Self-Assembly of Synthetic and Biological Polymeric Systems of ...

This oscillating dipole acts as an emitter of an electromagnetic wave with the same wavelength
as the incident one (the scattering process is elastic), which is emitted isotropically in all
perpendicular directions to the oscillator, as illustrated in Figure 2.4. The angle of observation
with respect to the direction of the incident light beam is called the scattering angle, and it
provides a measure of the accessible length scales by a light scattering experiment (23).
Figure 2.4. Oscillating dipole induced by an incident light wave, and emitting light (23).
For molecules or particles equal or larger than /20, being the wavelength of the incident
radiation, several of these oscillating dipoles are created simultaneously within one given
particle. As a consequence, some of the emitted light waves possess a significant phase
difference. Accordingly, interference of the scattered light emitted from such individual
particles leads to a non-isotropic angular dependence of the scattered light intensity. The
interference pattern of intra-particular scattered light, also called the particle form factor, is
characteristic of the geometry of the scattering particle. Hence, this provides a quantitative
means for the structural characterization of particles in very dilute solutions by light scattering.
On the other hand, for particles smaller than /20, only a negligible phase difference exists
between light emitted from various scattering centers within a given particle; in this case, the
detected scattered intensity will be independent of the scattering angle, and it only will
depend on the particle mass, which is proportional to the total number of scattering centers
one particle contains. The difference in the interference pattern of light scattered by small and
large particles, is illustrated in Figure 2.5 (23).
So far, we have considered light scattering as a purely elastic process where the emitted light
has exactly the same wavelength as the incident light. Particles in solution, however, usually
show a random motion (Brownian motion) caused by thermal density fluctuations of the
solvent. As a consequence of temporal changes in inter-particle positions and the
corresponding temporal concentration fluctuations, both the interference pattern and the
resulting scattered intensity detected at a given scattering angle also change with time,
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