A gelation phenomena has been observed in systems of two different fumed metal oxide nanoparticles that will form an extremely viscous thixotropic gel when mixed together in aqueous solutions. The thixotropic gels are formed only within certain pH ranges and at certain particle ratios. These gels form at relatively low concentrations of solids; between 1.5-2.5 vol%. It is currently theorized that these gels only form when the pH is between the two particle types' isoelectric points (also known as the point of zero charge or PZC), so that one type of particle has an overall positive charge and the other type of particles has an overall negative charge. The gels are broken when either particle crosses its PZC and switches its overall surface charge, creating a nanoparticle dispersion that behaves much like a dispersion of only a single type of nanoparticle at that total concentration and a viscosity very close to that of water. These gels form at relatively low volume percent solids, as low as 1.75 vol%, and have extremely short time constants for relaxation back from the liquid state to the gel state.The gels are stable against settling and do not flow under gravitational forces, behaving as a pseudo-solid under low stress conditions. Above a certain stress-level, however, the nanoparticle network is broken and the fluid's viscosity drops dramatically from its pseudo-solid state to a viscosity near that of water, i.e. between 1-10 cP. The gels' visco-elastic behavior can also responds to relatively small changes in solution pH, allowing for external control of the fluid's rheological properties via acid or base titration. The region of gelation can also be controlled by varying the type of metal oxide nanoparticles, nanoparticle size, size ratio, and solids concentration.

Extreme thixotropic behavior in pH-responsive gels of metal oxide nanoparticles

247th National Meeting (2014)

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