Bhuvaneswari Baskaran This isn't as easy a question as it may seem as the literature is confused. To look at this confusion then read a great summary from Gary Nichols:
Article A Review of the Terms Agglomerate and Aggregate with a Recom...
However, it's generally understood that aggregate means a tightly (chemically) bound collection of particles that cannot be separated easily (e.g. by sonication). Agglomerate is a loose collection of particles (or aggregates) that can be separated by relatively weak shear forces including sonication. 'Cluster' was a term favored at one time by ISO to refer to any assemblage of particles without defining whether they're loose or tight.
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Clusters are small collections of particles that are held together by physical forces like van der Waals force, electrostatic force, or hydrogen bonding. Aggregates are larger structures consisting of multiple clusters that are connected together through covalent bonds or other strong intermolecular forces. Agglomerates are the most loosely bound structures, formed when individual particles come into contact with each other and form weak interactions.
as R. Sagayaraj explained, clusters are composed of units, in this case particles, of a defined number range.
Additionally, clusters can be still stable in the dispersion medium.
On the other hand, aggregates are formed in an uncontrolled way and would have random number of nanoparticles and larger sizes closer to the micrometer scale. Therefore, they would not be optically clear and could be sedimented after certain time.