Writing Wrinkles on Polydimethylsiloxane (PDMS) by Surface

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Writing Wrinkles on Polydimethylsiloxane (PDMS) by Surface Oxidation with a CO Laser Engraver 2

Lin Qi, Cody Ruck, Griffin Spychalski, Brian King, Benxin Wu, and Yi Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17622 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 5, 2018

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ACS Applied Materials & Interfaces

Writing Wrinkles on Polydimethylsiloxane (PDMS) by Surface Oxidation with a CO2 Laser Engraver

Lin Qi1, Cody Ruck1, Griffin Spychalski1, Brian King1, Benxin Wu2 and Yi Zhao1,* 1

Laboratory for Biomedical Microsystems, Department of Biomedical Engineering The Ohio State University 2

School of Mechanical Engineering, Purdue University

*

To Whom All Correspondence Should Be Addressed: [email protected].

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Abstract Surface wrinkles formed by the buckling of a strained stiff layer attached to a soft elastomer foundation has been widely used in a variety of applications. Micropatterning of wrinkled topographies is, however, limited by process/system complexities. In this paper, we report an approach to write surface wrinkles with desired pattern geometries on polydimethylsiloxane (PDMS) elastomers using a commercial infrared laser engraver with the spot size of 127 µm. Wrinkled micropatterns with the wavelength from < 50 µm to > 300 µm were obtained in minutes without using special facilities or atmospheres. The minimal achievable pattern sizes of one-dimensional (1D) and two-dimensional (2D) patterns, and the change of the minimal achievable pattern size with the wrinkle orientation was investigated under a given set of operating parameters. Sub-spot size patterning was also demonstrated. In order to reduce surface cracking, a typical problem in large area wrinkle patterning, a patterning scheme that separates neighboring laser exposure areas by non-exposure gaps was developed. In addition, micropatterns with gradient wrinkles were created on the surface. This is the first report that patterns microscale surface wrinkles on elastomer surfaces using infrared laser irradiation. The simple and versatile approach is expected to provide a fast yet controllable way to create wrinkled micropatterns at low cost to facilitate a broad array of studies in surface engineering, cellular biomechanics, and optics.

Keywords: infrared laser irradiation; surface wrinkling; spatial gradient; elastomer; silica-like layer 2 ACS Paragon Plus Environment

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1. INTRODUCTION Surface buckling represents a simple yet effective approach to create periodic topographies at small scales without using pre-machined molds1-2. In this approach, wrinkled topography can be formed by straining two layers that have been firmly attached to each other (a stiff surface layer and a soft elastomer substrate) with a stiffness mismatch3-5. The geometries of resulting wrinkles vary with the stiffness mismatch between the two layers and the applied strain. The surface layer can be produced by depositing a rigid material on the pre-strained elastomer substrate via spin-coating, sputtering, or evaporation6-8. The adhesion between the deposited surface layer and the base elastomer substrate varies with the deposition parameters and the surface condition. If the adhesion is not strong enough, the two layers may delaminate upon strain releasing, which can significantly change the regularity of wrinkled topography9-11. Alternatively, the stiff surface layer can be produced by oxidizing the top surface of the elastomer substrate using plasma, UV irradiation, or UV/ozone (UVO) treatments12-15. In these oxidation approaches, the surface oxide layer adheres firmly to the bulk elastomer substrate via chemical bonds. The delamination issue associated with externally introduced surface layers is eliminated. Oxidation based wrinkling, however, is limited by several factors. In order to create an oxide surface layer with a desired thickness, the elastomer surface often needs to be exposed to ionized species. Such environment requires closed chamber environment supplied with certain gas atmospheres. For example, the most commonly used plasma oxidation process flows oxygen into a closed chamber16. The closed chamber configuration is unfortunately incompatible with most pre-straining devices not only because 3 ACS Paragon Plus Environment


these devices are too bulky to fit in the closed chamber but also because electromechanical components in these devices may fail when being exposed to the ionized species. UV ozonolysis, although can oxidize elastomer surfaces in an open area, is fairly slow. It takes about 10 to 60 min of UVO treatment to create surface wrinkles with the large wavelengths from a few to hundreds of µm13. To address this, we previously reported that atmospheric corona discharge can create surface wrinkles in an open environment without special atmosphere17. The ionized species around the discharge is used for surface oxidation. This approach, however, creates non-uniform wrinkle geometries: the peripheral area often has a smaller wrinkle wavelength than the center area due to the spatial concentration variation of the ionized species. In addition, it is difficult to create wrinkles with large wavelengths on the order of tens to hundreds of µm.

