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DOI: 10.1039/D4TA03444A (Paper) J. Mater. Chem. A, 2024, Advance Article

Condensation heat transfer enhancement through durable, self-propelling fluorine-free silane-treated anodized surfaces†

Mahmoud Mahlouji Taheri, Behzad Rezaee, Hossein Pakzad and Ali Moosavi*
Center of Excellence in Energy Conversion (CEEC), Department of Mechanical Engineering, Sharif University of Technology, Azadi Avenue, P. O. Box 11365-9567, Tehran, Iran. E-mail: moosavi@sharif.edu

Received 17th May 2024 , Accepted 13th August 2024

First published on 13th August 2024

Abstract

When two or more adjacent droplets coalesce, excess surface energy is generated, which can be converted into the kinetic energy of the merged droplet through a suitable nanostructure and the superhydrophobicity of the surface. This causes a self-propelled behavior on the condenser surface and, consequently, a much higher condensation heat transfer coefficient. Despite these advantages, developing a durable self-propelled condenser surface that is easy to fabricate and employs non-toxic and fluorine-free materials for its superhydrophobicity remains a challenge. Addressing this gap in knowledge, we introduce a durable yet versatile fluorine-free superhydrophobic surface through the modification of anodized aluminum, which can enhance the condensation heat transfer coefficient by up to 676.7% compared to a pristine aluminum surface at a subcooling temperature of 2.77 K. Furthermore, this surface can retain its superhydrophobic properties even after 320 hours of continuous condensation. Moreover, after undergoing 10 meters of abrasion, the superhydrophobicity of the surface remains unaffected. Additionally, a superhydrophilic surface obtained through anodizing aluminum has also proven to be effective only at low subcooling temperatures, improving the condensation heat transfer coefficient up to 16.15% compared with pristine aluminum at a subcooling temperature of 3.96 K. Both the superhydrophilic and superhydrophobic surfaces presented in this study show anti-corrosive behavior as well, reducing the corrosion current density by 2 and 4 orders of magnitude, respectively.

1. Introduction

Condensation, a well-known exothermic process that has many applications in industry, occurs in two different modes: filmwise condensation and dropwise condensation. The condensation process is widely used in power plants,1 water desalination systems,2,3 fog harvesting,4 heating, ventilation, and air conditioning (HVACs),5 heat pipes,6,7 and cooling of electronic components;8 therefore, the overall efficiency of industrial processes dealing with the condensation phenomenon could be enhanced by improving the efficacy of the condensation process. As a result, much research has been conducted on the improvement of the condensation process. In this regard, researchers have proposed active approaches such as the employment of an electrical field9 or vibration10 and passive methods like the modification of the condenser's surface wettability to enhance this process. However, efficacious passive methods are preferred over active ones, as they do not require an exogenous energy source to function. Thus, many studies have been published on altering the condenser's surface wettability, shifting the condensation mode from the low-efficiency filmwise mode to the high-efficiency dropwise mode. Through the filmwise condensation process, after the nucleation stage, the coalesced water droplets tend to form a thin water film on the surface.11 This water film acts as a thermal barrier due to the low thermal conductivity of water, which results in a lower condensation performance.12 On the other hand, in the dropwise condensation process, the nucleated water droplets grow and coalesce with each other to form larger droplets, and when their size reaches a specific amount, the gravitational force causes them to depart from the surface. After the departure of the condensed water droplets from the surface, the bare condenser surface is again subjected to water vapor for nucleation.13 Furthermore, previous studies have reported that some nanostructured superhydrophobic surfaces can lead to another mode of droplet departure in dropwise condensation, known as droplet jumping. In this mode, the coalescence of two or more adjacent water droplets generates excess surface energy, enabling small coalesced droplets to jump off the surface.14

