AEROSIL® fumed silica has proven to be an excellent rheology agent for many low-viscosity systems to improve parameters like thickening, shear thinning, thixotropy and yield value by using the right type and amount of AEROSIL® fumed silica.
But not every type of AEROSIL® product works best in every type of fluid!
In general hydrophilic AEROSIL® fumed silica, dispersed in non-polar fluids like mineral spirit converts the fluid into a gel quite easily, while hydrophobic AEROSIL® fumed silica thickens polar fluids.
The thickening effect of AEROSIL® fumed silica
Getting the best thickening effect with AEROSIL® fumed silica depends on the wetting properties between the surface of the AEROSIL® products and the liquid to be thickened, as well as on a good dispersion (see TI 1279: Successful Use of AEROSIL® Fumed Silica in Liquid Systems).
The particles absorb molecules from the liquid until the surface energy is minimized relative to the environment. This builds a three-dimensional structure which exhibits visco-elastic properties resulting in enhanced viscosity and more or less improved yield values, depending on concentration.
If the AEROSIL® fumed silica particles have a molecular skin identical to the surrounding fluid (e.g. hydrophilic AEROSIL® fumed silica in hydrophilic fluids like alcohols, acetone, polar solvents) they tend to float in the medium rather independent of each other, resulting in only minor increase of viscosity and rather quickly settlement of the particles.
The thixotropic effect of AEROSIL® fumed silica
When shearing occurs, the resistance of the three-dimensional network is overcome and it breaks into smaller aggregates.
The apparent reduction in viscosity is very large, but as the shear is reduced, immediately the AEROSIL® fumed silica network rearranges and the viscosity rises again.
This thixotropic behavior is of great importance wherever liquids are applied to substrates by brushing and spraying or on porous substrates.