Tuning wettability and getting superhydrophobic surface by controlling surface roughness with well-designed microstructures

This paper presents a method to fabricate the ordered arrays of microstructures on the metal surface using mechanical micro machining technology. With specific designed micro square pillar arrays, the wettability of a solid surface can be tuned. The samples were fabricated by high speed micromilling machine tool on the aluminum alloy surface. The apparent contact angle (CA) and microstructure of the surface were measured and the surface hydrophobic mechanism was also analysed. The results of the experiment indicate that the CA has been increased. The height-to-length ratio hr (h/a) is one of the key parameters to change the wettability of the material surface. Compared with the intrinsic CA (about 51°), the largest apparent CA is up to 102.88° when the height-to-length ratio hr (h/a) is 3/7. Furthermore, by controlling the size and layout of the micropillars, the aluminum alloy surface could be changed from hydrophilic to hydrophobic.

Inspired by nature, superhydrophobic surfaces have been widely studied. Usually the wettability of a superhydrophobic surface is quantified by the macroscopic contact angle. However, this method has various limitations, especially for precision micro devices with superhydrophobic surfaces, such as biomimetic artificial compound eyes and biomimetic water strider robots. These precision micro devices with superhydrophobic surfaces proposed a higher demand for the quantification of contact angles, requiring contact angle quantification technology to have micrometer-scale measurement capabilities. In this review, it is proposed to achieve micrometer-scale quantification of superhydrophobic surface contact angles through droplet adhesion characteristics (adhesion force and contact radius). Existing contact angle quantification techniques and droplet characteristics' measurement methods were described in detail. The advancement of micrometer-scale quantification technology for the contact angle of superhydrophobic surfaces will enhance our understanding of superhydrophobic surfaces.

It is necessary to theoretically research wettability in superhydrophobic surface fabrication. Here, a numerical calculation approach is proposed for determining the contact angle of the water droplets on array micropillars by wire electrical discharge machining (WEDM). A hierarchical model is employed for these array microstructures, including mechanical analysis for a water droplet placed on a smooth array and wettability evaluation on the morphology of the WEDM surface. On pillars, equations are listed to solve the apparent contact angle according to force balance of gravity, tension, and pressure. As for the WEDM morphology, temperature simulation and measurement are carried out, and then the effect of roughness on surface wettability is studied. Constructed formulas predict the contact angle, and then the effect of geometric dimensions is obtained. In order to verify the assumption, array micropillars with different cross-profiles are prepared using high-speed WEDM on the Al alloy surface. Through the results of contact angle determination, the numerical calculation is carried out. This theoretical prediction is beneficial for improving the fabrication of the superhydrophobic surface by WEDM.

Influence of temperature on wettability of micro hierarchical rough surfaces: A three-dimensional thermodynamic model

Erosion of materials is one of the major causes that lead to the malfunction of equipment and facilities, and surface texturing can be a solution for enhancement of erosion resistance. In this work, superwetting (superhydrophilic/superhydrophobic) titanium (Ti) alloy surface with periodic microstructure was prepared by a facile laser-based surface texturing approach which combines laser surface texturing and low-temperature annealing. The effect of laser-induced surface texture and wettability on the erosion resistance of the laser textured surface was studied. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to analyze the chemical surface microstructure and surface on the untreated and laser textured surfaces. The hardness and contact angle of the untreated surface, superhydrophilic surface and superhydrophobic surface were measured by microhardness tester and contact angle goniometer. Using an in-house built erosion experimental setup, the erosion resistance of the untreated surface, superhydrophilic surface and superhydrophobic surface was investigated. The experimental results demonstrate that micro-bumps are formed after laser surface texturing. In the meantime, the surface hardness for the laser textured surface with a step size of 150 μm is increased by 48% under the load of 1.961 N. Compared with the untreated surface, the erosion resistance is increased by 33.9%, 23.8% and 16.1%, respectively, for the superhydrophobic surface. The SEM results show that the untreated surface has large and deep impact pits, while the superhydrophobic surface only has small and shallow impact pits, indicating that the erosion process resulted in less damage to the substrate. The EDS results shows that superhydrophobicity plays a critical role in protecting the substrate from erosion. It is thus believed that the superhydrophobic surface has pronounced effects for improving the hardness and erosion resistance of Ti alloy.

THE large values given by Fogg1 for the contact angles of water with leaves suggest that these are apparent rather than true contact angles. Adam2 and Wenzel3 have shown that rough surfaces give an apparent contact angle which is greater than the true contact angle for the smooth material of the surface when the true contact angle is greater than 90°. We have recently extended this theory to porous surfaces4, and to surfaces so rough that much air is entrapped at the interface between the water and the solid; large apparent contact angles are then possible when the true contact angle is even less than 90°. The apparent contact angle is given by where θD is the apparent or observed contact angle, θ is the true angle, f1 is the area of solid – water contact and f2 is the area of air – water contact per unit superficial area of the interface.

