Template‐Free Scalable Fabrication of Linearly Periodic Microstructures by Controlling Ribbing Defects Phenomenon in Forward Roll Coating for Multifunctional Applications

Abstract

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.

A scalable microstructure photonic coating fabricated by roll-to-roll “defects” for daytime sub-ambient passive radiation cooling

“…Finally, the samples' water contact angles were measured by the low-bond axisymmetric drop shape analysis plugin provided by the ImageJ software. The surface energy calculation was based on Fowke's model and the Owens-Wendt model (SI.9) 20 .…”

“…1(b) and Video (for presenting convenience, 15 vol% Al2O3/PDMS (relative low viscosity) and low roller speed are utilized). Our previous simulation research demonstrated that surface energy γ, viscosity η, roller radius R, roller gap d, and roller speed U had strong correlations with the pressure gradient in the flow direction, which directly led to the formation of the spike "defects" 20 . To demonstrate the effects of η, U, γ, and R/d on the final spikes' peak periodicity (pspike), parametric experiments are conducted with U ranging from 20 rpm to 100 rpm, and R/d from 100 to 320.…”

“…The common ribbon and spike defects in traditional roll-to-roll manufacturing inspired researchers to develop a novel scalable way to fabricate periodical microstructures [14][15][16][17] . Specifically, the positive pressure gradient generated in the downstream meniscus sows the seeds of the spike "defects" on the surface [18][19][20] . These defects can be robustly kept when the material's rheological properties are modified by adding nanoparticles.…”

A Scalable Microstructure Photonic Coating Fabricated by Roll-to-Roll “Defects” for Daytime Subambient Passive Radiative Cooling

“…However, the performance and cost were limited by the metal substrate . The common ribbon and spike defects in traditional roll-to-roll manufacturing inspired researchers to develop a novel scalable way to fabricate periodical microstructures. − Specifically, the positive pressure gradient generated in the downstream meniscus sows the seeds of the spike “defects” on the surface. − These defects can be robustly kept when the material’s rheological properties are modified by adding nanoparticles. In this situation, the defects’ peak pitch length is correlated to material properties (i.e., surface energy and viscosity) and fabrication parameters (i.e., roller radius, roller gap, and roller speed). − The optimized spike microstructures for high cooling performance can be obtained by manipulating the embedded nanoparticles and fabrication parameters.…”

“…Our previous simulation research demonstrated that surface energy γ, viscosity η (or complex viscosity η*), roller radius R, roller gap d, and roller speed U were strongly correlated to the pressure gradient in the flow direction, which directly led to the formation of the spike "defects". 23 . The parametric roll-to-roll experiment results demonstrate that the higher η (higher particle content), U, and R/d would lead to a smaller p spike as shown in Figure 3g and Table S5.…”