Sedimentation is the end-result of severe nanoparticle clustering (whether in the form of hard aggregates or soft agglomerates). When a particle cluster exceeds a certain size limit, gravitational force dominates and the cluster starts to settle.

Visually, a gradual change in color from top (lighter) to bottom (darker) of your sample could be indicative of sedimentation. A uniform color change (with respect to the initial, presumably 'stable' sample) that is consistent all over your sample could be indicative of particle clustering without sedimentation.

Of course, a more decisive measurement technique would be necessary to have more conclusive results. For example, a shift in the plasmonic peak of the extinction (absorption + scattering) spectrum obtained from a UV-Vis measurement or the evolution of some absorption shoulders could be a sign of nanoparticles coming together into larger clusters. A strong drop in extinction could be a sign of particles sedimenting out of your dispersion.

I hope this somewhat helps - Omar

Shama Parwin

To answer this question you need to get acquainted with the theory of stabilization and sedimentation of dispersed systems in the course of colloid chemistry. The most famous theory of DLVO.

I think Omer provide good answer

According to its definition, particle sedimentation velocity is directly proportional to the density difference between nanoparticle and base fluid, and the square of the nanoparticle radius; and is inversely proportional to base fluid viscosity. This relation is mentioned by [Ghadimi et al. 2011. A review of nanofluid stability properties and characterization in stationary conditions, International Journal of Heat and Mass Transfer, 54(17–18), 4051-4068.] Therefore, dispersion state firstly depends on the types and natures of nanofluid components.

From colloidal stability and clustering points of view, checking the particle size distribution (e.g., via dynamic light scattering) to obtain average particle size values, and measuring the zeta potential of the suspension (to see the stability level) would provide valuable insights. But if there are visible changes in nanofluids appearance, as Omar Z. Sharaf mentioned, it is most probably an evidence of the lack of an appropriate dispersion, and stability enhancement measures are needed.