Enhancing Water Condensation on Hybrid Surfaces by Optimizing Wettability Contrast

Abstract

This study uses a hybrid concept to propose an optimal textured surface morphology for enhancing water condensation. The natural phenomenon-inspired morphology, which combined different degrees of wettability presented on the surface, documented their advantage in water harvesting compared to untreated surfaces. These superiorities might be explained by the appropriate combination of nucleation and water-driven ability facilitated by the superhydrophobic surrounding area. The uniform condensed droplets are effectively agglomerated to achieve the critical size. The best combination was found on a superhydrophobic-hydrophilic hybrid sample that improved water collection efficiency by up to 50% compared to bare Al. Condensation performance also illustrated an interesting tendency that revealed the great contribution of wettability on hydrophilic dots and the water-driven ability of the high-hydrophobicity area. The results were supported by a theoretical model which predicts the critical volume of a single droplet before it has departed from the surface. The findings reveal a good level of agreement between theory and real-time measurement, demonstrating the potential of combinations of hybrid samples to induce water collection efficiency.

1. Introduction

Water harvesting from the atmosphere is an interesting and valuable concept for sustainable solutions to issues such as water shortages in arid regions [1,2,3], energy consumption in HVAC (heating, ventilation, and air-conditioning) systems, and water harvesting via dewing [1,2,4,5], as well as for industrial applications. Water condensation on a functional surface consists of three main processes: nucleation, condensation, and coalescence. First, nucleation appears and attracts neighboring vapors. The small condensed droplets become large droplets via coalescence together. Finally, the water droplet falls due to the gravitational force [2,6,7,8].

The condensation process, therefore, is essentially manipulated by the morphology, such as the geology and surface energy, which might dominate the formation of water droplet condensation [6,7,8,9,10]. Recently, different surface morphologies fabricated by various techniques have been introduced to optimize collection efficiency [11,12,13,14,15]. Originality, the superhydrophobic solution had been believed to be the potential solution for water collection due to its unique water-driven ability characteristics [8,13,16,17,18]. However, a relatively high value in nucleation energy is possessed by the barrier, enhancing the condensation rate [19]. On the opposite side, the condensation process on the hydrophilic surface rapidly presents a water-thin film owing to the quite low energy required for nucleation. This thin film hinders heat transfer through the substrate for consecutive nucleation and reduces the collection amount [5,20].

Inspired by the effective fog-basking behavior of Namib Desert beetles [21,22] and water captured on heterogeneous spider web structures [23], many research studies have been conducted and demonstrated their advantages when compared with untreated surfaces [1,4,5,8,20,24]. Recently, Garrod et al. investigated the optical morphology for water collection on hybrid samples. Dorrer and Ruth obtained the critical volume of water to initiate the sliding of drops captured on a circular hydrophilic area surrounded by a superhydrophobic surface [17]. In addition, the heat transfer aspect has also been investigated to enhance the phase change through wettability control [25] or surface modification [26]. Moreover, the micro and nano features also demonstrate an advantage in enhancing condensation heat transfer [27].

However, current research is still divided between different approaches due to the lack of theoretical support [28,29]. Recently, Anna Lee et al. proposed water harvesting via dew in a saturated environment and found that the hybrid surfaces showed the lowest water collection due to the low condensation process of the water droplets on superhydrophilic spots, and almost all water collected resulted from the superhydrophobic area [16]. Recently, Tang et al. proposed the design of a superwetting surface with a self-driven droplet transport feature to enhance the condensation on hybrid surfaces [23].

We conducted water collection tests on hybrid samples with different combinations to investigate the controversy in water condensation. The spot’s wettability ranged from superhydrophilic (10°) to moderately hydrophilic (70°), while the surrounding area was varied from 50° to 150° to generate different contrasts. The spot size was 700 μm to mimic the Namib Dessert beetle’s back morphology. The results were compared with reference samples (as-received, superhydrophobic (SP), and superhydrophobic (SPh) Al) to demonstrate the advantage in water collection performance by an appropriate combination of morphology and wettability contrast. The hybrid samples could enhance the water collection, and hydrophilic spots contributed significantly to the condensation rate by reducing the critical droplet size.

