From Superamphiphobic to Amphiphilic Polymeric Surfaces with

Feb 25, 2011 - After etching/nanotexturing, the PMMA plates are amphiphilic and exhibit hierarchical (triple-scale) roughness with microscale ordered ...
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From Superamphiphobic to Amphiphilic Polymeric Surfaces with Ordered Hierarchical Roughness Fabricated with Colloidal Lithography and Plasma Nanotexturing K. Ellinas, A. Tserepi, and E. Gogolides* Institute of Microelectronics, NCSR “Demokritos”, 153 10, Aghia Paraskevi, Attiki, Greece

bS Supporting Information ABSTRACT: Ordered, hierarchical (triple-scale), superhydrophobic, oleophobic, superoleophobic, and amphiphilic surfaces on poly(methyl methacrylate) PMMA polymer substrates are fabricated using polystyrene (PS) microparticle colloidal lithography, followed by oxygen plasma etching-nanotexturing (for amphiphilic surfaces) and optional subsequent fluorocarbon plasma deposition (for amphiphobic surfaces). The PS colloidal microparticles were assembled by spin-coating. After etching/ nanotexturing, the PMMA plates are amphiphilic and exhibit hierarchical (triple-scale) roughness with microscale ordered columns, and dual-scale (hundred nano/ten nano meter) nanoscale texture on the particles (top of the column) and on the etched PMMA surface. The spacing, diameter, height, and reentrant profile of the microcolumns are controlled with the etching process. Following the design requirements for superamphiphobic surfaces, we demonstrate enhancement of both hydrophobicity and oleophobicity as a result of hierarchical (triple-scale) and re-entrant topography. After fluorocarbon film deposition, we demonstrate superhydrophobic surfaces (contact angle for water 168°, compared to 110° for a flat surface), as well as superoleophobic surfaces (153° for diiodomethane, compared to 80° for a flat surface).

1. INTRODUCTION Superhydrophobic surfaces (on which water drops roll and have a large contact angle, typically >150°, with a very small contact angle hysteresis, typically 90°. Oils in general have less surface tension and smaller contact angle than water; thus, according to Wenzel’s model most surfaces would be oleophilic. In contrast, the CassieBaxter equation allows for the possibility of an apparent contact angle of a roughened surface θCB > 90°, even when the initial θ0 < 90°.12 However, such states are “metastable” configurations.13-16 Thus, there exists an inherent difficulty developing robust surfaces for which θCB > 150° when in contact with oils, due to the low surface tension of oil in comparison with water, and the fact that in such a case the Wenzel state is more energetically favorable to the Cassie-Baxter state. Special re-entrant profiles of the microstructures or negative slopes are required to stabilize the oleophobic Cassie state.5-7,17 To achieve both mechanically robust and energetically favorable superhydrophobic states, dual scale topography (micro/nano usually) is preferred and is actually the method used by nature.18,19 However, little information is available as to whether dual-scale roughness may be beneficial for oleophobicity. Recently, it was shown that nanoscale ripples created by the Bosch etching process20 on the sidewall of Si microcolumns show superoleophobicity in contrast to smooth microcolumns,21 which do not show such a property; however, it was not clear whether oleophobicity was due to the dual-scale topography or the reentrant profile created by the Received: November 10, 2010 Revised: January 12, 2011 Published: February 25, 2011 3960

