Yu-Ru Huang1*, Pierre-Adrien Mante2, Chien-Cheng Chen2, Yu-Chieh Wen2,3, Hui-Yuan Chen2, Szu-Chih Yang2, I-Ju Chen2, Yun-Wen Chen4, Vitalyi Gusev5, Miin-Jang Chen6, Jer-Lai Kuo4, Jinn-Kong Sheu7, Chi-Kuang Sun1,2,3,8
1Molecular Imaging Center, National Taiwan University, Taipei, Taiwan
2Department of Electrical Engineering and Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei, Taiwan
3Institute of Physics and Research Center for Applied Science, Academia Sinica, Taipei, Taiwan
4Institute of Atomic and Molecular Science, Academia Sinica, Taipei, Taiwan
5Laboratoire d’Acoustique de l’Université du Maine, Université du Maine, Le Mans, France
6Department of Material Science and Engineering, National Taiwan University, Taipei, Taiwan
7Institute of Electro-Optical Science and Engineering and Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan, Taiwan
8Graduate Institute of Biomedical Electronics and Bioinformatics and Center for Optoelectronics Medicine, National Taiwan University, Taipei, Taiwan
* presenting author:Yu-Ru Huang,
Water molecules adjacent to solid surfaces (interfacial water) are known to play a key role in various natural phenomena ranging from surface wetting, electrolysis, to protein folding. For instance, the first few water molecular layers will govern the interface-mediated properties and energy transfer. Although a number of techniques, including atomic force microscopy, surface force apparatus, sum-frequency vibration spectroscopy, X-ray diffraction spectroscopy, etc., have been devoted to studying interfacial water, its true nature is still puzzling. For example, an ongoing debate concerns the physical properties of interfacial water at a hydrophilic surface, whether it is more solid-like, ice-like, or liquid-like. To answer this question, a complete picture of the distribution of the water molecule structure and molecular interactions has to be obtained in a non-invasive way and on an ultrafast time scale. In the present work, we developed a new experimental technique that extends the classical acoustic technique to the molecular level. Using nanoacoustic waves with a femtosecond pulsewidth and an ångström resolution to noninvasively diagnose the hydration structure distribution at ambient solid/water interface, we performed a complete mapping of the viscoelastic properties and of the density in the whole interfacial water region at hydrophilic surfaces. Our results point out that water in the interfacial region possesses mixed properties, depending on the layering of water molecules, and that the different pictures obtained up to now can be unified. Furthermore, we also discuss the effect of the interfacial water structures on the abnormal thermal transport properties, the so-called Kapitza anomaly, of solid/liquid interfaces.

Keywords: Interfacial water, hydrophilic, viscoelastic, nanoultrasonics, Kapitza anomaly