The force originates from the surface deformation (menisci) caused by the nanoparticles at the interface. The first is defined as a derivative of gravitational energy, and the latter as a derivative of wetting energy. The second one describes the particles partially immersed in a thin film or particles at the fluid-thin fluid-fluid three-phase system (thin fluid describes a layer with a thickness comparable with the nanoparticle diameter). Both can be attractive and repulsive. Increasing interfacial tension is correlated with the increase in immersion force and decrease in floatation force. The immersion force is attractive if the neighboring particles present the same meniscus (concave or convex) at the liquid interface. According to the literature, when the nanoparticle is too small (ca. <5 µm), the water surface remains flat, and no meniscus results in no capillary force. Then, the partial immersion of a particle depends on the wetting properties. The hydrophilic particles are more immersed in water than hydrophobic nanoparticles. On the contrary, Reynaert et al. studied sulfonated polystyrene microparticles (3.1 ± 0.2 µm diameter) at the decan-water interface. They showed that fractal aggregation is induced by dissolving NaCl in the subphase. The aggregation was faster for the higher salt concentration. NaCl was used to weaken the electrostatic repulsion of the particles.