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Langmuir ripples. Superoleophobicity and hierarchical topography were also observed with nanosphere stacking22 or spray casting.23 The methods to fabricate amphiphobic surfaces follow usually either stochastic or biomimetic bottom-up approaches22-26 or microfabrication top-down approaches.21,27,28 The latter produce ordered surfaces and are also useful for basic studies, while the former usually produce random, hierarchical surfaces presumably at a lower cost. We and others have proposed the use of plasma etching and plasma nanotexturing for the fabrication of random, dual-scale, stable, and robust superhydrophobic, superhydrophilic, and antireflective surfaces.29-39 However, it would be advantageous to be able to fabricate ordered dual-scale surfaces using cost-effective self-assembly techniques: On one hand, basic studies could be done on such surfaces having clearly separated roughness scales, and on the other hand, the method would be of low cost to be used for applications and could be combined with facile optical property control by tuning the spacing of ordered structures. Such processes are particularly important for polymeric open-area substrates or microfluidic channels. In this publication, we will be exploring the combination of colloidal lithography and plasma nanotexturing for fabrication of ordered, hierarchical (triplescale) topography, for amphiphobic and amphiphilic surfaces on polymers. Colloidal or nanosphere lithography is a well-established and flexible method of particle self-assembly organization40 as recently reviewed by Yang et al.41 A lot of processes for the assembly of colloidal micro/nano spheres have been studied up to now such as spin coating,42,40,43 electrophoresis,44 LangmuirBlodgett technique,45 and convective assembly.46,47 Colloidal particles have been used as etching masks to produce ordered pillars on silicon48,49 or as masks to selectively deposit metals on various substrates.50 Intensive research is taking place to use surfaces structured with colloidal lithography as superhydrophobic, self-cleaning surfaces.51-54 In addition, several efforts to fabricate ordered structures on surfaces have been reported using colloidal lithography and plasma etching on silicon or thin oxides on silicon substrates in order to produce superhydrophobic silicon pillars,55,56 while recently the technique has been applied for manufacturing antireflective and superhydrophobic glass.57 In this publication, we propose a fast and facile method for manufacturing ordered, hierarchical (triple-scale) topography, superamphiphobic, amphiphobic, and superamphiphilic surfaces using colloidal lithography of polystyrene (PS) microspheres followed by plasma etching. Our aim is to use colloidal lithography for creating ordered microscale structures, while simultaneously nanotexturing these structures during plasma etching34,35,37 with dual-nanoscale roughness, resulting in a hierarchical (triplescale) topography (ordered micro with random a few hundred nano/a few tens nano) with controllable undercut profiles. We stress that creating micropillars using the microparticles as a mask is not trivial for polymers, since shrinking of the microparticle is not independent of etching of the substrate as is the case for silicon, which is unetchable from oxygen plasma. We will demonstrate control of the spacing, height, nanotexturing, and reentrant profile of the fabricated pillars, resulting in controlled wetting properties from amphiphilic to superamphiphobic. To our knowledge, this is the first such effort on polymers and the first such effort using colloidal lithography for superamphiphobicity. We expect that our work can also be extended to control of optical properties based on the work of Li et al.,57 and can easily be applied for wetting control inside polymeric microfluidic

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channels (a nontrivial task, as shown in our recent work58), although these two emerging applications will not be a subject of the present publication. This paper is organized as follows: after the experimental details, we present the colloidal lithography and plasma etching processes to fabricate hierarchical (triple-scale) pillar-like topography on PMMA substrates. We then discuss the process design for superamphiphobicity. Subsequently, we present our results for superhydrophobic, superoleophobic, and superamphiphilic surfaces; discuss the role of hierarchical (triple-scale) roughness; and finally give an outlook from our work.

