Direct force measurement for the silica-plate system in nanoparticle suspensions by colloidal probe technique

Colloid stabilization is currently a subject of intense experimental and theoretical interest with many researchers focusing on charge stabilization and steric stabilization mechanisms. In recent years, the novel colloid stabilization mechanism of Nanoparticle Haloing has become a promising research trend. However, researchers have not explored the stabilization mechanism experimentally from the surface interaction perspective. The development of atomic force microscopy (AFM) has made it possible to directly measure the surface interaction forces between two colloidal particles in a solution. Prior to direct force measurements, MATLAB programs for the simulation of the Derjaguin, Landau, Verwey and Overbeek (DLVO) interaction forces were theoretically developed. Additionally, a cantilever calibration method was developed by fitting the experimental force curves to the theoretical force curves based on DLVO theory for the silica microsphere and silica flat interaction in a dilute KBr and nitric acid solutions in order to measure the interaction forces accurately. Upon finishing a cantilever's calibration, subsequent experiments were conducted to explore the colloid stabilization mechanism of nanoparticle haloing where negligibly charged silica microspheres can be stabilized by the addition of highly charged nanoparticles under an acidic environment near the isoelectric point of the microsphere. The transition force curves between the silica sphere and the silica flat-plate in different zirconia nanoparticle suspensions was firstly investigated and the transition from attractive to repulsive interactions can be untilized to explicitly show the nanoparticle haloing stabilization. The most prominent feature in these force curves is that there is meta-stabilization peak in the force curve at a zirconia volume fraction of 10-5 for the smallest zirconia nanoparticles (8 nm in diameter). On an increase (10-4 , 10-3) or decrease (10-6) of the volume fraction, a purely repulsive or attractive force was observed, respectively. Subsequently, an effective zeta potential fitting model was proposed to demonstrate the contribution of different forces to the mechanism including the van der Waals force, effective electrostatic force and depletion force. Finally, the surface force measurements were expanded to other nanoparticle suspensions. The size ratio of the nanoparticle to the sphere is extensively discussed. For systems with larger zirconia nanoparticles (55 nm and 136 mn in diameter), only repulsion appeared at a volume fraction of 10-4 and 10 -3 while purely attractive forces were observed in suspensions with a volume fraction of 10-6 and 10-5. Ultimately, the surface forces measured by colloidal probe technique can be used to predict the stabilization properties of the mixtures of nanoparticles and microspheres in an effort to directly engineer new complex fluid systems.