2. Materials and Methods

2.1. Materials

An Al sheet (A1100, 99.0%, 30 mm × 30 mm), purchased from Tisco Co., Ltd. (Vietnam Steel Corporation, Thai Nguyen, Vietnam), was used to prepare the substrate for water condensation. Acetone, Iso-Propanol (IPA), Ethanol, and Perfluoropolyether (PFPE) were purchased from Sigma-Aldrich (Singapore) and used to clean the substrate and for functionalization, respectively.

2.2. Method

The Al sheet (A1100, 99.0%, 30 mm × 30 mm) was sandblasted, followed by a wet etching step using hydrochloric acid (Merck, Sigma Aldrich, Ltd., St. Louis, MO, USA) to conventionally create a hierarchy on the surface (Figure 1b) [30,31]. After being immersed in an aqueous solution for 15 min, the samples were cleaned with acetone, IPA, and Ethanol each for 10 min in an ultrasonic bath, followed by being rinsed in deionized (DI) water and being naturally dried at room temperature. After wet etching, the surface became completely wetted (superhydrophilic), with a contact angle of about 9 degrees, i.e., water droplets adhered tightly to the surface. The superhydrophobic surface (SP) was finished by dipping etched samples inside Perfluoropolyether (PFPE) solution for 1 h and drying in ambient air for another 1 h.

After PFPE coating, the surface had reached high hydrophobicity with an equilibrium contact angle of 161° and a sliding angle of about 1°. To generate hybrid samples, the UVO exposure process was carried out with metal masks. After being attached to the surface, UVO was used to vertically degrade the SAM layer of PFPE. The SAM layer on the exposed area was selectively degraded by plasma to lower the hydrophobicity, while the covered area retained its properties. Exposure to the UVO slowly removes the PFPE layer and provides the -OH group, therefore enhancing the wettability of the exposed area while maintaining the surrounding area at the original superhydrophobic level. The mask pattern consists of a square array 700 $\mathsf{\mu }\mathrm{m}$ in size and 1400 $\mathsf{\mu }\mathrm{m}$ in pitch to mimic the Namib Dessert beetle’s back morphology [21,22].

To investigate the contribution of the hydrophobic–hydrophilic combination, the wettability of hydrophilic spots was also modified from 10° (#H10) to 70° (#H70) (

Table 1. Measurement of investigated surfaces.

Sample	CA	${\mathit{\theta }}_{\mathit{a}}$	${\mathit{\theta }}_{\mathit{r}}$	CAH	SA	Vc	Amount (L/h·m2)	Note
A-R Al	81	113	64		$15 \pm$ $0.5$	3.8	$0.52 \pm$ $0.02$
S.Philic	9	--	--		--	--	$0.48 \pm$ $0.05$
S.Phobic	$161 \pm$ $0.5$	$162 \pm$ $0.4$	$8 \pm$ $0.3$		$2 \pm$ $0.2$	$5.5 \pm$ $0.03$	$0.50 \pm$ $0.03$
#H10	$100 \pm$ $0.6$	$169 \pm$ $0.6$	$81 \pm$ $0.4$	$78 \pm$ $0.4$	$13 \pm$ $0.3$	$4.02 \pm$ $0.05$	$0.63 \pm$ $0.03$	$700 \mathsf{\mu }\mathrm{m}$ to mimic the Stenocara beetle’s back morphology
#H30	$110 \pm$ $0.5$	$165 \pm$ $0.4$	$80 \pm$ $0.4$	$75 \pm$ $0.4$	$11 \pm$ $0.5$	$3.92 \pm$ $0.02$	$0.66 \pm$ $0.04$
#H50	$115 \pm$ $0.4$	$161 \pm$ $0.4$	$78 \pm$ $0.4$	$73 \pm$ $0.5$	$10 \pm$ $0.4$	$3.51 \pm$ $0.03$	$0.72 \pm$ $0.04$
#H70	$125 \pm$ $0.5$	$170 \pm$ $0.3$	$72 \pm$ $0.5$	$79 \pm$ $0.4$	$8 \pm$ $0.6$	$4.27 \pm$ $0.03$	$0.55 \pm$ $0.03$

). It should be noted that the wettability, ranging from 10° to 70°, refers to the measurement on uniform surfaces, while the values in Table 1 correspond to the measurement of the hydrophilic region surrounded by the superhydrophobic area. The advancing and receding contact angles on treated surfaces were measured using a contact angle apparatus (DM-50, Kyowa Interface Science Co. Ltd., Niiza, Saitama, Japan) and calculated by the tangent method [32]. To mimic the behavior of the condensed droplet at the barrier, 5 μL of deionized (DI) water was dispersed on a hydrophilic spot before tilting the surface. Advancing and receding contact angles were recorded at the moment when sliding phenomena happened. Measured values provide important information for determining critical droplets for flowing in our calculation (Table 1).

