How Surfactants Affect Droplet Wetting on Hydrophobic Microstructures

Shardt, Nadia; Bigdeli, Masoud Bozorg; Elliott, Janet A.W.; Tsai, Peichun Amy

doi:10.1021/acs.jpclett.9b02802

Cited by 19 publications

(26 citation statements)

References 60 publications

(98 reference statements)

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“…This significant increase in the wetting area leads to the replacement of the insulating air pockets existing on the superhydrophobic coatings by liquid 35 and transition of the wetting state of the droplet from Cassie− Baxter to Wenzel. 52 Consequently, by replacing the insulating air pockets with liquid, which results in an increase in thermal conductivity as well as enhancement of the solid−liquid interfacial area, freezing is accelerated in the SDS solutions. To put it briefly, the energetic barrier for ice nucleation of SDS solutions on the superhydrophobic coatings is reduced at subzero temperatures of −20 and −30 °C.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

“…In the previous sections, it was shown that regardless of the concentration, SDS droplets easily spread on the substrate, inhibit the retraction, and enhance the droplets relaxing diameter (Figure ). This significant increase in the wetting area leads to the replacement of the insulating air pockets existing on the superhydrophobic coatings by liquid and transition of the wetting state of the droplet from Cassie–Baxter to Wenzel . Consequently, by replacing the insulating air pockets with liquid, which results in an increase in thermal conductivity as well as enhancement of the solid–liquid interfacial area, freezing is accelerated in the SDS solutions.…”

Section: Results and Discussionmentioning

confidence: 99%

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Impact Dynamics and Freezing Behavior of Surfactant-Laden Droplets on Non-Wettable Coatings at Subzero Temperatures

2021

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The present study investigates the impact and freezing behavior of the droplets of surfactant solutions on non-wettable coatings at very low temperatures of −10 to −30 °C. Our goal is to elucidate the critical role of concentration, molecular weight, and ionic nature of surfactants on these phenomena. To achieve this goal, we used sodium dodecyl sulfate (anionic), hexadecyltrimethylammonium bromide (cationic), and n-decanoyl-n-methylglucamine (nonionic) at four concentrations ranging from 0 to 2 × CMC (critical micelle concentration). We captured the impact-freezing of the droplets on superhydrophobic alkyl ketene dimer coatings using a high-speed camera at 5000 frames per second. The results show that the ability of the droplets to spread and retract on the coatings is a function of concentration, ionic nature, and molecular weight of the surfactants, as well as the temperature-dependent viscosity of the solutions. Additionally, surfactant-laden droplets generally demonstrated an accelerated freezing compared to pure water. This might be due to the fact that the presence of surfactants can promote both heterogeneous ice nucleation from within the liquid and a larger solid–liquid interfacial area by filling the air pockets of the surface, leading to enhanced heat transfer. The behavior of the cationic surfactant at certain concentrations was, however, an exception leading to a freezing delay, for which a mechanism will be proposed.

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Section: ■ Results and Discussionmentioning

confidence: 99%

Section: Results and Discussionmentioning

confidence: 99%

Impact Dynamics and Freezing Behavior of Surfactant-Laden Droplets on Non-Wettable Coatings at Subzero Temperatures

2021

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“…Although the modified CB eq 6 well predicted C S -dependent CA for CB drops, it does not explain the probability and the stability of the wetting states (shown in Figure 2). To get a better understanding concerning the stability and the metastability of the observed wetting behavior and to explain the occurrence of different wetting states depending on the C S on the different microstructures, we carried out an analysis starting from the Gibbsian thermodynamics, 23,38,53−57 following the work by Shardt et al 38 using SDS surfactants, and analytically estimated the free energy (E) for our composite system of DDAB-laden surfactant droplets sitting on a microstructured surface. The derived free-energy equation, E − E 0 , with respect to the assumed reference state has the form of Shardt et al 38 free energy…”

Section: Modified W and Cb Ca Equationsmentioning

confidence: 99%

“…We followed a model that a droplet transiting from the CB to W wetting state usually occurs through two main phases as described below. 23,38,58 In this first phase, after droplet deposition, the liquid is falling down along the pillars with an assumed flat LV interface as in the CB wetting state (with f = ϕ and f 1 = 1 − ϕ), until it wets the bottom of the surface. Here, we assume that only the cylinder's walls are wet and the bottom surface is not wet, so f increases and f 1 = 1 − f. At the end of the first phase, the value of f further increases as the solid−liquid contact area increases.…”

