Mechanical Engineering MS Thesis Defense by Sedat Doğru

KOÇ UNIVERSITY

GRADUATE SCHOOL OF SCIENCES & ENGINEERING

MECHANICAL ENGINEERING

MS THESIS DEFENSE BY SEDAT DOĞRU

Title: Viscoelastic Characterization of PDMS Thin Films

Speaker: Sedat Doğru

Time: June 7, 2018, 13.00

Place: ENG 208

Koç University

Rumeli Feneri Yolu

Sariyer, Istanbul

Thesis Committee Members:

Assoc. Prof.. B. Erdem Alaca (Advisor, Koç University)

Assoc. Prof. C. Can Aydıner (Boğaziçi University)

Asst. Prof. Halil Bayraktar (Istanbul Technical University)

Abstract:

Despite the fact that a small uncertainty in the 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 μm, a relevant size scale frequently employed in cell mechanics studies. Equally

important is the need for the consideration of the viscoelastic nature of PDMS, as no

mechanical property–including the 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 are

recorded at a rate of 18 Hz with a 40 μm spatial resolution. Microsphere-embedded PDMS

films as thin as 125 and 155 μm are 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-μm-thick PDMS is measured to

decrease from 0.483±0.034 to 0.473±0.040 over a period of 20 mins. For 125-μm-thick

PDMS, this reduction takes place from 0.482±0.041 to 0.468±0.038. Moreover, a

non-monotonic reduction is observed in both cases. This negative correlation between

Poisson’s ratio and relaxation time is found to be statistically significant for both thicknesses

with p<0.001. The viscoelastic behavior is further characterized through the Burgers model.

With a measurement field of 597×550 μm2 , 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 μm is 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. The technique can also be used to establish a link

between the conditions PDMS is exposed to during cell growth process and its mechanical

properties.