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
is honey more viscous than water?
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.
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.
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
physical unit of dynamic viscosity is the pascal-second (Pa·s),
(equivalent to (N·s)/m2, or kg/(m·s)).
for viscosity are the centipoise or poise but can be expressed
in other acceptable measurements as well. Some conversion
factors are as follows:
= 1 Poise
1 Centipoise = 1 mPa s (Millipascal Second)
1 Poise = 0.1 Pa s (Pascal Second)
Centipoise = Centistoke x Specific Gravity
is Viscosity Measured?
for measuring viscosity:
= shear stress/ shear rate
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
Stress = force applied/cross-sectional area of material
with area parallel to the applied force vector
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.
are expressed in centipoise (cP), which
is the equivalent of 1 mPa s (millipascal second).
Viscosities of Common Materials
(At Room Temperature-70°F) *
Viscosity in Centipoise
40 Motor Oil
vs. Non-Newtonian Fluids
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.
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
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.
are five types of non-Newtonian fluids: thixotropic, rheopectic,
pseudoplastic, dilatant, and plastic. Different considerations
are required when measuring each of these fluid types.
of Non-Newtonian fluids for more details.
commonly found substances are non-Newtonian fluids, such
as ketchup, custard, toothpaste, starch suspensions, paint,
blood, and shampoo.
of: sheer thickening where viscosity increases with increased
stress: cornstarch and water.: sand and water
of sheer thinning where viscosity decreases with increased
stress: whipped cream, ketchup, latex paint
in the Food Industry
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.
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.
Fluids in Food Design
found substances are non-Newtonian fluids, such as ketchup,
custard, mustard, whip cream and various food thickening agents.
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
Activity: Measuring Viscosity
doesn't the viscosity of water change much with temperature
like it does for other substances?
of Food Thickening Agents
testing reveals a world of difference between condiments of
similar viscosity values