IIE Lunchtime Seminar - Dr. Nima Rahbar (Worcester Polytechnic Institute)

Abstract:

This talk focuses on the fundamental ideas arising from understanding the mechanisms behind the superior mechanical and thermal properties of biological materials through four specific examples of nacre, bamboo, cartildge, teeth, and lipid bilayers.

The mechanical behavior and toughening mechanisms of abalone nacre-inspired multilayered materials are explored. In nacre's structure, the organic matrix, pillars and the roughness of the aragonite platelets play important roles in its overall mechanical performance. A micromechanical model for multilayered biological materials is proposed to simulate their mechanical deformation and toughening mechanisms. The modeling results are in excellent agreement with the available experimental data for abalone nacre. The highly nonlinear behavior of the proposed multilayered material is the result of distributed deformation in the nacre-like structure due to the existence of nano-asperities and nano-pillars with near theoretical strength. Finally, tensile toughness is studied as a function of the components in the microstructure of nacre. Bamboo, a fast-growing grass, has higher strength-to-weight ratios than steel and concrete. The unique properties of bamboo come from the natural composite structure of fibers that comprises mainly cellulose nanofibrils in a matrix of intertwined hemicellulose and lignin called lignin-carbohydrate complex (LCC). Here we have experimentally and numerically studied mechanical and fracture properties of bamboo at multiple scale. We have utilized atomistic simulations to investigate the mechanical properties and mechanisms of the interactions of these materials in the structure of bamboo fibers. It is shown that a control hemicellulose model has better thermodynamic and mechanical properties than lignin while lignin exhibits greater tendency to adhere to cellulose nanofibril. Therefore, the role of hemicellulose found to be enhancing the mechanical properties while lignin provides the strength of bamboo fibers. Lastly, given the amphiphilic nature and chemical structure, phospholipids exhibit a strong thermotropic and lyotropic phase behavior in an aqueous environment. We performed non-equilibrium molecular dynamics simulations for a range of different temperature gradients. The results show that the thermal properties of the DPPC bilayer are highly dependent on the temperature gradient. Higher temperature gradients cause an increase in the thermal conductivity of the DPPC lipid bilayer. We also found that the thermal conductivity of DPPC is lowest at the transition temperature whereby one lipid leaflet is in the gel phase and the other is in the liquid crystalline phase. This is essentially related to a growth in thermal resistance between the two leaflets of lipid at the transition temperature. These results provide significant new insights into developing new thermal insulation for engineering applications.

Bio:

Education:

B.S. Civil Engineering, Sharif University of Technology, Tehran, Iran – 1998

M.S. Civil Engineering, Northeastern University, Boston, MA - 2003

Ph.D. Civil Engineering, Princeton University, Princeton, NJ - 2008

Dr. Rahbar joined WPI in August 2012 as an Assistant Professor. His research interests are in the area of bioinspired design of materials with an emphasis on mechanical and thermal properties across multiple scales. Dr. Rahbar has also won several awards including TMS Young Leader’s award in 2013, Air Force Summer Faculty Fellowship Award in 2013 and NSF CAREER award in 2012. Dr. Rahbar recently received the consecutive 2015 and 2016 Outstanding Reviewer award form the ASCE Journal of Nanomechanics and Micromechanics. His graduate students have also received multiple best poster awards in national and international meetings. Currently, he is the chair of Biomaterials committee of TMS and Nano mechanics committee of EMI. 

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