Capillary-Driven Water Transport by Contrast Wettability-Based Durable Surfaces

Controlling water transport and management is crucial for continuous and reliable system operation in harsh weather conditions. Passive strategies based on nonwetting surfaces are desirable, but so far, the implementation of superhydrophobic coatings into real-world applications has been limited by durability issues and, in some cases, lack of compliance with environmental regulations. Inspired by surface patterning observed on living organisms, in this study we have developed durable surfaces based on contrast wettability for capillary-driven water transport and management. The surface fabrication process combines a hydrophobic coating with hard-anodized aluminum patterning, using a scalable femtosecond laser microtexturing technique. The concept targets heavy-duty engineering applications; particularly in aggressive weather conditions where corrosion is prevalent and typically the anodic aluminum oxide-based coating is used to protect the surface from corrosion, the concept has been validated on anodic aluminum oxide coated aluminum alloy substrates. Such substrates with contrast wettable characteristics show long-term durability in both natural and lab-based artificial UV and corrosion tests where superhydrophobic coatings tend to degrade.

S-2 Supporting Information S1. Single Droplet Absorption by Hydrophilic Track To further characterize the phobic-to-philic transition on the contrast wettability surface, droplets have been deployed close to the hydrophobic-hydrophilic junction. The goal of this analysis was to investigate up to which volume droplets could be absorbed at once by the hydrophilic track, without leaving water residues on the hydrophobic surface. Figure S1: Characterization of phobic-to-philic transition. a) Deployment of a droplet on the hydrophobic surface, close to the hydrophobic/hydrophilic junction. The droplet volume was gradually increased using a syringe pump and the droplet volume at the moment of absorption was recorded. b) 22 measurements were performed in total. Whenever the droplet was absorbed completely the bar is colored green, while a red bar represents the presence of water residues on the hydrophobic surface after absorption. The blue line represents the estimated volume limit for full absorption. Figure S1a shows how the water was deployed close to the junction: a syringe pump was used to deploy water at a constant volume rate on the hydrophobic surface. At a given instant, the droplet would get in touch with the hydrophilic surface due to its growth in volume and it would be absorbed by it. At that point, the volume was recorded from the syringe pump and the hydrophobic section was inspected for residual water. If the hydrophilic track did absorb the droplet completely, then a green bar is represented in Figure S1b, otherwise, it is represented in red. After a total of 22 measurements, the absorption limit was estimated to be close to the 50 µL mark.
S-4 S2. The geometry of the Hydrophilic Tracks

S3. Variation of the Apparent Contact Angle on the Hydrophilic Tracks in Different Conditions
It was observed that the water contact angle on the hydrophilic tracks is dependent on the water content that is inside the micro-structure of the tracks. To investigate how the contact angle changes depending on the quantity of water inside the micro-structure, the sample (test token) was exposed to vapor before taking the contact angle measurements. It was done by heating water inside a 250 mL baker to 80 °C and exposing the sample (token) to its vapors for a different amount of time (0, 2.5, 5, 10, 15, and 20 seconds).

Figure S4: Water contact angle dependence of the water content in the hydrophilic track. a)
Evolution of the droplet shape on the hydrophilic track for different vapor exposure times. It can be seen that to classify as "hydrophilic", the track needs to be exposed to a minimum amount of vapor, which in this work was found to be 2.5 seconds. b) Summary of the advancing contact angles measured on the hydrophilic tracks depending on the vapor exposure time. c) Measurement of the receding contact angle, which was found to be constant in all cases. Figure S4a shows the advancing contact angle and the shape of the droplet on a hydrophilic track as a function of the vapor exposure time. It can be noted that on a completely dry surface the advancing contact angle is > 90°, while for every other case, it is < 90°. The graph in Figure S4b summarizes the advancing contact angle values for different vapor exposure times. Once completely wetted, the tracks show a strong hydrophilic character, which can be seen from the measurements of the receding contact angle, which was found to be constant at 26° for all the vapor exposure times ( Figure S4c). S-7 S4. Surface Curvature Measurements Figure S5: The shape of the water accumulation on the hydrophilic tracks. Water was not removed from the contrast wettability surface, since the test was performed on the flat test sample (token). The radii of curvature R 1,L and R 2,L were used to calculate the Laplace pressure inside the water accumulation.
By recording the radii of curvatures and as a function of the accumulated volume, it was possible 1, 2, to derive the graph presented in figure 3b of the main text, plotting vs. Collected water. The radii of curvature were found by using the "Kappa -Curvature Analysis" plugin that comes as part of the "Fiji" application, available for free on https://imagej.net/software/fiji/downloads.

S5. Artificial and natural weathering tests
The samples followed the ASTM G7 outdoor test protocol. For this test, a direct exposure mounting method has been used. Each month a contrast wettability surface sample was collected and measured. S-8

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Starting from right to left; P is the pressure system (0.4 bar), CP is the centrifugal pump, the diameter of the nozzle is 3mm and B is the holder of the samples in this case of the sample (tokens). On the top left corner, BS is the bubble spirit. Due to the inclined base, the waterjet droplets hit the sample surface with a maximum droplet force of 3.44 N. In order to test the samples in the corrosion chamber, deionized water has been used from an external tank for the preparation of the solution. It is ensured that the water conductivity is less than or equal to 20 µS/cm at 25ºC ±2 ºC, whereas the concentration of sodium Chloride is 50g/L ±5g/L. The preparation method is the following: a clean graduate bucket is used in the testing room, the solution has to be mixed, S-11 this process shall last for a couple of minutes in order to dissolve the salt completely and a test of pH level has been performed on the solution. The values shall be between 6.5 and 7.2 at 25ºC and recorded. Next, the contrast wettability surfaces shall be placed inside the chamber and fixed accordingly to their mounts as shown in Figure S10. The temperature in the corrosion chamber during the experiment is 35ºC ±2 ºC, the concentration of sodium Chloride is 50g/L ±5g/L and the air pressure is 0.7-1.0 bar. After the third pass, the PFA coating has been removed and the exposed hard-anodized Al substrate is ready for the micro-texturing of the pillars. It is shown that due to the oscillating movement of the laser beam, it is possisble to find PFA residuals on top of the exposed hard-anodized Al substrate, which has not been removed successfully by the vacuum system.

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Wavelength ( Table S1. Process parameters of the SW femtosecond laser.

S-14
Figure S10: Pillars height profile. The image shows the pillar height profiles and measurements contacted by the laser.