Biomechanics: Principles - Bioengineering: A Conceptual Approach - page 188

A viscous glue was made from mistletoe berries and used for lime-twigs to catch birds.
Thus, water is “thin”, having a lower viscosity, while honey is “thick”, having a higher
viscosity. Viscosity describes a fluid's internal resistance to flow and may be thought
of as a measure of fluid friction [ 3 - 5 ]. For example, high-viscosity magma will cre-
ate a tall, steep stratovolcano, because it cannot low far before it cools, while low-
viscosity lava will create a wide, shallow-sloped shield volcano. Put simply, the less
viscous something is, the greater its ease of movement (fluidity). All real fluids
(except superfluids) have some resistance to stress, but a fluid which has no resis-
tance to shear stress is known as an ideal fluid or in viscid fluid. The study of viscos-
ity is known as rheology .
Viscosity Coefficients
Viscosity coefficients can be defined in two ways [ 1 ]:
• Dynamic viscosity , also absolute viscosity , the more usual one;
• Kinematic viscosity is the dynamic viscosity divided by the density.
Viscosity is a tensorial quantity that can be decomposed in different ways into
two independent components. The most usual decomposition yields the following
viscosity coefficients:
• Shear viscosity , the most important one, often referred to as simply viscosity ,
describing the reaction to applied shear stress; simply put, it is the ratio between
the pressure exerted on the surface of a fluid, in the lateral or horizontal direction,
to the change in velocity of the fluid as you move down in the fluid (this is what
is referred to as a velocity gradient).
• Volume viscosity or bulk viscosity , describes the reaction to compression,
essential for acoustics in fluids, see Stokes' law (sound attenuation).
Alternatively,
• Extensional viscosity , a linear combination of shear and bulk viscosity, describes
the reaction to elongation, widely used for characterizing polymers.
For example, at room temperature, water has a dynamic shear viscosity of about
1.0 × 10 −3 Pa s and motor oil of about 250 × 10 −3 Pa s.
Newton's Theory
Laminar shear of fluid between two plates. Friction between the fluid and the
moving boundaries causes the fluid to shear. The force required for this action is a
measure of the fluid's viscosity. This type of flow is known as a Couette flow [ 1 ]
(see Figs. 15.13 and 15.14 ).