Letter pubs.acs.org/NanoLett
Controlled Three-Dimensional Hierarchical Structuring by MemoryBased, Sequential Wrinkling Won-Kyu Lee,† Clifford J. Engel,‡ Mark D. Huntington,† Jingtian Hu,† and Teri W. Odom*,†,‡ †
Department of Materials Science and Engineering and ‡Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States S Supporting Information *
ABSTRACT: This paper describes how a memory-based, sequential wrinkling process can transform flat polystyrene sheets into multiscale, three-dimensional hierarchical textures. Multiple cycles of plasma-mediated skin growth followed by directional strain relief of the substrate resulted in hierarchical architectures with characteristic generational (G) features. Independent control over wrinkle wavelength and wrinkle orientation for each G was achieved by tuning plasma treatment time and strain-relief direction for each cycle. Lotus-type superhydrophobicity was demonstrated on three-dimensional G1−G2−G3 hierarchical wrinkles as well as tunable superhydrophilicity on these same substrates after oxygen plasma. This materials system provides a general approach for nanomanufacturing based on bottom-up sequential wrinkling that will benefit a diverse range of applications and especially those that require large area (>cm2), multiscale, three-dimensional patterns. KEYWORDS: Nanowrinkles, polymers, hierarchical texturing, polystyrene, superhydrophobicity, superhydrophilicity
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of materials limitations on h and large ratios of ES/EB. Thin Au skin layers (h = 5 nm) on polystyrene (PS) can only result in nanowrinkles with λ down to 200 nm because ES/EB ∼ 20.23 In addition, control of the silica skin layer thickness of mechanically stretched poly(dimethylsiloxane) (PDMS) after oxygen plasma treatment is difficult, although wrinkle wavelengths as small as 80 nm have been reported.24 Recently, we showed how the thickness of skin layers can be controlled down to several nanometers based on reactive ion etching (RIE) plasma gases on PS.22 CHF3-plasma treatment of PS resulted in fine control over h (from sub-10 nm to hundreds of nm) as well as very low ES/EB ( 0.3) can lead to the formation of self-similar hierarchical wrinkles having multiple wrinkle generations (G) spanning 5 orders of magnitude in wavelength. Upon compression, the first generation (G1) of wrinkles can act as an effective skin to form the second G (G2) of wrinkles having a larger λ as well as additional G such as G3 and G4 until the substrate is fully compressed. Sequential strain-relief of a stiff skin can also
ultiscale hierarchical structures show engineered interfacial properties that are important for controlled wetting,1 structural color,2−4 and selective filtration.5,6 In particular, bioinspired three-dimensional (3D) substrates have achieved such properties with superior mechanical stability over large areas (>cm2).7 For example, the structure of gecko feet have been mimicked for reversible adhesive properties,8 shark skin for reduced drag,9 and lotus leaves for self-cleaning surfaces.10 The two main design factors critical to realize 3D hierarchical structures are (1) patterning multiple length scales and (2) controlling order/disorder of features. Microscale sizes and microstructures dictate substrate mechanical properties such as stability, strength, and flexibility, while nanoscale sizes and nanostructures influence local and chemical properties such as adhesion and filtration.7 Also, controlling orientation of features that organize hierarchy enables unique physical properties such as anisotropic water spreading and directional adhesion.8,11−17 The fabrication of 3D patterns with length scales spanning several orders of magnitude (e.g., nm to μm), however, is usually done with complex top-down processes such as multistep photolithography or imprinting.7 Moreover, these tools cannot easily manipulate order/disorder of multiscale features over large areas. Strain-induced wrinkling of a stiff skin on a soft base layer is emerging as a powerful bottom-up method to realize ordered and disordered patterns across an entire surface.18,19 In the linear regime, wrinkles are characterized by a wavelength λ ≈ 2πh(ES/EB)1/3, where h is the thickness of the skin layer, ES is the Young’s modulus of the skin, and EB is the Young’s modulus of the substrate.20−22 Wrinkles with nanoscale control over either λ or orientation are challenging to produce because © XXXX American Chemical Society
