Abstract:
A systematically designed study has been conducted to understand and demarcate the degree of
contribution by the constituting elements to the surface tension of nanocolloids. The effects of elements such
as surfactants, particles and the combined effects of these on the surface tension of these complex fluids are
studied employing the pendant drop shape analysis method by fitting the Young-Laplace equation. Only
the particle has shown an increase in the surface tension with particle concentration in a polar medium
like DI water, whereas only a marginal effect of particles on surface tension in weakly polar mediums like
glycerol and ethylene glycol has been demonstrated. Such behaviour has been attributed to the enhanced
desorption of particles to the interface and a theory has been presented to quantify this. The combined
particle and surfactant effect on the surface tension of a complex nanofluid system showed a decreasing
behaviour with respect to the particle and surfactant concentration with a considerably feeble effect of
particle concentration. This combined colloidal system recorded a surface tension value below the surface
tension of an aqueous surfactant system at the same concentration, which is a counterintuitive observation
as only the particle results in an increase in the surface tension and only the surfactant results in a
decrease in the surface tension. The possible physical mechanism behind such an anomaly happening at
the complex fluid air interface has been explained. Detailed analyses based on thermodynamic, mechanical
and chemical equilibrium of the constituents and their adsorption-desorption characteristics as extracted
from the Gibbs adsorption analysis have been provided. The present paper conclusively explains several
physical phenomena observed, yet hitherto unexplained, in the case of the surface tension of such complex
fluids by segregating the individual contributions of each component of the colloidal system.
