Viscosity Measurements By Saja Mohammad Abdo

Out line: 1.Viscosity definition. 2.Viscosity forms. 3.Viscosity dependent factors. 4.Viscometer types. a. Rotational viscometers. b. Tube viscometers. 4. References.

Definition: *Viscosity is a quantitative measure of a fluid’s resistance to flow. *The viscosity of a fluid is a measure of its resistance to gradual deformation by shear stress or tensile stress.

Forms of shear viscosity of a fluid : Dynamic (or Absolute) Viscosity (Poises): The dynamic viscosity(η) of a fluid is a measure of the resistance it offers to relative motion(force over cross section area). where τ (Pa) and (1/sec) Kinematic Viscosity (Stokes): It is defined as the ratio of absolute viscosity to the density of fluid. ; ρ= (Kg/m3)density of fluid

Viscosity dependent factors. Viscosity is affected by different factors such as temperature, shear rate, catalyst, pressure, molecular weight,concentration and storage age. Viscosity as a function of temperate: The temperature dependence of liquid viscosity is the phenomenon by which liquid viscosity tends to decrease as its temperature increases. An exponential model for the temperature-dependence of shear viscosity (μ) was first proposed by Reynolds in 1886. µ(T) = µ̥ exp(-bT) where T is temperature and b and µ̥ are coefficients.

For example:

Viscosity as a function of pressure : viscosity increases, as a rule, with increasing pressure, provided the liquid is in stable or metastable equilibrium states.

Types of fluid: NEWTONIAN FLUIDS A Newtonian fluid is broadly defined as one for which the relationship between shear stress (τ) and shear rate (ᵞ) is a straight line. viscosity remains constant as the shear rate is varied. Typical Newtonian fluids include water and thin motor oils. Newtonian fluid

NON-NEWTONIAN FLUIDS A non-Newtonian fluid is broadly defined as one for which the relationship  is not a constant. In other words, “These fluids have different viscosity at different shear rates”. Non- Newtonian fluid

Viscometers: All viscometers apply a stress to the fluid and measure the resistance to flow. Most viscometers are in 2 major categories : 1. Rotational type viscometer “Dynamic viscosity”. .”kinematic viscosity“ 2. Tube type viscometer

Rotational viscometers: These viscometer give the value of the ‘dynamic viscosity’. It is based on the principle that the fluid whose viscosity is being measured is sheared between two surfaces. In these viscometers one of the surfaces is stationary and the other is rotated by an external drive and the fluid fills the space in between. Rotational viscometers use the idea that the torque required to turn an object in a fluid is a function of the viscosity of that fluid. They measure the torque required to rotate a disk or bob in a fluid at a known speed.

There are four main types of these viscometers: 1.Parallel plate cylinder viscometer. 2. Cone and plate viscometer. 3. Rotating cylinder viscometer. 4. Mixer viscometer

1.Parallel-plate viscometer: It is an instrument consisting of two circular parallel plates, the lower one stationary, the upper one rotatable, the disk-shaped specimen being confined between the plates. An example of a this type of viscometer is the Mooney Viscometer.

2. rotating cylinder viscometer. A rotating cylinder viscometer is described which depends for its operation upon measuring the frequency of a periodic flow “The terminal angular velocity is measured for a series of driving torques and of depths of liquid”. Compared with viscometers in common use, the results of this instrument are influenced to a smaller extent by end effects, difficulties of adjustment, or the requirement for high precision parts of special materials. This unit basically consist of two concentric cylinders and a small intervening annular space contains the test fluids whose viscosity is to be determined.

The viscous drag due to the liquid between the cylinders produce a torque on the inner cylinder. as the spring torque is proportional to the angle through which it turns, therefore the angular moment of the pointer on a fixed disk is used as a measure of viscosity

3. Cone and plate viscometer. The common feature of a cone-and-plate viscometer is that the fluid is sheared between a flat plate and a cone with a low angle. The cone-and-plate system produces a flow in which the shear rate is very nearly uniform. An example of a this type of viscometer is the Brookfield viscometer.

4. Mixer viscometer Mixer viscometer technique used to estimate mixer constants and viscosity using systems of complex geometry.

Tube viscometers: There are fore main types of these viscometers: These viscometer give the value of the ‘kinematic viscosity’. There are fore main types of these viscometers: Glass capillary viscometer. High pressure capillary viscometer . Pipe viscometer. Falling ball viscometer.

1.Glass capillary viscometer. Glass capillary viscometer consists of a U-shaped glass tube held vertically in a controlled temperature bath.  In one arm of the U is a vertical section of precise narrow bore (the capillary). Above this is a bulb, with it is another bulb lower down on the other arm. In use, liquid is drawn into the upper bulb by suction, then allowed to flow down through the capillary into the lower bulb. Two marks (one above and one below the upper bulb) indicate a known volume. The time taken for the level of the liquid to pass between these marks is proportional to the kinematic viscosity. By multiplying the time taken by the factor of the viscometer, the kinematic viscosity is obtained.

2.High pressure capillary viscometer . The high-pressure capillary viscometer has a measuring liquid high-pressure which passes through a heat-regulating bath which can be heated by a built-in heater and whose heat-regulating liquid is circulated. A high-pressure device for producing a high pressure in the measuring liquid comprises a high-pressure pump designed for continuous delivery and having its suction connection pipe connected to a measuring liquid stock container and its pressure connection pipe connected to the measuring liquid high-pressure. 

3.Pipe viscometer. pipe viscometers show better reliability and accuracy than rotational viscometers. viscometers are relatively expensive and not convenient for field applications. As a result, they are commonly used for research purpose and in-line viscosity measurement. A standard pipe viscometer system has flow rate and pressure loss measuring instrumentations. To obtain reliable and accurate measurements, these types of viscometers must have sufficiently long entrance and exit sections.

4.Falling ball viscometer. The Falling ball viscometer used to measure the viscosity of Newtonian liquid by measuring the time required for a ball to fall under gravity through a sample-filled tube that is inclined at an angle. The average time of three tests is taken; the result is converted into a viscosity value using a simple formula.

Refranses: Abdelraziq. I. R, "Concentration and Temperature Dependence of Shear Viscosity of the Critical Mixture of Nitroethane and 3- Methylpentane", An-Najah Univ. J. Res. (N. Sc), 16(2), 117-124(2002). Gwidon W. stachowiak and Andrew W. Batchelor, “Engineering Tribology”, 3ed edition(2005). J. A. Williams, “‘Engineering Tribology”, (2005). Jan .W. Gooch,“Encyclopedic Dictionary of Polymers”, 2ed edition(2011).  Reynolds O,Phil Trans Royal Soc London, 177, p.157(1886). PETROLEUM ENGINEERING DEPARTMENT FLOW LOOP EXPERIMENT .