On the depolarisation of Tyndall scattering in colloids

Abstract

In this paper, an attempt has been made to obtain theoretically the relationship between the depolarisations ρ _u, ρ _v and ρ _h of the Tyndall scattering in colloids where ρ _u, ρ _v and ρ _h are the depolarisations of the transversely scattered light when the incident light is (1) unpolarised, (2) horizontally polarised, and (3) vertically polarised respectively, the plane of observation being the horizontal plane. The total scattering observed in the case of colloids and emulsions is divisible into two parts, (1) the scattering arising from the finite size of the particles assumed to be spherical and isotropic and (2) the scattering arising from the varying orientations of the actual non-spherical anisotropic particles. It is assumed that the law of distribution in direction of the orientation scattering by larger particles is the same as that for very small ellipsoidal particles. The following numerical relationship is then derived between ρ _u, ρ _v and ρ _h, namely, ρ _u = (1 + 1/ρ _h)/(1 + 1/ρ _v) Using the experimental values for the depolarisation obtained with arsenic sulphide sols, castor oil emulsions and sulphur suspensions, it is found that the formula obtained is in satisfactory agreement with observations. In the case of protein solutions, however, certain anomalies are observed. The method of separating the observed scattering into the two parts as stated, and of calculating the size of the particles from the observations is also indicated.