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Superhydrophobic Micro- and Nano-structured Surfaces: Fabrication, Wetting, and Evaporation
- Author / Creator
- Aldhaleai, Ahmed Mahmood
Water-repellent, superhydrophobic (SH) surfaces have attracted a remarkable interest in researchers for both purely academic pursuits and industrial applications, including nanofluidics and microfluidics, coatings, and drag reduction, due to their unique self-cleaning and drag reduction properties. This thesis presents the synthesis, characterization, and application of superhydrophobic surfaces, which are inspired by lotus-leaf surfaces that repel water droplets. More specifically, we investigate the effects of nanograss structure and additive surfactant on both droplet wetting and evaporation dynamics. We furthermore use a thermodynamic model to design and fabricate SH surfaces using additive manufacturing.
We first investigated the initial wetting state, evaporating dynamics, and contact line movement of a naturally-evaporating water droplet on such SH surfaces of random nano-scale roughness, with an extremely low solid packing fraction, and surface roughness. Systematic measurements of the droplet contact angle, base diameter, height, and volume were performed for several SH nanograss surfaces. Our results show that all the droplets deposited initially form a gas-trapping, Cassie-Baxter state. Small droplets subsequently evaporate with a constant contact angle mode, followed by a mixed mode at the end of the droplet lifetime. On the contrary, for relatively large droplets, two distinct evaporation modes are found. Some of the larger evaporating droplets were initially in a constant contact angle mode and underwent a mixed mode, while others began with a mixed mode with slowly decreasing base diameter and contact angle. By increasing the droplet size, for the first time, stick-slip motions of the contact line for large droplets on SH nanograss surfaces were
studied. The experimental data of contact angle-dependent evaporative mass flux are found to nearly collapse onto one universal curve for different droplet sizes and initial contact angles, in agreement with an evaporative cooling model.
To study the effect of the aqueous surfactant solutions on the wetting and the contact angle behavior of SH microstructures with high and low roughness levels, we experimentally and theoretically examine the influence of a double chain cationic surfactant, didodecyldimethylammonium bromide (DDAB), on the wetting states and contact angles on SH surfaces made of hydrophobic micro-cylinders. We use two types of micro-patterns of different surface roughness, r, and packing fraction, ϕ, and vary nine surfactant concentrations
(CS) in the experiments. At low CS, some of the surfactant-laden droplets are in a gastrapping, CB state on the high-roughness microstructures. In contrast, some droplets are in a thoroughly 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 CS, however, all the DDAB drops wet on W mode. Based on a Gibbsian thermodynamic analysis, we find that for the two types of SH surfaces used, Wenzel state has the lowest thermodynamic energy and thus is more favorable theoretically. The CB state, however, is metastable at low CS due to a thermodynamic energy barrier.
Although the SH surfaces fabricated using nano- or micromachining technologies are well-repelled water and other fluids, their fabrication processes can be complicated, timeconsuming, and expensive. We report facile and simple (one- and two-step) approaches to fabricate SH surfaces with high contact angle (CA) and low roll-off angle (ROA) for self-cleaning and water-repellent applications. Using the one-step method, we are able to produce transparent superhydrophobic (TSH) surfaces with random roughness to study the effect of the regular patterns vs. the arbitrary structures in the SH CA and the wetting properties. In the two-step process, we incorporate a 3D-printing technique with the hydrophobic coating to produce robust SH textures composed of regular square pillars, which are designed according to a thermodynamic theory for a stable gas-trapping CB state. Our measurements of static and dynamic CAs of water drops on all the prepared SH surfaces agree well with a Cassie–Baxter model.
In summary, this thesis work contributes to a better understanding of the wetting and evaporation dynamics of pure water and surfactant drops on SH surfaces of different roughnesses and packing fraction levels. In particular, additive surfactants has strong effects on the wetting characteristics at high concentrations.
- Graduation date
- Fall 2020
- Type of Item
- Master of Science
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