52
3 Mechanical Aspects of Biosystems
In turn, surface forces can be divided into two types: tangential forces, also
called ‘shearing’ forces, and forces normal to the surface, called ‘pressure’ forces.
(Pressure forces will include negative ones, i.e. forces which pull out on a surface.)
Pressure Forces
A pressure is that part of a force per unit area applied perpendicular to the surface
of a substance. Pressure can arise from adjacent layers within a material, or from
other material interacting on a boundary layer. In the case of gases, a large part or
all of the force on a macroscopic surface comes from the average impulse per unit
time due to molecular collisions. Typically, at biological temperatures (i.e. from
the freezing to the boiling temperature of water), the energy carried by the gas
molecules (about 3k B T /2 ≈ 0.04 electron volt at room temperature) is insufficient
to cause excitation of the atoms or molecules in a material surface, so the collisions
are largely elastic. (Even so, energy can be transferred from or to the gas from the
surface.) A little Newtonian physics and statistics gives that the average pressure
due to an ideal gas (one with average molecular separations much larger than the
size of the molecules and with negligible long-range forces) to be proportional to
the average kinetic energy of the molecules in the gas: p = (1/3)ρ
v 2
, where ρ is
the mass density of the gas and
v 2
the average speed squared of molecules in the
gas.
Negative pressures can be created by pulling on a surface. Gases cannot do
this, but you can glue a plate onto a surface and then pull on the plate. This uses
either an attractive force between one layer of molecules and an adjacent layer, or
an interlocking of material at the interface. Barnacles use a fibrous protein to give
strength to their glue. Micro tentacles into surface cracks and holes is also effective.
For comparison, the pressure of the calm atmosphere at sea level is 1.01325 ×
10 5 Pa, which is 14.7 pounds per square inch. This is called one atmosphere. 28
Often, pressure is communicated in units of the height of a column of mercury held
up by air pressure. One atmosphere of pressure will support a column of mercury at
0 ◦ C (where the mercury has a density of 13.595 gm/cm 3 ) to a height of 76.00 cm.
The pressure to support 1 mm of mercury is called a ‘torr’. Another common
unit of pressure used in meteorology is the ‘bar’, from the Greek word baros, for
weight, and defined as 10 5 newton per square meter. This is close to the atmospheric
pressure at sea level. Processes carried out at constant pressure are called ‘isobaric’.
In physiology, the pressure needed to hold up a column of water rather than
mercury is often used for the unit of pressure. Since the density of water is 1/13.5695
that of mercury (both at 4 ◦ C), 1 mmHg is 1.36 cmH 2 O.
As the atmospheric pressure does not vary quickly, short term pressure changes
in our body are normally due to our own processes, such as our heart pumping blood
28 Before 1982, one atmosphere was called the ‘standard pressure’. In 1982, the International Union
of Pure and Applied Chemistry recommended the ‘standard pressure’ be taken as exactly 10 5 Pa.
3 Mechanical Aspects of Biosystems
In turn, surface forces can be divided into two types: tangential forces, also
called ‘shearing’ forces, and forces normal to the surface, called ‘pressure’ forces.
(Pressure forces will include negative ones, i.e. forces which pull out on a surface.)
Pressure Forces
A pressure is that part of a force per unit area applied perpendicular to the surface
of a substance. Pressure can arise from adjacent layers within a material, or from
other material interacting on a boundary layer. In the case of gases, a large part or
all of the force on a macroscopic surface comes from the average impulse per unit
time due to molecular collisions. Typically, at biological temperatures (i.e. from
the freezing to the boiling temperature of water), the energy carried by the gas
molecules (about 3k B T /2 ≈ 0.04 electron volt at room temperature) is insufficient
to cause excitation of the atoms or molecules in a material surface, so the collisions
are largely elastic. (Even so, energy can be transferred from or to the gas from the
surface.) A little Newtonian physics and statistics gives that the average pressure
due to an ideal gas (one with average molecular separations much larger than the
size of the molecules and with negligible long-range forces) to be proportional to
the average kinetic energy of the molecules in the gas: p = (1/3)ρ
v 2
, where ρ is
the mass density of the gas and
v 2
the average speed squared of molecules in the
gas.
Negative pressures can be created by pulling on a surface. Gases cannot do
this, but you can glue a plate onto a surface and then pull on the plate. This uses
either an attractive force between one layer of molecules and an adjacent layer, or
an interlocking of material at the interface. Barnacles use a fibrous protein to give
strength to their glue. Micro tentacles into surface cracks and holes is also effective.
For comparison, the pressure of the calm atmosphere at sea level is 1.01325 ×
10 5 Pa, which is 14.7 pounds per square inch. This is called one atmosphere. 28
Often, pressure is communicated in units of the height of a column of mercury held
up by air pressure. One atmosphere of pressure will support a column of mercury at
0 ◦ C (where the mercury has a density of 13.595 gm/cm 3 ) to a height of 76.00 cm.
The pressure to support 1 mm of mercury is called a ‘torr’. Another common
unit of pressure used in meteorology is the ‘bar’, from the Greek word baros, for
weight, and defined as 10 5 newton per square meter. This is close to the atmospheric
pressure at sea level. Processes carried out at constant pressure are called ‘isobaric’.
In physiology, the pressure needed to hold up a column of water rather than
mercury is often used for the unit of pressure. Since the density of water is 1/13.5695
that of mercury (both at 4 ◦ C), 1 mmHg is 1.36 cmH 2 O.
As the atmospheric pressure does not vary quickly, short term pressure changes
in our body are normally due to our own processes, such as our heart pumping blood
28 Before 1982, one atmosphere was called the ‘standard pressure’. In 1982, the International Union
of Pure and Applied Chemistry recommended the ‘standard pressure’ be taken as exactly 10 5 Pa.