Viscosity and density are important concepts when it comes to understanding behavior of liquids and gases. So let us look at the major differences between viscosity and density. Also we will define their roles in the world of science and engineering.
What is Viscosity?
Viscosity is resistance of a fluid (liquid or gas) to a change in shape, or movement of neighbouring portions relative to one another. Viscosity arises when there is relative motion between layers of the fluid. Viscosity denotes opposition to flow. Viscosity is a major factor in determining the forces that must be overcome when fluids are used in lubrication and transported in pipelines. It controls the liquid flow in such processes as spraying, injection molding, and surface coating.
In physics, viscosity is often expressed using Isaac Newton’s equation for fluids, which is similar to Newton’s second law of motion. This law states that when a force acts on an object, it will cause the object to accelerate. The larger the mass of the object, the greater the force will need to be to cause it to accelerate.
The viscosity of liquids decreases rapidly with an increase in temperature and the viscosity of gases increases with an increase in temperature. Thus, upon heating, liquids flow more easily, whereas gases flow more sluggishly.
One way is to measure the fluid’s resistance to flow when an external force is applied. This is known as Dynamic Viscosity. And the other way is to measure the resistive flow of a fluid under the weight of gravity. This measure of fluid viscosity is referred to as kinematic viscosity.
Also Read: Difference Between Fundamental Unit And Derived Unit
What is Density?
The density of material shows the denseness of that material in a specific given area. A material’s density is defined as its mass per unit volume. Density is essentially a measurement of how tightly matter is packed together. It is a unique physical property for a particular object. The principle of density was discovered by the Greek scientist Archimedes. It is easy to calculate density if you know the formula and understand the related units The symbol ρ represents density or it can also be represented by the letter D.
What density means in science is a comparison of the mass of an object to its volume. Density in science and math uses a formula to compare the two. In sciences, such as physics, the focus is on objects and matter and applies the concept to the laws of nature. In mathematics, it is a formula that can be calculated and manipulated. In other areas, such as social sciences, density deals with population. The number of people in a certain land area gives the population density for that region.
Iron, platinum, and lead are examples of dense materials. Many types of rock and minerals are examples of dense material. Materials that are dense are most likely to ‘feel’ heavy or hard. The opposite of dense is sparse and a few examples of sparse material are glass, bamboo, aluminium, and styrofoam.
In general, liquids are less dense than solids and gases are less dense than liquids. This is due to the fact that solids have densely packed particles, liquids are materials where particles can slide around one another, and gases have particles that are free to move all over the place.
Many applications of density are there in our real-life like a few examples are in pipe design, shipbuilding, diving of submarines, helium balloons, weight distribution in the airplane, and the floating of ice on water.
Viscosity vs Density: Key Differences
Definition
- Viscosity refers to a fluid’s resistance to flow or its internal friction. It measures how easily a fluid flows and how much resistance it encounters while flowing.
- Density is the mass of a substance per unit volume. It indicates how tightly packed the particles of a substance are.
Nature of Property
- Viscosity is related to the fluid’s internal resistance to shear stress and its ability to deform under shear forces.
- Density is a measure of how much mass is present in a given volume of substance.
Units
- Viscosity is typically measured in units like Poise (P) or Pascal-seconds (Pa·s).
- Density is measured in units like kilograms per cubic meter (kg/m³) or grams per cubic centimeter (g/cm³).
Influence on Flow
- Viscosity affects how easily a fluid flows. High viscosity fluids, like honey, flow slowly due to their high internal resistance.
- Density doesn’t directly affect flow speed but determines how buoyant an object is in a fluid.
Temperature Dependency
- Viscosity tends to decrease with increasing temperature for most fluids. Higher temperatures reduce the internal friction, making the fluid flow more easily.
- Density is less sensitive to temperature changes and may vary slightly due to thermal expansion effects.
Measurement
- Viscosity is measured using viscometers, which involve techniques like measuring the time it takes for a fluid to flow through a tube under gravity or under applied force.
- Density is typically measured using a hydrometer or densitometer, which relies on the principle of buoyancy.
Common Examples
- High viscosity fluids include honey, molasses, and motor oil.
- High-density substances include metals like lead, gold, and dense liquids like mercury.
Effect on Object Motion
- Viscosity affects how objects move through a fluid. In high viscosity fluids, objects encounter more resistance and slow down quickly.
- Density affects whether an object floats or sinks in a fluid. Objects with lower density than the fluid will float, while those with higher density will sink.
Shear Force Response
- Viscosity relates to how a fluid responds to shear forces, like those encountered when stirring or flowing around obstacles.
- Density doesn’t relate to shear forces directly but influences the overall behavior of a fluid in motion.
Mathematical Representation
- Viscosity is often represented by the symbol “η” and is included in equations that describe fluid flow, like the Navier-Stokes equations.
- Density is represented by the symbol “ρ” and is used in various equations involving buoyancy, pressure calculations, and more.
Density vs Viscosity: Takeaways
| Basis of Comparison | Density | Viscosity |
| Description | Density says how much mass occupies a certain volume | Viscosity is how well a liquid sticks together. |
| Nature of Property | Density is the space between its particles. | Viscosity is the thickness or thinness of fluid |
| Units | Density is measured in units like kilograms per cubic meter (kg/m³) or grams per cubic centimeter (g/cm³). | Viscosity is typically measured in units like Poise (P) or Pascal-seconds (Pa·s). |
| Mathematical Expression | Density is a scalar quantity and is used in various calculations, including calculating weight, volume, and specific gravity. | Viscosity is often represented as a coefficient in fluid flow equations, such as the Navier-Stokes equations. |
| Examples | Examples of high-density substances are metals like gold and lead. Examples of low-density substances are gases like helium and hydrogen. | Examples of fluids with high viscosity include honey, molasses, and motor oil. Examples of low viscosity fluids include water and air. |
| Measurement Techniques | Density is measured using devices like hydrometers or densimeters. | Viscosity is measured using instruments like viscometers or rheometers. |
| Effects of Temperature and Pressure | Density can be affected by changes in temperature and pressure, but the effect is typically smaller than that on viscosity. | Viscosity is influenced by temperature; generally, viscosity decreases as temperature increases. |
| Influence on Flow | Density affects how a fluid behaves in relation to other fluids. A less dense substance will float on a denser one. | Viscosity affects how easily a fluid flows. High viscosity fluids flow slowly (like honey), while low viscosity fluids flow more quickly (like water). |
More Takeaways
- Density can be explained as the relationship between the mass of the substance and the volume it takes up. In a qualitative term, it shows how much heavy an object is at constant volume.
- Viscosity is related to the internal friction and cohesion of a fluid’s molecules. It determines how much energy is required to make the molecules move past each other. Water has a low or “thin” viscosity while honey has a “thick” or high viscosity.
- The law of viscosity has important uses in such areas as inkjet printing, protein formulations and injections, and even food and beverage manufacturing.
- Density and viscosity decrease with temperature, but viscosity has an exponential relationship and density has a linear relationship with temperature.
- A fluid with higher density will transition to turbulent flow quicker, and fluid with higher viscosity will have that transition later. so in a sense, they are inversely proportional.