Does Viscosity Change With Pressure: Exhaustive Insights


Does viscosity change with pressure, and the answer is a yes and a no. Pressure does not affect viscosity, but in extreme cases, it does affect viscosity.

Generally, viscosity is independent of pressure but with maximum pressure applied, the liquid experiences a significant amount of increase in the viscosity. For most of the conditions we live in, pressure does not affect viscosity unless extreme circumstances.

Does viscosity change with pressure? The pressure does nothing but brings the molecules in a fluid closer to each other. It is a known fact that viscosity is independent of pressure, but under certain circumstances, like when extreme pressure is applied, the viscosity changes accordingly.

The flow of a fluid has some resistance, and that measure is given by viscosity. High viscous fluids usually flow slowly, and low viscous fluids flow at a faster rate. Another point to remember is the cohesive force is present in liquid and gas.

All molecules have a force that keeps another molecule of the same type intact. And this force is known as cohesive force or cohesion, likely to be an attractive force. Cohesive force is strong in solids and weak in gases.

In liquids, the cohesive force is strong and weak according to the circumstances. Meaning they are strong enough to keep the molecules together and weak enough to slide past each other.

Cohesion affecting the viscosity goes like this; flow rate plays a vital role in this. In liquids like honey or maple syrup, the viscosity is high, meaning the cohesive force increases, making the molecules slide past each other a difficult task.

In liquids like water, rubbing alcohol, cohesive force is low, and the flow rate is high; hence the molecules flow past each other easily. Therefore, pressure does not much affect the viscosity in such cases.

When extreme pressure is applied to liquid, then the viscosity is affected as in it deforms accordingly. Pressure increases, and also viscosity is affected. Liquids are compressed when extreme pressure is applied.

Coming to talk about does viscosity changes with pressure, the answer would be both a yes and a no according to the condition of the liquid in which the liquid or gas is in.

Is kinematic viscosity independent of pressure

Kinematic viscosity changes with compressible and incompressible liquids. And for each type, it has its own effects on them.

For compressible liquids (gases), kinematic viscosity depends on pressure, and for incompressible liquids, kinematic viscosity is independent of pressure. In both cases, the extremity plays a part in them. Depending on the conditions of both types of liquids, the dependency also changes accordingly.

Kinematic viscosity is basically the measure of the resistance of fluid flow. When the fluid flows with respect to its own weight then that is called kinematic viscosity.

Kinematic viscosity is defined as the ratio of the viscosity of the fluid to its density. And the formula is ν (kinematic viscosity) = η (dynamic viscosity) / ρ (density of the fluid). An instrument that is used to measure kinematic viscosity in fluids (compressible or incompressible) is called a capillary viscometer.

Kinematic viscosity for liquids will remain constant on an increase in pressure. Let’s see the reason behind this. We are aware from the general formula of kinematic viscosity, it is the ratio of dynamic viscosity of a fluid (compressible or incompressible) to its density.

Dynamic viscosity for liquids is independent of pressure but under extreme conditions, they change but on the whole, they remain unchangeable under any kind of applied pressure.

Next is the density, for an incompressible fluid that is basically the liquids, density is a constant here too. Therefore the kinematic viscosity for an incompressible fluid is always constant until extreme conditions arise.

Dynamic viscosity for compressible fluids such as gas changes with an increase in pressure. Also, density changes for a compressible fluid with an increase in pressure.

The reason behind this is the cohesive force action on both liquids and gases. Since both dynamic viscosity and density of gases change from time to time with pressure, the kinematic viscosity also changes according to pressure change.

Hence we come to know that kinematic viscosity is independent of pressure in the case of liquids and dependent on a pressure in the case of gases.

Does kinematic viscosity change

Kinematic viscosity does change with pressure and temperature. Dynamic viscosity will be a constant throughout but the density will change hence kinematic viscosity too will change accordingly.

Now let’s deal with the effects of pressure on kinematic viscosity. There are two kinds of fluid one is incompressible (liquids) and the other is compressible (gases). Kinematic viscosity will be a constant for liquids and will vary for gases.

For liquids, the dynamic viscosity will be constant since there would not be any deformation due to the tightly-packed molecules. Also, density will remain the same. Kinematic viscosity will stay constant for liquids.

For gases, the molecules are loosely packed and application of pressure will lead to deformation hence density of gases will change but the dynamic viscosity will remain the same. Therefore an increase in the pressure will decrease the kinematic viscosity, vice-versa.

Kinematic viscosity will decrease on an increase in temperature for liquids and kinematic viscosity will increase on a decrease in temperature.

Is kinematic viscosity dimensionless

Kinematic viscosity is not a dimensionless quantity. The reason is kinematic viscosity is defined as the ratio of the dynamic viscosity to its density. Hence it is a vector quantity.

Dynamic viscosity: force x time ÷ area; SI unit is pascal second. Density has formula is         ρ = m / V; SI unit is kilogram per cubic meter.

Hence the SI unit for kinematic viscosity is m2 s−1. Therefore this proves that kinematic viscosity has dimensions and it is denoted as L2T−1. Since kinematic is a combination of both vector quantities the resultant will also be a vector quantity. Therefore having dimensions of those quantities.

How does kinematic viscosity change with temperature

Kinematics viscosity increases with a decrease in temperature for gases and decreases with an increase in temperature for liquids.

In liquids dynamic viscosity decrease on increase in temperature and increase in density so kinematic viscosity increases. The reason behind this is that the molecules are affected due to the application of temperature.

In gases, the dynamic viscosity increases with an increase in temperature, and density decreases marginally so it is neglected. The concept behind this is that molecules in a gas are already loosely packed and at applied temperature, they furthermore get loose and collide. Hence kinematic viscosity will increase on increase in temperature for gases.

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Keerthana Srikumar

Hi...I am Keerthana Srikumar, currently pursuing Ph.D. in Physics and my area of specialization is nano-science. I completed my Bachelor's and Master's from Stella Maris College and Loyola College respectively. I have a keen interest in exploring my research skills and also have the ability to explain Physics topics in a simpler manner. Apart from academics I love to spend my time in music and reading books. Let's connect through LinkedIn-https://www.linkedin.com/in/keerthana-s-91560920a/

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