2. EXPERIMENTAL SECTION 2.1. Materials. Optically transparent 2-mm-thick PMMA plates were purchased from IRPEN (Spain) and cleaned in isopropyl alcohol (IPA) and in deionized (DI) water prior to plasma processing. PS particles of 0.98 and 3.21 μm diameter diluted in water solution were purchased from Mikropartikel Gmbh, Germany, and Triton X-100 was purchased from Sigma Aldrich. Deionized water, commercial soya oil, diiodomethane, hexadecane, and decane were used as test liquids and were bought from Sigma-Aldrich. 2.2. Spin Coating Method. Spin coating is used for the deposition of colloidal particles. Colloidal lithography is performed as follows: a PS bead solution is prepared (Triton X-100 is diluted with methanol by 1:400 and an equal volume of the PS particles aqueous solution) and is spin-coated on PMMA plates using a two-step spinning protocol:43 The first step is a slow one (200-300 rpm/min) with low acceleration approximately for 30 s in order to coat the whole sample uniformly. The second step is faster (1000 rpm/min) just for 10 s in order to remove the excess bead solution. Triton is used to make the substrate hydrophilic and increase the adhesion of the particles to the substrate. 2.3. Plasma Processing. After colloidal lithography, we perform an O2 plasma deep reactive ion etching step in a high-density plasma reactor (Helicon plasma reactor, Micromachining Etching Tool, MET, from Adixen-Alcatel). The conditions are as follows: 1900 W, 100 sccm O2, 0.75 Pa, and a moderate bias voltage (usually -80 V). This etching step increases the spacing among the spheres, and it also forms pillars on the PMMA substrate masked by the PS spheres. PS has an etching selectivity of 2 with respect to PMMA (etch rate of PMMA is twice that of PS), which means that one can etch the PMMA substrate for a depth roughly twice the PS particle diameter. When a better reentrant profile is required, a two-step etching process is performed: Starting with one anisotropic etching step as discussed above, we switch to an isotropic etching step using the following conditions: 0 V bias, 1900 W, 1.33 Pa, 100 sccm O2. Surfaces after oxygen plasma etching become amphiphilic. The same reactor was also used for conformal deposition of a thin fluorocarbon film after plasma etching using C4F8 gas at conditions (900 W, 0 V, 5.33 Pa C4F8, deposition rate 30 nm min-1) that conformally deposits a thin fluorocarbon (FC) film after plasma etching to render the surface superhydrophobic. 2.4. Surface Characterization. A JEOL JSM-7401F FEG SEM was used for observation of the PMMA surfaces after colloidal lithography and plasma treatment. Wettability of the fabricated surfaces was probed by static and dynamic water contact angle measurements, performed with a GBX Digidrop System. Typically, 5 μL droplets were used for static contact angle measurements. Advancing and receding CA were measured as the droplet volume was continuously increased and decreased, respectively, for estimation of CA hysteresis. A fitting procedure was used for the estimation of the contact angles through a third-degree polynomial from photographs of the drop (image resolution 768  572 pixels). The reported contact angles are the average of three measurements of CA of droplets at different places on the surface. 3961

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Figure 1. Spin coating has been used for particle self-assembly. Large-area uniformity can be achieved using a mesh-assisted process.60 (a) 1 μm PS particles on PMMA substrate (optical microscope magnification  100). (b) Higher magnification of the particles (SEM magnification  6000).

Figure 2. Ordered micropillars produced after colloidal lithography of 1 μm PS particles on PMMA substrate and one etching step in high-density oxygen plasma. (a) 1 min etch, -80 V bias; (b) 2 min etch, -80 V bias. [Other conditions: 1900 W,100 sccm O2, 0.75 Pa.]

3. RESULTS AND DISCUSSION 3.1. Fabrication of Ordered, Uniform Micropillars with Hierarchical (Triple-Scale) Roughness. Uniform, self-assembled,

closely packed arrays of PS spheres of 987 nm are fabricated as shown in Figure 1 after spinning the colloidal particles. The produced closely packed particle assembly exhibits some defects such as gaps or dislocations due to strong Brownian motion and capillary forces.59 These defects are of minor influence for our purpose, since they occupy only a small fraction of the entire surface. Thus, large, defectfree areas can be fabricated as Figure 1 illustrates. Large-area uniformity can be achieved using a mesh-assisted process.60 Highly ordered arrays of micropillars of PMMA are produced after O2 plasma etching as shown in Figure 2. Our method allows the fabrication of pillars of controlled height and cross-sectional diameter depending on etching time and bias voltage. The pillar diameter (pillars have a cylindrical shape) may be as low as 1/10 of the particle initial diameter depending on both etching time and bias voltage, which makes the method really flexible and adaptable to many applications. In Figure 3, a higher-magnification image is shown in order to observe the hierarchical (triple-scale) roughness and nanotexture. The same nanotexture is also produced on a PMMA plate without colloidal particles (see Figure 3b). This texture has been already used to produce robust superhydrophobic surfaces on PMMA with oxygen plasma34 and on poly(dimethyl siloxane)