Another limitation of current oxidation based surface wrinkling is the lack of patterning capability. In closed chamber plasma oxidation approaches, oxidation often occurs on the entire surface of the elastomer substrate, leading to homogeneous wrinkle geometries. Patterned wrinkles can be formed by masking selective areas with a physical mask18. This necessitates additional process complexity, high cost, and long preparation time associated with mask fabrication. Patterned wrinkles can also be produced by solvent induced strain mismatch7, 19-23. The wrinkle pattern depends on the dispensing locations and the diffusion process of the solvent. Since solvent diffusion is often isotropic and uncontrolled, the geometries of the wrinkled patterns are fairly limited. Alternatively, focused ion beam (FIB) 4 ACS Paragon Plus Environment

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offers a powerful solution of writing surface wrinkles24-26. Local oxidation occurs by producing a small and highly ionized space over the working surface. Desired surface patterns can be formed by moving the ion beam laterally on the subject surface. FIB has been shown to produce patterned wrinkles with controlled wavelengths. The minimal pattern as small as 500 nm was achieved24. The wrinkle wavelength ranged from tens of nm to several µm24-26. The wide adoption of FIB in surface wrinkling is, however, largely hindered by the high equipment cost and system complexity required for producing the focused ion beams. It is plausible to consider other lower-cost approaches that can generate high energy beams for creating wrinkle patterns. Laser irradiation has been widely used for surface ablation and modification27-30. Similar as in FIB, arbitrary patterns can be formed by moving the exposure spot on the surface following a pre-determined traveling path. Although both UV and infrared lasers are able to cause local oxidation in polydimethylsiloxane (PDMS) elastomers31-32, there is so far no report of producing patterned surface wrinkles using laser oxidization.

In this study, we demonstrate the writing of surface wrinkles with controlled wavelength and arbitrary patterns on PDMS surfaces using a commercial benchtop laser engraver with 10.6 µm CO2 laser source. The oxidization dose was modulated by changing the laser power density and the cycles of engraving. The resulting oxidation layer was examined using scanning electron microscopy with integrated energy dispersive X-ray spectroscopy (SEM-EDS) and attenuated total reflection spectroscopy (ATR). With a pre-strain (ε) from 10% to 20%, surface wrinkles with the wavelength of 300 µm were formed. The 5 ACS Paragon Plus Environment


resulting oxidation layer by infrared laser oxidation was compared to those by plasma and UVO treatments in terms of their different oxidation mechanisms. The minimally achievable pattern size with various wrinkle orientations was also investigated. The writing capability was validated by creating arbitrary patterns comprising crack-free wrinkles. Surface wrinkles with gradient wavelength were also demonstrated. This is the first report of surface wrinkle writing on elastomer surfaces by infrared laser oxidation. It provides a simple approach for generating patterned surface wrinkles in a fast, convenient and controllable way, and is expected to assist the wide adoption of surface wrinkles in various applications.

2. EXPERIMENTAL 2.1. Elastomer substrate preparation PDMS (Sylgard 184, Dow Corning, MI) was prepared by mixing the base pre-polymer and the curing agent at a weight ratio of 10:1. The mixed polymer solution was degassed in a vacuum chamber for 30 min. The prepolymer was then dispensed and spread on the bottom surface of a petri dish, resulting in a 2 mm thick thin film. The PDMS film was then baked on a hotplate at 65°C for 2 hours to cross-link. The cured PDMS film was cut into rectangular pieces with 70 mm in length and 30 mm in width.

2.2 Parameters of laser engraving CO2 laser treatment was performed using a commercial CO2 laser engraver (VSL2.30, Universal Laser Systems, AZ). A 25W continuous wave CO2 laser source with the 6 ACS Paragon Plus Environment

