Water is the most ubiquitous substance on earth and is known to have anomalous properties, which arise from the characteristic structure and dynamics of the hydrogen bond network in water. In this review, we focus on the dynamical aspects of water. Water dynamics is strongly controlled by the hydrogen bond network; there exist intermittent collective molecular motions associated with the rearrangement of the hydrogen bond network and concomitant fluctuation and relaxation in water. Five aspects of the water dynamics will be discussed: (1) intermittent collective motions and fluctuations with hydrogen bond network arrangements in water, (2) water properties at low and high temperatures, (3) molecular mechanism of water freezing, (4) role of water in biological systems, and (5) experimental methods for observing and analyzing the complex dynamics of water.

Water is a naturally circulating resource that is constantly recharged. Therefore, even though stocks of water in natural and artificial reservoirs help increase the amount of water resources available to human societies, the flow of water should be the main focus of water resources assessment. The climate system puts an upper limit on the circulation rate of available renewable freshwater resources (RFWRs). Although current global withdrawals are well below the upper limit, more than two billion people live in highly water-stressed areas because of the uneven distribution of RFWRs in time and space. Climate change is expected to accelerate water cycles, and thereby increases the amount of available RFWRs. This would slow down the increase in population under water stress; however, changes in seasonal patterns and the increasing probability of extreme events might offset this effect. Reducing current vulnerability is the first step in preparing for such anticipated changes.

Water is essential to our lives and is an extremely familiar presence to us. However, it has recently attracted significant attention worldwide. This is because the rise in population and economic development at a worldwide level have led to the growing demand for water and increase water pollution. In this paper, we report water treatment membrane technologies that contribute to solving global water problems, and most importantly the current state of water recycling technologies and the development of Japanese businesses and technologies in the world. We also report projects that appear to be particularly important,“the Mega-ton Water System” and “Water Plaza”, and their technical development issues.

The properties of water exhibit distinctive behaviors under supercritical conditions, which can be attributed to the temperature and pressure dependences of hydrogen bond networks. The relationships between the properties of supercritical water and hydrogen bond networks have been studied extensively by various experimental and theoretical techniques. This review presents the recent works on the microscopic and macroscopic properties of supercritical water and their applications.

As the world’s water market is expected to grow rapidly, the Japanese water industry, as well as municipal water utilities, is working hard to get into the market. In addition to the European major water companies, private companies in emerging and developing countries have also ventured into the water business. The Japanese companies should strengthen their technological competitiveness by developing and integrating their technologies into a total solution business in the world. It is expected that by building public-private partnership in Japan and in the world, the Japanese water industry will take hold of the world water market.

In this article, we describe requirements for ‘better tasting water’, proposed during ‘The Meeting on Better Tasting Water’. First, as items for indicating better taste, water temperature, evaporated residue, hardness and free carbon dioxide are discussed. On the other hand, as items for indicating worse taste, potassium permanganate consumption, threshold odor number and residual chlorine are explained. Total organic carbon (TOC), substances causing musty odor and trichloramine, which are related to potassium permanganate consumption, threshold odor number and residual chlorine, respectively, are also explained.

Finally, we introduce some of our activities for supplying better tasting tap water in Tokyo.

Recent studies of liquid water (H₂O) have supported a “water polyamorphism” notion that two distinct types of water exist at low temperatures. Water polyamorphism is related to a new critical phenomenon, such as the liquid-liquid phase transition of water. This new notion of liquid water may clarify the mysterious behaviors of liquid water, for example, the density maximum of liquid water at 4°C. Moreover, water polyamorphism is expected to be applied to various fields related to water in the future.

In liquid water, water molecules form a disordered three-dimensional hydrogen bond network that continuously changes with time. The physical and chemical processes in liquid water are intimately connected with the microscopic dynamics of the hydrogen bond network, such as structural fluctuation and energy transfer. In this article, we present the ultrafast processes of such microscopic dynamics in liquid water, particularly the vibrational energy relaxation studied by mid-infrared nonlinear spectroscopy. The vibrational energy relaxation in liquid water is a phenomenon that determines thermalization and affects the chemical reactions in aqueous environments. It has been revealed that the energy relaxations of the intramolecular vibrational modes occur at the time scale of about 200 fs. In addition, the vibrational relaxation rates of these modes have been found to increase with decreasing temperature. This anomalous temperature dependence sheds light on the role of hydrogen bonds for rapid energy transfer.

Recent progress on synchrotron radiation facilities and their experimental apparatus has enabled us to study liquids using soft X-rays. In this article, a recent study of liquid performed at SPring-8 BL17SU is described in terms of electronic state observations for solute molecules in aqueous solutions.

Water is believed to be the mother liquor or matrix of life. However, we still do not have a clear answer to why water is indispensable for life in terms of physics and chemistry. To answer this question, we have been investigating how protein molecules acting as molecular machines in the elementary processes of life are solvated and work in water. Here, we report our investigations on protein hydration through cryogenic X-ray crystallography and molecular dynamics simulation, and discuss the relevance of protein hydration to life viewed at the nanometer scale toward future research studies on protein hydration.

In recent years, Si photonics has attracted attention as a new integrated optical circuit technology. The impact of Si photonics is that the photonic integrated circuits used for optical communication can be fabricated with a conventional CMOS process. Optical transceivers for use in telecom or datacom networks are produced with Si photonic chips that integrate Si optical waveguides, optical MUX/DEMUXs, Si optical modulators, Ge pin photodiodes, and even light sources. In this article, I will describe “why Si is attractive for photonic devices”, “how Si is used in photonic devices”, and “how to integrate optical devices on Si wafers”.