New paradigm to design micro and nano-patterned membranes

Abstract

Access to drinking water is a growing issue and one of the key challenging of the twenty first century. The rapid depletion of current supply sources (aquifers, rivers, lake...) urges to find solutions, especially cost and energy efficient processes to desalinate seawater. Reverse osmosis is a membrane process for purification of seawater, invented in 1940's, which has evolved ever since, to become nowadays the most efficient process for desalination. We discuss the shortcomings of this technology, and identify bio-fouling to be the main cause of irreversibility (thus costs) in this process. After observation of solutions developed by Nature to deter bio-fouling (especially for marine species), surface micro-topography and chemistry have been identified as the two effective anti-fouling strategies. We introduce a brand new technology to create micro- and nano-patterned surfaces that is compatible with a wide variety of chemical compounds. A proof of concept is introduced with the first prototypes of wrinkled surfaces created with Initiated Chemical Vapor Deposition; Stiff polymeric coatings form wrinkles when deposited on pre-stretched soft elastomeric substrates. We are showing, both theoretically and experimentally, that the characteristics of these wrinkles can be tuned very easily. Mimicking Nature requires creating more complicated micro-topographies than the sinusoid-like pattern obtained with uniform coatings and substrates. We are showing with a numerical model that local stiffening of the substrate can be used to direct and control the buckling of the coating. In order to gain the full control of this design strategy, an inverse method is needed to establish how to treat the substrate in order to obtain a desired micro-topography. We set up the foundations of this inverse mechanical model, and develop an algorithm for a simple case.