Bioinspired Micro/Nano textured rough topography of a surface has many applications in super-hydrophobicity, self-cleaning surface, anti-icing coatings, anti-biofouling, and drag reduction surfaces. The surfaces of living organisms like plant leaves and insects, especially on lotus leaves and rose petals, consist of micro and nanoscale hierarchical structures. The role of hierarchical and complex surface topography in nature is to amplifies the hydrophobicity and maximize fouling resistance. These are evolved to optimal states to accommodate environments better after billions of years of natural selection. Thus, a similar micro- and nano-scale 3-D topographic surfaces inspired from the nature were fabricated using a simple and scalable two roll coating process. While there are various manufacturing methods to produce these rough topography surfaces, there are still overarching challenges including the cost, manufacturing scalability and durability attributed to the photolithography-based microfabrication processes. Microlithography, the most common method for producing such surfaces, is limited to small area substrates. Thus, in this study, we utilized an inexpensive and scalable roll-to-roll method to produce a durable surface based on the ribbing instabilities associated with the shearing of non-Newtonian fluids between two rollers. These instabilities possess complex surface textures similar to the surfaces of plants and animals consisting micro and nanoscale hierarchical structures. It was found that the formation of ribbing instability in the viscoelastic polymer, which is a 3-dimensional (3D) wavy deformation, was due to the large yield stress acting on the polymer composite between the two rollers and shear-thinning behavior of the polymer composite. The polymer composite retains the deformed shape due to the recovery of high-viscosity after removing the shear stress. The large yield shear stress of the polymer composite makes the composite behaves like a solid in the absence of shear stress.&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; The non-Newtonian viscoelastic polymer composite used was synthesized by dispersing 10 wt % of multiwalled carbon nanotube (MWCNT, 10-nm diameter, and 100-μm length) in polydimethylsiloxane (PDMS) through the centrifugal planetary mixer and the three-roll mill. The formation of the instability pattern is strongly related to the dynamic rheological property and the process parameter including the roller speed of the two rollers and the gap between the rollers. The 3D and 2D surface profiles were characterized by the non-contacting laser scanning confocal microscope. The SEM further analyzed detailed surface features. The relationship between the process conditions and the textured structure were studied with the shear rate (Shear Rate = [Velocity of roll 1 – Velocity of roll 2] / Roller gap), the dimensionless parameters, including the capillary number (Ca = viscosity × fluid velocity / surface tension) and the geometric factor (Γ= Roller gap/radius), and the surface roughness parameters (e.g., Wenzel factor and density of peaks). After a preliminary study of experiments, an experimental design with 4 levels of shear rate (-26.60 s-1, -42.56 s-1, -58.52 s-1, -79.80 s-1) and 3 levels of capillary number was designed. The sample with high roughness and the textured surface was obtained with a shear rate of -42.56 s-1 and a capillary number of 5475.60. The results showed that the samples' Wenzel roughness factor increased with the increase in shear rate up to a particular value (-58.52 s-1) and then decreased. Similarly, the density of peaks in the sample increased with an increase in capillary number up to a particular value (5500) and then decreased. These bioinspired surfaces with hierarchical textured patterns produced using two roll coating process show a tremendous potentiality to be used in super-hydrophobic, anti-biofouling, and drag reduction applications.

Fabrication of Bio-Inspired Micro/nano Textured Rough Surfaces Through the Scalable Roll Coating Process

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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