using SF6 plasma.35 We note that the nanotexture shown in Figure 3b is dual scale comprising approximately 200-nm-wide and 50-nm-wide columnar structures as discussed in detail by Vourdas et al.34 Thus, the surfaces shown in Figures 2 and 3a are hierarchical (triple-scale) with roughness in the micrometer, hundred nanometer, and ten nanometer range. In addition, these surfaces have micrometer-scale order (coming from the colloidal lithography) and nanoscale randomness coming from plasma nanotexturing. The much larger size of the pillars of Figure 2a is expected to provide better mechanical stability compared to the nanotexture shown in Figure 2b. In addition, we expect that improved superhydrophobicity will be shown on surfaces of Figure 2a compared to Figure 2b due to the clear separation of the scales. The nanotexture on top of the pillars is mainly formed due to the anisotropic etching conditions and the simultaneous codeposition of minute amounts of unetchable alumina molecules sputtered from the alumina Helicon reactor dome.31 Indeed, elemental XPS analysis after etching shows a small percentage of aluminum (1-10% depending on the etching time) in the form of oxyfluorides on the surface.31 We stress that aluminum does not form a film on the surface of the polymer; otherwise, etching would stop. Here, the etching rate is greater than 1 μm/min. Aluminum is unetchable in oxygen plasmas, and when present in small amounts, it initiates the formation of the nano “grass”-like structures locally, which grow higher since aluminum preferentially 3962

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Figure 3. (a) Hierarchical (triple-scale) roughness micropillars produced by the combination of colloidal lithography of 1 μm particles followed by plasma etching and dual-scale nanoroughness on top of the pillars formed during plasma etching (tilt 40°). (b) Nanotextured dual-scale PMMA surface fabricated by plasma etching, without colloidal microparticles (see also ref 34). Similar plasma etching conditions have been used for both cases [1 min etch, 1900 W, 100 sccm O2, 0.75 Pa, -80 V for part a and -100 V for part b].

sticks to the protruding parts of the topography causing a roughness instability, as discussed in detail in our recent simulation work.61 The protruding parts are etched at a smaller rate. The process is extremely reproducible in the same reactor, and it has to be tuned in a different plasma reactor by choosing the appropriate wall materials and plasma conditions. Similar nanotexturing results have been observed by other authors.38,39 The micropillars shown in Figure 2a and Figure 3a show a reentrant profile underneath the microparticle, which is expected to be beneficial for oleophobicity.6,62 This re-entrant profile is due to the spherical shape of the microparticle (which is by definition reentrant), and the anisotropic etching process, which creates a pillar of PMMA with the same diameter as that of the sphere (which functions as an etching mask). The re-entrant profile exists between the sphere and the column. However, more re-entrant profile may be required for superoleophobicity. To get a better re-entrant profile, a different two-step etching process was also developed for 3μm particles as discussed in section 2.3: (a) an anisotropic etching step with duration 3.5-4.5 min to create the pillar in the PMMA substrate and (b) an isotropic etching step with duration 1.5-2.5 min to enhance the re-entrant profile of the pillar. Apparently, during this second isotropic step not only is PMMA etched isotropically, but also the PS particle is etched faster in the bottom-half hemisphere, compared to the top half-hemisphere, which is roughened and partially covered by unetchable sputtered aluminum. The re-entrant profiles of the pillars are shown in Figure 4a,b for 3 μm particles in which one can observe that the reentrant shape that exists between the pillar and the particle is enhanced from the isotropic etching step. As Figure 4 suggests, it is important to control isotropic etch time, since longer etching would result in the removal of the PS sphere from the top of the pillar. This controlled undercut will probably not affect the mechanical stability of the PS particles and pillars, since even after the isotropic step, the thinnest neck is more than 500 nm wide. Furthermore, we have measured identical contact angles before and after immersing the surface in water and drying. Although the same process may be applied for a shorter time (e.g., 1.5 min and 30 s, respectively) for 1 μm particles, we found it easier to control the process with 3 μm particles, as will be explained in section 3.2. In addition, mechanical stability is better with 3 μm. 3.2. Design of Superamphiphobic Surfaces. In this section, we discuss the design of micronano structured superamphiphobic

Figure 4. Undercut, reentrant topography of a PMMA surface after 3 μm PS colloidal microparticle lithography followed by a two-step etching process in oxygen plasma. The first anisotropic etching step (3.5 min) produces the column, while the second isotropic etching (2.5 min) step enhances the re-entrant shape of the pillar, by etching isotropically both the PMMA and the bottom PS hemisphere. Notice the top nanotexture on the top half PS hemisphere. (a) tilt 25°, (b) tilted 90°.

surfaces. The micronano structures must have appropriate diameter and spacing (i.e., surface fraction “Φs’’), appropriate 3963

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Figure 5. Surface fraction versus the desired final contact angle for various initial contact angles. Design requirements to achieve a superoleophobic state in terms of surface coverage.