Superhydrophobic surfaces have gained sustained attention because of their extensive applications in the fields of self-cleaning, anti-icing, and drag reduction systems. Water droplets must have large apparent contact angle (CA) (>150°) and small CA hysteresis (<10°) on these surfaces. However, previous research usually involves complex fabrication strategies to modify the surface wettability. It is also challenging to maintain the temporal and mechanical stability of the delicate surface textures. Here, we develop a one-step solvent-free sand-in method to fabricate robust superhydrophobic surfaces directly atop various substrates with an apparent CA up to ∼163.8° and hysteresis less than 5°. The water repellency can withstand 100 Scotch tape peeling tests and remain stable after being stored under ambient humid conditions in Houston, Texas, for 18 months or being heated at 130 °C in air for 24 h. The superhydrophobic surfaces have excellent anti-icing ability, including a ∼2.6× longer water freezing time and ∼40% smaller ice adhesion strength with the temperature as low as -35 °C. Since the surface layers are fabricated by sanding the substrates with the powder additives, the surface damage can be repaired by a direct re-sanding treatment with the same powder additives. Further sand-in condition screenings broaden surface wettability from hydrophilic to superhydrophobic. The sand-in method induces the surface modification and the formation of the tribofilm. Surface and materials characterizations reveal that both microstructures and nanoscale asperities of the tribofilms contribute to the robust superhydrophobic features of sanded surfaces.

Due to their excellent performance, superhydrophobic materials have received a lot of attention and research in friction reduction and wear resistance. However, the effect of different contact angles of superhydrophobicity on friction and wear properties has not been thoroughly studied. In this paper, a nanosecond pulsed laser was used to realize the preparation of a superhydrophobic nickel surface, which indeed reduced the coefficient of friction but also increased the wear volume when compared to the unprocessed surface. As the contact angle of the superhydrophobic nickel surface increased, the coefficient of friction gradually increased, and the wear volume decreased gradually in superhydrophobic nickel surfaces. When the laser energy density was 1 J/cm2, the contact angle was 150.3° and the minimum friction coefficient was 0.4. However, when the laser energy density was 50 J/cm2, the maximum contact angle was 156.4° and the minimum wear volume was 4.23 × 107 μm3. The friction direction also influenced the tribological properties of the superhydrophobic-textured surface. This method makes it possible to process superhydrophobic surfaces with more suitable friction and wear properties.

The effect of drop volume on the apparent contact angle of hierarchical structured superhydrophobic surfaces

A super-hydrophobic aluminum alloy surface with decorated pillar arrays was obtained by hybrid laser ablation and further silanization process. The as-prepared surface showed a high apparent contact angle of 158.2 ± 2.0° and low sliding angle of 3 ± 1°. Surface morphologies and surface chemistry were explored to obtain insights into the generation process of super-hydrophobicity. The main objective of this current work is to investigate the maximum spreading factor of water droplets impacting on the pillar-patterned super-hydrophobic surface based on the energy conservation concept. Although many previous studies have investigated the droplet impacting behavior on flat solid surfaces, the empirical models were proposed based on a few parameters including the Reynolds number (Re), Weber number (We), as well as the Ohnesorge number (Oh). This resulted in limitations for the super-hydrophobic surfaces due to the ignorance of the geometrical parameters of the pillars and viscous energy dissipation for liquid flow within the pillar arrays. In this paper, the maximum spreading factor was deduced from the perspective of energy balance, and the predicted results were in good agreement with our experimental results with a mean error of 4.99% and standard deviation of 0.10.

Fabrication of durable superhydrophobic Mg alloy surface with water-repellent, temperature-resistant, and self-cleaning properties

Dynamic behavior of water droplets impacting on the superhydrophobic surface: Both experimental study and molecular dynamics simulation study

The core issue in the theoretical design of superhydrophobic surfaces is to elucidate the relationship between surface microstructure and wettability. In this paper, four two-scale array microstructure models with different shapes are proposed to predict the apparent contact angles (CAs) of EN-GJL-250 (Grey Cast Iron) surface under different wire-cut electrical discharge machining (WEDM) discharge parameters. The primary microstructure is the artificially designed semicircle, rectangular, sawtooth and square column array microstructure, and the secondary microstructure is the micro-nanostructure of the surface machined by WEDM. Firstly, the mechanical analysis of water droplets placed on the primary array microstructure is carried out by the force balance method. According to Newton’s third law, equations containing CA parameters are listed. Secondly, the finite element method (FEM) was used to analogue simulation the distribution of material surface temperature and flow field in the WEDM single-pulse discharge machining process. The surface equivalent geometric model of WEDM was established by a uniform arrangement of single pulse pits obtained by simulation, the surface roughness coefficients under different discharge parameters were calculated, and the influence of the surface roughness coefficients on the surface wettability was studied. The numerical results show that the CA is jointly determined by the surface roughness coefficient under different discharge parameters, and the shape and size parameters of the microstructure. Finally, semicircle, rectangular, sawtooth and square column array microstructures were fabricated on EN-GJL-250 surface by high-speed WEDM. Experimental results show: The CA of water droplets on the square column array microstructure is the largest, the predicted values of the CA were in good agreement with the experimental values, and the average relative error was 7.83%.

