Conference Programme

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I-03: Hierarchical, Biomimetic Materials
Tuesday, 20/Jun/2017:
10:30am - 12:00pm

Session Chair: Mark Jhon, Institute of High Performance Computing, A*STAR
Session Chair: Konstantin Volokh, Technion - Israel Institute of Technology
Location: Rm 305

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10:30am - 11:00am

Understanding the Strength of Soft Bioinspired Composites

Viacheslav SLEASARENKO1, Konstantin VOLOKH1, Jacob ABOUDI2, Stephan RUDYKH1

1Technion - Israel Institute of Technology, Israel; 2Tel Aviv University, Israel

Remarkable mechanical properties of biocomposites (bone, teeth, shell, antler etc.) are usually attributed to their special design where staggered mineral platelets are embedded in a protein matrix. Because of the platelet high aspect ratio the soft protein deforms in the shear mode predominantly providing the linkage for the hard inclusions. Mimicking Nature one might design materials with a similar architecture.

By employing a micromechanical analysis, we study in the present work the strength of a bio-inspired composite in which hard platelets are embedded in a soft matrix made of the vulcanized natural rubber. We perform simulations of uniaxial tension of the composite material based on a continuum mechanics formulation and the high-fidelity generalized method of cells. The use of the energy limiters in the constitutive model for rubber at finite strains allows us to model failure and arrive at the overall strength of the composite.

We find that the overall strength of the composite depends on the deformation and failure of soft matrix in tension and shear. Moreover, we find that the strength of the composite cannot exceed the strength of the matrix. The latter observation is noteworthy because it is qualitatively different from the experimental results with biocomposites which show a dramatic (ten times) increase of the strength of the material as compared to the strengths of its constituents. We illustrate these analytical and numerical findings by our experiments on 3D printed composite materials.

11:00am - 11:15am

Mechanical Behavior of Biomimetic Nanocrystalline TiO2/polyelectrolyte Nanolayered Composites

Bin ZHANG, Yu-Jia YANG, Hai-Feng TAN

Northeastern University, China

Biological materials with hierarchically laminated structures usually exhibit a good synergy between strength and fracture toughness. How to understand relationship between microstructures and mechanical properties of biomimetic hierarchical composites is quite important for the development of high-performance biomimetic materials. In the presentation, we will show experimental research on the mechanical behavior of biomimetic (TiO2/ polyelectrolyte (PE) )4 nanolayered composites consisted of nanocrystalline TiO2 and amorphous PE layers, which are prepared successfully by layer-by-layer (LBL) self-assembly and chemical bath deposition methods, alternately. For the PE layer, LBL method was used in the sequence of PEI/PSS/(PAH/PSS)2, respectively.Microstructures of the nanolayered composite were investigated by scanning electron microscopy, secondary ion mass spectroscopy, and high-resolution transmission microscopy. Mechanical performance of the composite was characterized by instrumented indentation method. The fracture toughness for the type I cracking of (TiO2/ polyelectrolyte (PE) )4 composite was determined, which is evidently larger than that of the bulk TiO2. The fracture behaviors of the composite for the type I and II cracking were characterized by TEM. The toughening behavior found in the present biomimetic nanocomposites was attributed to the low interfacial fracture energy and the large ratio of the crack deflection length to the crack penetration length. Furthermore, fatigue properties of the biomimetic nanolayered composites were examined. Our finding demonstrates that the crack deflection is an effective way to toughen the biomimetic nanocrystalline TiO2/PE nanolayered composites.

11:15am - 11:30am

Fabrication of Hierarchical Fibers for Dry Adhesive Applications

Avinash BAJI, Rahul SAHAY

Singapore University of Technology and Design, Singapore

This study use electrospinning combined with template wetting method to prepare hierarchical fibrous structures and shows its use for dry-adhesive applications. The electrospinning combined with template wetting method enabled us to fabricate nanopillars on the surface of micron sized fibres. These samples are adhered on a glass slide and pulled in shear mode to investigate their shear adhesion behaviour. For comparison, neat fibers with no surface substructures are also fabricated. A constant preload was applied on the samples during the measurement. The obtained force vs. displacement curves were then used to quantify the adhesion strength of the samples. Our results show that the shear adhesion strength of the hierarchical fibers is higher that the shear adhesion strength recorded for the neat fibers. Additionally, the shear adhesion strength can be improved when the aspect ratio of the nanopillars is increased. Adhesion strength as high as 0.161 N/cm2 was obtained for the hierarchical samples. Our results show the potential of using these samples for dry adhesive applications.

11:30am - 11:45am

Pull-off Force of Individual Electrospun Fibers through Novel Picoindenter/Scanning Electron Microscopy Technique

Rahul SAHAY, Ihor RADCHENKO, Hashina Parveen ANWAR ALI, Arief BUDIMAN, Avinash BAJI

Singapore University of Technology and Design, Singapore

We employed a novel picoindenter (PI)/scanning electron microscopy (SEM) technique to measure the pull-off force of individual electrospun Poly(vinylidene fluoride) (PVDF) fibers. Individual electrospun fibers were deposited over a channel in the custom designed silicon substrate, which is then attached to picoindenter. The picoindenter is then positioned firmly on the sample stage of the SEM. The picoindenter tip laterally pulls individual fibers to measure the force required to detach it from the surface of substrate. SEM was used to visualize and document the process. A mathematical model was developed to fit the experimentally obtained load vs. displacement curves to estimate pull-off force of individual PVDF fibers with respect to the silicon substrate. The pull-off force was estimated as high as ~17.8 ± 0.2 µN for individual electrospun PVDF fibers with average diameter as ~1.0 µm.

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