Received: June 16, 2015 Revised: July 23, 2015
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DOI: 10.1021/acs.nanolett.5b02394 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters produce in-plane micropatterns that consist of more than two different length scales.27 Deterministic control over λ and orientation of herringbone patterns was achieved by changing h, the direction of strain relief, and strain ratios in biaxially stretched PDMS. Neither one-step wrinkling under high strain for self-similar wrinkles nor sequential wrinkling, however, can generate 3D hierarchical architectures where both λ and orientation can be tuned independently. Either all generations of self-similar wrinkles form spontaneously under a single (continuous) strain-relief step26 or the previous-generation features are deformed under sequential wrinkling.28 Also, although the orientation of nanoscale features within hierarchical architectures could be tuned, at least one step of top-down patterning was required.21 Thus, there is a need for massively parallel, bottom-up strategies that can autonomously control λ and orientation of multiscale features, especially at the nanoscale. Here we present a memory-based, hierarchical wrinkling approach where both the wavelength and orientation of previous-generation wrinkles can be preserved as the next generation of wrinkles is formed. Multigenerational wrinkles were produced in prestrained, thermoplastic PS sheets by carrying out sequential cycles of plasma-mediated skin layer formation followed by global strain relief. Sequential wrinkling as a route to 3D hierarchical structures was possible because of two key characteristics of CHF3 plasma-treated PS: (1) strain can be manipulated by heating above the glass transition temperature Tg of PS; and (2) plasma-mediated skin layers can form conformally on any surface topography. To control wavelength of each generation (G) independently, we tuned the skin thickness h for each cycle by changing plasma treatment time. Our sequential wrinkling approach also allowed independent tuning of the orientation for each wrinkle G. Furthermore, we achieved superhydrophobic 3D hierarchical wrinkles via a three-cycle G1−G2−G3 process. These same substrate architectures were able to access all possible wetting states, from superhydrophobic to superhydrophilic, by selective oxygen-plasma treatment of different G features. Figure 1 depicts how multigenerational, hierarchical wrinkles can be produced by sequential wrinkling. The first skin layer (h1) was formed on a flat PS substrate via the first CHF3 plasma treatment (t1) in a reactive ion etching (RIE) system (Methods). The smallest G1 was generated after relieving the strain (ε1) by heating to a temperature (T = 125 °C) above the PS glass transition temperature (Tg = 100 °C). For the formation of hierarchy, generation G2 features were formed by growing a new skin layer on G1 wrinkles and then relieving strain to form a larger λ2. For G2 formation, the second CHF3 treatment time (t2) produced a layer of thickness h2 on G1 features, and the resulting effective skin (G1 + h2) generated a λ2 after strain relief ε2 greater than λ1. This process was then repeated until the desired generational hierarchy was reached. Because CHF3 plasmas resulted in CF2 polymerization, which formed the skin layer,29−31 G1 morphology was maintained even after h2 was deposited and ε2 applied (Figures S1 and S2). This preservation of smaller features as larger ones are formed is the basis for memory-based processing to create hierarchical 3D architectures. Unlike self-similar wrinkles,26 G2 formation by sequential wrinkling is no longer defined by the skin thickness for G1 features (h1). Our sequential process allows us to study effects of h2 and G1 geometry on the observed G2. To determine the conditions for forming hierarchical wrinkles, we first charac-
Figure 1. Sequential wrinkling can generate multigenerational, hierarchical structures. CHF3 RIE plasma treatment produced a thin skin layer on PS substrates. The thickness h was controlled by changing treatment time (t); h is linear proportional to wrinkle wavelength (λ). Substrates were heated in an oven at T = 125 °C. Sequential wrinkling could achieve multigenerational features (e.g., G1, G2, G3) with hierarchical structure. Orientation and periodicity of each G were controlled by changing t and strain (ε) direction (uniaxial or biaxial strain) for each cycle.