Superhydrophobic surfaces with droplet-jumping ability in the condensation process were introduced by Boreyko and Chen in 2009.15 They fabricated unique superhydrophobic surfaces by depositing carbon nanotubes (CNTs) on micropillars that were carved on a silicon wafer using micromachining. They concluded that the critical droplet diameter required for bouncing off the surface was 10 μm. Following this work, Miljkovic et al.14 investigated the condensation heat transfer performance of copper tubes by inducing knife-like nanostructures on them, which were modified with fluorinated alkylsilanes (FASs). Their results revealed that the heat transfer coefficient of the fabricated surface that enabled jumping condensation was about 30% higher than that of a hydrophobic surface with dropwise condensation. He et al.16 fabricated hierarchical nanostructures on an aluminum substrate by immersing aluminum in boiling water. The surface energy of these nanofeatures was reduced by using FASs, and eventually, the superhydrophobic surface with a condensed droplet jumping ability was acquired. Nonetheless, due to these nanostructures' perishable chemical and mechanical stability, Li et al.17 used the anodizing process instead of boiling water to achieve the required nanofeatures on the aluminum substrate. They also utilized FASs to modify the surface energy. Recently, Donati et al.18 studied the condensation heat transfer of an aluminum substrate with hierarchical nanofeatures that were modified using two different FASs, namely, trichloro-1H,1H,2H,2H-perfluorodecylsilane (FDTS) and 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTS), and another fluorinated material named poly-1H,1H,2H,2H-perfluorodecyl acrylate (pPFDA). They also performed a relatively short-term condensation durability test. Their findings showed that the stability of the surfaces modified with the two different FASs was easily compromised, despite the short-term droplet jumping mode on these surfaces. However, the surface modified with pPFDA had a heat transfer coefficient that was 9.6 times that of the surface in filmwise condensation mode and could maintain the jumping droplet mode for 72 hours.

Despite copious endeavors to find superhydrophobic aluminum substrates with appropriate nanostructures suitable for condensed droplet jumping, the heat transfer coefficient, which is the essential parameter that determines the condensation heat transfer performance of the surface, has not been measured in many studies reported in the literature.15–17,19–22 Moreover, besides the heat transfer measurements, the fabricated surfaces must undergo stability tests to evaluate their applicability in industry.23 However, in contrast to the importance of this point, stability has not been considered in most of the studies in the literature.14–17,19–22,24–30 Furthermore, most of the studies have proposed fabrication methods that were costly and complicated, which sometimes required clean rooms14 or photolithography.22,25 Moreover, according to the literature, FASs have been used in the vast majority of articles published on the jumping mechanism of condensation phenomena. However, these materials are expensive, and because of their degradation, the water collected from the condenser's surface is prone to contamination with fluorine materials, adversely affecting human health and the environment.31 In turn, the stability of these materials on the condenser's surface should be considered. With this in mind, there is a knowledge gap in the literature regarding nanostructured superhydrophobic surfaces that enable jumping condensation with durable yet non-fluorinated coatings. In addition, while many studies attempt to introduce varied superhydrophobic surfaces to ensure effective droplet departure, no previous studies attempted to scrutinize the effect of lower nucleation barriers on superhydrophilic surfaces on their condensation heat transfer performance. Nonetheless, a previous study has shown that when moisture capture becomes more important than condensate removal, more wettable surfaces perform better than non-wettable ones.32

In the present study, we experimentally show that anodized aluminum that is silanized using octadecyltrichlorosilane (OTS) can profoundly enhance condensation heat transfer. Moreover, the numerous stability tests performed reveal the splendid durability of the fabricated surface against mechanical wear, corrosion agents, and long-term exposure to environments filled with hot vapors. Furthermore, to get a deep understanding of the condensation performance of the fabricated surface, the condensation heat transfer coefficient is measured at relatively broad subcooling temperatures (difference between the saturated and surface temperatures) ranging from approximately 3 °C to about 30 °C. The condensation performance of the anodized aluminum as a superhydrophilic surface before the silanization process is also investigated herein, which has not been addressed in any previous studies.

2. Experimental section

2.1. Materials

The materials utilized in the fabrication of the nanoporous anodized aluminum surfaces encompass ortho-phosphoric acid (H3PO4, 85%, Merck, Germany), sulfuric acid (H2SO4, 95–98%, Dr Mojallali Industrial Chemical Complex Co., Iran), chromium(VI) oxide (CrO3, 99.99%, Ochem, Iran), and deionized water (H2O, with an electrical resistance of 18.2 MΩ, obtained from a water purification system, ZU101-C, Zolalan, Iran). Acetone (C3H6O, 99%, Dr Mojallali Industrial Chemical Complex Co., Iran) and ethanol (C2H5OH, 99%, Merck, Germany) are used as cleaning agents. The surface energy of the anodized surface is reduced using octadecyltrichlorosilane (OTS, CH3(CH2)17SiCl3, 90%, Sigma-Aldrich, USA) solution prepared in normal hexane (n-hexane, CH3(CH2)4·CH3, 99%, Oxford Lab Fine Chem LLP., India). Distilled water is used to generate steam for the condensation test process. Ethylene glycol (C2H6O2, 99%, Neutron Pharmachemical Co., Iran) is utilized in the coolant composition flowing in the heat exchanger of the condensation test apparatus.

2.2. Fabricating self-propelling fluorine-free silane-treated anodized surfaces

In the present study, aluminum 6061 sheets with dimensions of 40 mm × 40 mm × 3 mm are used as substrates. Prior to any surface treatments, the aluminum surfaces are ultrasonically cleaned in acetone and ethanol, each for ten minutes. Afterward, the surfaces are electropolished to remove the oxide layer and any remnant contamination, leading to a mirror-like smooth surface required for anodizing.

