Jagannathan Rajagopalan

Plastic Deformation Recovery in Freestanding Nanocrystalline Aluminum and Gold Thin Films

In nanocrystalline metals, lack of intragranular dislocation sources leads to plastic deformation mechanisms that substantially differ from those in coarse-grained metals. However, irrespective of grain size, plastic deformation is considered irrecoverable. We show experimentally that plastically deformed nanocrystalline aluminum and gold films with grain sizes of 65 nanometers and 50 nanometers, respectively, recovered a substantial fraction (50 to 100%) of plastic strain after unloading. This recovery was time dependent and was expedited at higher temperatures. Furthermore, the stress-strain characteristics during the next loading remained almost unchanged when strain recovery was complete. These observations in two dissimilar face-centered cubic metals suggest that strain recovery might be characteristic of other metals with similar grain sizes and crystalline packing.

A self-propelled biohybrid swimmer at low Reynolds number

Many microorganisms, including spermatozoa and forms of bacteria, oscillate or twist a hair-like flagella to swim. At this small scale, where locomotion is challenged by large viscous drag, organisms must generate time-irreversible deformations of their flagella to produce thrust. To date, there is no demonstration of a self propelled, synthetic flagellar swimmer operating at low Reynolds number. Here we report a microscale, biohybrid swimmer enabled by a unique fabrication process and a supporting slender-body hydrodynamics model. The swimmer consists of a polydimethylsiloxane filament with a short, rigid head and a long, slender tail on which cardiomyocytes are selectively cultured. The cardiomyocytes contract and deform the filament to propel the swimmer at 5-10 μm s(-1), consistent with model predictions. We then demonstrate a two-tailed swimmer swimming at 81 μm s(-1). This small-scale, elementary biohybrid swimmer can serve as a platform for more complex biological machines.

A phase reconstruction algorithm for Lamb wave based structural health monitoring of anisotropic multilayered composite plates

Platelike structures, made of composites, are being increasingly used for fabricating aircraft wings and other aircraft substructures. Continuous monitoring of the health of these structures would aid the reliable operation of aircrafts. This paper considers the use of a Lamb wave based structural health monitoring (SHM) system to identify and locate defects in large multilayered composite plates. The SHM system comprises of a single transmitter and multiple receivers, coupled to one side of the plate that send and receive Lamb waves. The proposed algorithm processes the data collected from the receivers and generates a reconstructed image of the material state of the composite plate. The algorithm is based on phased addition in the frequency domain to compensate for the dispersion of Lamb waves. In addition, small deviations from circularity of the slowness curves of Lamb wave modes, due to anisotropy, are corrected for by assuming that the phase and group velocity directions coincide locally. Experiment...

MEMS sensors and microsystems for cell mechanobiology

Forces generated by cells play a vital role in many cellular processes like cell spreading, motility, differentiation and apoptosis. Understanding the mechanics of single cells is essential to delineate the link between cellular force generation/sensing and function. MEMS sensors, because of their small size and fine force/displacement resolution, are ideal for force and displacement sensing at the single cell level. In addition, the amenability of MEMS sensors to batch fabrication methods allows the study of large cell populations simultaneously, leading to robust statistical studies. In this review, we discuss various microsystems used for studying cell mechanics and the insights on cell mechanical behavior that have resulted from their use. The advantages and limitations of these microsystems for biological studies are also outlined.

Linear High-Resolution BioMEMS Force Sensors With Large Measurement Range

We present a set of displacement-based high-resolution (50 pN) micromechanical force sensors with a large force measurement range (1 μN). Typically, force sensors that have high resolution have a limited force measurement range and vice versa. The force sensors presented here overcome this limitation and, in addition, have a highly linear force-displacement response. The sensors (≈3 mm × 4 mm × 150 m)are composed of a series of flexible beams attached to a rigid probe that deform when subjected to an external force. The force is obtained by optically measuring the displacement of the probe with respect to a fixed reference beam. The force sensors are fabricated using a simple two-mask process that allows for their stiffness to be varied over a wide range. Furthermore, we have developed a novel scheme to avoid capillary forces during the immersion and removal of these sensors from aqueous environments, which makes them highly suited for biological studies. We illustrate the capability and versatility of these sensors by measuring the in vivo force-deformation response of axons in Drosophila melanogaster (fruit fly).

