Nanofluid Surface Wettability Through Asymptotic Contact Angle

This investigation introduces the asymptotic contact angle as a criterion to quantify the surface wettability of nanofluids and determines the variation of solid surface tensions with nanofluid concentration and nanopartide size. The asymptotic contact angle, which is only a function of gas-liquid-solid physical properties, is independent of droplet size for ideal surfaces and can be obtained by equating the normal component of interfacial force on an axisym metric droplet to that of a spherical droplet. The technique is illustrated for a series of bismuth telluride nanofluids where the variation of surface wettability is measured and evaluated by asymptotic contact angles as a function of nanopartide size, concentration, and substrate material. It is found that the variation of nanofluid concentration, nanopartide size, and substrate modifies both the gas liquid and solid surface tensions, which consequently affects the force balance at the triple line, the contact angle, and surface wettability.