In the electropolishing process, as shown in Fig. S1a,† the substrate is used as an anode, and another aluminum substrate with the same dimensions is used as a cathode. The electropolishing solution is composed of 222.30 g of phosphoric acid, 39.38 g of sulfuric acid, 18.00 g of chromium(VI) oxide, and 15.82 g of deionized water. During electropolishing, the electrolyte solution is immersed in water and ice baths to maintain the solution temperature at 0 °C.33 The distance between two electrodes within the electrolyte solution is set to 1.5 cm, and direct current is supplied after immersing these electrodes within the electrolyte. The applied voltage is 24 V, which is maintained for 7 minutes. Then, to induce a porous anodic surface on the aluminum substrate, the electropolished aluminum is submerged into the anodizing electrolyte as an anode, whereas a steel sheet with the same size as the anode is used as a cathode, 2.5 cm away from the anode. The electrolyte solution is a mixture of phosphoric acid and deionized water with a concentration of 0.2 M. The anodizing electrolyte is surrounded by a water bath at 50 °C (Fig. S1b†). The applied voltage and time of the anodizing process are 130 V and 3 minutes, respectively.33

After the anodic layer is formed on the surface of the aluminum substrate, the substrate is submerged in a 0.003 mM solution of OTS in n-hexane.33 The reaction between the hydroxyl groups on the anodic layer on the substrate and OTS molecules inside the solution is exhibited in Fig. 1. Accordingly, low amounts of HCl are produced due to this reaction, and a long chain consisting of CH2 and CH3 with low surface energy covers the surface, leading to its superhydrophobicity. The reaction shown in Fig. 1 takes about a day to occur and does not require heating.


image file: d4ta03444a-f1.tif
Fig. 1 Schematics of silanization of the anodic layer formed on the aluminum substrate, resulting in a superhydrophobic surface and a small amount of HCl.

2.3. Surface morphology and wetting characteristics

The surface morphology and texture of the nanostructures of the samples are examined with a scanning electron microscope (SEM, TESCAN Inc., MIRA3, Czech Republic). Before conducting this test, the samples are coated with a nanometer layer of gold to inhibit charging accumulation. The accelerated voltage in the SEM tests is 15 kV. In addition, to detect the elements on the surface, an energy-dispersive X-ray spectroscopy (EDS, X-MaxN 80, Oxford Instruments, United Kingdom) module attached to the SEM device has been utilized. The contact angle and contact angle hysteresis of the samples are measured through a contact angle meter (JIKAN CAG-20, Iran). The accuracy of the contact angle measurement of this device is 0.1°. The contact angle is obtained by adding a 5 μL droplet of deionized water onto the surface and analysing the photographs taken of it. However, since the droplet is prone to attach to the needle tip rather than the superhydrophobic surface, the hanging droplet method has been employed to obtain the contact angle of the superhydrophobic surface.34 Additionally, contact angle hysteresis is calculated using the “add and remove volume” method by subtracting the receding contact angle from the advancing contact angle. These measurements are performed at three different points on the surface in order to obtain a reliable value for the contact angle and contact angle hysteresis of the surfaces, and their average has been reported.

2.4. Condensation test

The condensation apparatus provided for this work is exhibited in Fig. 2. Accordingly, the 40 mm × 40 mm × 3 mm sample is attached vertically to the front of the heat exchanger through the thermal paste. The heat exchanger is fed through a thermostatic bath with a coolant fluid that is composed of ethylene glycol and deionized water with a volume ratio of 20%.
image file: d4ta03444a-f2.tif
Fig. 2 Schematics of the condensation test apparatus, including a description of its various parts and the measurements throughout the system, including temperature, pressure, and humidity sensors.

Steam is introduced into the condensation chamber with a constant mass flow rate (ṁv = 14 g min−1) by means of a steam generator. The surface and condensation chamber temperatures are evaluated via type K thermocouples. The difference between the surface temperature (Tsur) and the vapor temperature (Tv) is reported as the subcooling temperature (Tsub = Tv − Tsur). The relative humidity inside the condensation chamber is also obtained through a hygrometer (AM2301, AOSONG, China). The tests are conducted under atmospheric pressure, with a relative humidity of 99.9% and a saturation temperature of 96.5 °C. In addition, a camera (Nikon, J4) is used to understand the interaction among vapor, condensate water, and the sample surface.

