Regulating particle morphology during a spray freeze drying ...

LIGHT‐SCATTERING 21of large particles more dependent on the location of the detector, i.e. the scatteringangle (see Figure 11) [Malvern Instruments 2009].a) b)Figure 11. Rayleigh scattering of small and large particles: interaction of a lightbeam with a) a small particle with a diameter D < λ/20 results in an angle‐independentisotropic scattering. b) When the particle´s size increases (D > λ/20), the scatteringintensity will increase upon decreasing scattering angles [Wyatt 2006a].If two or more light beams are scattered from the same particle, they can interact witheach other. If the phase relationship between the two electromagnetic waves is random,then the two light sources are called incoherent with respect to each other. If definitephase relationships between two light beams exist they are said to be “coherent”. Ifthere is no phase difference between E 1 and E 2 (equal to 0°), the two fields“constructively interfere”. The intensities are added, so the scattering intensity is twiceas large as that for two incoherent sources. When the different phases E 1 and E 2 differby 180°, then the observed intensity is reduced to 0 (Equation 2 and Equation 3; Figure12). If two electric fields completely cancel each other out, it comes to a “destructiveinterference” [Wyatt 2006a].a) b)c)Figure 12. Interacting light beams. a) incoherent scattering; b) coherent,constructive interference; c) An interference is called coherent destructiveinterference, when two electric fields cancel each other [Wyatt 2006a].Equation 2. Incoherent sum: I Total ∝ |E 1 | 2 + |E 2 | 2 = I 1 + I 2Equation 3. Coherent sum: I Total ∝ E 1 + E 2 2 ≠ I 1 + I 2