Figure 7. Surface fraction versus etching time for two different particle diameters (1 μm, 3 μm) assuming that we have cylindrical pillars and isotropic shrinking taking place at 400 nm/min.

Where a and b are functions of time, R is the etching rate of PS, and S is the etching rate selectivity (PMMA/PS)

Figure 6. Geometrical characteristics of the structure of microparticlecoated and etched surface: cross section (a) and top view (b) for hexagonal arrangement; ω is the angle between the tangent to the surface at the triple point and the horizontal.

height h, and appropriate re-entrant surface curvature.5 First, we start with the optimization of the surface fraction Φs. If one uses the Cassie equation and solves for the surface fraction assuming that a superamphiphobic contact angle of 150° is desired, then Φs ¼

cos θCB þ 1 cos θo þ 1

ð1Þ

Φs becomes a function of the Young contact angle as shown in Figure 5, where we plot Φs versus the final desired contact angle. Figure 5 shows that for Φs = 0.25 and initial contact angle 110° (e.g. water on teflon) one gets a final contact angle of 150°, while for an oil of 60° initial contact angle, one gets a final contact angle of 130°. To achieve contact angle of 150° for oils, Φs must become less than 0.1. We now calculate the etching time needed to achieve Φs e 0.1 values. We assume that the final topography is as shown in Figure 6. For these cylindrical pillars in the hexagonal arrangement, the surface fraction is given by πa2 Φs ¼ pffiffiffi 2 3ða þ bÞ2

ð2Þ

2a ¼ 2a0 - 2Rt=S

ð3Þ

2b ¼ 2Rt=S

ð4Þ

Thus, surface fraction becomes a time-dependent equation, as depicted in Figure 7. To achieve Φs e 0.1 with 1 μm particles, we need approximately 2 min using anisotropic etching, while with 3 μm particles, we need approximately 6 min anisotropic etching. Despite the fact that using 1 μm particles seems to be more effective, since the etching process is faster, Figure 7 shows that by using larger particles (3 μm) we can have better control over the process since the rate of change (slope) of Φs is smaller. Another advantage in using 3 μm particles is that bigger particles help the preservation of the re-entrant profile of the pillars, since the undercut will be higher and the particle shape less damaged by anisotropic etching and more re-entrant. In conclusion, both particle sizes can produce textured pillars, but the 3 μm particles are better since their processing can be easily controlled. Second, we discuss the height of the microstructure and the reentrant profile. The height of the microstructure should be enough so that the water meniscus in between the pillars does not touch the PMMA surface. The height is controlled by the selectivity of etch rates of particles to substrate. In addition, the surface should be as re-entrant as possible to provide a more robust metastable Cassie state especially with low-surface-tension liquids. Suppose that θ0 is the initial contact angle of a liquid on a Teflon flat surface and ω the angle between the tangent to the surface at the triple point and the horizontal (see figure 6): If ω e θ0, the net force is directed upward; in this case, the liquid-vapor interface tends to recede to the top of the pillars, creating a composite solid-liquid-air interface.63 On the contrary, surfaces that exhibit ω g θ0 such as pillar arrays or spikes will more easily transit to a fully wetted interface. The surfaces possessing re-entrant texture facilitate high apparent contact angles for liquids, even if θ0e90°. Unfortunately, using anisotropic etching, ω, although less than 90°, is not smaller than θ0 for several oils. 3964

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This happens because the particle is etched at the edges by the ions. This is why a two-step process is used. We can define a dimensionless height H* and sagging angle T* for the water meniscus as done by Tuteja et al.5 For a hexagonal arrangement shown in Figure 6, these dimensionless parameters can be derived and are given below normalized to the capillary pressure: H ¼