The inlet (Tin) and outlet (Tout) temperatures of the coolant flowing inside the heat exchanger are measured using two PT-100 sensors. Having the mass flow rate of the coolant (ṁ = 0.07 kg s−1), the specific heat capacity of the coolant (cp = 3.93 kJ kg−1 K−1), the area of the sample surface (As = L2 = 40 mm × 40 mm), and the temperatures measured by the sensors, the heat transfer coefficient of each sample can be obtained through eqn (1).

 
image file: d4ta03444a-t1.tif(1)

2.5. Durability tests

Various factors can have adverse effects on the provided dropwise condensation-enabling surface. Harsh condensation conditions, abrasion, and corrosion are some of these factors. Accordingly, three durability tests have been conducted to ensure surface stability against these factors. One of these tests is performed to assess the surface stability when exposed to condensation conditions to determine whether the surface characteristics, such as contact angle and contact angle hysteresis, deteriorate under these conditions. The contact angle and contact angle hysteresis of the sample surface are measured after 8 cycles of exposure to condensation test conditions, each test cycle lasting 40 hours. After these cycles, the surface is visually investigated for its performance during condensation. An abrasion test is conducted as well by attaching a 100 g weight to the back of the coated surface and dragging the coated surface on #1000 sandpaper in 10 cm cycles (Fig. S2†).35,36 The cycles are repeated 100 times. Following each abrasion cycle, the surface's contact angle and contact angle hysteresis are measured to assess the effect of mechanical wear on the surface. An electrochemical polarization test is conducted on the surface via a potentiostat (Compactstat, Ivium, Netherlands) to study how corrosion affects the surface. This test utilized a three-electrode cell consisting of a standard calomel electrode as the reference electrode, a platinum electrode as the counter electrode, and the sample as the working electrode. The polarization test is performed in the potential range of −250 to +250 mV relative to the open circuit potential and at a scan rate of 1 mV s−1.

The self-cleaning ability of the surface is also investigated by depositing dirt on the coated surface and then adding water droplets onto that surface to scrutinize their interaction and determine whether the surface characteristics degrade due to the dust.35

3. Results and discussion

3.1. Surface morphology, wetting characteristics, and elemental composition

The morphology of the anodic film that is formed on the aluminum surface is shown in Fig. 3. Accordingly, Fig. 3a, with 50k× magnification, exhibits the fabricated porous nanostructure of aluminum oxide with a pore diameter of 134.36 ± 9 nm, encompassing sharp nanoscale edges on the surface. In addition, Fig. 3b, with magnification of 10k×, shows the side view of the anodic layer that has a thickness of 6.99 ± 0.26 μm. Furthermore, a small window of Fig. 3b is magnified to a magnification of 25k× to catch a better glimpse at the sharp edges of the structure and its porous form.
image file: d4ta03444a-f3.tif
Fig. 3 Anodic film morphology including: (a) SEM image of the nanostructures on the anodized surface from top and (b) side views. SEM image of the micrometric thickness of the anodic layer formed on the aluminum surface.

After the silanization process, the morphology of the surface would not change; however, the elements on the surface ensure that the reaction between OTS molecules and the surface hydroxyl groups takes place. As mentioned, the EDS test results have been utilized to identify the precise chemical elements present on the sample surface. With this in mind, Fig. 4 reveals that carbon and silicon are also present on the superhydrophobic sample alongside aluminum and oxygen, resulting from the reaction between OTS and the hydroxyls on the anodized surface.


image file: d4ta03444a-f4.tif
Fig. 4 EDS results of the superhydrophobic OTS treated surface: (a) EDS map consisting of aluminum in red, carbon in green, oxygen in blue, and silicon in magenta. (b) Map sum spectrum analysis of the scanned superhydrophobic specimen. EDS maps showing (c) aluminum, (d) carbon, (e) oxygen, and (f) silicon atoms on the surface, separately.

Table 1 shows the contact angle and contact angle hysteresis of the pristine aluminum, superhydrophilic, and superhydrophobic samples. According to Wenzel,37 the rougher the hydrophilic surface, the more hydrophilic it gets. Consequently, by performing the anodization process on the hydrophilic pristine aluminum, a superhydrophilic surface is acquired. In addition, the silanization of the superhydrophilic surface results in a superhydrophobic surface with a low contact angle hysteresis and high contact angle that could lead to efficient droplet shedding in the condensation process.

