Confined water in complex networks: unifying our understanding of pore water and solutes at all scales
Résumé
The arid regions experience geopolitical and sociological problems which are related to the lack of water. Yet, it is well-known that some residual water remains inside the thinner part of the porous space in the soil, possibly being supported by a non-rainfall recharge linked to dew formation. This residual non-evaporating liquid resides in an unsaturated zone which is occluded in pores and channels, can be stabilized in various ways, such as by capillary bridges concavely curved towards the atmosphere, by tiny filled porous spaces (solid-liquid-solid confinement) or by deposition on solid surfaces as adsorbed thin films (solid-liquid-air confinement). The properties of the liquid in each of these situations are of special interest for the management of the mobility and retention of this residual water. Indeed, the pathway to recharge water down to the aquifers through this unsaturated zone is notoriously complex, also because structural and textural heterogeneities occur that result in preferential flow coexisting with almost immobile water. Even deeper, in the saturated zone, it remains still a challenge to safely predict the reactive pathway of water-rock-pollutant interactions in natural settings, because of a similar complexity in the relationship between the structure of the hosting porous network, and the reactive and migrating properties of the guest fluids. With such a wide background, the general aim of the workshop was: - To define and discuss the different angles of view towards confinement and capillarity, with particular emphasis on the practical consequences in terms of mass balance and flux in real channels and pores, either of synthetic or of natural materials; - To bring together a number of leading scientists in the field of the theoretical description, experimental investigation, and/or phenomenological modeling and to have an inter-disciplinary exchange around the good, the bad, and the ugly in each conceptual frame. Altogether 30 participants from 11 countries attended the 5 day workshop. Some additional colleagues working at the University Ben Gurion of the Negev, quite close to the workshop venue, attended some of the sessions. The main feature of this group was its large interdisciplinarity: the scientific fields encompassed nano/micro-fluidics, fundamentals of surface forces, fluid physics, porous materials science, tree physiology, soil hydrodynamics and chemistry, reactive transport in aquifers. This scientific scope allowed the group to tackle the "confinement, phases, network, triangle" at different scales with very different approaches. The main goal of the present book of proceedings is to reflect the inter-disciplinary exchanges experienced by the participants: sharing common knowledge from such different points of view that, sometimes, it could be felt like different topics. The book is organized thematically into four parts. The first is a general introduction to the surface forces which are clearly the nutshell of the special physics arising around interfaces and inside restricted spaces. The second part focuses on pore water flow at different scales, either to describe its special features in restricted spaces or to define the properties that cause the liquid to remain immobile ("Flow: from nano- to mega-scale"). The third part deals with the corresponding topic of the solute movement and behavior under the same surface forces fields, introducing the strong effect of coulombic forces in the discussion ("Ions, hydration and transport"). The fourth part starts the study of phase transitions with the liquid-vapor phase transition of superheated liquid. Superheating is a common event in natural thin channels and pores, especially under arid conditions, and the end of this metastable state is reached through an explosive boiling (known as cavitation), which is of fundamental and naturalistic value ("In-channels/pores cavitation"). The fifth and last part goes beyond to the solid-liquid phase transitions and their specific behavior when a growing crystal is confined in a thin channel or pore and brought in nanometric proximity to the host solid ("Crystallization under confinement"). Each part is briefly introduced by commenting the logics of the different contributions with respect to the workshop guidelines and the global questioning in the corresponding communities. In the rest of this foreword, we would like to describe how the workshop itself was organized and which lessons we learnt from this experience.
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