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Complex wetting phenomena

Final Report Summary - COWET (Complex wetting phenomena)

Static and dynamic wetting of solid surfaces by liquids is a focus of numerous theoretical, computational and experimental investigations. Most of the research and training activities in the past have been focused on the wetting of simple solids by one-component liquids. However, a wide variety of industrial processes include wetting/spreading of complex multiphase liquids over heterogeneous, structured or (nano)porous solids. The latter include deposition of active substances from foams and nanoemulsions on human hair and skin and deposition of functional nanoparticles on textile fibers and flexible polymer films. The mechanisms of those complex fluid-solid processes are still not completely understood. The development and optimization of the industrial processes and products is based on purely empirical trial and error methods.
CoWet supra-disciplinary project was aimed at bridging the gap between the industrially relevant processes involving the complex fluid-solid processes, from one side, and the high resolution experiment, as well as physically sound modelling and direct computer simulations, from the other side. The CoWet fellows were trained to study the systems of practical importance rather than focusing on model systems only. They learnt the cutting edge scientific methodology and application technology from an industrial perspective.
The CoWet research projects covered the full range of methodologies from experimental investigations and numerical simulations to the design of new ad-hoc complex liquids and complex surfaces. The project objectives included:
- understanding and prediction of the static wetting phenomena, especially the contact angles of complex liquids on complex surfaces
- understanding the spreading of complex liquids, with the focus on polymer and surfactant solutions, foams, emulsions, and functional inks
- understanding and prediction of wetting of complex surfaces, with the focus on deformable polymer surfaces, on chemically modified, microstructured or porous surfaces
- the development of strategies to fabricate ad-hoc complex surfaces and complex liquids for functional coatings
The scientific programme of CoWet embraced four interrelated research areas: (A) Wetting of complex substrates by liquids, (B) Spreading of complex liquids on substrates, (C) Development of complex liquids and substrates, and (D) Complex liquid-solid interactions and transport processes (
In the research area A “Wetting of complex substrates by liquids” (WP1 – WP5) the effect of the surface morphology, deformability and chemistry on the static and dynamic contact angle, as well as on the near wall liquid flow has been studied. WP1 was focused on the static and dynamic wetting of polymer substrates. The influence of polymer substrate rheology on forced wetting and on sessile drop retention force have been studied experimentally. In WP2 the theoretical foundations of wetting of elastic polymer substrates have been developed using the disjoining pressure concept. WP3 was devoted to the study of surface energy as well as electrokinetic and wetting properties of lipid layers, which constitute the model biological membranes. In WP4 a device for studying the wetting/dewetting of simple and complex substrates by liquids under body forces has been developed and built. The wetting of morphologically complex biological coatings has been characterized. A numerical model for the description of wetting flows on topographically complex substrates has been successfully developed and applied to description of rise of rivulets in capillaries in the framework of WP 5.
In the research area B “Spreading of complex liquids on substrates” (WP6 – WP9) the effect of the liquid composition on wetting phenomena has been studied. Surfactants and proteins solutions, liquid foams, and suspensions were considered. In WP6 the influence of the surface activity of surfactants and proteins on differences in surface tension and dilatational viscoelasticity of adsorbed layers as measured by bubble and drop analysis tensiometry have been analyzed and explained. In WP7 the surface tension and dynamic wetting of an industrially relevant high molecular weight surfactant Pluronic solutions have been investigated. In the same Work Package the dewetting process of complex industrial liquids (paints) has been studied experimentally. In WP 8 the penetration of foams (complex fluids) into porous substrates has been modelled theoretically and simulated numerically. The results have been compared with experimental data showing a good agreement. Additionally, spreading of blood (complex liquid) over porous substrates has been studied experimentally and numerically. WP 9 is devoted to particle-laden complex liquids as well as liquids containing both polymers and surfactants. Experiments have been performed to characterize the rheology and the surface tension of the adsorbed layers. The evaporation dynamics of complex liquid droplets under the influence of Marangoni stresses has been studied experimentally and theoretically/numerically.
Research area C “Development of complex liquids and substrates” (WP10 and WP11) employed the knowledge, obtained in the research areas A and B for the optimal design of new substrates and liquids applied for the coating technology and home care. In WP10 new functional and smart surfaces have been developed and designed through the formulation of specific inks. A process for coating of metallic surfaces by stimuli-responsive polymers has been designed. In WP11 an inorganic silica-based technology has been identified as a promising technology for creating fluorine-free oleophobic surfaces.
Research area D “Complex liquid-solid interactions and transport processes” (WP12 – WP14) included Work Packages in which the phenomena of wetting of complex surfaces by complex liquids is combined with micro- and nano-scale transport effects. In WP12 the membrane emulsification process for preparation of oil-in-water and water-in-oil emulsions has been studied. A hydrophobic surface treatment has been successfully developed and applied for water-in-oil emulsion production. WP13 was devoted to interaction between the nanoparticle-based inks with solid substrates for printing applications. New stable metallic nanoparticles with core/shell structure have been synthesized, conductive inks based on these particles characterized, and high quality coating obtained. In WP14 interaction of cationic multi-lamellar vesicles with anionic textile fibers has been studied. The influence of size and phase behavior of vesicles on deposition process has been investigated experimentally. A new theoretical model relating the streaming potential to the zeta-potential and the micro-geometry of porous media has been developed.
The work progress and achievements in the project have been facilitated by the high degree of synergy and collaboration within the consortium. The collaborations have been developed during the network-wide training events (like training schools and practical courses in Loughborough, Madrid, Mainz, Lublin, Thessaloniki and Jerusalem) and special sessions at established conferences (like 28th and 31st ECIS Conferences, 15th IACIS Conference) and further deepened during the secondments. The fellows benefited from the participation at the training events and from local training. Due to high and active participation of industrial members in the scientific and training program, the fellows have learnt to apply their fundamental knowledge to complex industrial processes. High and devoted industrial participation secure the high socio-economic impact of the project.
The dissemination of gained knowledge took place as well as individually by single participants, as well as in groups and in coordinated actions, including the Special Issue of “Current Opinion in Colloid and Interface Science” and of “Colloids in Surfaces A”.
The achieved understanding the complex wetting phenomena may result in ground-breaking improvement of efficiency in use of materials and in sustainable development in a chemical and food industry, in application to production of consumer goods, printed electronic devices and production of smart interfaces for energy harvesting and conservation. CoWet makes a significant contribution to the movement towards a more resource-efficient society.

Apl. Prof. Dr. Tatiana Gambaryan-Roisman
Technische Universität Darmstadt
Institute of Technical Thermodynamics
Alarich-Weiss-Str. 10