What is Viscosity?
Viscosity is a measure of a fluid’s resistance to flow. It is the property of a fluid that determines the resistance of the fluid to flowing and is dependent upon the temperature and pressure of the fluid. Viscosity is an important property for many fluids, including liquids and gases, as it affects the fluid’s ability to flow and mix with other substances. For example, high viscosity liquids, such as molasses, flow very slowly, while low viscosity liquids, such as water, flow more readily. The viscosity of a fluid can also change with temperature, pressure, and other factors, which can impact the fluid’s behavior and properties.
Viscosity is defined as the internal friction of a fluid that opposes the relative motion of different layers of the fluid. It is a measure of a fluid’s resistance to flow and is often described as the fluid’s “thickness” or “stickiness.” The greater the viscosity of a fluid, the slower it will flow, and the more difficult it will be to pour. Conversely, the lower the viscosity, the more readily the fluid will flow and the easier it will be to pour. The SI unit of viscosity is the Pascal-second (Pa·s), although other units such as the centipoise (cP) or poise (P) are sometimes used.
The viscosity of a fluid can be calculated using the formula:
η = shear stress / shear rate
where: η is the viscosity of the fluid in Pa·s (Pascal-seconds) shear stress is the force per unit area required to cause a fluid to flow, and is measured in Pascals (Pa) shear rate is the change in velocity of a fluid per unit distance, and is measured in reciprocal seconds (s^-1).
This formula is known as Newton’s law of viscosity, and it states that the shear stress applied to a fluid is proportional to the fluid’s shear rate. The constant of proportionality is the viscosity of the fluid.
It’s worth noting that the viscosity of a fluid can vary with temperature, pressure, and other factors, so the value of viscosity for a given fluid can change under different conditions.
There are several types of viscosity, including:
• Dynamic viscosity: This is the most common type of viscosity and is a measure of a fluid’s resistance to flow. Dynamic viscosity is related to the shear stress and shear rate, as described in the viscosity formula.
• Kinematic viscosity: This is a measure of a fluid’s resistance to flow per unit density. It is calculated as the dynamic viscosity divided by the fluid’s density. Kinematic viscosity is often used to compare the viscosities of different fluids, especially those with different densities.
• Absolute viscosity: This is a measure of the internal resistance of a fluid to flow. Absolute viscosity is equal to the dynamic viscosity multiplied by the fluid’s density.
• Brookfield viscosity: This is a measure of the viscosity of a non-Newtonian fluid, such as a gel or paste, which does not follow the simple relationship described by Newton’s law of viscosity. Brookfield viscosity is determined using a rotational viscometer, which applies a controlled shear stress to the fluid and measures its resulting shear rate.
• Plastic viscosity: This is a measure of the resistance of a non-Newtonian fluid to flow under a constant shear stress. Plastic viscosity is often used to describe the viscosity of fluids that do not flow easily, such as mud or clay.
• Apparent viscosity: This is a measure of the viscosity of a fluid that is affected by the presence of solids or other substances. Apparent viscosity is often used to describe the viscosity of suspensions or emulsions.
It’s important to note that the type of viscosity used depends on the specific application and the properties of the fluid being studied.
Newtonian and non-Newtonian fluids
Newtonian fluids and non-Newtonian fluids are two types of fluids that have different rheological (flow) properties.
• Newtonian fluids: These are fluids that follow Newton’s law of viscosity, which states that the shear stress applied to a fluid is proportional to the fluid’s shear rate. Newtonian fluids have a constant viscosity that is independent of the shear rate, so the fluid’s resistance to flow is always the same. Examples of Newtonian fluids include water, gasoline, and glycerol.
• Non-Newtonian fluids: These are fluids that do not follow Newton’s law of viscosity and have a viscosity that changes with the shear rate. Non-Newtonian fluids can have a variety of rheological properties, such as thixotropy (viscosity that decreases with shear), dilatancy (viscosity that increases with shear), or shear thinning (viscosity that decreases with increasing shear rate). Examples of non-Newtonian fluids include blood, ketchup, and paint.
The behavior of non-Newtonian fluids can be more complex than that of Newtonian fluids, and the type of viscosity measurement used (such as dynamic viscosity, kinematic viscosity, or Brookfield viscosity) can depend on the specific properties of the fluid.