Table 1 Contact angle and contact angle hysteresis of pristine, superhydrophilic, and superhydrophobic samples, along with an image of each one
Surface type Sample image Contact angle Contact angle hysteresis
Pristine aluminum image file: d4ta03444a-u1.tif 70.6° ± 3.7° 56.5° ± 5.9°
Superhydrophilic image file: d4ta03444a-u2.tif <10° —
Superhydrophobic image file: d4ta03444a-u3.tif 172.4° ± 2.1° 2.6° ± 2°

3.2. Performance in the condensation test

The apparatus must be validated before performing the heat transfer characterization test. For validation, the heat transfer coefficient of the pristine aluminum sample is compared with Nusselt's theory38 of filmwise condensation on a vertical surface. According to previous studies, a deviation of up to 10% from Nusselt's theory is acceptable.39 The deviation from Nusselt theory is due to its various approximations and simplifications.

Fig. S3† demonstrates the validation results of the pristine aluminum sample and Nusselt theory. Accordingly, the deviation of the apparatus results from Nusselt theory is less than the mentioned 10%, ensuring the validation of the results.

Fig. 5 reveals that the superhydrophilic surface is accompanied by a lower condensation heat transfer coefficient than that of the pristine aluminum sample for high subcooling temperatures. This is because, at these subcooling temperatures, the surface temperature is lower, which means that such a cold surface possesses a lot of suitable nucleation sites with low temperatures. Thus, utilizing a superhydrophilic surface not only does not improve the condensation heat transfer coefficient but also reduces the condensation heat transfer coefficient due to its adhesive properties to water and, consequently, weakens the shedding rate. On the other hand, at lower subcooling temperatures, the pristine surface is not readily suitable for nucleation due to the high surface temperature. Thus, the lower nucleation energy barrier of the anodized superhydrophilic surface can lead to a higher condensation heat transfer coefficient than that of the pristine aluminum surface at low subcooling temperatures. With this in mind, a 16.15% increment in the heat transfer coefficient is observed for the superhydrophilic sample at a subcooling temperature of 3.96 K. In addition, after rendering the superhydrophilic surface superhydrophobic using the OTS solution, the heat transfer coefficient drastically increases as a result of dropwise condensation. According to Fig. 5b, which shows the effect of the superhydrophobic surface on the condensation heat transfer coefficient, the sample is not very effective at high subcooling temperatures. This decline in the sample performance is attributed to a phenomenon known as surface flooding. At high subcooling temperatures, the surface temperature is relatively lower, leading to various low-temperature and suitable nucleation locations on the surface. By the coalescence of condensed droplets on the surface, large droplets cover the surface, leading to surface flooding and a decrease in the heat transfer coefficient.40 Moreover, high subcooling temperatures can cause water vapor condensation inside the air-filled cavities of the superhydrophobic surfaces. Thus, the condensed water fills the surface's nanostructures, pushing out trapped air. This transition from the Cassie state to the Wenzel state impairs the surface's effectiveness in repelling water.41 On the other hand, due to effective droplet shedding at low subcooling temperatures, the superhydrophobic surface demonstrates exceptional performance, augmenting the heat transfer coefficient by 676.7% compared to the pristine aluminum sample at a subcooling temperature of 2.77 K. To get a better understanding of the underlying phenomenon that leads to this high heat transfer coefficient, the self-propelling attribute of the superhydrophobic sample at a subcooling temperature of 2.77 K is shown in Fig. 5c and e (Video S2†). Accordingly, after the droplets grow and reach the borders of the other adjacent droplets, they merge. At this point, due to the reduced contact area of the resulting droplet and the sample surface, excess surface energy is freed by turning into the kinetic energy of the merged droplet. Nonetheless, when the subcooling temperature is high, the coalescence of the droplets does not produce enough energy for the resultant droplet to jump off the surface.


image file: d4ta03444a-f5.tif
Fig. 5 Comparison of the condensation heat transfer coefficients of the pristine aluminum sample and (a) superhydrophilic sample and (b) superhydrophobic sample. Visual investigation of a droplet jumping off the surface at a subcooling temperature of 2.77 K, including (c) the growth of adjacent droplets to a certain radius, (d) the coalescence of the adjacent droplets and jumping off the sample surface, and (e) an empty surface ready for nucleation. The length of the scales provided in (c–e) is 2 mm. The error bars are the results of 3 repetitions of the tests.

Moreover, different time frames of the condensation process on the superhydrophobic and pristine samples are compared in Fig. 6 to scrutinize the effect of the self-propelling ability of the superhydrophobic surface on the different stages of this process. On this account, the initial nucleation stage is shown in the first row of Fig. 6 for both samples; however, this stage occurs around 20 s and 40 s after the test's beginning for the pristine and superhydrophobic samples, respectively. The delay in the nucleation stage on the superhydrophobic surface is logical due to the reduced contact area between droplets and the solid surface (Cassie–Baxter mode), leading to fewer initial droplet formation sites and a higher nucleation energy barrier.32


image file: d4ta03444a-f6.tif
Fig. 6 Comparison of different time frames of the condensation process between the pristine aluminum and the superhydrophobic samples. The length of the scale bar is equivalent to 2 mm.

