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Research Papers

Design of a Compact Biaxial Tensile Stage for Fabrication and Tuning of Complex Micro- and Nano-scale Wrinkle Patterns

[+] Author and Article Information
Sourabh K. Saha

Laboratory for Manufacturing and Productivity,
Department of Mechanical Engineering,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: sourabh@alum.mit.edu

Martin L. Culpepper

Laboratory for Manufacturing and Productivity,
Department of Mechanical Engineering,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: culpepper@mit.edu

1Present address: Materials Engineering Division, Lawrence Livermore National Laboratory, P.O. Box 808, L-781, Livermore, CA 94551.

2Corresponding author.

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MICRO- AND NANO-MANUFACTURING. Manuscript received December 23, 2014; final manuscript received August 18, 2015; published online September 14, 2015. Assoc. Editor: Stefan Dimov.

J. Micro Nano-Manuf 3(4), 041004 (Sep 14, 2015) (11 pages) Paper No: JMNM-14-1077; doi: 10.1115/1.4031382 History: Received December 23, 2014; Revised August 18, 2015

Wrinkling of thin films is a strain-driven process that enables scalable and low-cost fabrication of periodic micro- and nano-scale patterns. In the past, single-period sinusoidal wrinkles have been applied for thin-film metrology and microfluidics applications. However, real-world adoption of this process beyond these specific applications is limited by the inability to predictively fabricate a variety of complex functional patterns. This is primarily due to the inability of current tools and techniques to provide the means for applying large, accurate, and nonequal biaxial strains. For example, the existing biaxial tensile stages are inappropriate because they are too large to fit within the vacuum chambers that are required for thin-film deposition/growth during wrinkling. Herein, we have designed a compact biaxial tensile stage that enables (i) applying large and accurate strains to elastomeric films and (ii) in situ visualization of wrinkle formation. This stage enables one to stretch a 37.5 mm long film by 33.5% with a strain resolution of 0.027% and maintains a registration accuracy of 7 μm over repeated registrations of the stage to a custom-assembled vision system. Herein, we also demonstrate the utility of the stage in (i) studying the wrinkling process and (ii) fabricating complex wrinkled patterns that are inaccessible via other techniques. Specifically, we demonstrate that (i) spatial nonuniformity in the patterns is limited to 6.5%, (ii) one-dimensional (1D) single-period wrinkles of nominal period 2.3 μm transition into the period-doubled mode when the compressive strain due to prestretch release of plasma-oxidized polydimethylsiloxane (PDMS) film exceeds ∼18%, and (iii) asymmetric two-dimensional (2D) wrinkles can be fabricated by tuning the strain state and/or the actuation path, i.e., the strain history. Thus, this tensile stage opens up the design space for fabricating and tuning complex wrinkled patterns and enables extracting empirical process knowledge via in situ visualization of wrinkle formation.

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Figures

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Fig. 1

Schematic of wrinkle formation via compression of a bilayer film

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Fig. 2

Sequence of process steps and the equipment used in fabrication of wrinkle patterns

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Fig. 3

Photograph of the designed system that consists of the tensile stage and the microscope stage. Holes on the table are on a 25.4 mm square spacing.

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Fig. 4

Fixtures for clamping the PDMS films onto the tensile stage. Dashed arrows indicate the degree-of-freedom for clamp bottoms.

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Fig. 5

Solid model rendering of the designed motion stage. Dashed arrows indicate the degree-of-freedom for the platforms and the clamps.

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Fig. 6

Stick figure model depicting the layout of constraints for a single stretch axis. Circles represent rigid connectors. All straight lines, except the PDMS film, represent rigid members. The actuator is capable of only pushing onto the moving platform; it cannot pull the moving platform.

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Fig. 9

Spatial variation in the period of wrinkles: (a) along the actuated direction and (b) orthogonal to the actuated direction. Error bars on period quantify the standard deviation of measurements over a single image frame that spans 163 μm × 122 μm.

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Fig. 8

Repeatability of registration between the stage and the vision system during successive engagement–disengagement cycles

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Fig. 7

Accuracy of mechanical stretching as quantified by the observed displacement of the stage versus the actuated displacement of the micrometer head for the (a) X axis and (b) Y axis. Insets are zoomed-out views close to the zero stretch point that illustrate the presence of residual stretch due to the stress relaxation effect.

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Fig. 10

(a) Effect of stretch release on the period of 1D wrinkles as measured by tracking a material point on a plasma-oxidized and uniaxially stretched PDMS film. (b) Close-up optical images of the defect zone that was tracked during experiments; scale bar is 15 μm long.

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Fig. 11

Emergence of period-doubled mode at high compressive strains during stretch release of a plasma-oxidized and uniaxially stretched PDMS film. The film was stretched by 20.9% prior to plasma oxidation. Estimate is based on the approximation of constant number of wrinkles. Inset at the higher stretch-release is an AFM profile scan; inset at the lower stretch-release is an optical image, scale bar is 15 μm long.

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Fig. 12

Tuning of 2D wrinkle modes via biaxial strain state and strain history. (a) Strain states and history of strain. (b) Wrinkle mode shapes observed on a PDMS/titanium bilayer. Inset alphanumeric labels in the photographs correspond to labeled strain states along the actuation path. Scale bars are 30 μm long.

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