Spreading of a nanodroplet over isothermally heated smooth and nanostructured surfaces: A molecular dynamics study

Abstract

Owing to scale effects, spreading of nanodroplets over heated textured surfaces displays distinct features from that of millimetric-sized droplets. In this work, spreading behaviors of a water nanodroplet on isothermally heated smooth and nanostructured surfaces are investigated for various surface temperatures, wettability, roughness via molecular dynamics (MD) simulations. The simulations show that, in the early spreading stage, the spreading radius of the nanodroplet on the textured surface is smaller than that on the smooth surface, whereas a reverse result was reported for millimetric-sized droplets. Because the nanodroplet size is comparable to the feature size of nanostructures, the pinning effect becomes prominent, leading to a large local dissipation in the vicinity of the contact line and hence hindering the nanodroplet spreading over the nanostructured surface. However, more drastic evaporation on the nanostructured surface in the later spreading stage facilitates the adsorption and desorption of water molecules, weakening the pinning effect. As a result, the difference in the spreading radius over the two surfaces becomes less obvious in the later spreading stage. The simulations also demonstrate that a higher surface temperature increases the spreading coefficient and promotes the evaporation, thereby accelerating the nanodroplet spreading; the nanodroplet spreading is significantly suppressed when the intrinsic wettability of the nanostructured surface changes from hydrophilic to moderate wettability; because of stronger pinning effect, the nanostructured surface with a larger aspect ratio exhibits a slower spreading. Moreover, owing to the influence of disjoining pressure, an abnormal evaporation phenomenon is observed for the intrinsically moderate wettability surface, on which the nanodroplet shows a much faster evaporation rate in the later spreading stage than that on intrinsically hydrophilic surfaces.