Chapter 4
Fluid Mechanics Applied to Biosystems
I keep the subject constantly before me and wait till the first
dawnings open little by little into the full light.
—Isaac Newton
Summary Life on Earth could not be active without fluids. The behavior of
ordinary fluids, whether in use by a life form, or otherwise in nature, follows
Newtonian principles. Here we look at the consequences of mechanics applied to
fluids, including the oceans, the atmosphere, and our own blood.
4.1 Nature of Fluids
A fluid is a material which undergoes macroscopic movement under stress or
reaches a limiting boundary. Both liquids and gases are fluids. Apart from weak
long-range forces such as gravity, gases expand by outward thermal pressure until
they reach a bounding surface. Even without a container, liquids have an equilibrium
volume produced by their own internal attractive molecular interactions. We can
understand the distinction between a gas and a liquid by the interaction of the
molecules therein.
For a gas, the motion of its molecules carries, on average, more kinetic energy
for a pair than the energy which might be released by the binding of that pair of
molecules. If the slower ones happen to stick together, the faster ones break them
apart by collisions, thus maintaining the gaseous state at the given temperature.
The collisions of molecules with the walls containing the gas produce the observed
pressure.
By contrast, for a liquid, intermolecular forces tend to hold molecules as
neighbors. Any shearing force acting on the liquid causes movement of layers of
fluid against adjacent layers. A liquid at rest can exert pressure on its containing
walls, this pressure coming from the repulsive forces acting between molecules
when they are pushed against a wall and from molecules thermally jostling against
the walls. However, because fluids move under stress, a liquid at rest cannot exert
© Springer Nature Switzerland AG 2020
W. C. Parke, Biophysics, https://doi.org/10.1007/978-3-030-44146-3_4
77
Fluid Mechanics Applied to Biosystems
I keep the subject constantly before me and wait till the first
dawnings open little by little into the full light.
—Isaac Newton
Summary Life on Earth could not be active without fluids. The behavior of
ordinary fluids, whether in use by a life form, or otherwise in nature, follows
Newtonian principles. Here we look at the consequences of mechanics applied to
fluids, including the oceans, the atmosphere, and our own blood.
4.1 Nature of Fluids
A fluid is a material which undergoes macroscopic movement under stress or
reaches a limiting boundary. Both liquids and gases are fluids. Apart from weak
long-range forces such as gravity, gases expand by outward thermal pressure until
they reach a bounding surface. Even without a container, liquids have an equilibrium
volume produced by their own internal attractive molecular interactions. We can
understand the distinction between a gas and a liquid by the interaction of the
molecules therein.
For a gas, the motion of its molecules carries, on average, more kinetic energy
for a pair than the energy which might be released by the binding of that pair of
molecules. If the slower ones happen to stick together, the faster ones break them
apart by collisions, thus maintaining the gaseous state at the given temperature.
The collisions of molecules with the walls containing the gas produce the observed
pressure.
By contrast, for a liquid, intermolecular forces tend to hold molecules as
neighbors. Any shearing force acting on the liquid causes movement of layers of
fluid against adjacent layers. A liquid at rest can exert pressure on its containing
walls, this pressure coming from the repulsive forces acting between molecules
when they are pushed against a wall and from molecules thermally jostling against
the walls. However, because fluids move under stress, a liquid at rest cannot exert
© Springer Nature Switzerland AG 2020
W. C. Parke, Biophysics, https://doi.org/10.1007/978-3-030-44146-3_4
77