  In nanoparticle haloing, charged nanoparticles have been found to enhance the
stability of colloidal suspensions by forming a non-adsorbing layer surrounding
neutral colloids which induces an electrostatic repulsion between them.
However, there has been some debate that nanoparticles may directly deposit
onto the colloidal surfaces and that the stabilization mechanism relies on
nanoparticle adsorption. In this study, we have found that these two mechanisms
control the stability of colloidal suspensions across a continuum over
increasing nanoparticle concentrations. AFM force measurements showed that
highly charged zirconia nanoparticles built up an electrostatic repulsion
between negligibly charged silica surfaces, preventing them from aggregating.
The follow-up adsorption measurements and force modeling indicated that minor
adsorption of nanoparticles is expected at volume fractions of 10-5 to 10-3,
but the amount of nanoparticle adsorption dramatically increases with
increasing the nanoparticle volume fraction beyond 10-3. Based on these
results, we propose that the fundamental mechanism of nanoparticle-regulated
stabilization is nanoparticle haloing at low nanoparticle concentrations which
transitions to adsorption at higher concentrations. Accordingly, at a
nanoparticle volume fraction of around 10-3 where the transition happens, the
stabilization can be influenced by both nanoparticle haloing and adsorption.
