1,720,971 research outputs found
Rain-repellency and droplet bouncing properties of Bauhinia leaves in natural and laboratory environmental conditions
No full textSelf-Replenishing, Wear-Resistant and Anti-Sticking Surfaces Based on Liquid Impregnation of Microstructured Mesoporous α-AI₂O₃ [Alpha Alumina] Matrices
Full text linkFabrication of wear-resistant omniphobic surfaces is a persistent scientific challenge and essential for various applications such as in paint industries, fluid transport, antifouling, reduction of friction drag on ship hulls, and stain resistant textiles. However, there are very limited efforts to address this problem, especially because the conventional omniphobic surfaces are intrinsically wear-sensitive, due to the loss of either non polar coating materials or surface roughness elements during wear. Consequently, there is a need to employ novel approaches to fabricate such surfaces. The present research work mainly focuses on fabricating anti-sticking and wear-resistant materials using liquid impregnation of mesoporous alumina (MPA) matrices. Secondly, and equally important is to study their wetting properties post wear. To achieve the outlined objectives, different impregnating liquids such as water, Hexadecane, Dodecane, Fomblin® oil and 3M HFE 7200 were employed in conjunction with various alumina matrix densities ranging from 70 to 99.5 %. Fomblin® oil was found to be the most efficient impregnating liquid. Indeed, anti-sticking and pinning free sliding behaviors were observed for water, Hexadecane, Dodecane, water based and oil based paints atop Fomblin® oil impregnated/lubricated composites. Subsequently, the friction coefficient (FC) and the wear-resistance of Fomblin® impregnated MPA composites were evaluated by varying the normal load and alumina matrix density. The optimum alumina density of 90 %, leading to a low FC and high wear-resistance was determined. Ultra-low FC values of 0.025 and small wear coefficients of 10-8 mm3 N-1 m-1 were measured. The wear mechanism is mild abrasion majorly by intergranular fracture and/or third body abrasion. Finally, the wetting characterizations of Fomblin® impregnated MPA composites post wear were evaluated using Hexadecane and water. The worn composite surfaces lost anti¿sticking properties to water and Hexadecane immediately after wear, exhibiting superhydrophilicity and oleophilicity. Nevertheless, given a sufficient self-replenishing/healing time, they exhibited enhanced omniphobicity. The contributing factors towards self¿replenishment are the strong capillary forces of the impregnating liquid, coupled with its surface diffusivity on the polar alumina matrix. Further, the replenishing efficiency may be improved by providing surface microstructuring to MPA samples. Consequently, a fabrication process of large area surface microstructuring the bulk MPA samples was developed using replication and slip casting techniques. The microstructures are first replicated into polydimethylsiloxane (PDMS) membranes using excimer laser ablated polycarbonate sheets and are subsequently employed as molds in the alumina slip casting process. The developed process can produce microstructured areas up to 120 cm2, but can be further extended to 3 m2 and on samples as thick as 10 mm. The drying of the ceramic slurry occurs as a one dimensional process and a linear mass diffusion model developed predicts that the drying times are linearly proportional to the slurry height, PDMS membrane thickness and independent of the microstructured areas. The 3D geometries include tear cavities, micro bowls, conical micro pillars, bone pits and are not limiting. Such microstructured liquid impregnated MPA composite surfaces may also lead to anisotropic wetting and tribological properties if desired.LPMA
Robust Rain-Repellency and Droplet Bouncing Properties of Bauhinia Fresh and Aged Leaves Up to 6 Months
No full textRobust rain-repellent surfaces are useful in roofs, solar panels, windshields, etc. Herein, excellent rain-repellency and droplet bouncing properties of Bauhinia Variegata leaves are presented. They possess surface microbumps (l ∼ 13 μm, w ∼ 8 μm, h ∼ 3 μm), which in turn comprise nanoplatelets (l ∼ 741 nm, t ∼ 59 nm) and Wenzel roughness (rw) of ∼2.2. The leaf’s surface energy was estimated to be 9.47 ± 0.03 mJ·m-2 by incorporating rw into the van Oss-Good-Chaudhary theory. The leaves exhibited static contact angle of 157 ± 1°, roll-off angle of 9 ± 1°, and contact angle hysteresis of 12 ± 4°, which retained as they aged up to 186 days in the natural weather and laboratory conditions. The water droplets (10 μL, 40 μL) bounced off for free-fall heights from 5 cm to ∼13 m (Weber no. 36 to ∼2990) and displayed robust rain-repellency (Weber no. ∼4500), similar to that of a lotus leaf. Also, Bauhinia leaves survived pressurized water jets (Weber no. ∼4240). Nevertheless, underwater hydrophobicity has been persistent only for up to 3 h when submerged in 20 cm (∼1.96 kPa gauge pressure) deep water, while lotus leaves retained for >7 h. Such robust Bauhinia leaf’s nanoplatelets and wax chemistries can be replicated onto glass/metals for preparing rain-repellent surfaces
Can Microcavitated Slippery Surfaces Outperform Micropillared and Untextured?
