Research Interests:

At the fundamental level, we are interested in the multi-physics of nanoscale (nanotubes and nanowires) and biological (cellular, extra and subcellular) materials. The dimensional and micro-structural constraints of these materials impose pronounced inter-play of multi-fields. Therefore it is more relevant to study them in coupled domain (for example electro-mechanical or chemo-mechanical) rather than traditional individual domains. Perhaps the best example comes from the cells - they are so sensitive to electrical, mechanical, themal and bio-chemical stimuli, that single domain studies are less appealing.

At the applied level, we are interested in making micro and nanoscale devices. These are sensors and actuators that involve strong multi-disciplinary design, involving solid/fluid mechanics, heat transfer and other mechanical engineering phenomena. Our main interest in developing sensors and actuators is to perform experiments at the smaller scales otherwise difficult or not possible.

The theme of our research group is "smearing all the boundaries". To justify it, we often spend time on diverse application areas. Examples are, man-machine dynamics (human vigilance monitoring) and bio-medical instrumentation (development of a minimally invasive self-propelled colonoscope in collaboration with Dr. Meah). Our core tools are solid mechanics, micro/nano device design, nanofabrication and curiosity.

Sample Projects:

NSF ECS #0545683: CAREER: In-situ Monitoring of Opto-electro-mechanical Responses of Single Cells to External Stimuli using MEMS

The objective of this research is to investigate how individual biological cells react to mechanical (coupled normal and shear forces) and bio-chemical (pH level and epsilon toxin) stimuli. The approach is to monitor their mechanical, electrical and bio-chemical responses, in-situ, by combining fluorescence/confocal microscopy with novel micro-electro-mechanical testing devices. The simultaneously qualitative (direct visualization) and quantitative (real-time measurement of force, displacement and bio-impedance) data on the mechanical and chemical assays will enable research in two unexplored areas: (i) cellular structural and adhesion mechanics under more realistic, combined normal and shear loading and (ii) relationship between cell health and its electrical impedance.

This proposal will provide fundamental understanding of multi-physics of cellular stimuli coupling and transduction. Relating cell health to its electrical impedance will result in automated and label-less electrical detection (electrical pathology) that may replace conventional manual labeling and microscopy-based schemes for faster diagnostics and drug discovery applications. The proposed research will impact the society by integrating multi-disciplinary research with education at all levels while promoting diversity. The accompanying educational plan is to (a) provide research experience to undergraduates in collaboration with the Penn State Multicultural Engineering Program (b) bring electron microscopy in local elementary, middle and high-school students through an outreach program . Microscopy for K-12 and (c) conduct summer workshop for local school K-12 teachers. Schools throughout the nation will be benefited from the outreach website with the streaming videos of these outreach sessions and associated multi-media materials.

NSF ECS #0501436: Thermo-Mechanical Effects on Electrical Transport in Carbon Nanotubes

Carbon nanotubes (mono/multi-layers of carbon atoms rolled into seamless tubes) are known to have superior electrical, mechanical, thermal and chemical properties. Interestingly, the same reasons behind their superiority also make them very sensitive to electrical, mechanical, thermal and chemical fields. Practical applications, as in electronics, sensors and actuators, composites and bio-medical, are likely to involve nanotubes that are mechanically strained and chemically treated during fabrication - and not the pristine ones. The very high power density of such ultra-miniaturized devices will cause higher operating temperatures (observed even in the existing computer chips), which will drastically alter the transport properties of the highly confined nanotube electrons. Studies involving the sensitivity of electrical properties under mechanical or thermal fields (such as tuning electrical properties with mechanical displacement or vice versa) have been mostly theoretical. Experimental studies focusing on the simultaneous effects of temperature (>300 K) and mechanical force-displacement on the electrical properties of the nanotubes are rare in the literature, an observation that motivates this research proposal.