Furthermore, Fig. 6 reveals that after minimal growth, droplets on the superhydrophobic surface begin to shed off through jumping, leaving some areas devoid of droplets, which could initiate another round of nucleation. In contrast, the pristine surface is covered with pinned and stationary droplets, with no empty regions present. Additionally, after 5 minutes of full surface nucleation, a film of condensate water has formed on the pristine surface; however, the droplets on the superhydrophobic surface are simultaneously nucleating, growing, and shedding off the surface.

According to eqn (1), the heat transfer coefficient is a function of 7 different variables that must be accounted for in the uncertainty evaluation.

 
image file: d4ta03444a-t2.tif(2)
where m denotes the mass of the flowed coolant within measurement time t. Moreover, to calculate the uncertainty of the heat transfer coefficient (Uh) at each subcooling temperature, eqn (3) is utilized. The measurements of these 7 variables (xi) have inherent uncertainties (Uxi). The uncertainties in the measured time, the mass of the coolant liquid, the surface dimensions, the inlet and outlet temperatures of the coolant liquid inside the heat exchanger (each measured with a Pt-100 sensor), and the vapor chamber and surface temperatures (each measured with a K type thermocouple) are 0.01 s, 0.1 g, 0.001 m, 0.1 °C, and 0.42 °C, respectively.12,42
 
image file: d4ta03444a-t3.tif(3)

Correspondingly, the range of uncertainty for the superhydrophilic and superhydrophobic surfaces is 1.03 to 8.87% and 1.21 to 13.82%, respectively.

3.3. Performance in the condensation durability tests

As mentioned earlier, a coating that increases the condensation heat transfer coefficient must have adequate durability to be practical for industrial use, which is the ultimate goal of these studies. The first durability test that is performed for this application is a surface stability test under condensation conditions, meaning that the sample is placed inside the condensation test apparatus for prolonged hours. According to previous studies,8,12,43 in order to investigate its surface durability, the wetting characteristics of the surface, such as contact angle and contact angle hysteresis, are measured after each cycle. Furthermore, to get a better insight into the stability of the surface in this test, its surface performance is visually scrutinized as well (Video S3†). Accordingly, Fig. 7a shows the contact angle of the superhydrophobic surface after each cycle of the condensation durability test. It is also worth mentioning that the contact angle hysteresis of the surface remained under 5°.
image file: d4ta03444a-f7.tif
Fig. 7 (a) The change in the contact angle of the superhydrophobic surface throughout the 8 cycles of the condensation durability test. (b) Droplet boundaries approaching each other. (c) Shedding the merged droplet through the self-propelling attribute of the surface. The scales represent a length of 2 mm.

It is evident from Fig. 7a that the superhydrophobic sample is not degraded after 320 hours under condensation test conditions. This prolonged durability is much longer than that of many previous studies.18,43–46 Moreover, as previously mentioned in the literature, many studies have not investigated the durability of their proposed surfaces at all. After completing the endurance cycles, the condensation process is recorded to visually verify the surface's proper condensation performance. The results in Fig. 7b and c show that the superhydrophobic surface remains effective and bounces droplets off its surface. Consequently, the results of this test show that this surface is a stable superhydrophobic sample. Besides the condensation test conditions of this study, it is noteworthy that OTS is more thermally stable (up to 573 K) than fluorosilanes such as PTES (which starts decomposing at a moderate temperature between 373 K and 423 K).47

The superhydrophobic surface undergoes an abrasion test where #1000 sandpaper is utilized to get an insight into the mechanical durability of the superhydrophobic coating. After every meter that the surface is pulled on the sandpaper, the contact angle is reported, as shown in Fig. 8a. It is worth mentioning that the contact angle hysteresis of the surface remains lower than 5°. Fig. 8b and c show the superhydrophobic surface before the abrasion test and the wear on the surface after 10 meters of the abrasion test, respectively. Additionally, Fig. 8d shows the traces of sandpaper on the superhydrophobic surface, while Fig. 8e exhibits the effect of sanding on the nanostructures of the superhydrophobic surface. According to Fig. 8, even after a harsh abrasion test, the superhydrophobicity of the sample is kept intact. This is due to the porous structure of the anodized substrate and the fact that the OTS molecules are assembled in a thin layer in this porous structure, making them inextricably bound up.


image file: d4ta03444a-f8.tif
Fig. 8 (a) Contact angle variation after each meter of abrasion distance. (b) Optical micrograph of the superhydrophobic surface before abrasion. (c) Optical micrograph of the superhydrophobic surface after 10 meters of abrasion. (d) SEM image exhibiting the traces of sandpaper on the surface. (e) SEM image magnifying the effect of 10 meters of abrasion on the nanostructure of the surface.