The study of nickel–titanium shape memory alloy (NiTi SMA) surface wettability is important to improve the blood compatibility of medical implant materials. Herein, a femtosecond laser with a wavelength of 1030 nm is used to process the hydrophobic surface of NiTi SMA. A scanning electron microscope, an X‐ray photoelectron spectrometer, a surface profiler, and a contact angle meter are used to measure the microstructure, the chemical composition, the surface roughness, and the surface contact angle of NiTi SMA. The influence of the laser fluence on the NiTi SMA surface wettability is analyzed using the Wenzel–Cassie composite model. The results show that as the laser fluence increases, the surface roughness gradually increases and then fluctuates. Moreover, a rapid increase followed by a gradual rise is observed in the surface contact angle. The blood compatibility of NiTi SMA surface increases with the increase in the surface contact angle. Specifically, the superhydrophobic surface with high contact angle and low rolling angle can significantly improve hemolysis and coagulation while reducing platelet adhesion and deformation. The study of the superhydrophobic microstructure of NiTi SMA surface will provide a new approach for improving the blood compatibility of medical implant materials.

Steam condensation with non-condensable gas exists widely in petro-chemical, power plant and seawater desalination processes. Super-hydrophobic surfaces with high contact angle could be an promising condensing surface to promote DWC and enhance heat transfer. As pure steam condensing droplet completely occupies micro/nano-structures of super-hydrophobic surfaces, super-hydrophobic function becomes “invalid”. For stream-air mixture vapor, the non- condensable gas would occupy the micro/nano-structures instead of water. Droplets form Cassie or transition wetting mode, holding or partly holding super-hydrophobic feature. Inclination angle of the tube can influence the shape, size and dynamic characteristics of the condensing droplets, thus changing the heat transfer performance. Therefore, for getting deeply insights of the effect of the non-condensable gas on condensation heat transfer, it is necessary to study the condensing drops characteristics on super-hydrophobic tube for different inclined angles. According to dropwise condensation model, increasing contact angle hysteresis, departure droplet size, sweeping period would reduce the condensation heat transfer efficiency. In this paper, dropwise condensation is performed on super-hydrophobic copper tube, departure droplet size, sweeping period and the phenomenon of contact angle hysteresis are thoroughly studied and are compared with the case of hydrophobic surface or pure steam to explore the influence of inclined angle and surface wettability on droplet dynamics to achieve condensation heat transfer enhancement. The experimental result shows: (1) On the top of copper tube, for the condensation of steam with 10% non-condensable gas, the contact angle hysteresis on super-hydrophobic surface is smaller than that on hydrophobic surface. Compared with pure steam condensation on super-hydrophobic surface, the contact angle hysteresis of mixed steam is smaller because the non- condensable gas fills the micro/nano-structures. However, on the middle and bottom parts of the tube, the contact angle hysteresis on super-hydrophobic surface is greater than that on hydrophobic surface for the condensation of both pure steam and mixed steam. This phenomenon indicates that the contact angle hysteresis is determined by surface wettability instead of inclination angle. (2) The departure droplet size on super-hydrophobic surface increases with the inclination angle and the opposite trend is found on hydrophobic surface. With the increase of inclination angle, the support force provided by the tube would increase and change direction. So the wettability of the surface and departure droplet size would be changed. (3) The sweeping period of droplet on super-hydrophobic surface is longer because the micro/nano-structures increase the adhesion between the surface and the droplets. With the increase of the inclination angle, the sweeping period decreased slightly, shorter than the sweeping time when the tube is vertical. The sweeping period of vapor with non-condensable gas condensation is greater than that of pure steam. The presence of non-condensable gas increases thermal resistance, thus slowing down the drop growth rate and increasing the sweeping period. To sum up, on the top of copper tube, the contact angle hysteresis on super-hydrophobic surface is smaller than that on the hydrophobic surface. However, the totally opposite trend is found on the middle and bottom parts of the tube. The departure droplet size on super-hydrophobic surface increases with the inclination angle. The sweeping period of droplet on super-hydrophobic surface is longer. This work offers a new avenue to further enhance heat transfer of condensation with non-condensable gas.