Dynamic viscosity, also known as absolute viscosity, is a crucial property in understanding the behavior and flow characteristics of fluids. It represents the internal resistance of a fluid to flow when a force is applied, and is an essential parameter in various fields, including fluid mechanics, engineering, and material science.
Understanding Dynamic Viscosity
Dynamic viscosity, denoted by the symbol η, is a measure of a fluid’s resistance to flow. It is an intensive property, meaning it does not depend on the amount of matter present. The dimensional formula for dynamic viscosity is ML^1T^1, where M represents mass, L represents length, and T represents time.
The formula for the coefficient of dynamic viscosity is given by:
η = F . dx / A . dv
Where:
– F is the viscous force
– dx is the distance between two fluid layers
– A is the surface area
– dv is the velocity gradient between the two fluid layers
Dynamic viscosity is typically measured in units such as:
– Pascalsecond (Pa·s)
– Millipascalsecond (mPa·s)
– Poise (P)
– Centipoise (cP)
It is important to note that 1 mPa·s is equal to the dynamic viscosity of water at 20°C.
Factors Affecting Dynamic Viscosity
The dynamic viscosity of fluids is influenced by several factors, including:

Temperature: The viscosity of liquids generally decreases as the temperature increases, while the viscosity of gases increases with temperature. This is due to the changes in the intermolecular forces and the kinetic energy of the fluid molecules.

Pressure: Increasing the pressure can lead to an increase in the dynamic viscosity of fluids, particularly for liquids. This is because higher pressure can cause the fluid molecules to be more closely packed, resulting in increased intermolecular interactions.

Molecular Structure: The chemical composition and molecular structure of a fluid can significantly impact its dynamic viscosity. Larger, more complex molecules tend to have higher viscosities compared to smaller, simpler molecules.

Concentration: The concentration of solutes or suspended particles in a fluid can affect its dynamic viscosity. Higher concentrations typically result in increased viscosity due to the increased interactions between the fluid and the dissolved or suspended components.
Measuring Dynamic Viscosity
There are several methods and instruments used to measure dynamic viscosity, including:

Rotational Viscometers: These instruments measure the torque required to rotate a probe or spindle immersed in the fluid sample at a constant speed. The measured torque is then used to calculate the dynamic viscosity.

Capillary Viscometers: These devices measure the time it takes for a fixed volume of fluid to flow through a calibrated capillary tube under the influence of gravity or a controlled pressure difference.

Falling Ball Viscometers: In this method, the time it takes for a small, dense ball to fall through a fluid sample is measured. The dynamic viscosity can then be calculated using the Stokes’ law.

Resonating Micromachined Tubes: As mentioned in the previous answer, these instruments use a micromachined resonating tube to measure both the dynamic viscosity and density of a fluid, enabling the calculation of kinematic viscosity.
The choice of measurement method depends on factors such as the fluid properties, the required accuracy, and the available equipment.
Applications of Dynamic Viscosity
Dynamic viscosity is a crucial parameter in various applications, including:

Fluid Mechanics: Dynamic viscosity is essential in understanding the behavior of fluids, such as their flow patterns, pressure drops, and the forces acting on objects moving through the fluid.

Engineering: Dynamic viscosity is used in the design and analysis of fluid systems, such as pumps, valves, and pipelines, to ensure efficient and reliable operation.

Material Science: Dynamic viscosity is important in the study and characterization of complex fluids, such as polymers, suspensions, and emulsions, which are widely used in industries like paints, cosmetics, and pharmaceuticals.

Biomedical Applications: Dynamic viscosity plays a crucial role in the study of blood flow, the design of medical devices, and the understanding of physiological processes.

Automotive and Aerospace: Dynamic viscosity is a key parameter in the selection and performance of lubricants, hydraulic fluids, and other fluids used in vehicles and aircraft.

Environmental Studies: Dynamic viscosity is important in the analysis and modeling of the transport and behavior of pollutants, oils, and other fluids in the environment.
Numerical Examples and Calculations
To illustrate the practical application of dynamic viscosity, let’s consider a few examples:
 Calculating the Viscous Force: Suppose a fluid with a dynamic viscosity of 0.5 Pa·s flows past a flat plate with a surface area of 0.2 m^2 and a velocity gradient of 10 m/s. Calculate the viscous force acting on the plate.
Using the formula: η = F . dx / A . dv
Rearranging for F:
F = η . A . dv / dx
F = 0.5 Pa·s × 0.2 m^2 × 10 m/s
F = 1 N
 Determining the Pressure Drop in a Pipe: Consider a fluid with a dynamic viscosity of 0.8 Pa·s flowing through a pipe with a length of 10 m and a diameter of 0.05 m. The volumetric flow rate is 0.2 m^3/s. Calculate the pressure drop along the pipe.
Using the HagenPoiseuille equation:
ΔP = 32 . η . L . Q / (π . d^4)
ΔP = 32 × 0.8 Pa·s × 10 m × 0.2 m^3/s / (π × (0.05 m)^4)
ΔP = 1280 Pa
 Measuring Dynamic Viscosity with a Rotational Viscometer: A rotational viscometer is used to measure the dynamic viscosity of a fluid. The instrument measures a torque of 0.5 N·m when the spindle is rotated at a speed of 60 rpm. The spindle has a radius of 0.02 m and is immersed in the fluid to a depth of 0.05 m. Calculate the dynamic viscosity of the fluid.
Using the formula: η = T / (2 . π . r^2 . h . ω)
Where:
– T is the torque (0.5 N·m)
– r is the spindle radius (0.02 m)
– h is the immersion depth (0.05 m)
– ω is the angular velocity (2 . π . 60 / 60 = 2 . π rad/s)
η = 0.5 N·m / (2 . π . (0.02 m)^2 . 0.05 m . 2 . π rad/s)
η = 0.5 Pa·s
These examples demonstrate how dynamic viscosity can be used to calculate various fluid mechanics parameters, such as viscous forces, pressure drops, and the measurement of viscosity using different techniques.
Conclusion
Dynamic viscosity is a fundamental property that plays a crucial role in understanding and predicting the behavior of fluids. By mastering the concepts of dynamic viscosity, including its measurement, factors affecting it, and its applications, you can gain a deeper understanding of fluid mechanics and its practical implications in various fields.
References:
 Dynamic and Kinematic Viscosity Measurements with a Resonating Micromachined Tube, metersolution.com
 What is the Difference Between Dynamic and Kinematic Viscosity?, cscscientific.com
 Dynamic Viscosity Formula, geeksforgeeks.org
 Dynamic Viscosity – an overview, sciencedirect.com
 Guide to Viscosity Measurement, bykinstruments.com
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