What is Viscosity? | Definition, Examples and Measurement

Viscosity is a measure of a fluid’s resistance to flow. It describes how thick or thin a fluid is. For example, honey has a higher viscosity than water because it flows more slowly and is thicker. Viscosity is an important property in both liquids and gases and affects how they move and interact with surfaces and other fluids.

Definition

Viscosity is a physical property of fluids (liquids and gases) that measures the internal resistance of the fluid to flow. It can be thought of as a measure of the fluid’s thickness or a measure of how much the fluid resists attempts to move through it.

Types of Viscosity

1. Dynamic Viscosity (Absolute Viscosity):

Definition: Dynamic viscosity ($\eta$) measures a fluid’s internal resistance to shear forces when an external force is applied. It is expressed in units of poise (P) or centipoise (cP). Example: Honey has high dynamic viscosity, resisting flow more than water.

2. Kinematic Viscosity:

Definition: Kinematic viscosity ($\nu$) is the ratio of dynamic viscosity to fluid density. It is measured in units of stokes (St) or centistokes (cSt). Example: Motor oil exhibits varying kinematic viscosity based on temperature and impurities.

3. Saybolt Viscosity:

Definition: Saybolt viscosity measures the time for a fixed volume of fluid to flow through a calibrated tube under standardized conditions, often used in the petroleum industry. Example: Saybolt Universal Seconds (SUS) indicates the time for oil to flow through a standard orifice.

4. Absolute Viscosity:

Definition: Absolute viscosity is a general term encompassing both dynamic and kinematic viscosity, providing an overall measure of a fluid’s resistance to deformation. Example: Crucial in understanding material behavior in various applications.

5. Apparent Viscosity:

Definition: Apparent viscosity measures a non-Newtonian fluid’s resistance to flow under applied stress. It varies with the rate of deformation. Example: Molten chocolate’s apparent viscosity changes with the rate at which it is poured or sheared.

These viscosity concepts are fundamental in understanding fluid properties in different industries and scientific disciplines.

Units

Please note that different industries and regions may use different units, so it’s essential to be aware of the specific context in which viscosity is being discussed.

Examples of Viscosity

Liquids

1. Water: Water has a relatively low viscosity, which means it flows easily and quickly. It’s often used as a baseline for comparing the viscosity of other liquids.
2. Honey: Honey is an example of a liquid with high viscosity. It flows slowly compared to water, demonstrating greater resistance to movement.
3. Motor Oil: Motor oil has a higher viscosity than water, which is crucial for its role in lubricating engine parts. The viscosity of motor oil is designed to provide a protective film under varying temperatures and conditions.
4. Glycerin: Glycerin, a thick, colorless liquid, has a high viscosity and is used in pharmaceutical formulations, as a sweetener in the food industry, and in cosmetic products.
5. Air: While air is not a liquid, it’s worth noting that gases also have viscosity. Air’s viscosity increases with temperature, which is opposite to the behavior of most liquids.

Gases

1. Carbon Dioxide (CO2): Gases have viscosity, although it’s much lower than that of liquids. The viscosity of gases increases with temperature, which is a key consideration in processes involving gas flow at high temperatures.
2. Helium: Helium, being a lighter gas, has a lower viscosity compared to heavier gases. Its low viscosity is one reason why helium can quickly escape through small openings or leaks.

Examples in Daily Life

• Pouring Syrup on Pancakes: Syrup has a higher viscosity than water, which is why it pours slowly and coats the pancakes evenly without running off too quickly.
• Using Lubricants: Lubricants like oil or grease have specific viscosities to ensure they can provide a protective layer between moving parts, reducing wear and overheating.
• Paint: The viscosity of paint determines how it flows and spreads on a surface. Thicker (high-viscosity) paints may require thinning before application, while thinner (low-viscosity) paints are easier to spread but may require multiple coats.
• Flight: In the context of flight, viscosity refers to air viscosity, influencing aerodynamics by affecting the flow of air around an aircraft. Dynamic viscosity, measured in units like Pascal-seconds, plays a crucial role in determining the nature of the boundary layer and impacts lift, drag, and other aerodynamic forces. Engineers use parameters like the Reynolds number to assess the significance of viscous effects in the airflow around an aircraft.

These examples highlight how viscosity affects the behavior of fluids in various contexts, from industrial applications to everyday phenomena.

How to Measure Viscosity?

Measuring viscosity is crucial in various industries and scientific research to understand how fluids flow under different conditions. There are several methods to measure viscosity, ranging from simple and direct to more sophisticated and precise techniques. Here are some common methods used to measure the viscosity of liquids and gases:

1. Capillary Viscometers

• Example: Ostwald viscometer.
• How It Works: The fluid is allowed to flow through a thin tube (capillary), and the time it takes for a specific volume of fluid to pass through is measured. The viscosity can be calculated based on the flow time, the density of the fluid, and the geometry of the viscometer.

2. Rotational Viscometers

• Example: Brookfield viscometer.
• How It Works: These devices measure the torque required to rotate an object, such as a spindle or cylinder, at a constant speed in the fluid. The resistance the fluid offers to this movement is used to calculate its viscosity.

3. Falling Sphere Viscometers

• Example: Höppler viscometer.
• How It Works: A sphere is allowed to fall through the fluid, and the time it takes to fall a certain distance is measured. The viscosity is determined based on the sphere’s size, the density of the sphere and the fluid, and the fall time.

4. Vibrational Viscometers

• Example: Tuning fork viscometer.
• How It Works: These viscometers measure the damping of a vibrating element (like a tuning fork) when immersed in a fluid. The change in vibration frequency or amplitude is correlated with the fluid’s viscosity.

5. Rheometers

• How It Works: Rheometers are used for more complex fluids that have properties that change under different flow conditions or over time. They can apply various types of stress (shear, extensional) and measure the corresponding strain, allowing for a detailed analysis of the fluid’s rheological properties.

Factors Affecting Measurement:

• Temperature: Viscosity is highly temperature-dependent. Measurements are usually conducted at a controlled temperature to ensure accuracy.
• Shear Rate: Some fluids, like non-Newtonian fluids, change their viscosity depending on the applied shear rate. It’s important to consider this when choosing a measurement method.

Choosing the Right Method:

The choice of method depends on several factors, including the type of fluid (Newtonian or non-Newtonian), the viscosity range, the temperature and pressure conditions, and the required accuracy. For simple, quick measurements, capillary or rotational viscometers are often used. For more detailed analysis, especially with complex fluids, rheometers may be necessary.

Each method has its advantages and limitations, and the choice often depends on the specific requirements of the application, including the precision needed, the nature of the fluid, and the conditions under which the measurement is to be made.

Viscosity of Different Fluids

This example will include hypothetical viscosity values for common fluids at room temperature (approximately 20°C or 68°F). Remember, the actual viscosity can vary based on factors like temperature, pressure, and the specific composition of the fluid.

Notes:

• Viscosity (mPa·s or cP): The unit millipascal-second (mPa·s) is equivalent to the centipoise (cP), commonly used to express viscosity. The values provided are approximate and can vary.
• Temperature: Viscosity decreases as temperature increases for liquids and increases with temperature for gases. The table provides values at or near room temperature for simplicity, except for blood, which is given at body temperature (37°C) to reflect its physiological condition.
• Fluid Variability: The viscosity of substances like honey can vary significantly depending on their composition, temperature, and other factors.