Electron Beam Induced Artifacts During in situ TEM Deformation of Nanostructured Metals.

A critical assumption underlying in situ transmission electron microscopy studies is that the electron beam (e-beam) exposure does not fundamentally alter the intrinsic deformation behavior of the materials being probed. Here, we show that e-beam exposure causes increased dislocation activation and marked stress relaxation in aluminum and gold films spanning a range of thicknesses (80–400 nanometers) and grain sizes (50–220 nanometers). Furthermore, the e-beam induces anomalous sample necking, which unusually depends more on the e-beam diameter than intensity. Notably, the stress relaxation in both aluminum and gold occurs at beam energies well below their damage thresholds. More remarkably, the stress relaxation and/or sample necking is significantly more pronounced at lower accelerating voltages (120 kV versus 200 kV) in both the metals. These observations in aluminum and gold, two metals with highly dissimilar atomic weights and properties, indicate that e-beam exposure can cause anomalous behavior in a broad spectrum of nanostructured materials, and simultaneously suggest a strategy to minimize such artifacts.

Fabrication of Freestanding 1-D PDMS Microstructures Using Capillary Micromolding

A method to create freestanding, biocompatible polydimethylsiloxane (PDMS) microstructures is presented. First, capillary flow through micro channels on silicon (Si) substrates is used to create high fidelity PDMS structures that are a few μm wide and deep but several mm long (length to width/depth ≈ 500:1). Next, an improvised procedure is employed to remove the cured PDMS microstructures from the Si substrate without damaging them. The method is used to create extremely sensitive cantilever beams with stiffness less than 0.1 pN/μm, and micro platforms for cell biology studies. The PDMS microstructures created using this method have applications in cell mechanobiology as force and mass sensors.

Local, atomic-level elastic strain measurements of metallic glass thin films by electron diffraction.

A novel technique is used to measure the atomic-level elastic strain tensor of amorphous materials by tracking geometric changes of the first diffuse ring of selected area electron diffraction patterns (SAD). An automatic procedure, which includes locating the centre and fitting an ellipse to the diffuse ring with sub-pixel precision is developed for extracting the 2-dimensional strain tensor from the SAD patterns. Using this technique, atomic-level principal strains from micrometre-sized regions of freestanding amorphous Ti0.45Al0.55 thin films were measured during in-situ TEM tensile deformation. The thin films were deformed using MEMS based testing stages that allow simultaneous measurement of the macroscopic stress and strain. The calculated atomic-level principal strains show a linear dependence on the applied stress, and good correspondence with the measured macroscopic strains. The calculated Poissons ratio of 0.23 is reasonable for brittle metallic glasses. The technique yields a strain accuracy of about 1×10(-4) and shows the potential to obtain localized strain profiles/maps of amorphous thin film samples.

MEMS-based testing stage to study electrical and mechanical properties of nanocrystalline metal films

We have developed a MEMS-based testing stage that can quantitatively characterize both the electrical and mechanical properties of nanocrystalline metal films. This stage, which is SEM and TEM compatible, is a modified version of an earlier MEMS-based tensile testing stage (M. A. Haque and M. T. A. Saif, Proc. Natl. Acad. Sci., 101(17), 6335-6340 (2004)). This modified stage requires a simpler fabrication procedure, involving fewer lithography and etching steps, and has higher yield compared to the earlier version. It allows for 4-point electrical resistivity measurement, and in-situ tensile testing in SEM and TEM of free-standing nano-scale metal films. The stage was used to perform a tensile test on a 100-nm-thick aluminum film and electrical resistivity measurement on a 110-nm-thick aluminum film, the results of which are described.

In Situ TEM Straining of Ultrafine-grained Aluminum Films of Different Textures Using Automated Crystal Orientation Mapping.

Several studies have shown that metal films with similar thickness and grain size but dissimilar texture show significant differences in their mechanical behavior. For instance, ultrafine-grained (UFG) Al films with no preferred texture show lower flow stress and more pronounced nonlinear behavior during unloading compared to films with a bicrystalline microstructure. [1] [2] However, no systematic study has been focused on understanding the deformation mechanisms responsible for such differences in mechancial behavior.