Transition Force Measurement between Two Negligibly Charged Surfaces: A New Perspective on Nanoparticle Halos

Since 2001, silica microspheres have been reported to be stabilized by highly charged hydrous ZrO2 nanoparticles which form halos around the microspheres at pH 1.5. However, the exact mechanisms behind this novel stabilization method in terms of the relevant interaction forces remain unclear. In order to gain a greater insight into this mechanism, the interaction between a silica flat and a silica sphere in different ZrO2 nanoparticle suspensions was investigated by the colloid probe technique. The interaction force between a silica flat and a 600 nm silica sphere was first investigated in a ZrO2 nanoparticle (D approximate to 8 nm) suspension with volume fractions of 10(-3), 10(-4), 10(-5), and 10(-6). When the volume fraction of ZrO2 is 10(-6), only a purely attractive van der Waals force was observed between the silica surfaces. With an increase in the ZrO2 nanoparticle volume fraction, a peak was detected on the transition force curve at a ZrO2 volume fraction of 10(-5) while a purely repulsion force was observed for ZrO2 volume fractions of 10(-4) and 10(-3). The average distance difference between the peak and the zero distance point on the transition force curve which should define the distance between the halo on the microsphere is approximately 2.3 nm. Additionally, the repulsion increases with the effective zeta potential of the binary composite sphere (BCS, the entity of the silica sphere and the surrounding zirconia particles) on an increase of the nanoparticle volume fraction while the adhesion force decreases, which indicates a denser nanoparticle halo.