Final Report Summary - HYDROFAKIR (Roughness design towards reversible non- / full-wetting surfaces: From Fakir Droplets to Liquid Films)
The HYDROFAKIR project aimed at the design and fabrication of solid surfaces with ‘programmable’ wettability, ranging from superhydrophobicity to hydrophilicity, without modifying the material chemistry. Such surfaces are valuable for many technological applications like liquid handling in the micro-scale i.e. Lab-on-Chip devices, self-cleaning on demand, liquid lenses, tunable flow resistance and water collection through fog harvesting.
Wettability enhancement was performed through the application of electric fields (electrowetting); Hydrophobicity enhancement was performed by suitably designing the surface morphology in nano- or micro-scale i.e. designing surface roughness. Tunable wettability is, however, limited by thermodynamic limitations related to irreversible wetting transitions and the project aimed to illuminate the underpinning mechanisms, to study their dependence on surface roughness features or electric field distribution, and to finally suggest and fabricate surfaces that facilitate the reversibility of real-time wettability modification.
The implementation of the project faced several technical and scientific challenges; however we finally managed to integrate all the efforts towards the aim of achieving electrically tunable and reversible wettability modification. Detailed computational analysis of the wetting transitions on patterned surfaces (especially the Cassie- Wenzel transition) helped us to illuminate the underpinning mechanisms, compute the energetics and draw conclusions on the feasibility of the spontaneous reverse transition e.g. the Wenzel - Cassie one. One of the most important findings of the project is that the energy barriers of the reverse transition are fairly insensitive on the geometry features of the surface patterns. Thus the only way to achieve reversible wettability modification is to prevent the forward transition i.e. the Cassie-Wenzel one, thus to design a surface which is resistant to this transition. In this spirit, we developed a novel technique based on laser induced creation of high speed droplets for testing the thermodynamic robustness of superhydrophobic surfaces. And since, there are plenty of techniques for fabricating surfaces that resist to the Cassie-Wenzel transition when high hydrostatic or dynamic (through high speed droplets) pressure is applied, however this is not true for electrowetting actuation which is the basic requirement for electrical and real-time tunability of wetting. Electric stresses inducing high curvature modification of the liquid menisci at the grooves of the patterned surface are so high that can easily cause the Cassie – Wenzel transition. We found for the first time, as our novel computations showed and our experiments verified that the only way to avoid the high liquid curvature in electrowetting is to use high thickness of the main dielectric material. This way, the range of reversible wettability modification is substantially enlarged.
Combining the above research developments we managed to design and fabricate a surface with a contact angle modification capability from 155 to 99 degrees. This is particularity important since up to now there is no reported reversible contact angle modification with a range larger than 30 degrees (from 160 to 130 degrees) when the liquid is surrounded by air.
We have also managed to cheaply fabricate a robust polymeric superhydrophic surface and it is our next step to commercialize the above idea.
Wettability enhancement was performed through the application of electric fields (electrowetting); Hydrophobicity enhancement was performed by suitably designing the surface morphology in nano- or micro-scale i.e. designing surface roughness. Tunable wettability is, however, limited by thermodynamic limitations related to irreversible wetting transitions and the project aimed to illuminate the underpinning mechanisms, to study their dependence on surface roughness features or electric field distribution, and to finally suggest and fabricate surfaces that facilitate the reversibility of real-time wettability modification.
The implementation of the project faced several technical and scientific challenges; however we finally managed to integrate all the efforts towards the aim of achieving electrically tunable and reversible wettability modification. Detailed computational analysis of the wetting transitions on patterned surfaces (especially the Cassie- Wenzel transition) helped us to illuminate the underpinning mechanisms, compute the energetics and draw conclusions on the feasibility of the spontaneous reverse transition e.g. the Wenzel - Cassie one. One of the most important findings of the project is that the energy barriers of the reverse transition are fairly insensitive on the geometry features of the surface patterns. Thus the only way to achieve reversible wettability modification is to prevent the forward transition i.e. the Cassie-Wenzel one, thus to design a surface which is resistant to this transition. In this spirit, we developed a novel technique based on laser induced creation of high speed droplets for testing the thermodynamic robustness of superhydrophobic surfaces. And since, there are plenty of techniques for fabricating surfaces that resist to the Cassie-Wenzel transition when high hydrostatic or dynamic (through high speed droplets) pressure is applied, however this is not true for electrowetting actuation which is the basic requirement for electrical and real-time tunability of wetting. Electric stresses inducing high curvature modification of the liquid menisci at the grooves of the patterned surface are so high that can easily cause the Cassie – Wenzel transition. We found for the first time, as our novel computations showed and our experiments verified that the only way to avoid the high liquid curvature in electrowetting is to use high thickness of the main dielectric material. This way, the range of reversible wettability modification is substantially enlarged.
Combining the above research developments we managed to design and fabricate a surface with a contact angle modification capability from 155 to 99 degrees. This is particularity important since up to now there is no reported reversible contact angle modification with a range larger than 30 degrees (from 160 to 130 degrees) when the liquid is surrounded by air.
We have also managed to cheaply fabricate a robust polymeric superhydrophic surface and it is our next step to commercialize the above idea.