You need to get acquainted with the section of colloid chemistry "stability of colloids". Check out our Student Dictionary. Colloidal disperse systems are stable due to the double electrical layer on the surface of the nanoparticles. You washed the double layer and therefore the dispersed system became unstable. Add a lot of electrolyte and it will become stable again. However, to determine the size of nanoparticles in microscopes, it is necessary to take from the sediment.
Data Yu.A.Mirgorod, R.V. Grebennikova. Glossary on Colloid Chemistry.