The change in the wetting regime of a nanodroplet on a substrate with varying wettability: A molecular dynamics investigation

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The effect of the triple-phase contact line (TPCL) on the wetting phenomenon has been extensively discussed during the past decade. Numerous attempts have also been made to quantify its characteristics based on thermodynamic or mechanical definitions. In this research, molecular dynamics simulation was used to define the term “vicinity of the TPCL” and its effect on the hydrophilic and hydrophobic behaviors of a water nanodroplet. A nanodroplet was placed on a substrate that was modified in a stepwise manner by growing a patch of heterogeneity from either the center of the substrate or from the sides. The relative direction of motion of the TPCL and the patch determined the pathway that the nanodroplet chooses in order to change its wetting regime from hydrophilic to hydrophobic and vice versa. A gradual change occurs when the TPCL and the heterogeneity move in the same direction, and an abrupt change takes place otherwise. In addition to the insights into the wetting phenomenon, the width of the TPCL is also discussed. The obtained data suggest that the effective width of the TPCL, δ, is different inside the perimeter of the nanodroplet from outside of it. Moreover, the value of δ for the abrupt pathway is twice as large as the gradual one. In conclusion, the width, or vicinity, of the TPCL depends on the type of the pathway and the configuration of the substrate-patch system and cannot be treated similarly in both cases.