Seong Min Kangab
aGlobal Frontier Center for Multiscale Energy System, Seoul National University, Seoul 151-744, Korea. E-mail: henry86@snu.ac.kr; Fax: +82-2-887-8762; Tel: +82-2-887-8626
bDepartment of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151-742, Korea
First published on 25th July 2016
In this study, we have demonstrated a structural advantage of mushroom-like micropillar arrays for making superrepellent surfaces. We derived and plotted theoretical graphs based on the Cassie–Baxter equation to find out optimal conditions in order to repel liquid droplets with a wide range of surface tensions on desirable surfaces. Superrepellency experiments were carried out using two types of microstructure, namely, a mushroom-like shape and a cylindrical shape, for comparison with the theoretical analysis. The results show that only mushroom-like arrays can maintain the metastable Cassie state even with a liquid droplet of low surface tension such as ethanol, owing to the structural advantage of their wide-tip shape. As a result, we identified the smart function of the wide-tip structure by theoretical and experimental approaches for designing superrepellent surfaces that can be used for various potential applications.
A number of groups and research papers have proved that the metastable Cassie state should be generated in order to obtain superomniphobic properties for fabricating the aforementioned superrepellent surfaces.5,16,17 The surfaces should possess air pockets underneath liquid droplets with a low surface tension to maintain their repellency to any type of liquid, in the same way as superhydrophobic surfaces do with air pockets that support water droplets in composite solid–liquid–air interfaces. In terms of the shape of pillars, re-entrant structures have generally been employed.18–20
To fabricate re-entrant structures, we have developed a fabrication process for polymeric superrepellent surfaces with mushroom-like shapes. There are various methods of preparing mushroom-like micropillar structures, such as the microfabrication of T-shaped pillars by patterning a SiO2 cap on silicon structures,21 a cost-efficient molding method using conventional photolithography,22,23 and a direct technique for the formation of a mushroom shape by electrically induced polymer deformation.24,25 Recently, we have reported a method of utilizing an over-etching process and a simple replica molding approach and made robust polymeric mushroom-like micropillar arrays for making superrepellent surfaces.26 Surfaces with these fabricated arrays, in our results, displayed improved repellency, with a decreased liquid–solid contact fraction (i.e., solid fraction, fs) for a suspended liquid droplet in the Cassie state. However, without the wide-tip shape of these structures, the surfaces easily underwent a transition to the energetically favorable Wenzel state.
To investigate further the effects of these mushroom-like micropillar structures in order to construct optimized superrepellent surfaces, we have compared them with cylindrical micropillar arrays and investigated their structural advantages both in experiment and theory. We have fabricated two types of sample, namely, with re-entrant and cylindrical micropillar structures, of the same dimensions (in diameter and height) with seven different spacing ratios (SR; i.e., the separation of pillars divided by their diameter). The possibility of having liquid-repellent properties of each fabricated structure was theoretically studied by deriving the Cassie–Baxter equation in terms of the critical SR, which is closely related to fs. Compared with the cylindrical micropillar structures, the mushroom-like micropillar arrays demonstrated superrepellency to both water and ethanol droplets, which was mainly due to the structural advantage of their wide tip.
We designed two types of sample of the same dimensions in terms of pillar diameter (2R) and height (h) to carry out an experiment in controlled conditions, as shown in the cross-sectional diagrams illustrated in Fig. 1(a). It is important to note that the diameter of the wide tip of a mushroom-shaped structure was the same as that of a cylindrical micropillar structure. The symbol d represents the diameter of a column of the mushroom-like structure and s represents the distance between two neighboring micropillars. In this work, we have used samples with 2R ∼ 8.5 μm, h ∼ 10 μm, d ∼ 5 μm and seven different SRs (= s/d, from an SR of 1 to an SR of 7).
Representative scanning electron microscopy (SEM) images of two different fabricated types of PDMS array, namely, with mushroom-like and cylindrical micropillar structures with an SR of 2 are shown in Fig. 1(b) and (c). (The detailed procedure of the fabrication method is described elsewhere.26) The side view of the SEM images in Fig. 1(b) and (c) proves that the morphology of both fabricated structures is well defined with high fidelity and well ordered with a defect-free arrangement in large areas.
To find out the optimal design conditions in order to make superrepellent surfaces from both fabricated mushroom-like and cylindrical arrays, we derived the Cassie–Baxter equation using the given parameters of the micropillar structures and plotted the results in Fig. 2(a). The apparent contact angle θc for a liquid droplet supported by an air pocket at composite solid–liquid–air interfaces in the Cassie state is given by:27
cos![]() ![]() ![]() ![]() ![]() | (1) |
The solid fraction factor, fs, of both fabricated structures of the same dimensions, as shown in Fig. 1(a), is given by:
![]() | (2) |
Finally, eqn (1) and (2) are combined to express the Cassie–Baxter equation, as below:
![]() | (3) |
From the relationship between θc and fs in eqn (1), it is proved that the value of fs needs to be low in order to obtain a high θc value of a liquid on a surface. In other words, θc can be increased by maximizing the spacing s of the designed micropillar arrays. It is noted that the column diameter d of both microstructures is the same in this experiment, which can use SR (= s/d) instead of the spacing s. We examined the effect of SR on θc by plotting eqn (3) with SR and θo as parameters in Fig. 2(a). As shown in this graph, both structured surfaces may repel any kind of liquid droplet within a wide range of θo values above an SR of 5.
