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Research Papers

Geometric Prepatterning-Based Tuning of the Period Doubling Onset Strain During Thin-Film Wrinkling

[+] Author and Article Information
Sourabh K. Saha

Materials Engineering Division,
Lawrence Livermore National Laboratory,
PO Box 808,
Livermore, CA 94550
e-mail: saha5@llnl.gov

1Corresponding author.

Contributed by the Applied Mechanics Division of ASME for publication in the JOURNAL OF APPLIED MECHANICS. Manuscript received February 17, 2017; final manuscript received March 20, 2017; published online April 5, 2017. Assoc. Editor: Yihui Zhang.The United States Government retains, and by accepting the article for publication, the publisher acknowledges that the United States Government retains, a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for United States government purposes.

J. Appl. Mech 84(5), 051010 (Apr 05, 2017) (10 pages) Paper No: JAM-17-1089; doi: 10.1115/1.4036325 History: Received February 17, 2017; Revised March 20, 2017

Wrinkling of thin films is an easy-to-implement and low-cost technique to fabricate stretch-tunable periodic micro and nanoscale structures. However, the tunability of such structures is often limited by the emergence of an undesirable period-doubled mode at high strains. Predictively tuning the onset strain for period doubling via existing techniques requires one to have extensive knowledge about the nonlinear pattern formation behavior. Herein, a geometric prepatterning-based technique is introduced that can be implemented even with limited system knowledge to predictively delay period doubling. The technique comprises prepatterning the film/base bilayer with a sinusoidal pattern that has the same period as the natural period of the system. This technique has been verified via physical and computational experiments on the polydimethylsiloxane (PDMS)/glass bilayer system. It is observed that the onset strain can be increased from the typical value of 20% for flat films to greater than 30% with a modest prepattern aspect ratio (2·amplitude/period) of 0.15. In addition, finite element simulations reveal that (i) the onset strain increases with increasing prepattern amplitude and (ii) the delaying effect can be captured entirely by the prepattern geometry. Therefore, one can implement this technique even with limited system knowledge, such as material properties or film thickness, by simply replicating pre-existing wrinkled patterns to generate prepatterned bilayers. Thus, geometric prepatterning is a practical scheme to increase the operating range of stretch-tunable wrinkle-based devices by at least 50%.

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Figures

Grahic Jump Location
Fig. 1

Schematic representation of the formation and growth of wrinkles with strain during compression of (a) flat bilayers and (b) prepatterned bilayers. Onset of period doubling is delayed in prepatterned bilayers.

Grahic Jump Location
Fig. 2

Demonstration of delayed onset of period doubling via prepatterning. (a)–(c) Fabricated wrinkles on top of flat and prepatterned bilayers. Finite element modeling of wrinkles on (d) flat and (e) prepatterned bilayers. The prepattern amplitude, strain, and natural period were 168 nm, 0.249, and 2.0 μm for both experiments and simulations. Scale bars are 20 μm long for (a), 5 μm for (b) and (c), and 2.5 μm for (d) and (e).

Grahic Jump Location
Fig. 3

Wrinkle patterns on three different bilayer samples, each with a pair of flat and prepatterned regions. (a)–(c) Period-doubled wrinkles on flat regions and (d)–(f) single-period wrinkles on the corresponding prepatterned regions. λn, εp, and ε for (a) and (d) were 2.3 μm, 0.04, and 0.255; for (b) and (e) were 2.2 μm, 0.07, and 0.249; and for (c) and (f) were 2.3 μm, 0.06, and 0.245. Scale bars are 10 μm long.

Grahic Jump Location
Fig. 4

(a) Finite element modeling of wrinkle formation during compression of a prepatterned bilayer. Color information (green-to-red) represents increasing first principal strain in the system. (b) Growth in amplitude of wrinkles with increasing strain. The prepattern amplitude, natural period, thin-film thickness, and Young’s moduli ratio were 168 nm, 2.0 μm, 42 nm, and 1690.4. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article)

Grahic Jump Location
Fig. 5

Effect of strain on the ratio of the strain energy in the base for two competing wrinkle modes. The smaller period (λ) was 2.25 μm, and the prepattern amplitudes were 0.084 times the periods.

Grahic Jump Location
Fig. 6

Effect of prepattern size on the period doubling onset strain quantified using finite element simulations. Onset strain was evaluated for different natural periods by separately varying film thickness (h) and moduli ratio (η). The natural periods were 1 μm, 2.25 μm, and 4.5 μm. The parameter ho was 47.5 nm and ηo was 1690.4.

Grahic Jump Location
Fig. 7

Increased in amplitude and aspect ratio due to increase in prepattern size. (a) Amplitude versus strain. (b) Aspect ratio versus strain. The natural period was 2.25 μm, the film thickness was 47.5 nm, and the moduli ratio was 1690.4 for the two bilayers.

Grahic Jump Location
Fig. 8

Effect of prepattern size on the maximum aspect ratio of wrinkles. Maximum aspect ratio is the aspect ratio at the period doubling onset strain. Aspect ratio was evaluated for different natural periods by separately varying film thickness (h) and moduli ratio (η). The natural periods were 1 μm, 2.25 μm, and 4.5 μm. The parameter ho was 47.5 nm and ηo was 1690.4.

Grahic Jump Location
Fig. 9

Finite element model of (a) prepatterned bilayer and (b) flat bilayer for the case when the prepattern and natural periods are not identical. The natural period of the bilayers was 2.25 μm, and the prepattern period and amplitude were 3 μm and 36 nm. The onset strain for the flat bilayer is 0.19, whereas the onset strain for the prepatterned bilayer is 0.225. The strain in the two bilayers is 0.205. Color information (green-to-red) represents increasing first principal strain in the system. Scale bars are 2 μm long. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article)

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