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wavelength of 10.6 µm was used. The laser spot size using the objective lens of lens 2.0 was ~127 µm. The lateral speed of the laser head was ~ 0.2629 m/s. The laser repetition rate was kept at 5000 Hz. The spatial resolution of laser exposure was calculated as ~500 PPI (Point Per Inch). The laser power was modulated by an external TTL (Transistor-Transistor Logic) signal. Two operating parameters, namely Gray Scale (GS) and Raster Intensity (RI) allowed independent control of the laser power temporally and spatially. In this study, GS ranged from 10% to 25% and RI ranged from -50% to 0%. The time-averaged laser power at each GS/RI combination was experimentally measured using a laser power meter (Mahoney, Bell Laser LLC, Seattle, WA). The pattern was formed in the raster mode, where the defined area was exposed through line-by-line scanning. Given that every two neighboring scanning lines were spaced at ~ 50 µm, the majority of the patterned area (except for those at the edges) was subject to ~ 5 laser pulses during each engraving cycle. The number of the engraving cycles (n) and the time-averaged laser power density (refers to power density P hereafter) were used to modulate the total laser dose.

2.3. Surface wrinkling by infrared laser The laser-induced surface wrinkling process is depicted in Fig. 1a. The rectangular PDMS substrate was stretched lengthwise and held at a pre-stain ε. The substrate is then mounted on an acrylate plate and placed within a laser engraver. The pattern geometries were generated in CorelDraw X6 and engraved on the PDMS substrate. After engraving, the PDMS substrate was released to the original length at a slow unloading rate (< 5 mm/s). Surface wrinkles 7 ACS Paragon Plus Environment


formed on the surface (Fig. 1b). In order to determine the range of operating parameters that can produce wrinkled topographies, laser engraving with systematically varied GS and RI was performed within a 10 mm × 10 mm area.

2.4. Characterization of the oxidized PDMS and the wrinkle geometries The laser induced oxidation layer in the PDMS samples without pre-straining was examined. The top surface and the cross-sectional surface were observed using a SEM (Zeiss Ultra 55 Plus FE, Carl Zeiss AG, Oberkochen, Germany) with an integrated EDS module (EDS, Oxford Instruments, Abingdon, UK) under the magnification of 2000×. The working distance was 8.5 mm, and the accelerating voltage was 6 kV. All the samples were sputtered with Au:Pd alloy before the observation to avoid surface charging. Quartz and pristine PDMS were also examined as the controls. The ATR spectra were obtained from a 50 µm × 50 µm area at the center of the exposure area using a Raman-Fourier-transform infrared microspectrometer (IlluminatIRTM, Smith Detection, MD, USA).

The thickness of the oxidation layer was also examined by hydrothermal dissolution of silica-like layer. This is based on the fact that silica is soluble in hot and concentrated basic solution while PDMS has very little solubility33-36. The samples with the engraving cycles n100 were immersed in the same basic solution for six days. The samples were then rinsed in water for three times and blow dried. The surface profile was 8 ACS Paragon Plus Environment

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measured by z-axis scanning using an upright measuring optical microscope (Nikon Eclipse LV 100, 5× to 100× objectives, Tokyo, Japan).

The wrinkle wavelength was examined by optical microscopy (Nikon Eclipse LV 100, 5× to 100× objectives, Tokyo, Japan). For each sample, the wavelength was measured at eight different locations in the center of the exposure area (5 mm × 5 mm). The measurement was represented by the average value ± the standard deviation. An optical stereoscope (AmScope, V-SM-3, CA) was used to examine the surface topography of laser-induced wrinkles under the magnification from 3.5× to 5×.

3. RESULTS AND DISCUSSION 3.1. Wrinkling conditions The ranges of the operating parameters that can generate wrinkled topography were experimentally determined by systematically varying three operating parameters (GS, RI, and n) (Fig. 1c). The results were categorized into three groups: blue dots represent the conditions where no obvious topography change was observed within the exposed area; green dots represent the conditions with obvious wrinkled topographies, and red dots represent the conditions with obvious surface burning. The pre-strain (ε=10%) was along the raster scanning direction of the laser beam. The results showed that surface wrinkles can be formed most repeatedly and reliably at GS=21% and RI=-2%. The power density under this condition was ~16.77kW/cm2. This condition was used for engraving hereafter unless otherwise stated. 9 ACS Paragon Plus Environment