terized the skin thickness ratio (R) = h1/h2 (Figure 2). In a twocycle G1−G2 process, if strain was relieved uniaxially (1D strain) for both cycles, we designate the cycle as 1D−1D; strain relieved biaxially (2D strain) in two different cycles would be 2D−2D. We found that hierarchical wrinkles formed after 1D− 1D and 2D−2D cycles only when h2 > h1 (Figure 2 shows the specific case h2 ≈ 80 nm and h1 ≈ 20 nm). The second strain (ε2) was relieved and formed G2 features while maintaining G1 features, and hence λ2 of G2 was larger than λ1 for both 1D− 1D and 2D−2D cycles. For h2 < h1, however, ε2 was relieved by decreasing λ1 instead of forming G2 features, similar to nonlinear strain absorption of single-generation wrinkles (Figure S3).25 We investigated whether there was a structural transition from a single-wrinkle generation to hierarchical wrinkles by fixing h1 and gradually increasing h2. As shown in Figure S4, once R was smaller than a threshold value (Rt), hierarchical wrinkles formed. Rt was 0.67 for both 1D−1D and 2D−2D cycles. Figure 3 and Figure S5 show that once the condition R < Rt was satisfied, by further decreasing R (increasing h2 while fixing h1), λ2 can linearly increase from 500 nm (5 times larger than λ1) to 1.5 μm (15 times larger than λ1) with the same λ1 = 100 nm for a 1D−1D cycle. Similarly, for the 2D−2D cycle, λ2 was controlled from 700 nm to 2 μm with a fixed λ1 = 150 nm. Because λ1 can be tuned by changing h1, hierarchical wrinkles with all possible combinations of λ1 and λ2 (λ1 < λ2) can be designed. In contrast, self-similar wrinkles formed under high strain (ε > 0.9) showed a fixed λ2 that was roughly five times that of λ1, regardless of treatment time.25 To design hierarchical wrinkles with different combinations of h1 and h2, we quantified λ2 with respect to wrinkle wavelength of a single generation (λsingle) because λsingle is easy to determine based on h. By studying the correlation between B
DOI: 10.1021/acs.nanolett.5b02394 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters ⎛ E ⎞1/3 λsingle = 2π (h1 + h 2)⎜ S ⎟ ⎝ EB ⎠
(1)
while λ2 for two-cycle, G1-G2 hierarchical wrinkles can be expressed as ⎛ E ⎞1/3 λ 2 = 2πg (h1 + h2)⎜ S ⎟ ⎝ EB ⎠
(2)
where g is a parameter for the geometric contribution of G1 on the effective skin for G2 structures. With the λ2−λsingle experimentally measured in Figure 3, and calculated ES/EB from previous work,22 we extracted g values under different h1 + h2. As a result, we found that the geometric contribution of effective skin decreased as h1 + h2 increased both for 1D−1D and 2D−2D cycles. This decrease in g can be attributed to the decreased amplitude of G1 features relative to h1 + h2 (Figure S7). Notably, the geometric contribution under sequential wrinkling was larger under 2D strain than 1D strain. Figure 4 shows how sequential wrinkling can control the orientation of hierarchical structures. Hierarchical nanowrinkles with different orientation combinations cannot be realized in one-step wrinkling because the strain direction for each cycle can change the orientation of each G features. Using sequential wrinkling, we achieved independent control over the orientation for each G by relieving strain in 2D or 1D after each cycle. For the simplest case with two cycles (G1−G2), the direction of ε1 and ε2 can be 1D−1D, 1D⊥1D (perpendicular directions between ε1 and ε2), 2D−1D, and 2D−2D. Interestingly, the orientation of G2 features was different for 1D−1D, 1D⊥1D, and 2D−1D even though the applied strain direction for G2 was the same. That is, the orientation of G2 is no longer defined only by the direction of ε2 but by a combination of G1 orientation and ε2. The quasi-ordering of hierarchical wrinkles can be analyzed using an order parameter S based on the fast Fourier transform (SFFT) (Figure S8), where SFFT = 0 corresponds to completely random features and where SFFT = 1 has features that are perfectly ordered. For single-generation wrinkles, SFFT ≈ 0.77 for 1D wrinkles (1D strain) and 0.04 for 2D wrinkles (2D strain).25 SFFT of microscale G2 1D wrinkles were 0.73, 0.53, and 0.39 for 1D−1D, 2D−1D, and 1D⊥1D cycles, respectively. The orientation of G1 features acted as a directional guide for orienting G2 in 1D−1D; however, in 2D−1D and 1D⊥1D cycles, the G1 orientation perturbed 1D ordering of G2 under 1D strain (ε2). The number of bifurcation points or disclination defects of global 1D wrinkles (G2) was higher in 2D−1D and 1D⊥1D structures compared to those in 1D−1D, increasing disorder of G2. Plasma-mediated skin formation enables superior adhesion with no delamination between the newly deposited CF2 polymer layer and the underlying wrinkled substrate (Figure S9); therefore, previous patterns are invariant to subsequent processing cycles. To demonstrate the versatility of our memory-based processing, we designed 3D hierarchical architectures based on three cycles of wrinkling (G1−G2− G3); to highlight design flexibility, we kept λ1, λ2, and λ3 roughly fixed at 220 nm, 1 and 12 μm, respectively (h1 ≈ 20 nm, h2 ≈ 100 nm, h3 ≈ 1.8 μm). Notably, the threshold for hierarchy formation also held for G1−G2−G3 structures (R2 = h1/h2 = 0.2 < 0.67 for G2 formation and R3 = (h1 + h2)/h3 = 0.67 for G3 formation).