½ð1 - cos θ0 Þ þ Ha2 πlcap pffiffiffi ½2 3ða þ bÞ2 - a2 πb

ð5Þ

aπ sinðθ0 - ωmin Þlcap pffiffiffi 2 3ða þ bÞ2 - a2 π

ð6Þ

T ¼

where a and b come from eqs 3 and 4, ω is approximately calculated from SEM cross section images, and lcap is the capillary length. Increasing the magnitude of the dimensionless robustness parameters H* and T* creates a robust composite interface, with a high energy barrier between the metastable composite interface and the fully wetted interface. Notice that, for oils (θ0 < 90°) and for pillars (ω = 90°), T* becomes negative suggesting a lack of oleophobicity. For the anisotropic etching process, ωmin is approximately 60°, which gives small positive values of T* for diiodomethane (θ0 = 80.6°) and soya oil (θ0 = 62°), while it predicts negative values for hexadecane (θ0 = 41°) and decane (θ0 = 35°). On the contrary, for the two-step etching process ωmin becomes approximately 34° as seen in Figure 4 ensuring better oleophobicity. Note, however, that the above analysis considers smooth surfaces (see Figure 6). For our nanotextured surfaces, a different apparent θ0 value may be required in eqs 5 and 6 rather than the initial value on a smooth surface; if the nanoscale is not wetted, this apparent θ0 value increases and thus enhances oleophobicity. The opposite happens if it is wetted. We discuss this issue in section 3.4.2. 3.3. Superamphiphilic Surfaces. Appropriate conditions of highly anisotropic etching (section 2.3) were sought for the creation of micro/nano scale topography. All the fabricated surfaces are superhydrophilic and superoleophilic (i.e., contact angles are practically zero) immediately after O2-plasma treatment for plasma treatment durations even as short as 10s. Liquids spread on such surfaces as shown in Figure 11b. Amphiphilicity is a result of oxidation, as well as of roughness formation on polymers in O2 plasma. Hydrophilic treatments are known to age with time after plasma treatment. We have, however, observed a significant delay of hydrophobic recovery on plasma nanotextured surfaces, the delay being greater for longer etch times.31 Given this previous knowledge, no further experiments are presented in this publication for amphiphilicity. 3.4. Superamphiphobic Surfaces. 3.4.1. Superhydrophobic Surfaces. After exposure to O2 plasma, the structured surface is exposed to C4F8 plasma in the same reactor for the conformal deposition of a thin FC film (30 nm).59 The combination of the FC coating with the surface micro and nano texturing enhances the surface hydrophobicity leading to a high contact angle of water droplets on the surface. All surfaces after the fluorocarbon deposition become superhydrophobic, from the first minute of etching. The results of the static contact angles and hysteresis of the surfaces fabricated are presented in Figure 8 and are compared with plasma-only nanotextured surfaces. For a given etching time (1 min), the optimum bias voltage is -80 V. In comparison with the superhydrophobic PMMA surfaces fabricated with plasma treatment without the use of the colloidal

Figure 8. Water contact angle comparison between PMMA plasma nanotextured surfaces and the surfaces manufactured with colloidal lithography of 1 μm PS particles and plasma treatment under similar etching conditions. The plasma etching time was 1 min. Optimum voltage bias is -80 V. Sec Supporting Information.

