MEAM Seminar Series Summer 2010

Multi-scale composites are material systems exhibiting heterogeneity  at two or more well-separated length-scales. Semi-crystalline polymers,  such as polyethylene, polypropylene, Nylon-6, etc., are prominent examples of two-scale systems and constitute the largest class of polymers used in technological applications. At the larger length-scale, a semi-crystalline polymer is an aggregate of a large number of randomly distributed grains that, at the smaller length-scale, are made up of alternating layers of an amorphous and a crystalline phase, both of which exhibit nonlinear material behavior. During processing, these materials are often subjected to large deformations leading gradually to highly anisotropic mechanical properties as a result of the evolution of the underlying sub-structure.

In the first part of this talk we will propose generalizations of the linear comparison composite? (LCC) homogenization methods for the determination of the effective behavior of nonlinear multi-scale composites with random sub-structures. These methods make use of an LCC with the same sub-structure as the actual nonlinear composite and local properties that are optimally chosen through suitably designed variational procedures. The effective properties of the resulting multi-scale LCC are in turn obtained by employing appropriate estimates that are available from the linear homogenization theory. The nonlinear estimates delivered by the LCC methods incorporate fine sub-structural  information at all length-scale levels, they depend on both the first- and second-moments of the local  stress field and they have the distinguishing feature of being either bounds or exact to second-order in the heterogeneity contrast.

Utilizing the LCC methods, in the second part of this talk we will propose a constitutive model for the macroscopic response and texture evolution in semi-crystalline polymers at large plastic deformations. In the context of this model, both the amorphous and the crystalline phase are taken to be elasto-viscoplastic materials. Furthermore, the present model accounts explicitly (on average) for the current state of the crystallographic, lamellar and morphological texture as well as for its evolution due to the finite changes in geometry as a result of the large applied strains. The predictions of the LCC model for the macroscopic stress-strain response and texture evolution in high-density polyethylene will be compared with corresponding experimental results as well as with the predictions of earlier models that have been proposed thus far in the literature.

The transmission electron microscope (TEM) and scanning transmission electron microscope (STEM) are among the most powerful nanoscale imaging tools available to the scientific community today, producing detailed images with resolution in the nanometer or even sub-nanometer range. These high resolution imaging tools, however, cannot readily be used to observe dynamical processes occurring in liquid media without addressing two experimental hurdles: sample thickness and sample evaporation in the high vacuum microscope chamber. Liquid cell in situ TEM/STEM is a burgeoning experimental technique that addresses these issues and makes it possible to view processes taking place in liquid media in a standard TEM or STEM. There are many processes, such as colloidal crystal formation, aggregation, nanowire growth, electrochemical deposition, and biological interactions, whose understanding would benefit greatly from real-time, direct imaging with a TEM/STEM. The ability to perform in situ electron microscopy of liquid samples is likely to enhance our knowledge in diverse branches of science and technology. In this talk I will present our liquid cell TEM/STEM device, dubbed the nanoaquarium. I will describe the fabrication process, with an emphasis on wafer bonding, one of the critical steps. Experimental results involving direct observation of nanocrystal growth in a solution of gold nanoparticles will be presented, along with quantitative analysis of the growth process.

A lab-on-a-chip (LOC) device integrates on a single substrate one or several laboratory functions. Often, it is necessary to move fluids around and to control fluid flow.  This talk covers two propulsion methods. First, due to the high surface-to-volume ratio associated with microfluidic devices, surface tension force is more important than in the larger scales. We designed, fabricated and tested a microfluidic chip with non-uniform conduit cross-sections to induce surface tension imbalance and thus liquid motion. Second, since the samples are usually weakly conducting electrolytes, under imposed electric and magnetic fields, Lorentz body force is generated and could propel the liquid. The magneto-hydrodynamic (MHD) pumping requires only externally applied electric and magnetic fields without a need for moving elements. We studied the electro-kinetics, flow characteristics, electric current injection and Taylor-Aris dispersion associated with MHD flow in a uniform conduit and around pillar arrays. This work will guide the design of MHD driven, cyclic chromatographic devices, which could provide any desirable column length and real-time detection and of liquid gyroscope.

We present a model that describes mechanical unfolding behavior in rod-like macromolecules. We propose that unfolding in these molecules occurs via the motion of a folded/unfolded interface along the molecule. We predict the speed of this interface as a function of the pulling velocity such that the resulting force-extension curve replicates the overstretching transition typical of DNA and coiled-coil proteins. We model the molecules as one-dimensional continua capable of existing in two metastable states under an applied tension. The interface separates these two metastable states and represents a jump in stretch which is related to applied force by the worm-like-chain relation with different persistence lengths in the two phases. We use the Abeyaratne-Knowles theory of phase transitions in continua to describe the motion of the interface.

Using multiple robots in place of a single complex robot to accomplish a task has many benefits, including simplified system repair, less down time, and lower cost. Combining groups of these multi-robot systems allows addressing multiple subtasks in parallel, reducing the time it takes to address many problems, such as search and rescue and automated warehouse systems.

I will address the synthesis of controllers for groups of multi-robot systems that enable them to automatically create desired labeled formations and maintain those formations while traversing an environment with obstacles. The robots have constraints on communication, both within and across groups. In a group, individuals are capable of close coordination via high bandwidth communication, since they are within a specified distance of the other robots. Coordination across groups must be limited because communication links can be sporadic or more expensive.

I will describe a method for developing feedback controllers for reconfiguring groups of robots that is entirely automatic, and provably correct by construction. I will also describe a method for navigating a large group of robots while managing complexity by using an abstraction. This work provides a framework with which navigation of multiple groups in environments with obstacles is possible, and enables scaling to many groups of robots.

