Biomedical Engineering Reference
In-Depth Information
Despite the above de
nitions, we need to clarify that many of the systems described as
'
, such as shower gels and pain relief or topical gels, are often not gels even in terms
of the various descriptions above. For example, in the cosmetic and pharmaceutical area,
the term
gels
'
'
hydrogel
'
is widely employed. The
'
hydro
'
part of this again shows that the
solvent is water or electrolyte, but the
part, by contrast, does not always require that
the underlying structures are of the types of network de
'
gel
'
ned above. Instead, in some of
these application areas, this term has been applied rather more broadly to just
'
viscous solutions. The importance of this distinction is introduced below, and detailed in
Chapters 2 , 3 and 4 . Noting how widely the term is applied in the external literature, we
will still use the term
'
simple
in this volume, but underline the necessary caveat that
we are using it implicitly in a more restricted sense than some workers in these
application areas.
'
hydrogel
'
1.2.1
Solids versus liquids
What should be clear from almost all of the de
nitions given above is that a gel has some
solid or solid-like characteristics, but the
field is not made easier by the number of
different terms employed. These include hydrogel, as already noted, microgel, nanogel,
strong gel, weak gel,
fluid gel, topical gel and liquid gel.
A further question must be considered: if a gel is solid-like, then what constitutes a
solid? This is not as straightforward a distinction as might at
rst be thought. For
example, some materials, such as pitch or bitumen, appear to be solids, but are actually
flow is obviously extremely small. In the famous pitch
drop experiment, for example, one drop falls approximately every decade. 1 At the same
time, some unfortunate jumping off a very high river bridge dies not of drowning but of
multiple fractures, since over very short times water responds like a solid. This underlies
the reality that almost all materials are actually
flowing, although their rate of
they combine both liquid
(viscous) and solid (elastic) properties, and which of these dominates tends to depend on
factors such as temperature (and pressure) and the duration of observation. In practice
most conscious human activities correspond to time scales from, say, 0.1 s up to tens of
seconds, so specialist instruments that sample over much shorter or much longer time
scales ( Chapter 2 ) tend to be much better at making distinctions such as
'
viscoelastic
'-
'
solid
'
versus
'
liquid
.
In fact the transition from liquid to solid
'
has been quite
extensively studied. This is examined in Chapter 3 , including a discussion of the best way
to determine the characteristics of the transition. In food applications, a very traditional
area of use for gels, there are many methods recommended for beginners when preparing
jams, marmalades or creamy desserts, for example, to produce soft creams or wobbling
solids rather than a viscous, sticky, unpleasant, lumpy texture. Every country has its own
recipes, because gel textures are appreciated in food preparation everywhere.
For a long while it was thought that simply inverting a tube was an appropriate
laboratory technique. Indeed, even today this approach is sometimes still used, not
-
the sol
-
gel transition
-
1
http://en.wikipedia.org/wiki/Pitch_drop_experiment
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