Advantage 4 of Electrostatic Flocking
Published Date
The short fibers on the surface create an uneven, velvety texture, which increases the roughness of the fiber-free areas. Increased roughness enhances surface friction and modulates wettability, improving wetting on hydrophilic surfaces and non-wetting on hydrophobic surfaces.
I. Enhanced Surface Friction
After conducting experiments and summarizing the traditional rules of friction in 1699, Amontons logically suggested Equation (7) as the term for friction force:[51,52] μ= Ff/P
where μ represents the coefficient of friction, Ff is the friction force, and P is the normal force. An apparent result is that fric tion force primarily depends on the coefficient of friction and the normal force. The increased roughness contributes to an increase in the coefficient of friction.[53] In other words, the fric tion coefficient can be effectively improved by enhancing the sur face roughness. The electrostatic flocking technology offers a promising method to change materials surface physical state. Electrostatic flocking can help wearable sensors overcome the problems of motion artifacts and high contact pressure in real world applications (Figure d).[8,54] The force Fx, a shear mea surement, was recorded as the stage with the flocked electrode moved along the x-axis.[7] Experimental results demonstrate that the roughness of the surface can be increased by utilizing silver coated fiber flocked textiles, thereby improve the coefficient of friction between the sensor and the skin (Figure e). This helps reduce the contact pressure needed to prevent displacement between the skin and the electrode, effectively minimizing motion artifacts.[54]
II. Modified Wettability
Based on the ideal thermodynamic model, in the condition of fully wetted rough surface of Figure f, Wenzel[55] was the first to analyze the contact angle of such surfaces and proposed the relationship between the roughness factor Rf of the solid surface and the apparent contact angle,[56] Rf is defined as the ratio of the true surface area to the projected area, finally indicate Wenzel equation as Equation (8) ：Cosθ*= Rf cosθ
(8) Here, θ indicates the apparent contact angle at this time, θ is the flat surface contact angle given in Young’s equation. Cassie and Baxter modified the form of the Wenzel equation to determine the contact angle for rough surfaces, as shown in Figure 6g, in cases when the liquid droplet does not completely wet the surface.[57] In the Cassie–Baxter model, the Wenzel equa tion is rewritten as Equation (9) ：Cosθ*= Rf cosθ- f LA（ Rf cosθ+1）
Where LA represents the proportion of the liquid-air interface in the total projected composite interface. Formula (9) demonstrates that altering surface roughness can modulate the material’s wettability. Zhang et al. [58] fabricated a hair-like surface material using electrostatic flocking combined with surface modification. As shown in optical images (Figure. h), this material exhibits outstanding superhydrophobicity. The electrostatically flocked rough surface significantly increased the friction coefficient Rf while optimizing liquid film thickness fLA. Consequently, hydrophobic fibers achieve enhanced superhydrophobic performance [56]. Feng et al. [59] designed a cutting tool with nylon fibers embedded in textured patterns on its surface. Contact angle images of different surfaces in Fig. i reveal rapid wicking through honeycomb structures on flocked textiles, visually demonstrating substantially improved wettability [59–62]. Collectively, surfaces with hydrophilic textile fibers exhibit more pronounced hydrophilic properties than flat substrates, whereas hydrophobic textile fibers yield superior hydrophobic characteristics [63].
d) Cross-sectional view during electrode contact; Reprinted with permission. [54] Copyright © 2019 The Author(s).
e) Static and dynamic friction characteristics of flocked electrode (fiber length: 1.2 mm); Reprinted with permission. [7] Copyright © 2024 The Author(s).
f) Wenzel model.
g) Cassie-Baxter model.
h) Optical images of water droplets on PDMS-modified sample surfaces and related schematic diagrams; Reprinted with permission. [58] Copyright © 2022 Elsevier B.V.
i) Contact angle comparison: Non-textured surface vs. mesh-patterned surface vs. flocked surface; Reprinted with permission. [59] Copyright 2023, The Author(s), under exclusive licence to Springer-Verlag London Ltd., part of Springer Nature.
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Excerpt from the paper "Electrostatic Flocking: Reborn to Embrace Multifunctional Applications"