The specific aims of this research are, (i) Development of a micro-electro-mechanical characterization instrument with co-fabricated freestanding single carbon nanotube specimens. The 1mm x 1mm size device will be compatible with any type of microscopy (Optical/SEM/TEM/STM). It will measure force and displacement with 20 pico-Newton and 5 nm resolutions respectively, using an optical microscope. The mechanical sensors (electrically isolated and metallized silicon micro-beams) will also work as electrical connectors to measure current-voltage signals. (ii) Study the effects of elevated temperature on the mechanical properties (Young’s modulus, fracture stress and strain) of individual carbon nanotubes, in-situ inside the transmission electron microscope. (iii) Study the electrical properties of carbon nanotubes for a wide range of temperature (300-500K) and mechanical strain (up to 30%), for which data is not yet available in the literature.

The instrument will allow in-situ atomic resolution experiments on individual carbon nanotubes for simultaneous qualitative (direct visual information on deformation, defect generation and failure of the nanotubes) and quantitative information. The wealth of data on the separate and coupled effects of thermal, mechanical and chemical environmental factors will help researchers gaining insight to the coupling of these fields.

NSF CMS # 0555420 Nano-mechanics of Carbon Nanotube-Polymer Interfaces

The objectives of this research are to experimentally study and model deformation and failure of nanoscale interfaces using nanowires and nanotubes as reinforcing agents. To meet these objectives, nanoscale 'pull-out' specimens and high resolution micro-electro-mechanical (MEMS) force and displacement sensors will be fabricated. Single nanowire or nanotube pull-out experiments will be performed in-situ inside the Scanning Electron Microscope (SEM) at up to 100,000x magnification while interfacial forces and displacements are measured. The simultaneously qualitative and quantitative information on the nanoscale interfaces will be used to model the load bearing, deformation and failure mechanics after exploring the effects of surface functionalization and accounting for adhesion and friction, which are known to be dominant at the nanoscale. The advances in the fundamental understanding in the mechanics of nanoscale interfaces will help develop nano-composites with novel properties.

NSF CMS # 0625650 Nano-mechanical Properties of Grain Boundaries

More than 50 years of research on grain boundaries has established their impact on the overall strength of materials, yet experimental studies on their yield strength or Young's modulus are rare in the literature. This is because grain boundaries are random networks of interfaces that are only a few nanometers wide and cannot be isolated and characterized by conventional tensile, bending, indentation tools. This project aims to address this challenge by developing an experimental setup using micro-electro-mechanical (MEMS) force and displacement sensors and in-situ transmission electron microscopy (TEM). Since TEM renders the microstructures visible, the experimental setup will allow simultaneous characterization of crystallographic orientations, crack propagation and plastic deformation mechanisms with atomic resolution. Models based on interfacial fracture mechanics will be developed to extract the grain boundary properties from the experimental data. The fundamental understanding on the interfaces in materials will impact grain boundary engineering, an evolving research direction towards optimized materials design.

Honda Initiation Grant: Driver Vigilance Monitoring

Alertness of individuals operating vehicles, aircrafts and machinery is a pre-requisite for safety of the individual and for avoiding economic losses. Exploiting the interactions at the operator-machine interface, we are developing a technique that is (i) non-intrusive (ii) non image processing based (iii) small and simple to install, (iv) accurate and mainly (v) cheap. Our hypothesis is that the operator-machine interface data can be processed so that it becomes a fingerprint of the operator. This means, it will not only be possible to discern different levels of alertness in an individual, but also to distinguish among different individuals. Our preliminary results corroborate this hypothesis. We have developed a model to quantify the alertness during car driving. By feeding an artificial neural network with training data, we envision intelligent vigilance monitoring.  The low cost, ruggedness and low-volume data processing requirements of the proposed technique give it a competitive edge over existing predominantly image processing based vigilance monitoring systems.

