Roll-to-roll UV nanoimprint lithography has superior advantages for high-throughput manufacturing of micro- or nano-structures on flexible polymer foils with various geometries and configurations.
A recent advancement in nano-pattern replication using roll-to-roll extrusion coating (R2R-EC) shows potential for many biomimetic applications. For further development of the technique a study of the micro-replication regime is executed. In this study a full and partial replication in polypropylene (PP) of micro-sized pillars has been demonstrated using the extrusion coating process. The replication fidelity of the pillars is investigated in a systematic variation of different process parameters: the line-speed of the rolls, the extruder output, the cooling roll temperature and the pressure on the cooling roll. The parameter making biggest impact on the replication is the temperature of the cooling roll. The micro-sized pillars show replication behavior opposite to what was found previously for the nano-patterns. The larger structures take the time to fill in and replicate best at the lower speeds. In this article a direct comparison of the speed at the same coating thickness is conducted.
Super-hydrophobic natural surfaces usually have multiple levels of structure hierarchy. Here, we report on the effect of surface structure hierarchy for droplet evaporation. The two-level hierarchical structures studied comprise micro-pillars superimposed with nanograss. The surface design is fully scalable as structures used in this study are replicated in polypropylene by a fast roll-to-roll extrusion coating method, which allows effective thermoforming of the surface structures on flexible substrates. As one of the main results, we show that the hierarchical structures can withstand pinning of sessile droplets and remain super-hydrophobic for a longer time than their non-hierarchical counterparts. The effect is documented by recording the water contact angles of sessile droplets during their evaporation from the surfaces. The surface morphology is mapped by atomic force microscopy (AFM) and used together with the theory of Miwa et al. to estimate the degree of water impregnation into the surface structures. Finally, the different behavior during the droplet evaporation is discussed in the light of the obtained water impregnation levels.
The rain impact and ice accretion on different aerodynamic constructions represent a large problem to their safety and operation. Most current de-icing systems include either physical or chemical removal of ice, which is resource-and energy-intensive as well as environmentally polluting. A more desirable approach to prevent initial ice formation from water droplets on a surface is to employ highly ordered super-hydrophobic materials, which mimic lotus leaves and other natural dirt-and water-repelling surfaces and reproduced in many laboratory tests. The ability to fend off water droplets could lead to prevention of icing as an inherent material property, which further would prevent ice formations, rather than fighting its build-up. Our objective is to draw attention to problems of an extension of this effect to technical applications. The main idea of the icing or rain protection of the different aerodynamic constructions may be the use of superhydrophobic thermoplastic polymers with the water-repellent properties to prevent corrosion, improve aerodynamics or add self-cleaning properties to the material. The purpose of the present study is to explore the possibility of using a fast and cheap technology of R2R-EC to enable the fabrication of nanostructures on polymer foils of sizes ranging from 50 nanometers and up to 100 micrometers with the aim of improving the ice-cleaning and water-repellent properties of the coating. This high-speed and low-cost lithography method is developed at DTU and Danapak Flexibles A/S. These superhydrophobic surfaces with water-repelling structures provide rebound actions on the impacting droplets with the large contact angle θ. For the foils with the different micro-and nanostructures, the wetting properties are measured and impact with single droplet is presented. These coatings can be used for different engineering tasks, which need to establish and test new anti-icing/rain solutions for complex aerodynamic flows. We will start the present investigation from the initial point to include main aspects of the previous water-repellent investigations and test a single droplet impacts on the surface with the thermoplastic foil for the coating.
The review reflects physical solutions for de-icing, one of the main problems that impedes the efficient use of wind turbines for autonomous energy resources in cold regions. This topic is currently very relevant for ensuring the dynamic development of wind energy in the Arctic. The review discusses an effective anti-icing strategy for wind turbine blades, including various passive and active physical de-icing techniques using superhydrophobic coatings, thermal heaters, ultrasonic and vibration devices, operating control to determine the optimal methods and their combinations. After a brief description of the active methods, the energy consumption required for their realization is estimated. Passive methods do not involve extra costs, so the review focuses on the most promising solutions with superhydrophobic coatings. Among them, special attention is paid to plastic coatings with a lithographic method of applying micro and nanostructures. This review is of interest to researchers who develop new effective solutions for protection against icing, in particular, when choosing systems for protecting wind turbines.
Experiments were carried out in a water-filled elongated cup of a “kitchen scale,” where motion was created by a rotating disk with various micro- and nano-roughness in the top of the cup. The obtained results have shown that for some patterns of nanostructures, there is a noticeable growth of a vortex, generated by the disk, while other roughnesses do not make visible changes in the flow structure. The results are of interest in assessing the efficiency of surfaces with nanoscale roughnesses. Indeed, the first type of nano-roughness may become useful for enhancing soft mixing in chemical and bio-reactors, including in the preparation of special food delicacies. On the other hand, the use of nanostructured surfaces that do not affect the main flow can help to solve some industrial problems of water and ice erosion, for example, in wind turbines or any other objects where disturbances of the main flow are undesirable.
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