# Relationship between fluid viscosity and density

### Viscosity - Wikipedia

The viscosity of a fluid is the measure of its resistance to gradual deformation by shear stress or .. Newton's law of viscosity is a constitutive equation (like Hooke's law, Fick's law, and Ohm's law): it is not a fundamental .. is typically inversely proportional to density and increases with temperature, μ {\displaystyle \ mu } \mu. The density of a fluid is obtained by dividing the mass of the fluid by the volume of the fluid. Density is Kinematic Viscosity and Dynamic Viscosity Relationship. There is no relationship between the viscosity and density of a fluid. While viscosity is the thickness or thinness of a fluid, density refers to the space between its.

The obvious case is that of a liquid droplet on a horizontal surface that is not wetted by the liquid—mercury on glass, or water on a surface that also has a thin oil film on it. For small droplets, such as those on the left of Fig.

For larger droplets, the shape becomes somewhat flatter because of the increasingly important gravitational effect, which is roughly proportional to a3, where a is the approximate droplet radius, whereas the surface area is proportional only to a2. Overall, the situation is very similar to that of a water-filled balloon, in which the water accounts for the gravitational effect and the balloon acts like the surface tension. A fundamental property is the surface energy, which is defined with reference to Fig.

A molecule I, situated in the interior of the liquid, is attracted equally in all directions by its neighbors. However, a molecule S, situated in the surface, experiences a net attractive force into the bulk of the liquid. The vapor above the surface, being comparatively rarefied, exerts a negligible force on molecule S.

Therefore, work has to be done against such a force in bringing an interior molecule to the surface. An equivalent viewpoint is to consider the surface tension T existing per unit distance of a line drawn in the surface, as shown in Fig. Suppose that such a tension has moved a distance L, thereby creating an area WL of fresh surface. In more detail, there are two different types of forces to be considered: At equilibrium, these two forces are equated, giving: Think about what it would be like to paint with water or honey for comparison.

The former is always too runny and the latter is always too sticky.

Toothpaste is another example of a material whose viscosity decreases under stress. Toothpaste behaves like a solid while it sits at rest inside the tube. It will not flow out spontaneously when the cap is removed, but it will flow out when you put the squeeze on it.

### Fluid viscosity and density: A pump user’s guide | Grundfos

Now it ceases to behave like a solid and starts to act like a thick liquid. You don't have to worry about it flowing off the brush as you raise it to your mouth. Shear-thinning fluids can be classified into one of three general groups.

A material that has a viscosity that decreases under shear stress but stays constant over time is said to be pseudoplastic. A material that has a viscosity that decreases under shear stress and then continues to decrease with time is said to be thixotropic.

## Fluid viscosity and density: A pump user’s guide

If the transition from high viscosity nearly semisolid to low viscosity essentially liquid takes place only after the shear stress exceeds some minimum value, the material is said to be a bingham plastic. Materials that thicken when worked or agitated are called shear-thickening fluids. An example that is often shown in science classrooms is a paste made of cornstarch and water mixed in the correct proportions.

The resulting bizarre goo behaves like a liquid when squeezed slowly and an elastic solid when squeezed rapidly. Ambitious science demonstrators have filled tanks with the stuff and then run across it.

### Density Vs. Viscosity | Sciencing

As long as they move quickly the surface acts like a block of solid rubber, but the instant they stop moving the paste behaves like a liquid and the demonstrator winds up taking a cornstarch bath. The shear-thickening behavior makes it a difficult bath to get out of.

Viscosity of Fluids & Velocity Gradient - Fluid Mechanics, Physics Problems

The harder you work to get out, the harder the material pulls you back in. The only way to escape it is to move slowly. Materials that turn nearly solid under stress are more than just a curiosity.