NSF CMS #0411603: SGER: Interfacial Mechanics of Carbon Nanotube-Polymer Composites (Completed):

Even though the superior electrical, mechanical and thermal properties of carbon nanotubes open up possibilities for novel composite materials, they do not guarantee superior composites because the composite properties strongly depend on the strength of the nanotube-matrix interfaces. Existing studies on polymer-nanotube interfaces are indirect and/or qualitative, which motivates this exploratory research to develop a novel experimental setup that will allow, for the first time, simultaneously qualitative and quantitative nanotube pull-out testing in-situ in the Transmission Electron Microscope. The PI will design and fabricate a MEMS device with pico-Newton force and nanometer displacement resolutions. Physical processes of polymer-nanotube interfacial deformation and failure will be identified and quantified to model the interfacial mechanics at the nanoscale without relying on simple scaling down approach. The findings will also advance our understanding of two closely related interface effects, adhesion and friction, in context of the nanotubes.

Direction Sensitive MEMS Shear Stress Sensor with High Spatial and Temporal Resolution

We present design and fabrication of a micro-electro-mechanical (MEMS) floating element type direct shear stress sensor with a resolution of 0.01 Pa and 50 KHz bandwidth. The sensor is capable of measuring shear stress along the flow and also across to the flow, which is essential for characterizing turbulent flows. The total contact area (with the fluid flow) is 100 microns by 20 microns. This provides higher spatial and temporal resolution of shear stress compared to the existing sensors. The space and time resolved data will help us understand the mechanics of boundary layers better than space and time averaged properties. The small size of the sensor (3 mm x 2 mm), high spatial and temporal resolution, and wide range of measurement of the sensor makes it useful in a wide variety of civil and military applications like aerospace, automotive, marine and biomedical. Two materials, silicon and silicon carbide are being used for fabrication.

A Novel MEMS Device for High Resolution Force and Displacement Measurement

The goal of this project is to study mechanical properties of one dimensional materials (nanotubes and nanowires). Gripping, and loading of these micrometer long and few nanometers diameter materials are challenging. coupling of electrical, mechanical We present design and fabrication of a novel MEMS device for high resolution force and displacement measurements. The design philosophy exploits the mechanical and geometrical amplification of displacement and attenuation of structural stiffness (spring constant) of slender silicon beams to obtain pico-Newton force and nanometer displacement resolution before any signal processing. Using deep reactive ion etching (DRIE) on a silicon-on-insulator (SOI) wafer and subsequent hydro-fluoric (HF) acid etch release, we have fabricated a device with overall dimension 2 mm x 2 mm. The small device size allows in-situ testing in scanning, transmission and tunneling electron microcopy (SEM, TEM and STM), where the small chamber size makes it challenging to integrate conventional force-displacement sensors. The device will have a broad impact on mechanical testing nanostructures, and a demonstration on tensile testing of individual carbon nanotubes will be presented.

Nanoscale Materials Behavior

Extreme dimensional (thickness) and micro-structural (grain size) constraints and the large proportion of interfaces at the nano-scale cause deviation in the materials behavior. Fundamental understanding in nano-scale materials behavior will impact nano-technology, MEMS and micro-electronics. In this research, we study the effect of all these factors in failure of nano-grained materials. One outcome of this research is probably the world's smallest nanotensilometer. This chip integrates the specimen with force and displacement sensors. We performed, for the first time, tensile testing of nano-scale thin films in-situ in TEM and SEM. The advantage of using TEM is the ability to directly observe deformation mechanisms. This picture shows crack propagation in a 100 nm thick aluminum specimen with no dislocation activities ahead of the tip. This video shows crack nucleation and failure in a 200 nm thick freestanding aluminum film. Here is an example of electrostatic comb drive actuators with nano-N force resolution we designed and fabricated for materials studies. We performed bending experiments on both end clamped and cantilever 100 nm thick aluminum films to study strain gradient effects on plasticity at the nano-scale.

My group is interested in development of MEMS/NEMS sensors and actuator to study length scale effects in typical mechanical engineering disciplines. Specifically, we aim to measure grain boundary properties of materials at different scales. The motivation comes from the observation that even after decades of research, mechanical properties of grain boundaries (alone) have been rarely studied quantitatively. Our in-situ TEM testing tool allows us to measure forces and displacement in electron transparent specimens with a interfacial fracture running through the grain boundaries. Another technique would be to apply elastic theories of dislocations in estimating the strength of grain boundaries as we observe dislocation pileups in TEM and measure the corresponding stress and strain values.