Another valuable attribute that a condensation surface can have is the ability to delay and resist corrosion. This attribute is essential since it can prolong the use of such surfaces under condensation conditions that encompass high humidity. It is widely recognized that superhydrophobic surfaces can potentially protect the surface against corrosion.48–50 With this in mind, the present surface is forecast to perform well in the corrosion test. In order to evaluate the performance of the implemented superhydrophobic surface, an electrochemical polarization test is performed, and the Tafel curves of the pristine aluminum sample, the superhydrophilic sample (anodized aluminum sample), and the superhydrophobic sample are compared.

It should be noted that the lower the corrosion current density, the higher the resistance of the surface against corrosion. By scrutinizing the acquired Tafel curves of Fig. 9, the corrosion current density and the corrosion potential for each of the samples are described in Table 2. Accordingly, the corrosion current densities of the superhydrophilic and superhydrophobic surfaces are respectively 2 and 4 orders of magnitude lower than that of the pristine aluminum sample, exhibiting their higher resistance against corrosion. Thus, the fabricated surfaces not only improve the condensation heat transfer coefficient but also protect the surface against corrosion.


image file: d4ta03444a-f9.tif
Fig. 9 Tafel curves resulting from the electrochemical polarization test of the pristine aluminum sample, superhydrophilic sample, and superhydrophobic sample, where superior corrosion resistance is observed for the superhydrophobic sample.
Table 2 Corrosion current density and corrosion potential of the different fabricated samples acquired from the Tafel curves of each surface resulting from the electrochemical polarization test
Samples Corrosion current density [mA cm−2] Corrosion potential [V cm−2]
Pristine aluminum sample 2.246 × 10−3 −0.6758
Superhydrophilic sample 5.109 × 10−5 −0.4798
Superhydrophobic sample 2.177 × 10−7 −0.5992

Another potential benefit of the superhydrophobic surfaces is their self-cleaning capability. Fig. 10 shows that the presented superhydrophobic surface possesses this property, as it can clean the deposited dirt off the surface of this sample with each drop of water that is dropped on it. Furthermore, Fig. 10 reveals that the deposited dirt cannot compromise the superhydrophobicity of the surface. By dropping a droplet on the dirt-deposited surface, the droplet can collect dirt from where it collides with the surface and then bounce off the surface with the dirt (Video S4†). Past studies have also utilized fine-grain all-purpose flour to cover their proposed surface and tried to wash it off with water to show the self-cleaning feature of their surface.51


image file: d4ta03444a-f10.tif
Fig. 10 The self-cleaning test consists of six frames: (a) a dirt-deposited surface, (b) a water droplet falling onto the surface, (c) the water droplet colliding with the dirty surface, (d) the water droplet collecting dirt and bouncing off the surface due to its low surface energy, (e) the water droplet, now containing dirt, bouncing off the surface into the air, leaving the superhydrophobic surface, and (f) dirt-deposited superhydrophobic surface with less dirt in the collision area.

Overall, to highlight the superiority of the presented work compared to previous studies, where fluorinated materials or mercaptans were utilized, Table 3 encompasses a comparison between the results of some previous studies and this study.

