Fresh water availability is becoming a global challenge in the context of global warming. Recently, efforts have been made to harvest atmospheric water vapour by the use of active or passive collectors that trigger water to condense on a surface. Passive collectors usually display an inclined planar surface at 30° to horizontal, a compromise angle for radiative exchange with the atmosphere for passive surface cooling and for the shedding of the formed droplets by gravity. Thus, two major physics domains are concerned: (i) Thermal processes that drive the collecting surface below the dew point temperature and (ii) adhesive properties to rapidly remove and collect the condensed water by gravity. Indeed, the formed droplets need to be rapidly removed from the collecting surface in order to avoid the evaporation of the droplets upon sunrise and subsequent water loss, and to promote re-nucleation on the collecting surface to increase the condensed water mass. The present talk will focus on the adhesive properties of the collecting surface by the presentation of two systems under development: The first system supresses the pinning of water droplets by the introduction of a sub-micron layer of lubricant at the surface of the collecting material, which promotes rapid droplets growth and shedding. The second system frees oneself from surface adhesive properties by modifying the collection fashion with the introduction of surface micro-grooves: the condensed water fill in the micro-grooves and continuously supply water to few major growing droplets at the bottom of the collecting surface. Both systems can reduce the latency time of water collection by an order of magnitude.
Nicolas Lavielle graduated with his PhD in physics and physical-chemistry at Empa, ETH (Switzerland) / University of Strasbourg (France) in the domain of materials science and interfaces for biomedical applications. He, then, was a postdoctoral fellow at the University of Toronto, ESPCI Paris and ESIEE working on non-adhesive materials for water and bacteria and on the collection of atmospheric water vapour. His expertise fields are physical-chemistry of polymers, soft matter and physics at interfaces.