Skin Supersolidity Matters the Performance and Functionality of Water Droplets

Even more fascinating than the bulk parent, water droplets possess the extraordinary catalytic capability, chemical reactivity, elastic adaptivity, hydrophobicity, hydro-voltaicity, sensitivity, thermal stability, and so forth, with an elusive underlying mechanism. We show that molecular undercoordination polarizes the surface molecules and lengthens 8% of the O—O distance by shortening the intramolecular H-O bond and lengthening the intermolecular O:H nonbond, making the skin a supersolid phase and surface self-electrified. The sequential events follow the HBCP (hydrogen bond cooperativity and polarizability) and the BOLS-NEP (bond order-length-strength correlation and nonbonding electron polarization) regulations. The size dependence of the H-O bond stiffness or its phonon frequency shift H, the freezing temperature TN depression or the O:H cohesive energy loss, the polarizability, and the electronic and phonon lifetimes vary with the skin-volume ratio to the entire R-sized droplet. We determined that the H-O bond is 10% shorter and vibrates at 3450 cm-1 frequency in the 0.3 nm thick skin of 75% mass density by refining the skin phonon abundance F(R) population. The skin supersolidity and surface self-electrification could entitle the performance and functionality of the droplets. Further extension of the findings could impact the undercoordinated aqueous systems, including bubbles and molecular clusters.