VISCOSITY is a measure of the resistance of a fluid to flow.  This resistance arises from the attractive forces between the molecules of the fluid.  A fluid will only flow if enough energy is supplied to overcome these forces.

For a body to be able to move through a fluid, the fluid has to flow around or across it.

Therefore, the energy required to move a body through a fluid is directly related to the degree to which that fluid resists flow, i.e. its viscosity.

In scientific use, viscosity is the tendency of a liquid or gas to resist, by internal friction, the relative motion of its molecules and hence any change of shape; the magnitude of this, as measured by the force per unit, area resisting a flow in which parallel layers unit distance apart have unit speed relative to one another, also called absolute or dynamic viscosity; kinematic viscosity is the dynamic viscosity divided by the density of the fluid.

Newton defines viscosity

The Newtonian Law derived a mathematical formula relating the viscosity to the resistive (drag) force experienced by a thin flat plate “cutting” its way (“shearing”) through the fluid. The definition was based on two quantities:

“Shear rate”: this is the speed of the thin plate divided by its distance from some reference surface, such as the wall of the container.

 “Shear stress”: this is the drag force experienced by the thin plate divided by its surface area.

Viscosity is defined as the shear stress divided by the shear rate.

Non-Newtonian fluids

Newton law assumed that viscosity changed with temperature and assumed that viscosity was always independent of shear rate, i.e. the viscosity would remain the same no matter how quickly the plate was shearing through the fluid.

Fluids which exhibit this type of behavior are now called “Newtonian”:

  • Water
  • Alcohol
  • Thin motor oil

Many fluids, however, are “non-Newtonian” at a given temperature, the viscosity depends on the shear rate and/or the length of time during which the fluid is subjected to shear.

There are several types of non-Newtonian fluids; they are classified on the basis of the way in which their viscosities change.

Time independent:

  • Pseudoplastic (shear-thinning): the viscosity falls as the shear rate increases.

Examples: paints, shampoo.

  • Dilatant (shear-thickening): viscosity rises as the shear rate increases.

Example: mixtures of sand and water.

  • Plastic (Bingham): they behave as a solid until a certain force (the “yield value”) is applied, after which they may display either Newtonian or non-Newtonian flow characteristics.

Example: ketchup (catsup).

Time dependent, constant shearing liquids: Thixotropy and rheopexy

The viscosity of some fluids changes over time even if the shear rate remains constant.

  • Thixotropic: If the viscosity falls, the fluid is said to be “thixotropic”
  • Rheopectic: if it viscosity increases, the fluid is said to be “rheopectic”.

Both thixotropy and rheopexy may occur together with other flow characteristics, or only at certain shear rates.

While rheopectic fluids are rare, thixotropic fluids are very common.

The best-known examples are non-drip paints and heavy printing inks.

Why measuring viscosity?

There are many reasons for making rheology measurements. Often, the objectives of rheology testing or viscosity testing are to identify and quantify rheological properties that correlate to consumer perceived product attributes.

These attributes can include:

How does it look in its container on the store shelf?

How easily does the product dispense (squeeze, pump or spray) from the container?

Does the lotion easily spread on the face, hands or nails?

How does it apply to a hard or soft surface, and remain on the surface?

One of the objectives of Rheology Central is to quantify consumer perception of product attributes that are rheology driven. We complete comprehensive rheology studies of consumer properties specifically for correlation to consumer sensory studies.

Are your products pseudoplastic, viscoelastic? Do they exhibit time dependent shear effects such as thixotropy or rheopexy?

Many products are non-Newtonian pseudoplastic fluids. Emulsions and other complex dispersions are frequently viscoelastic.

What can the strain sweeps and frequency sweeps tell us about your product? And how does this relate to consumer product perceptions and preferences?

Fungilab is the key company to give you all of the answers in Viscosity filed.