Section: Modified W and Cb Ca Equationsmentioning

confidence: 99%

Effect of a Cationic Surfactant on Droplet Wetting on Superhydrophobic Surfaces

Aldhaleai

Tsai

2020

Langmuir

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We experimentally and theoretically examine the influence of a double-chain cationic surfactant, didodecyldimethylammonium bromide (DDAB), on the wetting states and contact angles on superhydrophobic (SH) surfaces made of hydrophobic microcylinders. We use two types of micropatterns of different surface roughness, r, and packing fraction, ϕ, and vary nine dimensionless surfactant concentrations (C S), normalized by the critical micelle concentration (CMC), in the experiments. At low C S, some of the surfactant-laden droplets are in a gas-trapping, Cassie–Baxter (CB) state on the high-roughness microstructures. In contrast, some droplets are in a complete-wetting Wenzel (W) state on the low-roughness microtextures. We found that the contact angle of CB drops can be well predicted using a thermodynamic model considering surfactant adsorption at the liquid–vapor (LV) and solid–liquid (SL) interfaces. At high C S, however, all of the DDAB drops wet in a Wenzel mode. Based on a Gibbsian thermodynamic analysis, we find that for the two types of superhydrophobic surfaces used, the Wenzel state has the lowest thermodynamic energy and thus is more favorable theoretically. The CB state, however, is metastable at low C S due to a thermodynamic energy barrier. The metastable CB wetting state becomes more stable on the SH microtextures with greater ϕ and r, in agreement with our experimental observations. Finally, we generalize this surface-energy analysis to provide useful designs of surface parameters for a DDAB-laden surfactant droplet on the SH surface with a stable and robust CB state.

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“…The gas-trapping CB wetting state, contributing to a large contact angle and a low CA hysteresis, is critical to resilient superhydrophobicity, which is beneficial for various applications in surface engineering. However, the long-term stability of the preferred CB state on SH surfaces is still challenging and can be lost through an irreversible wetting transition to Wenzel state, when exposed to a high-temperature environment, droplet evaporation, , or surfactant additives. − …”

Section: Introductionmentioning

confidence: 99%

Fabrication of Transparent and Microstructured Superhydrophobic Substrates Using Additive Manufacturing

Aldhaleai

Tsai

2020

Langmuir

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We report facile one- and two-step processes for the fabrication of transparent ultrahydrophobic surfaces and three-dimensional (3D)-printed superhydrophobic (SH) microstructures, respectively. In the one-step method, polydimethylsiloxane (PDMS) solution is treated thermally at 350 °C for 4 h, while PDMS-soot is generated and deposited on a glass slide to obtain a transparent SH surface without further chemical modification. For the two-step approach, SH surfaces are obtained by incorporating a 3D printing technique with a convenient hydrophobic coating method. Herein, we first 3D-print microstructured substrates with particular surface parameters, which are designed to facilitate a stable gas-trapping Cassie–Baxter (CB) wetting state based on a thermodynamic calculation. We subsequently coat the 3D-printed microstructures with candle soot (CS) or octadecyltrichlorosilane (OTS) solution to make superhydrophobic surfaces with mechanical durability. These surfaces exhibit an ultrahigh static water contact angle (CA, θ ≃ 158 ± 2 and 147 ± 2° for the CS and OTS coating, respectively) and a low roll-off angle for water droplets. Both static and dynamic (in terms of the advancing and receding) contact angles of a water droplet on the fabricated SH surfaces are in good agreement with the theoretical prediction of Cassie–Baxter contact angles. Furthermore, after a one-year-long shelf time, the SH substrates fabricated sustain good superhydrophobicity after ultrasonic water treatment and against several chemical droplets. All of these methods are simple, cost-effective, and highly efficient processes. The processes, design principle, and contact angle measurements presented here are useful for preparing transparent and superhydrophobic surfaces using additive manufacturing, which enables large-scale production and promisingly expands the application scope of utilizing self-cleaning superhydrophobic material.

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How Surfactants Affect Droplet Wetting on Hydrophobic Microstructures

Cited by 19 publications

References 60 publications

Impact Dynamics and Freezing Behavior of Surfactant-Laden Droplets on Non-Wettable Coatings at Subzero Temperatures

Impact Dynamics and Freezing Behavior of Surfactant-Laden Droplets on Non-Wettable Coatings at Subzero Temperatures

Effect of a Cationic Surfactant on Droplet Wetting on Superhydrophobic Surfaces

Fabrication of Transparent and Microstructured Superhydrophobic Substrates Using Additive Manufacturing

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