Imparting scalephobicity with rational microtexturing of soft materials

Abstract

Crystallization fouling, a process where scale forms on surfaces, is widespread in nature and technology, negatively affecting energy and water industries. Despite the effort, rationally designed surfaces that are intrinsically resistant to it remain elusive, due in part to a lack of understanding of how microfoulants deposit and adhere in dynamic aqueous environments. Here, we show that rational tuning of coating compliance and wettability works synergistically with microtexture to enhance microfoulant repellency, characterized by low adhesion and high removal efficiency of numerous individual microparticles and tenacious crystallites in a flowing water environment. We study the microfoulant interfacial dynamics in situ using a micro-scanning fluid dynamic gauge system, elucidate the removal mechanisms, and rationalize the behavior with a shear adhesive moment model. We then demonstrate a rationally developed coating that can remove 98% of deposits under shear flow conditions, 66% better than rigid substrates.

Fig. 1.. Microfoulant dynamics under shear-driven water flow.

Bottom-view image sequence showing calcium carbonate crystallites on (A) uncoated glass, (B) 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTES)–coated glass, (C) polydimethylsiloxane (PDMS 10:1)–coated glass (coating thickness, δ ≈ 10 μm), and (E) poly(ethylene glycol) diacrylate (PEG-DA 10)–coated glass (δ ≈ 10 μm) immersed in water and subjected to a shear flow (starting at t = 0 s, the flow rate increases from 7 to 103 ml/min in a channel of 80-μm height, resulting in a bulk velocity, $\overline{v}$ = 0.2 to 6 m s⁻¹). The inset image reveals the crystallite diameter to be approximately 5 to 15 μm. Zoom images showing the removal of single crystallites from the compliant substrates (D) PDMS 10:1 and (F) PEG-DA 10. We define the number of visible crystallites on the surface to be n, and its initial value, n₀. (G) Temporal evolution of n/n₀ for various coatings on glass substrates. Lines representing the mean values and shaded regions are the SD for e ≥ 9 experiments on N = 3 independent samples. (H) Influence of stiffness and wettability on n(t = 20 s)/n₀ for various coatings on glass substrates. Scale bars, (A) to (C) and (E) 100 μm; inset: (A) 10 μm and (D) and (F) 10 μm.

Fig. 2.. μ-sFDG enables insight into compliance-induced microfoulant removal.

(A) Schematic of the μ-sFDG test section: The glass capillary is connected to a fluidic system, which provides a radial laminar Poiseuille flow in the channel (h = 80 μm; Re_gap = $2\mathrm{\rho }\dot{V}/\mathrm{\mu }(2\mathrm{\pi }{R}_{\mathrm{I}}+h)$ < 1400) between the nozzle and tested material. We define the point of a given particle, “i”, on the surface beneath the nozzle as a function of time, t, as s_i(r,ψ,z = 0,t) = s_r,i(t)e_r + s_ψ,i(t)e_ψ, (r ∈ (R_I, R_O) at time zero and i ≤ n₀). (B) Epifluorescent image of the coating surface (z = 0), fouled with polystyrene microparticles (diameter D = 10 μm). Dashed lines show inner, R_I = 560 μm, and outer radii, R_O = 1000 μm, of the nozzle. (C) Experimental procedure showing the ramp-up of the volume flow, $\dot{V}$ versus t. The gray line represents the mean value of $\dot{V}$ of five experiments, and black and blue lines represent fitted data. Background colors indicate the different stages of the experiment: blue, nozzle at position h = 2000 μm; red, approach h = 80 μm; orange, linear ramp up of $\dot{V}$. Bottom-view image sequences showing the position of the microfoulants beneath the nozzle and bulk flow velocities $\overline{v}$, at r = R_I and R_O, on (D) PDMS 10:1, (F) CY52-276, and (H) PEG-DA 10 samples (δ ≈ 10 μm). The image at time zero shows the projected (r-ψ plane, z = 0) trajectories of removed microfoulants (orange). We define the initial number of visible microfoulants on the surface beneath the nozzle to be n₀(r,ψ,z = 0,t = 0 s), [r ∈ (R_I, R_O)], and its temporal value to be n(r,ψ,z = 0,t) ⊂ n₀. (E, G, and I), n/n₀ versus t for the different coatings. Bold lines represent mean value and transparent lines are individual experiments for e ≥ 28 experiments on N = 5 independent samples. Scale bars, (B), (D), (F), and (H) 200 μm.

Fig. 3.. Compliance and wettability alter microfoulant adhesion in shear flow environments.