To prevent the disadvantage of hydrophilic dots owing to long-time condensation, we used an Al template with quite good thermal conductivity and a lower setting temperature to accelerate nucleation behavior, which might enhance the contribution of hydrophilic spots in the condensation process [28].

3. Results and Discussion

The water condensation behaviors on surfaces with different wettability are shown in Figure 2. During the water condensation process, the wettability and morphology of the surface influence each step, illustrating the most important factors for heat exchange systems, and even energy conversion [16,33]. Indeed, on high-hydrophobicity samples, tiny droplet nuclei grow slowly, coalesce, and, finally, achieve critical size before leaving the surface. Conversely, thin-film condensation was rapidly found in the hydrophilic one [5,34].

As mentioned before, a good water collection surface should be well balanced between nucleation rate and anti-friction for water drainage. In that regard, the best combination should belong to the hydrophobic surrounding area—hydrophilic spots combine due to the high nucleation rate and low friction. Hence, the hybrid sample has been introduced in order to exhibit a potentially superior performance.

Figure 2 introduces the condensation evolution on different morphologies and documents the significant contribution of hydrophilic spots to the total condensation process. The water droplets initially generated from the hydrophilic area grew up rapidly and coalesced with nearby droplets’ nuclei to form a “hybrid droplet” which corresponded to water volume, covering the hydrophilic spot and underneath area [7]. After reaching critical size, it was dragged downward owing to gravitational force and left behind a dry surface for consecutive nucleation processes.

Figure 3 presents the collection rate for surfaces with different morphologies. The measurement was calculated solely after the 1st, 2nd, and 3rd hours to investigate the contribution of surface wettability. Interestingly, a quite high collection rate was found on the S.Philic surface after the 1st hour, and it could be explained by the rapid formation of water in film-wise state. However, this water layer inhibited the heat transfer from the cooling module to the surface for consecutive nucleation; therefore, it almost prohibited the collection process. In contrast, the collection observed on the S.Phobic surface revealed stable progress over the three hours, demonstrating the advantage of low-friction surfaces for water harvesting.

The combination of the two above-mentioned properties have been integrated in hybrid samples, and the combination’s outstanding performance can be clearly described. All hybrid surfaces demonstrated a higher harvesting rate compared to the uniform surfaces. This emphasizes the appropriate combination of two wetting states generated on one surface. The lower friction allows for easier movement of water directly to the container after a single droplet reaches critical volume. Further investigation revealed that the best combination for the 150°–50° hybrid sample included an enhancement of up to 50% compared with a bare surface and a surface with a 20% higher rate than the uniform S.Phobic surface.

It is worth noting that this condensation efficiency proposes an inconsistent trend when the highest one belongs to 50° of spot wettability and gradually decreases by 30°, 10°, and 70°, respectively. This presented an explanation for the optimal contrast at the boundary that should be supported by a theoretical model.

To quantitatively investigate the wettability contrast contribution, we proposed a calculation for the critical size and volume of a single water droplet forming on a hybrid dot (Figure 4). It should be noted that the model might be applied to any wettability boundary. The condensation and evolution of a single droplet at different amounts of wettability were captured using a high-speed camera to support the calculation.

