Deepika Saini

Direct measurement of shear properties of microfibers.

As novel fibers with enhanced mechanical properties continue to be synthesized and developed, the ability to easily and accurately characterize these materials becomes increasingly important. Here we present a design for an inexpensive tabletop instrument to measure shear modulus (G) and other longitudinal shear properties of a micrometer-sized monofilament fiber sample, such as nonlinearities and hysteresis. This automated system applies twist to the sample and measures the resulting torque using a sensitive optical detector that tracks a torsion reference. The accuracy of the instrument was verified by measuring G for high purity copper and tungsten fibers, for which G is well known. Two industrially important fibers, IM7 carbon fiber and Kevlar(®) 119, were also characterized with this system and were found to have G = 16.5 ± 2.1 and 2.42 ± 0.32 GPa, respectively.

Impact absorption properties of carbon fiber reinforced bucky sponges

We describe the super compressible and highly recoverable response of bucky sponges as they are struck by a heavy flat-punch striker. The bucky sponges studied here are structurally stable, self-assembled mixtures of multiwalled carbon nanotubes (MWCNTs) and carbon fibers (CFs). We engineered the microstructure of the sponges by controlling their porosity using different CF contents. Their mechanical properties and energy dissipation characteristics during impact loading are presented as a function of their composition. The inclusion of CFs improves the impact force damping by up to 50% and the specific damping capacity by up to 7% compared to bucky sponges without CFs. The sponges also exhibit significantly better stress mitigation characteristics compared to vertically aligned CNT foams of similar densities. We show that delamination occurs at the MWCNT-CF interfaces during unloading, and it arises from the heterogeneous fibrous microstructure of the bucky sponges.

Photoresponse of a Single Y-Junction Carbon Nanotube.

We report investigation of optical response in a single strand of a branched carbon nanotube (CNT), a Y-junction CNT composed of multiwalled CNTs. The experiment was performed by connecting a pair of branches while grounding the remaining one. Of the three branch combinations, only one combination is optically active which also shows a nonlinear semiconductor-like I-V curve, while the other two branch combinations are optically inactive and show linear ohmic I-V curves. The photoresponse includes a zero-bias photocurrent from the active branch combination. Responsivity of ≈1.6 mA/W has been observed from a single Y-CNT at a moderate bias of 150 mV with an illumination of wavelength 488 nm. The photoresponse experiment allows us to understand the nature of internal connections in the Y-CNT. Analysis of data locates the region of photoactivity at the junction of only two branches and only the combination of these two branches (and not individual branches) exhibits photoresponse upon illumination. A model calculation based on back-to-back Schottky-type junctions at the branch connection explains the I-V data in the dark and shows that under illumination the barriers at the contacts become lowered due to the presence of photogenerated carriers.

Harmonic detection of resonance method

Electromechanical resonators in the micro (MEMS) and nano (NEMS) regimes have emerged as promising tools for use in diverse applications such as ultrasensitive physical, chemical, and biological sensors, with detection limits as low as a single molecule. The advent of state-of-the-art micro-fabrication techniques has enabled a high throughput platform for commercialization. However, the sensitivity and reliability of such devices are highly dependent on the employed detection technique. We present a highly useful yet simple electrical detection scheme: the Harmonic Detection of Resonance (HDR) method. The prominent HDR features will be discussed and applications ranging from the use of micro-cantilevers as sensors to probing mechanical properties in nano-cantilever systems will be presented.

Ringdown sensing method

Miniaturization of devices into lab-on-chip designs is a dominating field of current scientific research. While the technology to build these devices is continuing to develop, the practical realization of such devices remain elusive, mainly due to lack of techniques that bridge the macroscopic world to the mechanical motion and/or the electronic signals generated on the micro- or nano-sized scale. Hence, there is an increasing need for sensitive detection techniques that can not only be implemented on such small scale systems but also be integrated with the current CMOS technology. In this regard, a fully electrical microcantilever-based ringdown method will be presented. We show that detection technique can be employed to precisely analyze the composition of gas mixtures. The viscosity and density can be measured simultaneously, which is illustrated for multiple gases yielding viscosities within ± 2% and densities within ± 6% of NIST values.