Figure 2. Conditions for formation of hierarchical wrinkles. (a) Hierarchical wrinkles formed with sequential wrinkling when the h2 (≈ 80 nm) was larger than h1 (≈ 20 nm). t1 and t2 were 40 and 160 s, respectively. (b) Normal wrinkles formed in the opposite condition of h1 > h2 (t1 = 160 s and t2 = 40 s). SEM images of hierarchical wrinkles and normal wrinkles under (c,d) 1D−1D and (e,f) 2D−2D cycles.
Figure 3. Quantifying hierarchical wrinkles with respect to singlegeneration wrinkles. Second wavelengths (λ2) of the hierarchical wrinkles formed with 1D−1D and 2D−2D processes. Hierarchical wrinkles show λ2 larger than wrinkle wavelength of single generation (λsingle) under the same total RIE time (t1 + t2). For the calculation of geometric contribution, h1 + h2 values were measured using variableangle spectroscopic ellipsometry.22 t1 was fixed at 20 s, generating h1 ≈ 10 nm for all samples. Slopes of line-of-best-fit and R2 values are summarized on Table S1.
λ2 and λsingle, we can quantify the geometric contribution of G1 features on the effective skin layer for G2 formation. Singlegeneration wrinkles have the same total skin thickness (h1 + h2) but only a single strain-relief step (ε = ε2). Figure 3 and Figure S6 shows that λ2 of the hierarchical wrinkles is larger than that of λsingle, which supports that the G1 morphology affects the effective skin layer. For single-generation wrinkles, λsingle can be described by C
DOI: 10.1021/acs.nanolett.5b02394 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 4. Controlled quasi-ordering of hierarchical wrinkles having different combinations of orientation. SEM images of hierarchical wrinkles formed with (a) 1D−1D, (b) 2D−1D, and (c) 1D⊥1D cycles. SFFT of microscale 1D wrinkles decreased from (a) to (c). t1 and t2 were 20 and 160 s for all samples. Yellow arrows indicate bifurcation points or disclination defects of G2 features.
Figure 5. Building up 3D hierarchical wrinkles with tunable orientation. The 3D wrinkles formed with (a) 1D−1D−1D (SFFT ≈ 0.72), (b) 2D−1D− 1D (SFFT ≈ 0.61), (c) 2D−2D−1D (SFFT ≈ 0.46) and (d) 2D−2D−2D (SFFT ≈ 0) sequential wrinkling. The insets show FFT patterns of the third generational wrinkles (G3) for the calculation of SFFT.
cycle was 0.72, similar to that of single-generation 1D wrinkles (SFFT = 0.77), which indicated highly aligned physical assembly of G3 features. In contrast, the SFFT of G3 in 2D−1D−1D and 2D−2D−1D hierarchical structures was 0.61 and 0.46, respectively, where the most disordered arrangement of G3 features was from the 2D−2D−2D cycle, where SFFT = 0. Furthermore, other G1−G2−G3 structures such as 1D−2D− 2D and 2D−1D−2D can be fabricated with controlled orientation (Figure S10). The SFFT of G3 of 1D−2D−2D was around 0, which is similar to 2D−2D−2D (completely disordered features), while the SFFT of G3 2D−1D−2D was 0.2 (quasi-ordered features).