Modern industrial heat treating of metallic alloys is driven by the cost effective processing of large scale production lines. Typically these involve using traditional 'heat and beat' techniques combining annealing and rolling stages. Such fabrication methods have been
developed to optimize the selection of final microstructures, and therefore material properties, across the product as a whole. However as engineering problems become more complex and require multi-functional solutions it is desirable to fabricate 'architectured' or designed components whose microstructure and properties may vary across the sample. This talk will present an innovative processing technique designed to control the location of
formation and growth rate of precipitates within metallic alloys, primarily the binary aluminum-copper alloy system, in order to create multiple patterned areas of unique microstructure within a single sample. Control over precipitation is achieved by altering the conventional heat treatment process with an additional surface pressure applied to selected locations during the precipitate heating period. The applied pressure changes both the enthalpic and chemical
potential landscapes which in turn control the rate and directionality of atomic diffusion in regions close to the loaded surfaces. Experimental results displaying a dependence on quench rate and thermal history will be presented, as well as a qualitative discussion of practical aspects associated with implementing the technique on an industrial scale.

Microbeads are frequently used as a solid support to capture targets of interest, such as proteins and nucleic acids, from a biological sample. The integration of microbeads into microfluidic systems for biological testing is an area of growing interest. Such 'lab-on-chip' systems are designed to integrate several functions of a conventional laboratory onto a single chip. As a platform to capture targets, beads offer several advantages over planar configurations including large surface areas to support biological reactions (increasing
sensitivity), the availability of a library of bead types from many vendors, and arrayed assemblies capable of detecting multiple targets simultaneously (multiplexing). This talk will present the development and characterization of our microbead-based biosensing devices. A customized hot embossing technique is used to stamp microwells in a thin plastic substrate where appropriately functionalized agarose microbeads are selectively placed within a conduit. Functionalized quantum dot nanoparticles are pumped through the conduit and used as a label to monitor binding to the bead. Three-dimensional finite element simulations are carried out to model the mass transfer and binding kinetics on the bead's surface and within the porous bead. The theoretical predictions are compared and favorably agree with experimental observations carried out with a confocal microscope. In addition to agarose beads, the talk will discuss other types of bead arrays and demonstrate alternative bead-based target capture and detection strategies. This work enhances our understanding of bead-based microfluidic systems and provides a design and optimization tool for developers of point-of-care, lab-on-chip devices for medical diagnosis, food and water quality inspection, and environmental monitoring.

The electronics industry and in particular the CMOS technology will shortly face a major roadblock associated with the inability to further reduce standby and operating power consumption of computing devices. This obstacle is due to the fundamental physics of operation of the CMOS transistor and no alternatives or roadmap for the solution has yet been identified.

A promising approach to overcome this power crisis that is afflicting the CMOS industry is the use of fully mechanical elements for achieving switching (mechanical transistors (MTs)) in place of semiconductor transistors. This talk will cover the basic design, fabrication and implementation of Aluminum Nitride (AlN) based piezoelectric MEMS MTs. These devices have been used as prototypes for demonstrating the advantages that miniaturized NEMS MTs would provide. They have shown nearly zero standby leakage and very abrupt switching characteristics, which will enable the design of systems that require very low operating power. A unique way of operating the device as a 4 terminal transistor  has been implemented, and this technique has enabled the tuning of the threshold voltage and has provided with the ability to change the nature of the switch (i.e. make it behave like an n-type or p-type device) according to the need of the configuration. Thus, these MTs enable complementary operation without any change in design or fabrication. Basic and complex logic elements like inverters, NANDs and NORs have been demonstrated using these body-biased MEMS MTs.

Design equations to project scaling of the piezoelectric MT technology into the nano-regime and initial feasibility tests will be presented. The development of the thinnest piezoelectric actuators demonstrated to date and formed by 50 and 100 nm AlN piezoelectric films will be discussed. Finally, the main challenges that still need to be overcome to synthesize nanomechanical computing components will be briefly presented.

New manufacturing technologies have allowed engineers to fabricate  materials with nanometer-scale features such as planar superlattices and quantum dot superlattices.  Inclusions and nanostructures influence the propagation of mechanical waves that transport heat. These waves exist in quantum units of phonons.  The ability to scatter these vibrations, without preventing electron transport, could improve thermoelectric materials which are used to provide compact, high-reliability cooling devices or to convert heat to electrical power.  The formation and dissipation of hot spots in integrated circuits is also highly influenced by the details of phonon scattering events.

To simulate phonon-inclusion scattering events I have developed a new computational method using Molecular Dynamics (MD).  It is capable of determining the amount and spatial distribution of vibrational energy scattered by granular nanometer-scale inclusions in anisotropic bulk crystals.  This information is then incorporated into a new Kinetic Monte Carlo model which performs statistical tracking of phonon scattering to calculate a material's thermal conductivity at and beyond the micron scale.  I will also present a straightforward MD-based method to compute optical phonon lifetimes, simulating phonon-phonon collisions in hot spots on computer chips.

I will then discuss the fabrication methods I use to build micron-scale silicon devices for the creation and study of vibrations.  The presentation will show new flexural plate wave devices and the associated MEMS manufacturing procedure.  These devices are made from single Si crystals using cleanroom lithographic processes originally developed for microelectronics.  The devices function using surface-mounted piezoelectric transducers that generate mechanical waves.  The waves propagate through a thin silicon membrane, scatter off of carefully-etched perforations, and are detected by output transducers.  This allows creation and detection of a transient mechanical wave scattering event.  I will briefly discuss my recent laboratory measurements using these sensors.