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The current research activities of the Chen group focus on:
As a nonwetting liquid is forced to infiltrate into nanopores, the solid-liquid internal friction and surface tension over the ultra-large specific surface area can lead to a nanoporous energy absorption system of unprecedented performance. Moreover, the confined liquid in the nanoenvironment and the enormous specific surface area of nanoporous materials can also amplify the mechanoelectric, thermoelectric, and photoelectric effects when functional liquids are employed. Eventually the nanoporous system may simultaneously harvest electricity from ambient mechanical motions, heat, and solar energy. With appropriate system design, the reverse process leads to thermally or electrically controllable actuators that possess the volume memory characteristics. The nanoporous system is thus very attractive as the building blocks of advanced multifunctional protective, active, and adaptive materials and structures, with wide potential consumer, military, and national security applications. The research of nanoscale energetics also opens up a new scientific area of nanofluidics. The fundamental studies provided useful insights on developing nanoporous systems for energy absorption, harvesting, and actuation with ultrahigh performances (orders of magnitude higher than conventional systems) that contributes to enhanced energy efficiency and sustainbility. We use multiscale interdisciplinary solid/fluid mechanics approaches to study a variety of problems in environmental engineering, including new ways of carbon sequestration in deep ocean sediments, carbon capture using nanotubes, water movement and liquid mixing, water purification and ethanol purification using nanoporous materials, liquid drop-liquid drop impact with application in weather and energy, liquid-solid impact with application in turbines and rain drop erosion, wet flue gas desulfurization, among others. Biomolecules and their assemblies, such as mechanosensitive channels and molecular motors, which are very sensitive to external and internal mechanical perturbations, change their configurations to realize certain biological functionality. We have pioneered a new hierarchical multiscale approach, the molecular dynamics-decorated finite element method (MDeFEM), which is essentially a coarse grained-continuum mechanics framework that incorporates key insights from molecular simulations. The critical components of biomolecules are modeled as integrated structures, and their effective properties and interaction parameters are derived from MD simulations. The framework has been used to simulate the conformational transitions of large biomolecule systems under complex loading with a very low computational cost (comparing to the conventional all-atom simulations), while retaining some of the most important structural details via mechanochemical coupling. The successful application of the MDeFEM approach was demonstrated on the mechanosensitive channels in several bacteria, including E.Coli and Tuberculosis. Self-assembly involving thin films has become the new cornerstone for fabricating small material structures. Controlled mechanical failure may be utilized to generate highly ordered buckling, cracking, and buckle delamination patterns in thin films. The mechanical self-assembly offers a quick, simple, and economical alternative to the conventional techniques based on photolithography and physical-chemical reactions, thus having the potential to receive wide applications in microelectronics, MEMS, and biomedical engineering, such as microcontact printing, microfluidic channels for drug delivery and detection, protein/cell patterning and migration, and fabrication of nanowires, to name a few. Recently, we have combined mechanical self-assembly with mechanobiology, and have shown that spontaneous buckling plays an essential role in the morphogenesis of quite a few natural and biological systems due to differential growth in model core/shell structures, and the simple mechanical principle (anisotropic stress-driven buckling) may interact with the more complex biological and biochemical processes at deep levels. Using such "simple" principle, we have successfully reproduced the distinct undulation morphologies observed in quite many kinds of fruits and vegetables, as well as those in dehydrated fruits, nuts, animal skins, cells, tissues, etc., where the inspiration from earth/Nature is unlimited. Nanotubes, nanowires, and nanofilms have been subjects of intensive research thanks to their excellent physical properties and wide potential applications. We have systematically investigated the mechanics of carbon nanotubes, silica nanotubes, ZnO and metal nanowires and nanofilms. We have established a new structural mechanics model for carbon nanotubes which accounts for chirality and its critical parameters were derived from atomistic simulations. We investigated the thermal vibration behaviors of carbon nanotubes and employed them as the basis for nanostrain sensors. We studied the buckling mechanisms of nanotubes in detail, including those encountered during compression, bending, and indentation, which are not only critical for their mechanical integrity but also for the applications of nanotubes (e.g. nanofluidic conduits). We elucidated the size effects of mechanical properties of ZnO nanostructures and for the first time proposed that the higher modulus at smaller dimension is due to a weakened surface and a strengthened core of the nanostructure. We have also studied the size-dependent plastic properties of metal nanowires and nanoporous materials and developed new micromechanics models, to name a few Nanoindentation is one of the most widely used techniques of measuring the mechanical properties of materials, especially for materials of small volume. A primary goal of the nanoindentation analysis is to relate the material properties (constitutive relationship, toughness, microstructure, etc.) to the indentation response. In the forward analysis, the indentation behaviors are measured or computed when the material/indentation parameters are varied in a wide range, to establish relevant functions that correlate material and indentation behaviors; next, the desired material properties can be identified by solving these established relationships through a reverse analysis. We have systematically investigated the uniqueness of indentation analysis. We have pioneered the use of the substrate effect, instead of avoiding it, to measure the intrinsic mechanical properties of thin films deposited on substrates. We have developed new indentation theories and techniques to account for residual stress, microstructure, and fracture in the specimen, as well as different indenter tip geometries. We have applied the indentation technique to understand the properties and structures of a variety of small material systems, including those in geology, aerospace structures, microelectronics and biomedical engineering.
Funding Resources
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National Science Foundation