Figure 5 displays 3D hierarchical wrinkles with four different combinations, 1D−1D−1D, 2D−1D−1D, 2D−2D−1D, and 2D−2D−2D. Similar to hierarchical wrinkles formed by a G1G2 process, the orientation of G3 features was defined by G2 orientation and ε3 direction. In a G1-G2 process, only two options for G1 orientation (i.e., SFFT of 0 (2D) or 0.77 (1D)) could influence G2 under ε2. In a G1−G2−G3 process, however, G2 features with more diverse SFFT of 0 (2D−2D), 0.57 (2D−1D), and 0.73 (1D−1D) can be selected to form G3 under ε3. In this manner, the potential range of SFFT (of G3) was increased in a G1−G2−G3 process compared to that of G2 in a G1-G2 process. SFFT of G3 features from a 1D−1D−1D D
DOI: 10.1021/acs.nanolett.5b02394 Nano Lett. XXXX, XXX, XXX−XXX
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substrate with an O2 RIE plasma to increase wettability.35 We hypothesize that some of the largest G3 structures masked smaller G1 or G2 features in a self-shadowing effect;36,37 hence, masked regions remained hydrophobic while those exposed to the oxygen plasma became hydrophilic. This type of selective chemical modification is unique to 3D hierarchical wrinkled substrates, and a starting 2D−2D−2D substrate with θw > 160° could be continuously tuned down to 0° as O2 plasma time increased (to ca. 10 min). We found that the chemical modification resulting in superhydrophilicity was stable38 at least up to 10 days (the time tested) after O2 treatment (Figure S14). In contrast, θw = 89° on bare PS decreased to 16° after only 30 s of O2 plasma (Figure S15), and a superhydrophilic state was not possible even after long O2 plasma treatment times. In the Wenzel state, surface roughness always enhances wettability,34 and thus hierarchical wrinkle topology was critical to generate the superhydrophilic state (θw ∼ 0°). Because RIE plasma times can be controlled within ±1 s, nearly any wetting state is possible on the same 3D wrinkled substrate. In summary, we demonstrated a memory-based, sequential wrinkling process able to transform PS sheets into 3D, hierarchical architectures over large areas (>100 cm2). Independent, nanoscale control over the wavelength of each wrinkle generation was achieved by tuning skin thickness for each cycle, where average values for G1, G2, and G3 features ranged from λ1 = 100−200 nm, λ2 = 500 nm−2 μm, and λ3 > 10 μm. Beyond control over λ, we tuned the orientation of each wrinkle generation by manipulating direction of strain relief between ordered 1D and disordered 2D for each cycle. Because each wrinkle generation spontaneously forms across the entire surface, order and disorder can be controlled at all length scales. Hierarchical generations of nanowrinkles and microwrinkles within the same substrate also spanned superhydrophobicity to superhydrophilicity. Our memory-based hierarchical nanowrinkling provides a general approach to construct polymeric, 3D patterns that have potential in unidirectional liquid transport, antibiofouling substrates, and omniphobic surfaces.
One important application of large-area, 3D hierarchical nanostructures is mimicking of lotus-type superhydrophobic surfaces for self-cleaning,7 anti-icing,32 or dew collection.33 The 3D wrinkles with multiple length scales can control static water contact angle (θw) and contact angle hysteresis (θH) to generate stable superhydrophobicity that satisfies both θw > 150° and θH < 5°. Figure 6a depicts how a flat PS sheet (θw =
Figure 6. Controlled wetting properties of hierarchical wrinkles. (a) Measured static contact angles (θw) and contact angle hysteresis (θH). All samples were treated with 2 min SF6 gas. (b) Measured θw with increased O2 plasma time. Insets show optical images of corresponding water droplets (volume = 30 μL).
89°) can be converted into a superhydrophobic 3D substrate (θw > 150°). To maximize hydrophobicity, we treated hierarchical wrinkles with RIE SF6 plasmas. SF6 treatment on bare PS or CF2 skin layers increases hydrophobicity by saturating the C atoms at the surface with F atoms.29−31 Indeed, θw of both flat PS and CHF3-treated PS (but no wrinkling) increased after SF6 plasma treatment, verifying surface saturation with F atoms (Figure S11a). The θH of the flat CF2 skin layer also increased from 40° to 52° after SF6 treatment, which indicates that wetting transformation from the sticky Wenzel state (θH > 30°) to the rollable Cassie−Baxter state (θH < 5°) state was not only because of changes in surface chemistry (Figure S11b). To achieve a Cassie−Baxter state, where air pockets exist at the solid and liquid contact line, surface roughness is necessary.34 SF6 treatment on singlegeneration wrinkles or hierarchical wrinkles also increased θw (Figures S11c and S12). The 2D (G1) wrinkles had θw = 140°, which increased to 147° on 2D−2D (G1−G2) hierarchical wrinkles. Adding G3 features increased θw significantly to 162°, where superhydrophobicity was achieved on the 2D−2D−2D (G1−G2−G3) surfaces. At the same time, θH on 2D wrinkles (26 ± 6°) decreased to 2° on 2D−2D−2D surfaces, which represents a transformation from the Wenzel state to the Cassie−Baxter state. Figure S13 indicates that a water droplet on the 2D−2D−2D surface showed an extremely low roll-off angle (
