Mass Transport and Heat Transfer in the Microcirculation - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 236

plasma proteins, such as albumin, have a permeability that is significantly lower than the
permeability of water.

7.4

The critical adhesive forces that play a role in biological applications are the molecular
forces, the electrostatic forces, and the capillary forces. Molecular forces are the weakest, act
over the smallest range, and are sometimes transient. The mechanical force associated with
these types of adhesive forces can be quantified with

" #

12 6

d 0
d

d 0
d

- - 0

Ψ ðdÞ 5 Ψ

2

Electrostatic forces may arise in biological situations, if there is a net charge on one of the
molecules. These forces can be calculated from Coulomb's law:

1 q 1 q 2

r 2

F 5

4 πε 0

A charged surface would have some potential associated with it, which is termed the Nernst
potential. Due to this net charge, the ions in solution or water can organize near the charged
surface. This highly organized layer is termed the Stern layer and the potential at which the
layer ends is termed the Stern potential. The electric double layer describes the remaining
distance that has some organization due to the presence of the charged surface. The Debye
length quantifies the electric double layer:

s

ε l ε 0 k B T
e 2 P i ðc i χ

λ D 5

2

i Þ

Capillary forces are composed of adhesion, cohesion, and surface tension. Adhesion occurs
when two different molecules tend to stick to each other, due to an attractive force. Cohesion
is the likelihood for two of the same molecules to stick together. Surface tension is a fluid
property that arises due to the intermolecular forces within the fluid that occur at the bound-
ary of a fluid and another substance.

7.5

Porous media are solid materials that are composed of pore structures, which are typically
fluid filled in biological applications. Porous media can be described by the porosity, which
is

volume assoicated with voids
total volume
The porosity of the interstitial space, neglecting blood vessels and cells, approaches 0.9,
whereas the porosity of general biological tissues (including blood vessels and cells) would
be closer to 0.25. Porous media can also be classified based on the body's tortuosity, which is
a measure of the random orientation and random spacing of pores and can be defined as

ε 5

L actual
L
Fluid flow through porous media is most typically quantified via Darcy's law, which is

T 5

- r -

52 K