Table 3 A review of the results of studies on the fabrication of self-propelling condensation surfacesa
Reference Surface roughness fabrication method Surface modifier Subcooling temperature range [K or °C] Condensation heat transfer [kW m−2 K−1] Durability [h] Additional information
a Cells labeled with N/A indicate “Not Available”.
He et al. (2012)16 Immersion in hot water 1H,1H,2H,2H-perfluorodecyltrimethoxysilane (FAS-17) N/A N/A N/A Heat transfer performance and durability were not addressed. The coating consisted of fluorinated materials
Miljkovic et al. (2013)14 Immersion into a hot alkaline solution Trichloro(1H,1H,2H,2H perfluorooctyl)-silane N/A ∼92 N/A The coating consisted of fluorinated materials. Durability is not addressed
Li et al. (2015)17 Anodized aluminum Heptadecafluorodecyltrimethoxysilane N/A N/A N/A Heat transfer performance and durability were not addressed. The coating consisted of fluorinated materials
Qu et al. (2016)27 Immersion into an aqueous solution for in situ growth of ZnO nanopencils Heptadecafluorodecyltrimethoxysilane ∼2–∼12 ∼21 at Tsub = ∼2 °C N/A The coating consisted of fluorinated materials. Durability was not addressed
Wen et al. (2017)25 Photolithography and electroplating processes + anodized aluminum N-octadecanethiol 0.4–30 ∼170 at Tsub = ∼0.4 °C N/A Complicated substrate fabrication. Durability was not addressed
Wang et al. (2020)29 Electrodeposition for in situ growth of copper nanocones Hexadecyl mercaptan ∼1.5–∼10 =29.1 at Tsub = 1.5 °C, Tv = 20 °C N/A Durability was not addressed. Condensation heat transfer increased by 98% and 51%, and decreased by 43% at Tv = 20 °C, 40 °C, and 80 °C, respectively
=39.8 at Tsub = 1.5 °C, Tv = 40 °C
∼43 at Tsub = 1.5 °C, Tv = 80 °C
Wang et al. (2023)30 Electrodeposition + immersion in alkaline solution 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane 0.5–20 ∼115 at Tsub = ∼0.5 °C N/A The coating consisted of fluorinated materials. Durability was not addressed
Donati et al. (2024)18 Etching + immersion in hot water Trichloro-1H,1H,2H,2H-perfluorodecylsilane (FDTS) and 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTS), and poly-1H,1H,2H,2H-perfluorodecyl acrylate (pPFDA) ∼2.5–∼30 ∼240 72 The coating consisted of fluorinated materials. A mechanically perishable nanostructure (induced through hot water immersion) was used
This work Anodized aluminum OTS 2.77–31 ∼147 at Tsub = 2.77 °C 320 Facile, durable, and fluorine-free

According to Table 3, compared to previous studies, this work has shown suitable durability and heat transfer performance. In addition, unlike most studies on self-propelled surfaces, this work uses fluorine-free substances to modify the facile nanostructures formed on the surface. Notably, only a few studies have used fluorine-free materials such as mercaptan as their surface modifier, and according to Table 3, these studies either used complex fabrication methods or their application was limited to specific conditions. Moreover, the durability of these surfaces was not addressed, which makes their applicability questionable.25,28,29 Furthermore, the utilization of mercaptans to modify surface energy results in a coating with sulfur atoms, while OTS-modified surfaces do not consist of chlorine atoms. Consequently, there is a risk that if a mercaptan-modified surface degrades, sulfur compounds can contaminate the condensed water. Thus, the high potential of this study is clear. Additionally, OTS has proved to be thermally more stable than its rivals, such as 1H, 1H, 2H, and 2H-perfluorooctyltriethoxysilane (PTES).47

4. Conclusions

In this work, a novel fluorine-free and self-propelling superhydrophobic surface was fabricated to effectively shed condensate droplets off the condenser surface and improve the condensation heat transfer coefficient. Additionally, the effect of an anodized aluminum surface, which is a superhydrophilic surface, on condensation heat transfer is investigated. The condensation tests covered a wide range of subcooling temperatures to encompass various applications. Moreover, various durability tests were performed to ensure the stability of the superhydrophobic surface. The main conclusions are as follows:

• The chemical composition formed on the surface, leading to the superhydrophobicity of the sample, is not hazardous due to the lack of fluorine in the coating.

• Due to the lower nucleation energy barrier, the superhydrophilic surface could improve the condensation heat transfer coefficient at low subcooling temperatures, leading to a 16.15% improvement at a subcooling temperature of 3.96 K.

• The high droplet shedding efficiency of the self-propelled superhydrophobic surface led to a much higher condensation heat transfer coefficient, which was 676.7% higher than that of the pristine surface at a subcooling temperature of 2.77 K.

• In addition to the condensation improvement, the utilization of the presented superhydrophilic and superhydrophobic surfaces could decrease the corrosion current density by 2 and 4 orders of magnitude, respectively, compared to the bare aluminum surface, providing higher resistance against corrosion.

• The superhydrophobic surface was shown to be quite durable under harsh condensation conditions, not losing its superhydrophobicity and self-propelling properties after prolonged testing for 320 h. Additionally, due to the texture of the superhydrophobic surface, it could endure 10 m of abrasion before degrading. Moreover, tests revealed that the superhydrophobic surface possesses self-cleaning properties and could repel water even after being covered with dirt.

Data availability

Data will be made available on request.

Author contributions

M. M. T. conceived and designed the experiments and analyzed the data. M. M. T. and B. R. carried out the experiments and performed the validation. M. M. T. and H. P. prepared the original draft, visualized the data, and contributed to the writing of the paper. A. M. supervised the project and commented on the manuscript.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

We would like to thank the members of the micro-/nano-fluidic lab at the Mechanical Engineering Department of the Sharif University of Technology for their kind help.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta03444a
This journal is © The Royal Society of Chemistry 2024