We offer an original technique to study non-transparent disperse systems, when the opacity makes particle size measurements by light scattering very difficult. Suitable light scattering geometry is presented. The problem involves two related aspects: strong light absorption and a complex refractive index creating difficulties for correct interpretation of the data. For some very dilute but still opaque solutions the conventional 90° geometry with the scattering volume placed at the corner of the sample cell can also be used.
The optical scheme for particle sizing in opaque dispersions
- A rectangular sample cell is placed at an angle of 45° with respect to the incident laser beam.
- The optical axis of the photodetecting system is normal to the front plane of the sample cell. The scattered light is collected from the region where the laser beam enters the sample.
- A special asymmetric (non-coaxial) holder allows one to change the position of the scattering volume in accordance with the opacity of the sample and other experimental conditions.
- The laser beam is attenuated by an optical neutral glass to avoid the "thermal lens" effect.