lithography, the micro/nano structured surfaces exhibit higher static contact angle and lower contact angle hysteresis (Figure 8). This can be explained from the fact that the surfaces fabricated by the combination of colloidal lithography and plasma etching are taller and present hierarchical (triple-scale) roughness with clearly separated micro/nano components in the height distribution function. The plasma-only textured surfaces at small etching times also show dual-scale roughness (see Figure 3 in Vourdas et al.34), but the components are not clearly separated and are smaller than 300 nm. Nevertheless, at higher bias voltages (-100 V) differences in contact angles and hysteresis are minimal, and thus, -100 V is optimum for PMMA surfaces without colloidal lithography. Nanotexturing happens to some extent in all bias voltage conditions. There is, however, an interplay between etch time and bias voltage for optimum geometry and wetting behavior. This is obvious given that both affect etch depth. A detailed optimization is presented in the Supporting Information. It is also worthwhile to point out that the superhydrophobic state on these surfaces is thermodynamically stable, due to the fact that the estimated Cassie-Baxter CA is actually smaller than the corresponding Wenzel CA; thus, inhomogeneous (solid-air) wetting is energetically favored.35 The contribution of the plasmainduced surface nanoscale structures on the enhancement of surface hydrophobicity is actually crucial in achieving robust superhydrophobicity, which otherwise would be possible only through the fabrication of high-aspect-ratio microstructures. This has also been demonstrated on plasma-treated SU-8 micropillars.64 In addition to thermodynamically stable superhydrophobicity, we anticipate a better mechanical stability, due to the fact that the micropillars obtained on the PMMA after a short O2 plasma treatment (1-2 min) are larger compared to the random PMMA nanopillars created by plasma etching (micrometer versus hundred nanometer wide). 3.4.2. Superamphiphobic Surfaces. We tested the wetting properties of the surfaces discussed in section 3.4.1 with various low-surface-tension liquids, hoping to observe oleophobicity due to the pillar re-entrant profile. The first result after 1 min etching was that our surfaces exhibit oleophobic behavior, but not superoleophobic behavior (Figure 9). The reason that the surfaces are not superoleophobic is that the surface fraction Φs is relatively high, 0.37 (i.e., we have dense structures). The reduction of the surface fraction can be fulfilled by 3965

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increasing the etching time (e.g., for the optimal bias voltage -80 V). Our design (see Figure 7) suggests that the etching time should be greater than 1.5 min for 1 μm particles and greater than 5 min for 3 μm particles. However, although anisotropic etching initially improves the contact angle by decreasing the surface fraction, it eventually damages the particle and destroys the reentrant pillar profile at times equal to or greater than those needed from the process design. Thus, in order to get a better re-entrant geometry and a better process control we used 3 μm particles and the two-step process discussed above. The evolution of contact

angles for 1 μm, 3 μm, and different etching processes is shown in Table 1. Table 1 indicates that the optimized process leads to superoleophobicity for diiodomethane (drops easily roll off the surface), while soya-oil drops do not roll off so easily (higher contact angle hysteresis), although they exhibit relatively high static contact angle of 134° (starting contact angle on flat fluorocarbon coating is 62°). Hexadecane shows a contact angle of 101° (starting contact angle 41°), while decane does not spread on the surface anymore (small increase of contact angle from 35° to 41°). We can compare our results with the results

Figure 9. Contact angle versus bias voltage for several oils and water for 1 min etching after colloidal lithography of 1 μm PS microspheres. The highest contact angle for all liquids is again achieved when the voltage bias is -80 V as in Figure 9 (see also Supporting Information). Symbols at 0 V represent the initial contact angle (θ0) of the liquids on a flat fluorocarbon-coated surface.

Figure 10. Static contact angle for five different kinds of liquids with varging surface tension, according to the etching process used. The maximum improvement in contact angle from process A to B is 25°. Improvement for process A (one step) to B (two step) is due to surface fraction decrease and preservation/enhancement of the re-entrant profile.

Table 1. Optimization of Static Contact Angle and Geometrical Characteristics of PMMA Surfaces after Colloidal Lithography of 1 or 3 μm Particles Followed by Plasma Etchinga colloidal 3 μm colloidal 1 μm, PMMA plasma Teflon flat liquid water

colloidal 1 μm,

only nanotextured followed by 1 min surface (-80 v) anisotropic plasma

Φs, H*, T* for the colloidal

followed by 4.5 min

3 μm followed by

followed by

colloidal 3 μm,

anisotropic þ1.5

4.5 min anisotropic

1.5 min anisotropic

followed by 4.5 min anisotropic plasma

isotropic plasma etching

þ1.5 isotropic plasma etching (optimal process)

surface (1)

1 min (2)

etching (3)

plasma etching (4)

etching (5)

(optimal process) (6)

110°

155°

162° (e5°)

165° (e5°)

164° (e5°)

168° (2°)

Φs e 0.1 H* = 6022 T* = 1403

diiodomethane

80°

123°

127°

148°

152° (9°)

153° (9°)

Φs e 0.1 H* = 1743 T* = 463

soya oil

62°

104°

108°

125°

109°

134° (>15°)

