Effect of nanoparticles on sessile droplet contact angle.

This paper investigates the change in contact angle of droplets of fluid containing dispersed nanoparticles (nanofluid) functionalized with thioglycolic acid molecules as a function of the concentration and size of nanoparticles, and the quality and composition of the substrate material. Bismuth telluride nanoparticles with an average size ranging from 2.5 to 10.4 nm and functionalized with thioglycolic acid groups were grown by a microemulsion method and dispersed in water. Experimental measurements of the contact angle of nanofluid droplets cast on smooth glass and silicon substrates show that the contact angle depends strongly on nanoparticle concentration. Moreover, smaller size nanoparticles lead to larger changes in contact angle at the same mass concentration. These findings contribute to understanding the role of functionalized nanoparticles in surface wettability.

Analysis of the relationship between liquid droplet size and contact angle.

The purpose of this paper is to present a consistent theoretical concept that can explain the various physical phenomena associated with the effect of droplet size on contact angle for droplets on solid surfaces, and with the geometry of the liquid/gas/solid contact line in general. Two droplet geometries have been considered: uniformly elongated droplets and axisymmetric droplets. It has been shown that the contact angle for elongated droplets is size-independent and, thus, satisfies the Young equation for constant material and interfacial properties. On the other hand, whereas the contact angle for axisymmetric droplets is size-dependent and does not satisfy the original Young equation, it is shown that this contact angle can still be predicted for any combination of droplet and substrate materials, and a given mass of the droplet. The theoretical work has been combined with the development of numerical schemes of solving the Laplace-Young equation for various droplet geometries. The proposed approach has been applied to different material/substrate combinations and validated against several sets of experimental data. As a result, a method has been developed for predicting the contact angle of both long and axisymmetric sessile droplets of arbitrary sizes for given liquid/solid/gas properties.