Viscosity | SSC CGL Tier 2 - Study Material, Online Tests, Previous Year PDF Download

Understanding Viscosity

Viscosity is a property exhibited by most fluids, representing their resistance to motion. This resistance emerges when different layers of the fluid experience relative motion. Specifically, viscosity quantifies the resistance to flow that results from internal friction between fluid layers as they slide past each other during motion. Conceptually, viscosity serves as a measure of a fluid's thickness or its ability to impede the passage of objects through it.

A fluid with high viscosity demonstrates substantial resistance to motion, primarily due to robust intermolecular forces causing significant internal friction. This resistance hinders the smooth movement of fluid layers. Conversely, a fluid with low viscosity flows more easily, experiencing minimal friction during motion. It's worth noting that gases also exhibit viscosity, although it may be less noticeable in everyday scenarios.

Definition of Viscosity

Viscosity is formally defined as the measure of a fluid's resistance to flow. The SI unit for viscosity is the poiseiulle (PI), with additional units including newton-second per square metre (N s m⁻²) or pascal-second (Pa s). The dimensional formula of viscosity is [ML⁻¹T⁻¹].

Temperature Dependency:

The viscosity of liquids undergoes a rapid decrease with a rise in temperature. In contrast, the viscosity of gases tends to increase as temperature rises. Consequently, heating liquids promotes easier flow, while gases exhibit slower flow with increased temperature. Importantly, viscosity remains constant regardless of the quantity of matter, establishing it as an intensive property.

Understanding Viscosity Measurement:

Viscosity, representing a fluid's resistance to flow, is quantified by a ratio of shearing stress to the velocity gradient within the fluid. The formula to determine viscosity in the context of a sphere dropped into a fluid is as follows:

Here, ∆ρ is the density difference between the fluid and the tested sphere, a is the radius of the sphere, g is the acceleration due to gravity, and v is the velocity of the sphere.

Types of Viscosity

Viscosity is categorized into two main types based on the measurement approach:

1. Dynamic Viscosity (Absolute Viscosity):

   • This measures a fluid's resistance to flow when an external force is applied.
   • It is represented by the formula mentioned above, capturing the dynamic nature of fluid flow resistance.

2. Kinematic Viscosity:

   • This measures the resistive flow of a fluid under the influence of gravity.
   • It is a distinct measure of fluid viscosity, providing valuable insights into fluid behavior.

Distinguishing Between Dynamic and Kinematic Viscosity

While both dynamic and kinematic viscosity are measures of fluid resistance, they differ significantly in their applications and measurement approaches. Dynamic viscosity focuses on external force-induced resistance, whereas kinematic viscosity considers resistive flow under gravity.

Understanding these distinctions is essential as each type of viscosity serves specific purposes. In certain applications, kinematic viscosity proves more practical and relevant than dynamic viscosity, showcasing the nuanced nature of fluid behavior analysis.

Newtonian and Non-Newtonian Fluids

The viscosity of a fluid is significantly influenced by temperature and pressure. In liquids, viscosity tends to increase with rising temperature, while the pressure can elevate the viscosity of liquid molecules due to increased resistance to flow.

1. Newtonian Fluids:

   • In Newtonian fluids, viscosity remains constant regardless of changes in pressure, stress, or temperature.
   • Water is a classic example of a Newtonian fluid, displaying consistent viscosity under various conditions.

2. Non-Newtonian Fluids:

   • Non-Newtonian fluids exhibit variable viscosity in response to alterations in stress or temperature.
   • Toothpaste is a practical example of a non-Newtonian fluid, showcasing changes in viscosity under different conditions.

Understanding whether a fluid is Newtonian or non-Newtonian is crucial for predicting its behavior in diverse situations.

Viscosity Measurement Techniques

The measurement of viscosity is a critical aspect of understanding fluid behavior. While a basic method involves timing the fall of a sphere through a fluid, a more precise measure is provided by a viscometer. One common type is the U-tube viscometer, also known as a glass capillary viscometer or Ostwald viscometer.

U-Tube Viscometer:

• It comprises two reservoir bulbs and a capillary tube forming a U shape.
• The upper bulb draws the liquid by suction, allowing it to flow down through the capillary into the lower bulb.
• Marks above and below the upper bulb indicate a known volume, and the time taken for the liquid to pass between these marks is proportional to the kinematic viscosity.
• A conversion factor is often provided to obtain the kinematic viscosity by multiplying the measured time.

Using sophisticated instruments like viscometers ensures accurate viscosity measurements, providing valuable insights into fluid characteristics.