No full textSurface features’ morphology is crucial in designing lubricant-infused slippery surfaces (LIS). Microcavities were hypothesized to provide lower physical pinning, reduced droplet normal adhesion, and superior lubricant retention as compared to micropillars and untextured surfaces. Micropillars and microcavities (h = 10 ± 3 μm, d = 8 ± 1 μm, p = 17 ± 3 μm, rw = 1.4 ± 0.2) were replicated on polydimethylsiloxane from Lotus leaf and were coated with 1000 cSt silicone oil films (530 nm-27 μm thick). Water wetting, water-oil thermodynamic stability, droplet’s normal adhesion and oil shear drainage properties were investigated to evaluate the relative performance of microcavitated, micropillared and untextured LIS. For ≤7 μm thick oil films, cavitated and untextured LIS displayed superior slippery properties than micropillared LIS (16 ± 1°, 7 ± 1°, 30 ± 4° slide-off angles respectively). Also, normal adhesion is of the order: cavities cavities > untextured. Thus, it can be concluded that microcavitated LIS can outperform micropillared and untextured LIS
Unravelling the anisotropic wetting properties of banana leaves with water and human urine
No full textWe report the heterogeneous and anisotropic wetting behaviors of Banana (Musa) leaves on adaxial (top) and abaxial (bottom) surfaces with water and human urine. Both surfaces comprise of varying number densities of micro ridges, micro stomata, micro flakes and nano hairs while the adaxial surface also constitutes discrete visible crest lines. The Wenzel roughness on abaxial and non-crest adaxial are 1.5 ± 0.05 and 1.18 ± 0.008, respectively as measured by combining optical profilometry and atomic force microscopy. A key scientific development in this work is to incorporate the measured Wenzel roughness in the surface energy estimations using Neumann's, Owens-Wendt-Rabel-Kaelble (OWRK) and Wu's models. The surface energy is 29 ± 2 mJ/m2 (OWRK) and 26 ± 1 mJ/m2 (Wu) on the adaxial surface. Nevertheless, the abaxial surfaces are characterized by smaller surface energy values of 20 ± 4 mJ/m2 (OWRK) and 24 ± 3 mJ/m2 (Wu). The measured static contact angles with water and urine are 110 ± 4° and 88 ± 4° (crest line), 130 ± 4° and 107 ± 5° (non-crest adaxial), 121 ± 4° and 120 ± 2° (abaxial) indicating hydrophobicity and urine-phobicity. The water droplets (2.5 to 10 µl) exhibited anisotropic dynamic wettability as they only rolled off parallel to the micro-ridges but stuck when slid perpendicular to the ridges even for specimen tilts up to 90°. Thus, this study unravels the anisotropic wettability of banana leaves due to the presence of micro-ridges which can be biomimicked for providing anisotropic liquid gliding. Also, this can aid in designing the spraying conditions of urine as a natural pesticide/fertilizer in the banana orchards
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