Researcher:
Aksoy, Bekir

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Master Student

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Bekir

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Aksoy

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Aksoy, Bekir

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2017 - 20182

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Researcher Of Publications: search.filters.isAuthorOfPublication.854038bf-a5af-4121-af43-53c247172920

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    High-resolution spatiotemporal strain mapping reveals non-uniform deformation in micropatterned elastomers
    (Iop Publishing Ltd, 2017) N/A; N/A; Department of Chemistry; Department of Mechanical Engineering; Aksoy, Bekir; Rehman, Ateeq Ur; Bayraktar, Halil; Alaca, Burhanettin Erdem; Master Student; PhD Student; Faculty Member; Faculty Member; Department of Chemistry; Department of Mechanical Engineering; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Sciences; College of Engineering; N/A; N/A; 201764; 115108
    Micropatterns are generated on a vast selection of polymeric substrates for various applications ranging from stretchable electronics to cellular mechanobiological systems. When these patterned substrates are exposed to external loading, strain field is primarily affected by the presence of microfabricated structures and similarly by fabrication-related defects. The capturing of such nonhomogeneous strain fields is of utmost importance in cases where study of the mechanical behavior with a high spatial resolution is necessary. Image-based non-contact strain measurement techniques are favorable and have recently been extended to scanning tunneling microscope and scanning electron microscope images for the characterization of mechanical properties of metallic materials, e.g. steel and aluminum, at the microscale. A similar real-time analysis of strain heterogeneity in elastomers is yet to be achieved during the entire loading sequence. The available measurement methods for polymeric materials mostly depend on cross-head displacement or precalibrated strain values. Thus, they suffer either from the lack of any real-time analysis, spatiotemporal distribution or high resolution in addition to a combination of these factors. In this work, these challenges are addressed by integrating a tensile stretcher with an inverted optical microscope and developing a subpixel particle tracking algorithm. As a proof of concept, the patterns with a critical dimension of 200 mu m are generated on polydimethylsiloxane substrates and strain distribution in the vicinity of the patterns is captured with a high spatiotemporal resolution. In the field of strain measurement, there is always a tradeoff between minimum measurable strain value and spatial resolution. Current noncontact techniques on elastomers can deliver a strain resolution of 0.001% over a minimum length of 5 cm. More importantly, inhomogeneities within this quite large region cannot be captured. The proposed technique can overcome this challenge and provides a displacement measurement resolution of 116 nm and a strain resolution of 0.04% over a gage length of 300 mu m. Similarly, the ability to capture inhomogeneities is demonstrated by mapping strain around a thru-hole. The robustness of the technique is also evaluated, where no appreciable change in strain measurement is observed despite the significant variations imposed on the measurement mesh. The proposed approach introduces critical improvements for the determination of displacement and strain gradients in elastomers regarding the real-time nature of strain mapping with a microscale spatial resolution.
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    Poisson's ratio of PDMS thin films
    (Elsevier Sci Ltd, 2018) Bayraktar, Halil; N/A; Department of Mechanical Engineering; Department of Mechanical Engineering; TRUE, Sedat; Aksoy, Bekir; Alaca, Burhanettin Erdem; Master Student; Master Student; Faculty Member; Department of Mechanical Engineering; N/A; N/A; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 115108
    Despite the fact that a small uncertainty in PDMS Poisson's ratio leads to significant errors in traction force microscopy, there is a clear lack of data for PDMS films at the scale of 100 mu m, a relevant size scale frequently employed in cell mechanics studies. Equally important is the need for consideration of the viscoelastic nature of PDMS, as no mechanical property - including Poisson's ratio - can be taken as a time-independent constant. The foremost challenge for addressing these issues is the difficulty of carrying out stress relaxation tests on miniature PDMS samples accompanied by non-contact strain measurement with a very high spatiotemporal resolution. This study introduces such a stress relaxation platform incorporating i) the proper means for the application of necessary boundary conditions, ii) a high-precision in load measurement, and iii) a non-contact, local strain measurement technique based on single particle tracking. During stretching, images were recorded at a rate of 18 Hz with a 40 mu m spatial resolution. Microsphere-embedded PDMS films as thin as 125 and 155 mu m were prepared to study the Poisson's ratio by a local strain microscope. After tracing the displacement of microspheres by a single particle tracking method and using a strain mapping, Poisson's ratio for 155-mu m-thick PDMS was found to decrease from 0.483 +/- 0.034 to 0.473 +/- 0.040 over a period of 20 min. For 125-mu m-thick PDMS, this reduction took place from 0.482 +/- 0.041 to 0.468 +/- 0.038. Moreover, a non-monotonic reduction was observed in both cases. This negative correlation between Poisson's ratio and relaxation time was found to be statistically significant for both thicknesses with p < 0.001. The viscoelastic behavior was further characterized through the Burgers model. With a measurement field of 597 x 550 mu m(2), this study emphasizes the importance of the local investigation of mechanical properties. Furthermore, the dependence of transverse strain on a film thickness difference of 30 mu m was measured to determine the sensitivity of local strain tracking. The inherent high resolution of the proposed approach enables one to measure deformations more precisely and to observe the temporal evolution of the Poisson's ratio that has not been observed before. In addition to the high-precision determination of PDMS Poisson's ratio, this work also offers a promising pathway for the accurate and time dependent determination of the mechanical properties of other soft materials, where similar ambiguities exist regarding the mechanical behavior.