*3.5.2 Rheology*

Nanoparticles have high surface energy, resulting in agglomeration. In particular, aliphatic polyester-based nanoparticles rapidly agglomerate due to the strong interactions between the hydroxyl, carboxylic acid, and ester groups of the polymeric structures interacting with other nanoparticles. This behavior promotes the nanoparticles to be dispersed in media, affecting the rheological properties, such as viscosity and yield stress, of the suspension. Rheological characteristics can be examined with a Brookfield viscometer. The rheological properties show the nanoparticle region and its separation from other regions and play a principal role in stability [37, 38].