The water droplet, after coalescing, only adhered to the hydrophilic area [17]. The droplet’s maximum diameter at the hybrid area comprising hydrophilic dot and hydrophobic area can be determined by balancing capillary force and gravitational force [14,33]:

$$a\gamma \left(\cos {\theta }_r^h-\cos {\theta }_a^h\right)+\left(L-a\right)\cos {\theta }_r^b-L\cos {\theta }_a^b=\rho gV$$

(1)

with a being the critical diameter of the spherical cap droplet; $\gamma ,\rho ,g$ being surface tension, water density, and gravitational acceleration; ${\theta }_r^h$ and ${\theta }_a^h$ being the water receding and advancing angles on the spot surface; and ${\theta }_r^p$ and ${\theta }_a^p$ being the water receding and advancing angles on the surrounding area, respectively. Assuming that the maximum drop stays on the surface before the sling has a spherical cap shape, the contact length and volume can be determined through the derivation [35].

$$V={\displaystyle \frac{\pi {\left(1-\cos \theta \right)}^2\left(2+\cos \theta \right)a^3}{24{\sin }^3\theta }}$$

(2)

Since the spherical cap expands due to condensation, the static contact angle should be considered as the advancing contact angle at the sliding moment. From (1) and (2), we can express the critical diameter of the droplet condensed on a hybrid area.

$$a={\left[{\displaystyle \frac{24\gamma {\sin }^3{\theta }_a}{\pi \rho {\left(1-\cos {\theta }_a\right)}^2\left(2+\cos {\theta }_a\right)}}\right]}^{1/2}$$

(3)

And, the critical volume of droplet is as follows:

$$V={\left[{\displaystyle \frac{24\gamma {\sin }^3{\theta }_a}{\pi \rho {\left(1-\cos {\theta }_a\right)}^2\left(2+\cos {\theta }_a\right)}}\right]}^{1/2}{\displaystyle \frac{\gamma \left(\cos {\theta }_r-\cos {\theta }_a\right)}{\rho g}}$$

(4)

The critical droplet size forming on the hydrophilic spot is meaningful due to the presence of the surrounding hydrophobic area. Condensed water formed a dropwise behavior owing to the main contact area belonging to the hydrophobic surrounding area. The result reveals the correlation is proportional with the calculation (Table 1) when higher critical volume leads to lower condensation efficiency, demonstrating the substantial contribution of wettability contrast at the border. A hybrid droplet with a smaller critical size can introduce a better water-sliding effect due to low wettability hysteresis and is facilitated by the hydrophilic area inducing the driven force needed to drain the water downward.

Figure 5 describes the proportional relationship between the critical diameter of volume before departure from the surface and the advancing contact angle. The dots indicate the real-time measurement of critical diameter, while the solid line was plotted according to its theoretical calculation and proposes quite good agreement. The high advancing contact angle might ensure a high diameter, and so on the departed volume of a single droplet, regardless of the equilibrium, or a receding contact angle.

Furthermore, a model of critical volume has been constructed based on the diameter measurement (Figure 6). The result demonstrated good agreement with the proposed calculation when critical volume showed the proportional trigonometric relation with contact angle hysteresis and proposed an interesting tendency. The hybrid droplets accumulated on #H50 (150°–50°) samples demonstrate the lowest critical size for movement (Figure 6). Due to the lowest wettability hysteresis being presented on a 50° surface, surface tension force acting on hybrid droplets can lower the critical volume for water falling to induce water falling and consecutive nucleation processes (Figure 7).

The order of collected water also agreed well with the critical size calculation when a smaller critical volume was found on a higher-efficiency collection hybrid sample, indicating the importance of hydrophilic wettability hysteresis in relation to sliding ability. The lower the hysteresis of a single droplet a hybrid dot might facilitate, the higher the observable mobility of the droplet. It is worth noting here that the hysteresis can be different from other materials or surface roughness. A lower static contact angle can exhibit low hysteresis, while a higher static contact angle can introduce a quite large hysteresis. The most important consideration in hybrid samples is the minimizing of wettability hysteresis to minimize capillary force for enhancing water drainage ability.

4. Conclusions

In this work, we have presented a water harvesting effectiveness investigation on uniform and hybrid samples with different contrasts. The results demonstrated the superiority of hydrophobicity in improving collection performance due to the low adhesion with water, which might lead to easier water drainage in both uniform and hybrid surfaces. An appropriate wettability contrast between textured spots and superhydrophobic surrounded area showed their advantage in enhancing condensation efficiency via dewing. The experimental results were facilitated by a model designed for calculating the critical diameter and volume of a single droplet before departure. The high advancing contact angle and low critical volume ensure higher collection efficiency on hybrid samples, proposing an understanding and effective strategy for harvesting surface design.