Φs e 0.1 H* = 1938 T* = 440

hexadecane

41°

72°

76°

96°

66°

101°

Φs e 0.1 H* = 2394 T* = 104

decane

35°

e5°

e5°

∼10°

35°

41°

Φs e 0.1 H* = 1909 T* ≈ 0

a

Comparison among plasma only etched surfaces, and colloidal lithography followed by one or two etching steps is shown. The hysteresis is given in parentheses. 3966

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Figure 11. (a) Fluorocarbon-coated, oxygen plasma treated superamphiphobic surface at optimal conditions (see Table 1). (b) Oxygen plasma treated PMMA superamphiphilic surface. Liquid drop volume 10 μL.

available in the literature: Zimmermann et al.65 obtained a contact angle of 140° for hexadecane and 165° for diiodomethane, while both liquids showed small hysteresis. However, note that they used PFOTS (1H,1H,2H,2H-perfluorooctyltrichlorosilane) self-assembled monolayers, which on a planar surface exhibit initial contact angles of 70° for hexadecane, 98° for diiodomethane, and 115° for water (compared to 62°, 80°, and 110°, respectively, for our 30 nm plasma deposited fluorocarbon coatings). Improved coatings were also used by Tuteja et al.6 who used fluorodecyl POSS (heptadecafluorodecyl polyhedral oligomeric silsesquioxane) with initial contact angles of water larger than 120°. They also found a critical initial contact angle of 67° in order for a liquid to go to the Cassie State. We are thus confident that if a different low-surface energy coating is used similar results would be obtained with our structures as well. An interesting phenomenon is observed by looking at column 2 of Table 1 where it is shown that plasma-only nanotextured PMMA surfaces (without colloidal lithography; see Figure 3b) show oleophobicity with all liquids except decane. This suggests that these surfaces with dual nanoscale roughness are not fully wetted by the liquids hinting on the beneficial role of the nanoscale for both water and oils as long as the nanoscale is not wetted. In addition, the contact angle of the hierarchical (triple-scale) surfaces of colloidal lithography followed by plasma etching (see columns 3,4,5) is enhanced compared to plasma-only nanotextured PMMA (column 2) also for all liquids except decane. The improvement of contact angle from column 2 to column 3 is probably due to the combination of dual-scale roughness created by plasma processing and micropillar geometry (hierarchical (triple-scale) roughness), as well as the reentrant profile of the microcolumns. Figure 10 shows the final contact angle versus the starting contact angle for two processes highlighting the process improvement evolution in fabricating superamphiphobic surfaces. Process A uses 1 μm particles anisotropically etched for 1 min. Process B guarantees low surface fraction and well re-entrant profile by using 3 μm particles etched for 4.5 min anisotropically and 1.5 min isotropically. Note that for process B liquids do not fully wet the surface and points are above the diagonal. Finally, in Figure 11 we demonstrate the superamphiphobic and the superamphiphilic surfaces fabricated on PMMA for three different liquids, namely, water, diiodomethane, and soya oil. In the superamphiphilic sample (Figure 11b, nonfluorocarbon coated), the static contact angle is below 5° for all liquids tested, while liquids are on the Cassie State for the fluorocarbon-coated surface (Figure 11a).

4. CONCLUSIONS AND OUTLOOK We have presented an easy-to-implement technique for rapid (and potentially large-scale) fabrication of amphiphobic and amphiphilic surfaces with ordered hierarchical (triple-scale) topography. We produced pillar arrays of different height and diameter with one-step etching and mildly re-entrant profiles that exhibit oleophobicity with low-surface-tension oils. Optimal surface topography design requirements were discussed to achieve superoleophobicity. As a result, we developed a two-step etching process to produce a better re-entrant topography on polymeric substrates for superoleophobicity. We also present evidence that this hierarchical (triple-scale) topography is beneficial also for oils. The presented technology is environmentally friendly, flexible, and rapid, and could easily be extended to control of optical properties or of wettability inside polymeric microchannels for applications in polymeric microfluidics. Although our results for hexadecane and decane do not show superoleophobicity, we are confident that the use of a better low-surface-energy coating will make the surfaces superoleophobic also for these liquids. ’ ASSOCIATED CONTENT

bS

Supporting Information. Additional information and table on the role of blas voltage and etch time. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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