developing multifunctional surfaces with unique capabilities, including superhydrophobicity, [1,2] self-cleaning, [3,4] anti-icing, [3,5] anti-biofoulin, [6,7] biological sensors, [6] and radiative cooling. [6,8] For example, Namib Desert beetles use the superhydrophobicsuperhydrophilic micropatterned skin to harvest water from fog. [12][13][14] Plants such as lotus utilize microstructures on leaves to influence wetting behavior and enable self-cleaning. [12,15] Sharks' unique skin morphology augments the near-wall vorticity during turbulent flow. This augmentation reduces the skin friction and enables sharks to be one of the most efficient swimming species in the ocean. [16][17][18] These dragreducing microstructures are particularly interesting to the scientific community. Thus, researchers have focused on periodic microstructures aligned in the flow direction, similar to sharkskin. [19] In prior modeling and experimentation, researchers have decreased frictional drag by using different variations of linear periodic microstructures. For example, Xu et al. developed a series of linear microtrenches demonstrating a maximum drag reduction of ≈30% in high Reynolds number applications. [20] In addition, Park et al. studied the impact of grating parameters on drag reduction and demonstrated a max drag reduction of 75%. [21] Lithography is one of the standard processes for manufacturing superhydrophobic periodic microstructures. [20][21][22] Periodic micro/nanoscale structures from nature have inspired the scientific community to adopt surface design for various applications, including superhydrophobic drag reduction. One primary concern of practical applications of such periodic microstructures remains the scalability of conventional microfabrication technologies. This study demonstrates a simple template-free scalable manufacturing technique to fabricate periodic microstructures by controlling the ribbing defects in the forward roll coating. Viscoelastic composite coating materials are designed for roll-coating using carbon nanotubes (CNT) and polydimethylsiloxane (PDMS), which helps achieve a controllable ribbing with a periodicity of 114-700 µm. Depending on the process parameters, the patterned microstructures transition from the linear alignment to a random structure. The periodic microstructure enables hydrophobicity as the water contact angles of the samples ranged from 128° to 158°. When towed in a static water pool, a model boat coated with the microstructure film shows 7%-8% faster speed than the boat with a flat PDMS film. The CNT addition shows both mechanical and electrical properties improvement. In a mechanical scratch test, the cohesive failure of the CNT-PDMS film occurs in ≈90% higher force than bare PDMS. Moreover, the nonconductive bare PDMS shows sheet resistance of 747.84-22.66 Ω □ −1 with 0.5 to 2.5 wt% CNT inclusion.

Bioinspired, micro/nano-textured surfaces have a variety of applications including superhydrophobicity, self-cleaning, anti-icing, anti-biofouling, and drag reduction. In this paper, a template-free and scalable roll coating process is studied for fabrication of micro/nano-scale topographies surfaces. These micro/nano-scale structures are generated with viscoelastic polymer nanocomposites and derived by controlling ribbing instabilities in forward roll coating. The relationship between process conditions and surface topography is studied in terms of shear rate, capillary number, and surface roughness parameters (e.g., Wenzel factor and the density of peaks). For a given shear rate, the sample roughness increased with a higher capillary number until a threshold point. Similarly, for a given capillary number, the roughness increased up to a threshold range associated with shear rate. The optimum range of the shear rate and the capillary number was found to be 40-60 s-1 and 4.5×105- 6×105, respectively. This resulted in a maximum Wenzel roughness factor of 1.91, a peak density of 3.94×104 (1/mm2), and a water contact angle (WCA)of 128°.