usually aggregate refers to dense particle collectives and agglomerates to looser, larger structure. Unfortunately, in some fields and tradition both have just the opposite meaning.

Aggregation is a definite pattern of molecules which may be in any state, while agglomeration is only a process where in gathering into a mass in solid state. On the other hand, a agglomerate does not require a definite pattern, shape, size.

my recent publication mabe help for clarification of these two terminologies:

Effective Density Characterization of Soot Agglomerates from Various Sources and Comparison to Aggregation Theory; Jenny Rissler, Maria E. Messing, Azhar I. Malik, Patrik T. Nilsson, Erik Z. Nordin, Mats Bohgard, Mehri Sanati, Joakim Pagels; Aerosol Science and Technology, 47:792–805, 2013

Agglomeration is when particles are joined loosely together which can be simply broken by mechanical forces but aggregation is a definite pattern of moleclues which may be in any physical state

in addition see e.g.

http://goldbook.iupac.org/A00182.html

an agglomerate corresponds to the case when the dispersed particles are held together by

weak physical interactions ultimately leading to phase separation by the formation of precipitates of larger than colloidal size and the whole process is reversible. An aggregate is defined as comprising strongly bonded colloidal particles and the clustering process is irreversible. (Anal. Chem., 2015, 87 (19), pp 10033–10039)

According to IUPAC they are synonyms

https://www.degruyter.com/downloadpdf/j/pac.2013.85.issue-2/pac-rec-12-03-12/pac-rec-12-03-12.pdf

aggregation

agglomeration

Process whereby otherwise discrete molecules or particles form aggregates

.

Note 1

: See also association

.

Note 2

: Adapted from the definition in [2] to accord with the revised definitions of aggregate and agglomerate.

Aggregation is the collection (dense) of massive particles which are having strong bonding, where as in the case of agglomeration, the particles are loosely join together and they are break easily using mechanical forces.

Thank you

This is one of those things where there is no right answer. It can depend a lot on the scientific discipline/subject matter.

Within colloid science, the terms aggregate, agglomerate, flocculate and coagulate often appear. Some people consider them different, others interchangeable.

Personally, prefer just to use aggregate as a generic term for all of them. You can see this is true from the origins of the words. Flocculation, agglomeration and coagulation are specific forms of "coming together" - the definition of aggregate.

Scientific terminology should be consistent with the origin of the words.

aggregate (v.)

c. 1400, "bring together in a sum or mass," from Latin aggregatus, past participle of aggregare "attach, join, include; collect, bring together," literally "bring together in a flock," from assimilated form of ad "to" (see ad-) + gregare "to collect into a flock, gather," from grex (genitive gregis) "a flock" (from PIE root *ger- "to gather"). Intransitive meaning "Come together in a sum or mass" is from 1855. Related: Aggregated; aggregating.

flocculation (n.)

"the union of small particles into granular aggregates," 1875, from flocculate + -ion.

agglomerate (v.)1680s, "collect or gather in a mass" (trans.), from Latin agglomeratus, past participle of agglomerare "to wind or add onto a ball," from ad "to" (see ad-) + glomerare "wind up in a ball," from glomus (genitive glomeris) "ball of yarn," which is of uncertain origin (see glebe). Intransitive sense "grow into a mass" is from 1730. Related: Agglomerated; agglomerating.

coagulate (v.)

early 15c., "to clot, congeal, become curdled, change from a liquid into a thickened mass; to make to clot," from Latin coagulatus, past participle of coagulare "to cause to curdle," from cogere "to curdle, collect" (see cogent). The earlier verb was coagule, c. 1400, from Old French coaguler and directly from Latin. Related: Coagulated; coagulating.

(All definitions from https://www.etymonline.com/)