Particles interaction forces and their effects on soil aggregates breakdown

• Soil particle interaction forces and their effects on aggregates breakdown were quantitatively evaluated.
• Surface hydration force determines aggregates swelling, Coulomb force determines aggregates breakdown.
• Strong electric field in soil induces aggregates sudden breakdown (explosion), while weak electric field induces slow breakdown (dispersion).
• Sudden breakdown produces huge amount of small soil particles or Brownian particles.

Soil aggregates profoundly influence soil fertility and environmental problems, and usually improving soil aggregation is the central issue in soil management. Compared with external forces, the internal forces of soil, i.e., surface hydration force, electrostatic force and van der Waals force, may play a crucial role in aggregate formation and stability. However, there are few quantitative investigations on those fundamental issues. In the present work we aim to calculate surface hydration force, electrostatic force and van der Waals force of soil/clay particles in aqueous solution, and then quantitatively evaluate the effects of the three forces on soil/clay aggregates breakdown. There was critical surface potential in particles interaction pressure and aggregates breakdown, and if the surface potential exceeded this critical point, a further increase of the surface potential could not significantly increase particles interaction pressure and aggregate breakdown. The critical surface potentials for particle interaction pressure were 207.0 and 179.7 mV for the soil and montmorillonite, respectively. Our study suggested two steps in aggregate breakdown when dried aggregates were re-wetted: (1) separating soil particles in aggregates to a distance of 1.2–1.4 nm between two adjacent particle surfaces by the surface hydration forces (swelling process); (2) breaking soil aggregates in a way of explosion or dispersion under strong or weak electric field conditions. Surface hydration force played a crucial role in aggregate swelling, and without this repulsive pressure, a dried aggregate could not be dispersed again after re-wetting.