Viscosity is a property arising from collisions between neighboring particles in a fluid that are moving at different velocities. Viscosity can be described as the internal resistance to root canal irrigant flow deformed by either shear or tensile stress. The best wetting properties of the irrigants are obtained at this concentration. Above the CMC, the addition of surfactant provokes formation of micelles in the liquid, keeping constant the surface tension. Adding surfactants to irrigants, the surface tension decreases until reaching the critical micellar concentration (CMC). This may be accomplished by the use of heat or the addition of surfactant. Surface tension is a condition of intramolecular attraction, and it is decreased when this condition is destroyed. Wettability is correlated with surface tension on ideal surfaces (chemically homogeneous, flat, nonreactive, undeformable, and not swollen by the wetting liquid) and then with the surface properties of dentine. To obtain a suitable contact time of irrigants with root canal dentinal walls, a major role is played by the wettability. It has been demonstrated that high viscosity and high surface tension reduce the ability of sodium hypochlorite (NaOCl) to penetrate into dentine and its antibacterial effectiveness within dentinal tubules. Fluid surface tension and viscosity largely influences the ability of an irrigant to penetrate into dentine and its spreading property on dentin surfaces. Two of the most essential parameters related to fluid flow are its surface tension and its viscosity. Several conditions affect how these irrigants spread onto the root canal system and reach the noninstrumental areas. Endodontic irrigants can remove debris and microorganisms from the areas that cannot be mechanically approached. Due to the complexity of endodontic space, mechanical instrumentations alone cannot achieve this outcome. Although, after chemo mechanical treatment of root canals the population of microorganisms is significantly decreased, all the microorganisms are not eliminated. The ultimate goal of endodontic treatment is the complete eradication of microorganism from the root canal space, or at least their reduction to levels compatible with periradicular tissue health.

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