particle size. Minimum particle size and average diameter decrease very sharply during a very short period as soon as sonication begins. Then, at a certain time the trend suddenly changes and keeps decreasing with a much smaller slope for quite long sonica tion times up to 60 min. Maximum particle size starts decreasing with a very steep slope, but with continued sonication it follows a gradual decreasing pattern. The obtained results show that even 15 s of sonication can effectively reduce the minimum particle size average diameter and maximum particle size from 28.21, 54.92 and 164.20 nm to 18.17, 38.72 and 141.80 nm, respectively. With continued sonication up to 60 min, the minimum particle size average diameter and maximum particle size decrease to 8.72 17.54 and 58.77 nm, respectively Silica gel has a highly porous microstructure produced by many clustered particles of silica nanoparticles interlinked and con nected together [1] The abrupt decrease in the average diameter occurring immediately after applying sonication confirms that this specific microstructure is easily and fast broken down into a colloi dal solution consisting of both micro and nano size clustered par ticles made up of many single nanosilica particles The abrupt decrease in the minimum particle size also confirms that small clustersare easily broken down in the first few minutes of the Sonication so that the extent of cluster breakage depends on its strength in which it depends on the nature of bonds between particles 311. Cohesive strength of clusters can range from weak bonds (agglomerates) to strong bonds (aggregates) [25] The rela tively big clusters, however, do not undergo any abrupt size decrease, since they need much more energy to break down Agglomerates are easily broken down by ultrasonic irradiation and most clusters consist of strongly bonded aggregates can be reduced in size via fragmentation or erosion mechanism [25] The fragmentation can occur due to particle collisions with the ultrasonic horn tip. The agglomerates are broken down by thei interaction with collapsing of bubbles results from cavitation phe nomenon. Other mechanism introduced for agglomerate fragmen tation is the breakage by collisions among the particles [26]