Stability

The destabilizations can be classified into two major processes:

1.    Migration phenomena: whereby the difference in density between the continuous and dispersed phase, leads to gravitational phase separation:

• Creaming, when the dispersed phase is less dense than the continuous phase (e.g. milk, cosmetic cream, soft drinks, etc.)
• Sedimentation, when the dispersed phase is denser than the continuous phase (e.g. ink, CMP slurries, paint, etc.)

2.    Particle size increase phenomena: whereby the size of the dispersed phase (drops, particles, bubbles) increases

• Reversibly (flocculation)
• Irreversibly (aggregation, coalescence, Ostwald ripening) 

Multiple light scattering coupled with vertical scanning is the most widely used technique to monitor the dispersion state of a product, identifying and quantifying destabilization phenomena. It works on concentrated dispersions without dilution. When light is sent through the sample, it is backscattered by the particles/droplets. The backscattering intensity is directly proportional to the size and volume fraction of the dispersed phase. Therefore, local changes in concentration (creaming and sedimentation) and global changes in size (flocculation, coalescence) are detected and monitored.