92168.pdf

Optical properties S3

Microscopic

objective

Colloidal

dispersion

Cardioid

consenser

Figure 3.4 Principle of the cardioid dark-field condenser

be separately visible. Dark-field microscopy is, nevertheless, an

extremely useful technique for studying colloidal dispersions and

obtaining information concerning:

1. Brownian motion.


2. Sedimentation equilibrium.


3. Electrophoretic mobility.


4. The progress of particle aggregation.


5. Number-average particle size (from counting experiments and a
   knowledge of the concentration of dispersed phase).


6. Polydispersity (the larger particles scatter more light and therefore
   appear to be brighter).


7. Asymmetry (asymmetric particles give a flashing effect, owing to
   different scattering intensities for different orientations).

Light scattering^3 ^^40

When a beam of light is directed at a colloidal solution or dispersion,
some of the light may be absorbed (colour is produced when light of
certain wavelengths is selectively absorbed), some is scattered and
the remainder is transmitted undisturbed through the sample. Light
scattering results from the electric field associated with the incident
light inducing periodic oscillations of the electron clouds of the atoms
of the material in question - these then act as secondary sources and
radiate scattered light.