Techniques for the characterization of physical gels - Physical Gels from Biological and Synthetic Polymers - page 40

sinusoidal manner at its resonance frequency. The amplitude of this oscillation is
dampened when the tip approaches the sample, and information is relayed to the piezo-
electric tube, which keeps the vibration at constant amplitude by adjusting the vertical
position of the sample. This virtually eliminates lateral forces and allows imaging of
weakly absorbed samples. Further technological advancements have enabled the use of
this technique under pseudo-physiological conditions. In the latter case, samples can be
imaged under physiological buffer, with the result that the sample, although immobi-
lized, is kept

. This gives it a fundamental advantage over traditional electron
microscopy, where the sample is static and is usually partially destroyed during the
process of imaging. Thus, a portion of the

' alive '

flat surface can be constantly scanned for time-
evolving changes, from which some time information can be derived.
The resolution obtained with AFM very much depends on the size of the tip that is
used. Most commercial silicon nitride tips have a size of 10

20 nm, although carbon
nanotube technology has been used to enhance the resolution that is obtained. Here, the
tips can have a nominal size as low as 5 nm. In order to immobilize the sample, mica
(negatively charged) is frequently employed, although glass, gold and hydrophobically
surfaced highly orientated pyrolytic graphite (HOPG) have also all been used. Such
materials should have the

-

' atomically smooth ' surface necessary for such imaging. Often,

when a thin water film at the sample surface forms a capillary meniscus, capillary forces

reduce the resolution obtained with biological samples scanned in air. A number of
interesting applications have been partially developed including so-called ,

using the tip to cut or unfold and manipulate samples, and chemical contrast imaging,
which is adding speci

' nanotools '

c ligands or chemically active groups to the tip, which can help to
map chemical interactions.
Applications of the technique will be discussed in the relevant chapters, but work has
been published on both polysaccharide and protein gel precursors (Ikeda et al., 2001 ;
Gosal et al., 2002 ; Round et al., 2010 ). Although these studies have concentrated on
imaging sub-gelling concentrations of aggregates, particularly for ionic polysaccharide
gels ( Chapter 5 ), progress has now been made in imaging aqueous gels, by forming a thin
hydrated

film on the mica substrate (Morris, 2009 ). The imaging process compresses the
polymer network, but without obvious structural damage. This approach has been
particularly successful for gellan gels ( Chapter 5 ). At the moment, however, there appear
to be few such compressive gel studies for other systems such as protein gels.

2.5 Rheological characterization

The characterization of gels by rheological measurements has been carried out for many
years, since the viscous and elastic properties of gels are among the most signi

cant in
practical applications. This is not the place for a rigorous introduction to rheological
techniques, since there are many sources (Ferry, 1980 ), but it is important to consider a
few basics before discussing, in more detail, the characterization techniques employed.
As we noted in Chapter 1 , gels (and again, arguably all real systems) are viscoelastic -

that is to say, under various conditions of time, temperature or other factors they can be