In order to confirm the results of the theoretical analysis, we carried out an experiment to confirm the superrepellency of DI water (λlv = 72.1 mN m−1) and ethanol (λlv = 22.3 mN m−1), which have θo values of 125° and 65°, respectively, on a smooth (flat) PDMS surface, as shown in the inset images in Fig. 2(a). According to the plots in Fig. 2(a), surfaces with both mushroom-like and cylindrical arrays can be superhydrophobic above the critical SR of DI water (SRc,w). Also, they are expected to repel an ethanol droplet above an SRc,e of ∼4 owing to their lower θo values, which result from their surface tensions being lower than that of DI water.
The experimental results of CA measurements with DI water and ethanol on fabricated surfaces of both samples are plotted in Fig. 2(b) and (c). As shown in the graphs, theoretically calculated Cassie and Wenzel state curves were also plotted for comparison with the measured values of CA in the cases of both DI water and ethanol. Both surfaces exhibited superhydrophobicity, which was well matched with the theoretically derived results. On the other hand, it is noted that the ethanol droplet was easily wetted on the surface with cylindrical arrays (see the red dot-line curve in Fig. 2(c)). Although both structures have the same values of fs and θc in eqn (2) and (3), only the mushroom-like micropillars maintained the Cassie state with the ethanol droplet. Optical microscope images of DI water and ethanol droplets on each surface with seven different SR values are presented in Table 1. The ethanol droplet existed in the Wenzel state on the cylindrical arrays for all SR values, with the contact angle the same as the θo,e value on the smooth PDMS surface.
Achieving superrepellency on a surface with cylindrical arrays fails owing to the low liquid suspension force on their structure in the case of the ethanol droplet. In contrast, microstructures with a re-entrant topology such as mushroom-like arrays have enough suspension force to resist the penetration of liquid droplets with a wide range of surface tensions. Fig. 3 explains the relationship between the intrinsic CA of a liquid (θo) and the morphology of the surface of a structure (viewed as the structural angle αs) for successful suspension of the liquid. It should be noted that θo always represents the same value, as an inherent property, regardless of the position of the liquid meniscus.4,26,28 In other words, the meniscus of a liquid droplet may be changed by the shape of a microstructure. Also, we can control the direction of the capillary force (suspension force) of the droplet, along with the meniscus, with different structural topologies.
In order to achieve an upward suspension force to make a superrepellent surface, the structural angle (αs) should be smaller than the θo values of various liquids. In the cases of the truncated pyramidal and cylindrical shapes of micropillars, the intrinsic CA of DI water (θo,w = 125°) is larger than both αs values, and thus the capillary force of DI water is exerted upward. However, it is impossible to repel ethanol droplets on these structures owing to the downward direction of the capillary force caused by the low value of θo,e (65°) (see Fig. 3(a) and (b)). On the other hand, the mushroom-like re-entrant structures have extremely low αs values, as shown in Fig. 3(c) and (d), which enables both DI water and ethanol to be repelled on the surface. The meniscus of a liquid droplet is pinned to the edge of the wide tip of the structures and the capillary forces of both DI water and ethanol act upward. As a result, the wide-tip shape of the mushroom-like arrays provides the possibility of making a superrepellent surface even with a liquid with low surface tension, such as ethanol in this experiment.
As mentioned earlier, the mushroom-like profiles of microstructures are advantageous for maintaining the metastable Cassie state. Fig. 4 illustrates the theoretical relationship between the Cassie, Wenzel and metastable Cassie states in a graph from the calculated equations. It has been proven that a liquid droplet prefers a wetting state with a lower free energy. In this manner, as shown in Fig. 4, θcrit is the critical contact angle, which is a transient point between the Cassie state and the Wenzel state and is defined as:27,29
![]() | (4) |
In summary, we have revealed the advantage of the wide tip of mushroom-like arrays for making a superrepellent surface by using theoretical and experimental approaches. We have fabricated two types of micropillar surface, namely, a mushroom-like re-entrant shape and a cylindrical shape, of the same structural dimensions with the aim of comparison with theoretically calculated results. We have derived the Cassie–Baxter equation, which demonstrates the relationship between θc and SR, and plotted it using experimental data of the measured CA of both DI water and ethanol on both surfaces. In contrast to the theoretical prediction, an ethanol droplet was repelled only on the mushroom-like arrays. In the case of the cylindrical structures, the capillary force of the ethanol droplet acted downward, along with the meniscus, owing to both the low value of θo,e of the liquid and the structural topology of the micropillars. On the other hand, the wide tip of the mushroom-like arrays provided an extremely low αs value and thus they could maintain the metastable Cassie state even with the ethanol droplet. These results of both theoretical and experimental analysis have demonstrated the advantages of the wide tip of the mushroom-like shape. It is envisioned that these scientific studies could present useful insights for making superrepellent surfaces that can be employed in various fields.
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