Side-view micrographs showing the contact behavior of a foulant (D = 20 μm) on (A) glass, (B) PDMS 10:1, and (C) CY52-276 in a vacuum environment. For PEG-DA 10, we are not able to show micrographs as the sample would not survive the vacuum in the SEM or need to be freeze-dried, which would change the contact behavior. (D) Schematic of theoretical moment analysis at point O of the removal mechanism on compliant material in a shearwater flow environment. The shear force working on the microfoulant surface can be expressed as an effective hydrodynamic force F_hyd acting on the foulant at a lever length distance, 0.7D, from point O, generating the removal moment M_hyd. Similarly, the adhesion moment M_adh holding the microfoulant on the coating can be expressed as the adhesion force F_adh acting at a lever length distance a, which is equal to the contact radius. (E) Removal efficiency (1 − n/n₀) versus momentum ratio (M_hyd/M_adh) for PEG-DA 10 (blue, E = 79.5 kPa, W_adh ≈ 0.001 J/m²) (62) and PDMS 10:1 (green, E = 1112.2 kPa, W_adh ≈ 0.041 J/m²), coating thickness δ ≈ 100 μm (see Materials and Methods for material characterization). Circles represent the overall performance and semitransparent lines represent individual experiments, for e ≥ 32 experiments on N = 5 independent samples. Scale bars, (A) to (C) 10 μm.

Fig. 4.. The effect of microfoulant size and microtexture on removal dynamics.

Side-view micrographs showing the contact behavior of a microfoulant (D = 20 μm) on (A) smooth PDMS 10:1 and (B) microtextured PDMS 10:1 (δ ≈ 100 μm and surface texture, width w = 2 μm, height e = 2 μm, pitch p = 6 μm) coating in a vacuum environment. Epifluorescent image sequence showing the removal of microfoulants with D = 20 μm from a (C) smooth PDMS 10:1 and (D) microtextured PDMS 10:1 coating. Arrows display flow direction from top to bottom, microtexture aligned with the flow. Inset within sequence illustrates (C) representative rolling removal behavior indicated by the position markers and Δs_r matching the circumference length of the microfoulant and (D) shedding with initial small displacement on the coating. The last image shows the projected (r-ψ plane at z = 0) trajectories of the microfoulants in orange. (E) Removal efficiency (1 − n/n₀) versus momentum ratio (M_hyd/M_adh) for smooth PDMS 10:1 and (F) microtextured PDMS 10:1 with varying p, keeping w and e constant. Circles represent the overall performance and semitransparent lines represent individual experiments, for e ≥ 14 experiments on N = 5 independent samples. (G) Occurrence of microfoulant shedding events c_shed from the coating, represented as c_shed /n₀, versus smooth and microtextured coatings with varying p/w. Scale bars, (A) and (B) 10 μm; (C) and (D) first and last image, 200 μm; (C) and (D) inset image sequence, 20 μm.

Fig. 5.. Rational design of compliant scalephobic coatings with intrinsic scale-shedding properties.

(A) Schematic illustrates the volume flow $\dot{V}$ and gap height h versus t, colored points match the timestamps of the image sequences. Nozzle schematics indicate the nozzle position and the volume flow at specific times. Image sequence showing the removal of calcium carbonate crystallites from (B) smooth PEG-DA 50 and (C) microtextured (width w = 2 μm, height e = 2 μm, pitch p = 6 μm) PEG-DA 50 coatings. The first image in (B) and (C) indicates the reference state, the nozzle is far away (h = 2000 μm) from the surface, and white lines indicate the position of the nozzle for the image sequence. Yellow rhombus markers represent removed crystallites before the ramp-up of the volume flow. Scale bars, (B) and (C) first image, 200 μm; (B) and (C) image sequence, 100 μm.

Fig. 6.. Shear-driven crystallite removal from microtextured PED-DA 50 in a parallel plate flow chamber.

(A) Schematic (not to scale) of the test section. The polymethyl methacrylate chamber is composed of a parallel plate channel connected to a fluidic system (reservoir, pump and flowmeter), which provides a turbulent shear flow in the channel (height a = 3 mm, width b = 12 mm, length l = 120 mm, hydraulic diameter D_H = 4.8 mm; Re = ρuD_H/μ ≈ 6800; u ≈ 1.4 m s⁻¹). (B) Experimental procedure showing $\dot{V}$, over time, t. Bottom-view image sequence showing the removal of calcium carbonate crystallites from (C) microtextured (width w = 2 μm, height e = 2 μm, pitch p = 6 μm) PEG-DA 50 coating. Flow direction left to right. (D) Magnified image sequence showing crystallite removal. Flow direction left to right. Scale bars, (C) 200 μm and (D) 20 μm.