**2. Colloidal stability and interaction forces**

Colloidal materials in environmental and biological systems consist of small particles with very large surface area. Their typical sizes are in the range of 0.001– 10 μm as shown in **Table 1**. The stability of these colloidal materials when dispersed in fluid can be explained by their tendency to acquire electrostatic charges by adsorbing ions from their surroundings. Forces mediating particle-particle interactions can broadly be classified into the following categories depending


#### **Table 1.**

*Classification of particles in dispersion [16].*

on the chosen approach: contact forces due to the particle-particle collisions and non-contact forces due to molecular interactions at contact or interface. A brief description of these forces is hereby presented in the following sections.
