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.