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WHAT IS VISCOSITY?

Viscosity

The viscosity of a liquid is a measure of the difficulty which molecules have passing each other in a fluid. It is dependent upon the size and shape of the molecules and by the magnitude of the cohesive forces and intermolecular bonds such as hydrogen bonding. Spherical molecules have less flow resistance than long molecules which can become entangled. Very viscous liquids tend to be long chained molecules. For example, honey has a much higher viscosity than water. Viscosity is highly dependent on temperature, decreasing with increased temperature.

Why is honey more viscous than water?

Honey is a mixture of primarily of fructose, glucose and water and with the remaining ingredients composed of various saccharine, organic acids, minerals and enzymes. Honey in part is more viscous (thick) than water due to strong inter-molecular forces. However in addition the structure of glucose, fructose and other saccharine are large in size compared to water molecules and can become entangled.

Viscosity describes how a fluid resists forces, or more specifically shear forces. Shear is the type of force that occurs when two objects slide parallel to one another. Fluids with low viscosity have a low resistance to shear forces, and therefore the molecules flow quickly and are easy to move through.

A fluid that has no resistance to shear stress is known as an ideal fluid or inviscid fluid. Zero viscosity is observed only at very low temperatures. Otherwise, all fluids have positive viscosity, and are technically said to be viscous or viscid. However, a liquid is said to be viscous if its viscosity is substantially greater than water's, and may be described as mobile if the viscosity is noticeably less than water's. If the viscosity is very high, the fluid will appear to be a solid in the short term.

The SI physical unit of dynamic viscosity is the pascal-second (Pa·s), (equivalent to (N·s)/m2, or kg/(m·s)).

The units for viscosity are the centipoise or poise but can be expressed in other acceptable measurements as well. Some conversion factors are as follows:

100 Centipoise = 1 Poise
1 Centipoise = 1 mPa s (Millipascal Second)
1 Poise = 0.1 Pa s (Pascal Second)
Centipoise = Centistoke x Specific Gravity

How is Viscosity Measured?

The formula for measuring viscosity:

viscosity = shear stress/ shear rate

A shear stress is defined as the component of stress coplanar with a material cross section. Shear stress arises from the force vector component parallel to the cross section.

Shear Stress = force applied/cross-sectional area of material with area parallel to the applied force vector

The shear rate for a fluid flowing between two parallel plates, one moving at a constant speed and the other one stationary (Couette flow), is defined by:

shear rate = velocity of the moving plate, measured in meters per second/ the distance between the two parallel plates, measured in meters.

The units are expressed in centipoise (cP), which is the equivalent of 1 mPa s (millipascal second). 

Approximate Viscosities of Common Materials
(At Room Temperature-70°F) *

Material

Viscosity in Centipoise

Water

1 cps

Milk

3 cps

SAE 40 Motor Oil

650-900 cps

Castrol Oil

1,000 cps

Honey

10,000 cps

Ketchup

50,000 cps

Sour Cream

100,000 cps

Peanut Butter

250,000 cps

Newtonian vs. Non-Newtonian Fluids

Isaac Newton, the man who discovered the above formula, thought that, at a given temperature and shear stress, the viscosity of a fluid would remain constant regardless of changes to the shear rate. 

He was only partly right. A few fluids, such as water and honey, do behave this way. We call these fluids Newtonian fluids. Most fluids, however, have viscosities that fluctuate depending on the shear rate. These are called Non-Newtonian fluids.

A non-Newtonian fluid is a fluid whose flow properties differ in any way from those of Newtonian fluids. Most commonly, the viscosity (the measure of a fluid's ability to resist gradual deformation by shear or tensile stresses) of non-Newtonian fluids is dependent on shear rate or shear rate history.

There are five types of non-Newtonian fluids: thixotropic, rheopectic, pseudoplastic, dilatant, and plastic. Different considerations are required when measuring each of these fluid types. See Types of Non-Newtonian fluids for more details.

Many commonly found substances are non-Newtonian fluids, such as ketchup, custard, toothpaste, starch suspensions, paint, blood, and shampoo.

Example of: sheer thickening where viscosity increases with increased stress: cornstarch and water.: sand and water

Examples of sheer thinning where viscosity decreases with increased stress: whipped cream, ketchup, latex paint

Viscosity in the Food Industry

Viscosity measurements are used in the food industry to maximize production efficiency and cost effectiveness. It affects the rate at which a product travels through a pipe, how long it takes to set or dry, and the time it takes to dispense the fluid into packaging. The production process has to be designed with the viscosity of the product in mind, making sure that pipes are angled to optimize flow, or that dispensers provide the right amount of force to induce flow, but not so much that the packaging will overfill.

Viscosity is also a characteristic of the texture of food. The product’s viscosity must be measured and monitored during production to ensure that each batch is consistent and will go through the production process efficiently. 

Non-Newtonian Fluids in Food Design

Many commonly found substances are non-Newtonian fluids, such as ketchup, custard, mustard, whip cream and various food thickening agents.

Non-Newtonian shear thinning is an obviously desirable attribute that allows a condiment to be delivered easily from a bottle (low viscosity at "squeezing" shear rates) and yet retain a rich texture on the plate (high viscosity at low shear rates). Contrast this behavior to the Newtonian flow of honey which necessitates a hard squeeze on delivery yet flows under its own weight.

References and Readings

Hands-on Activity: Measuring Viscosity
Why doesn't the viscosity of water change much with temperature like it does for other substances?
Non Newtonian Fluids
Science of Food Thickening Agents
Rheology testing reveals a world of difference between condiments of similar viscosity values


 

 

 

 

 

 

 

 

 

 

 

Test your Understanding:
1. What statement is not true about viscosity
a) Viscosity describes how a fluid resists forces
b) Viscosity remains constain regardless of shear stress
c) viscosity is a measure of the difficulty which molecules have passing each other in a fluid
d) Viscosity is dependent upon the size and shape of molecules

2. Why is honey more viscous than water?
a) because honey contains large molecules and can become entangled

b) because many of the molecules are bonded to each other
c) due to strong inter-molecular forces

d) both a and c are correct

3. Why is the understanding of viscosity import for the food industry.
a) Many commonly found substances are non-Newtonian fluids, such as ketchup, custard, mustard, whip cream and various food thickening agents.
b) Non-Newtonian shear thinning allows fluids to easily be removed from containers yet retain shape on the plate
c) viscosity is important in developing food thickeners
d) all of the above

4. What are the units for viscosity in the SI System:
a) newton m/sec2
b) the newton-second per square meter or a Pascal second (Pa*s)
c) Centipoise or MilliPascal second
d) joules m/sec2

5. Under what conditions can we observe zero viscosity
a) at very high temperatures
b) at zero degrees Centigrade
c)
in the real world viscosity can only be observed at very low temperatures


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