Regulating particle morphology during a spray freeze drying ...

LIGHT‐SCATTERING 19perpendicular to an irradiated white light beam through a gold sol, known as theTyndall‐effect. After J.C. Maxwell (1831‐1879) published his electromagnetic theory tothe Royal Society in 1864, Rayleigh could explain how electromagnetic waves arescattered when they interact with small gas molecules. The intensity of the scatteredlight is proportional to the sixth power of the particle radius and to the inverse value ofthe fourth power of the wavelength of the incident light. These two discoveries couldexplain some well‐known phenomena like the blue colour of the daylight sky and redsunsets (see Figure 8). Debye (1884‐1966) extended Rayleigh´s theory to large particlesand also investigated the effect of intramolecular interactions in light scattering.3.2 Static light scattering (SLS)3.2.1 Static light scattering by small particlesEinstein (1879 – 1955) indicated that LS in liquids only occurs when non‐homogeneousregions are present [Debye 1944]. When light is radiated through a solution the lightbeam will be attenuated due to different phenomena (Figure 9).a) b) c) d)Figure 9. Overview of scattering phenomena that occur when a particle isilluminated with an incident light beam: a) absorption, b) diffraction, c) refraction andd) reflection. Reflection means the abrupt change in direction of a wave front at aninterface between two dissimilar media, so that the wave front returns into themedium from which it originated.