3.2. Surface oxidation layer SEM examination of the cross-sectional surface showed that the oxidation layer caused by CO2 laser engraving had a clear boundary with the untreated PDMS substrate (Fig. 2a). The oxidation layer thickness was significantly greater than those by plasma (a few to tens of nm)37-38 or UVO treatments (tens nm to a few µm)15, 39-41. The thick oxidation layer was experimentally confirmed by the hydrothermal dissolution experiments, which also showed that the thickness of oxidation layer increased with n (Fig. 2b). ATR spectra (Fig. 2c) showed the changes of a series of infrared bands associated with –CH3 rocking and ≡Si–C≡ stretching (Peak 1: 789 cm−1); ≡Si–OH stretching (Peak 2: 825–865 cm−1); asymmetric ≡Si–O–Si≡ stretching (Peak 5: 1055–1090 cm−1); symmetric –CH3 deformation (Peak 6: 1257 cm−1); and asymmetric Si–CH3 stretching (Peak 7, 2960 cm−1). With the increase of the engraving cycles (n), there was a decrease in the band intensity of ≡Si–O–Si≡ signal (Peak 5), indicating that some chain scission occurred in the PDMS network. Simultaneously, there was a decrease in –CH3 signals (Peaks 1, 6, and 7) (Fig. 2d) that was accompanied by an increase in –OH signal (Peak 2), suggesting oxidative conversion.

The result was supported by the EDS analysis on the top surface of PDMS, where the SiOx conversion ratio increased with n, from ~21.42% at n=10 to ~ 50.10% at n=100 (Fig. 2e). The EDS analysis on the cross-sectional surface of PDMS showed that the oxidation layer exhibited a spatial gradient along the depth direction (Fig. 2f): the near-surface region had a 10 ACS Paragon Plus Environment

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higher SiOx conversion ratio than the inner region. EDS examination also indicated that the CO2 laser induced oxidation layer can be as thick as a few tens of µm. The formation of the thick oxidation layer is probably due to the low degree of oxidation by CO2 laser, which is discussed in section 3.7.

3.3. Minimally patternable area under a given laser power density Wrinkle formation within a finite patterned area is subject to the edge effect42. Briefly, the strain mismatch is zero at the edge of an exposed area due to the lack of confinement from the stiff silica-like layer. The strain mismatch increases as the point of interest moves inwards the patterned area. If the patterned area is too small, the strain mismatch would be too low to cause wrinkling. The minimally patternable area under a given set of operating parameters (P= ~16.77kW/cm2, ε=10%, and n=20) was examined. To simplify the analysis, the edge effects in parallel to and perpendicular to the strain direction were investigated separately. Rectangular areas with the length of 10 mm were patterned. The width of the areas varied from 200 µm to 2000 µm. Since the length of the areas was much greater than the width, the patterned areas can be approximated as infinitely long so that the edge effect along the longitudinal direction of the rectangular areas was negligible. This is named one-dimensional (1D) patterning. The patterned areas were arranged to have the longitudinal direction of the rectangular areas be either in parallel to (Fig. 3a) or perpendicular to (Fig. 3d) the strain direction. The angles between the longitudinal direction of the patterned rectangular areas and the pre-strain direction (θ) were 0° and 90°, respectively. The results showed that when 11 ACS Paragon Plus Environment


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θ = 0°, the minimal width of the pattern that can cause periodical wrinkles was ~ 400 µm under the given set of operating parameters (Fig. 3b). The corresponding wrinkle wavelength was λ= 79.58 ± 5.78 µm. As the width of the patterned area increased to ~ 500 µm, λ increased to 112.95 ± 7.35 µm (Fig. 3c). The wrinkle wavelength change with the pattern width can be estimated by43: ߣ=

ଵ/ସ ଵ/ଷ ா೑ ଵ/ଷ ଵ଺ ாതೞ ௐ 2ߨℎ௙ ቀ ത ቁ tanh ൝ ቈ൬ ൰ ቉ ൡ, ଵହ ா೑ ௛೑ ଷாೞ

(1)

where E, h, W are the elastic modulus, the thickness of the stiff layer, and the pattern width, respectively. Subscripts f and s denote the stiff film and the elastomer substrate. Ē=E/(1-υ2), where υ is the Poisson’s ratio. It is seen that the wrinkle wavelength increased with the pattern width, while the changing rate of the wavelength decreased as the pattern width increased. Such trend was experimentally validated (Fig. 4).

