Rheological Approach to Non-Newtonian Fluids
Non-Newtonian simply means a fluid, like AFFF, will change viscosity when a shear stress is applied to the fluid.
Pseudoplasticity means the fluid will stretch
Thixotropy means the fluid will return to its original viscosity over time
The figure above presents a graph that illustrates the viscosity changes with applied and relaxed shear stress
Formulations that make foams need these properties to function. These properties are also used in spray on products like pesticides, paints, and coatings
Self aggregated PFAS microemulsions (non-Newtonian LNAPL) sheds PFAS to groundwater, which may continue for hundreds of years if not mapped and removed from the subsurface.
Microemulsion formulations like AFFF are “Surfactant-Water-Oil” systems not “Surfactant-Water” systems. Microemulsions form spontaneously upon mixture of certain compounds due to favorable thermodynamics. In other words, the Gibbs Free Energy is negative, a spontaneous process. This is why AFFF foam self heals when debris falls through the foam; self aggregation.
Formulators have long understood that the smaller the oil droplet size, the more curved the droplet is, which in turn provides more utility of the formulation. Blends of surfactants are used to design the formulation. Microemulsion droplets are smaller than a wavelength of light making them invisible to the naked eye. Microemulsions are clear isotropic thermodynamically stable emulsions with particle sizes ranging from 1 nm to 300 nm that will not separate with time. Macro-emulsions on the other hand, require shearing to formulate, have larger droplet size, are not thermodynamically stable and will separate with time.
Unique phenomena occur with smaller droplet size that are not addressed with traditional older technologies.
The complexity of PFAS science made its toxicity easy to hide in plain sight