a measuring bob or spindle in
a container filled with sample fluid. While the
driving speed is preset, the torque required for
turning the measuring bob against the fluid’s
viscous forces is measured.

20. Rotational Viscometer
• Servo Devices
• This viscometer type uses a servo motor to drive the main shaft. The spindle with
the measuring bob (rotor) is attached directly to the shaft. A high-resolution digital
encoder measures the rotational speed. The motor current is proportional to the
torque caused by the viscosity of the sample under test. The viscosity can be
computed based on rotational speed and current.
• Compared to models with a pivot bearing and spring systems, viscometers with a
servo motor cover a wider measuring range and are more robust. The electronic
decoder and motor allow for greater torque and speed ranges than is possible with
a mechanical spring.
• However, the accuracy for low speeds and low viscosity is lower than for spring
systems, as the friction of the motor and bearing influences the measurement.
1 … Servo motor with high-resolution current measurement
2 … High-resolution optical encoder
3 … Encoder disc
4 … Measuring bob (rotor)

21. Rotational Viscometer
Coaxial cylinder geometry

22. • The rolling-ball principle uses gravity as the driving force. A ball rolls through a
closed capillary filled with sample fluid which is inclined at a defined angle. The
time it takes the ball to travel a defined measuring distance is a measure for the
fluid’s viscosity. The inclination angle of the capillary permits the user to vary the
driving force. If the angle is too steep, the rolling speed causes turbulent flow. For
calculating the viscosity from the measured time, the fluid’s density and the ball
density need to be known.
• Instruments that perform at inclination angles between 10° and 80° are rolling-ball
viscometers.
• If the inclination angle is 80° or greater, the instrument is referred to as a falling-
ball viscometer.
• Another variety of this principle is the bubble viscometer, which registers the rising
time of an air bubble in the sample over a defined distance.
Rolling- / Falling-Ball Viscometer

23. Rolling- / Falling-Ball Viscometer
• The Rolling-ball principle
• FG … Effective component of gravity
FB … Effective component of buoyancy
FV … Viscous force
• The following forces have a dominating influence
on the rolling ball: While gravity pulls the ball
downwards, the buoyancy inside the liquid and
the liquid's viscosity oppose the gravitational
force. The stronger the viscous force is, the
slower the ball rolls.

24. Rolling- / Falling-Ball Viscometer
• To calculate the ball’s viscosity from the rolling time, the gravitational and
buoyancy influence have to be considered. While the influence of gravity (FG)
depends on the ball's density and volume, an object's buoyancy depends also on
the liquid's density. This is why both the density of the liquid and the density of
the ball need to be known to obtain a viscosity result

25. Online Viscosity Measurement

26. Online Viscosity Measurement
• There are two flow paths in the sensing unit - a purge flow path (solid line)
and a bypass flow path (dotted line).
• A control signal from the measurement unit to the 3-way valve opens the
purge path for a specified interval. While the purge path is open, process
fluid flows into the measuring tube and the falling piston is pushed
upward by the force of the flow.
• When a preset interval of time (purge time) elapses, the measurement
unit sends a control signal to the 3-way valve which closes off the purge
flow path and opens the bypass flow path. When the purge flow path is
closed, the flow of process fluid in the measuring tube ceases and the
falling piston begins to descend at a velocity proportional to the viscosity
of the liquid.

27. Online Viscosity Measurement
• A permanent magnet is incorporated in the falling piston.
• Two magnetic sensors are positioned at the side of the measuring tube.
• The upper magnetic sensor is referred to as the starting point sensor and the
lower magnetic sensor is referred to as the ending point sensor.
• The measurement unit computes the time it takes for the falling piston to transit
the starting and ending point sensors (‘fall time’). It then converts this ‘fall time’
into viscosity which is displayed in % or SI units and outputs current and voltage
signals in addition to various warning and status signals. 5. When the preset fixed
time (measurement time) elapses, the measurement unit sends a control signal to
the 3- way valve which closes off the bypass flow path and opens the purge path -
in other words, returning the cycle to step [1]

28. Online Viscosity Measurement

29. Online Viscosity Measurement

30. Effect of Temperature
• The viscosity of liquids decreases with increase the
temperature.
• The viscosity of gases increases with the increase the
temperature.
• The lubricant oil viscosity at a specific temperature
can be either calculated from the viscosity -
temperature equation or obtained from the
viscosity-temperature chart.

31. Viscosity Temperature equations

32. Effect of Pressure
• Lubricants viscosity increases with pressure.
• For most lubricants this effect is considerably largest
than the other effects when the pressure is
significantly above atmospheric.
• The Barus equation :

34. Applications
• Selection of lubricants for various purpose.
- we can choose an optimum range of viscosity for
engine oil.
- for high load and also for speed operation high
viscous lubricants is required.
• In pumping operation
- for high viscous fluid high power will require.
- for low viscous fluid low power will require.
• In making of blend fuel
- less viscous fuels easy to mix.
• In the operation of coating and printing.