When θ = 90°, the edge effect can be represented by the edge length (Ledge):42 ‫ܮ‬௘ௗ௚௘ = ℎ௙ ݃൫ߝ௣௥௘ , ߝ௖ ൯,

(2)

which is defined as the distance from the pattern edge to the midpoint between the first peak and the first valley closest to the pattern edge. g is a non-dimensional function of the pre-strain εpre and the critical strain εc, which decreases as εpre and εc increase. Recalling the equation that predicts critical stain εc: ଵ

ாത

ߝ௖ = ൬ ത ೞ ൰ ா ସ

೑

ଶ/ଷ

,

(3)

Ledge is proportional to the thickness of the top stiff film, and increases with the increase of the elastic modulus of the stiff film, or with the decrease of the elastic modulus of the 12 ACS Paragon Plus Environment

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elastomer substrate. The results showed that the minimal pattern width that can cause periodical wrinkles was ~ 500 µm under the given operating parameters (Fig. 3e&f). The image contrasts, however, were much lower than those in Fig. 3b&c. This indicated smaller wrinkle amplitudes. The measurements also showed that the wrinkle amplitude and the image contrast both decreased when θ increased from 0° to 90° (Fig. 3g-i). The reduction was because the edge effect became more dominant when the effective length of patterned area along the pre-strain direction

ௐ

ୱ୧୬ ఏ

decreased and continued to approach 2×Ledge as θ

increased. The edge effects in two-dimensional (2D) patterns were also examined by patterning square areas (Fig. 3j). Within a 2D square area, the strain mismatch reduction originated not only from the edges perpendicular to the pre-strain (Edges I and III), but also from the edges in parallel to the pre-strain (Edges II and IV). As a result, the length of the minimally patternable square area (2.4 mm) was greater than that in above 1D patterns under the same operating parameters (Fig. 3k&l).

3.4. Sub-spot size writing Despite the relatively large laser spot size (127 µm in this study), it is possible to obtain wrinkle patterns with the width smaller than the spot size. A laser has a typical Gaussian intensity distribution (Fig. 5a). During engraving, multiple laser spots may overlap with each other and make the local dose of a specific point under exposure higher than the dose under a single laser shot (Fig. 5b). It is thus possible to generate a small area in the center of the exposure region where the local dose exceeds that is required for wrinkle generation and the 13 ACS Paragon Plus Environment


width (T) is smaller than the laser spot size (D). For example, a 1D pattern with the design width of 100 µm was exposed to the laser with the power density of ~53.29kW/cm2 (ε=10%, n=40, and θ = 0°). The average width of the resulting wrinkled pattern was ~65 µm, smaller than the designed pattern width and the laser spot size (Fig. 5c). The resulting wrinkle wavelength was 48.09 ± 19.43 µm.

3.5. Writing crack-free wrinkled patterns Cracking is a common issue in surface wrinkling. Cracks usually lie perpendicular to linear wrinkles and may disrupt continuous wrinkle patterns. Cracks add additional topographical features to the wrinkled surface and may interfere with periodical wrinkles. The experiment showed that when θ = 0° and the width of the 1D patterns was close to the minimally patternable width, no cracks occurred (Fig. 3b&c). Similarly, the 2D patterns with the minimally patternable width exhibited no cracks (Fig. 3l). This is due to the fact that the elastic energy stored in the oxidation layer can be released by the free edges before it accumulated to the critical point. As the pattern area increased, however, the free edges may not be able to lower the accumulated elastic energy to a sufficient extent, especially in the center area of the pattern. As a consequence, cracks occurred. With ε=10%, no crack was formed in a 2.5 mm× 2.5 mm pattern (Fig. 6a) with n=8 (Fig. 6b). As the length of the patterned square area increased to 5.5 mm, several cracks formed (Fig. 6c). When n increased to 20, severe cracks were observed even in the pattern square with the length of 3.5 mm (Fig. 6d). In order to pattern a large crack-free wrinkled area, an array of closely spaced 1D 14 ACS Paragon Plus Environment

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patterns was written on the surface (Fig. 6e&i). The length of each 1D rectangle pattern was much larger than its width. The longitudinal edge of each 1D rectangle pattern was parallel to each other. The two neighboring 1D rectangle patterns were separated by a small gap that was not irradiated. The pre-strain was along the longitudinal axis of these 1D rectangle patterns, similar as in Fig. 3a. Upon laser irradiation and strain releasing under the same operating parameters as in Fig. 6d, linear wrinkles formed in the rectangles. For 3500 µm × 400 µm 1D rectangle pattern with the gap of 300 µm (Fig. 6e&f), wrinkles in the neighboring rectangles terminated in the non-oxidized gap, leaving a small non-wrinkled area (

Writing Wrinkles on Polydimethylsiloxane (PDMS) by Surface

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