= Solution
A useful five-part classification is \b[electrostatic double-layer, dispersion, steric, <depletion force> and solvent-mediated interactions]. It is a practical classification rather than a unique exhaustive list.
Electrostatic double-layer <forces> arise from surface charge and mobile-ion redistribution. Like-charged surfaces often repel; opposite charges can attract. Their range changes with salt screening. <London dispersion forces> comes from correlated electronic fluctuations and is one contribution to <Van der Waals force>; its sign and strength across a medium depend on dielectric response.
<steric stabilization> occurs when adsorbed or grafted layers overlap, increasing chain confinement and/or local osmotic <free energy>. <depletion force> arises when smaller nonadsorbing solutes are excluded from a narrow gap: their greater accessible <volume> outside favors bringing the large particles together. Structured-solvent <forces> include short-range <hydration force> and <hydrophobic attraction>; their microscopic origin differs from the <depletion force> of an added solute.
The original sketches show double-layer overlap, a polymer-coated gap, <depletion force> exclusion and solvent structure, alongside a dispersion-potential curve. A realistic suspension can experience several simultaneously. Charge neutrality of a particle does not remove dispersion, steric or solvent-mediated interactions.
\Image[/past-exam-of-the-mphil-in-micro-and-nanotechnology-enterprise-course-of-the-university-of-cambridge/2018/paper-2-colloid-rheology.png]
{title=Colloidal interaction mechanisms and viscoelastic material response}
{description=Original interaction cartoons, model potentials, step-response curves and parallel Maxwell–dashpot rheology. Potential parameters and rheological constants are chosen illustrative values, not fitted measurements.}
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