Characterizing neuromorphologic alterations with additive shape functionals

The complexity of a neuronal cell shape is known to be related to its function. Specifically, among other indicators, a decreased complexity in the dendritic trees of cortical pyramidal neurons has been associated with mental retardation. In this paper we develop a procedure to address the characterization of morphological changes induced in cultured neurons by over-expressing a gene involved in mental retardation. Measures associated with the multiscale connectivity, an additive image functional, are found to give a reasonable separation criterion between two categories of cells. One category consists of a control group and two transfected groups of neurons, and the other, a class of cat ganglionary cells. The reported framework also identified a trend towards lower complexity in one of the transfected groups. Such results establish the suggested measures as an effective descriptors of cell shape

Ultrafast Dynamics of Vibrational Symmetry Breaking in a Charge-ordered Nickelate

The ability to probe symmetry breaking transitions on their natural time scales is one of the key challenges in nonequilibrium physics. Stripe ordering represents an intriguing type of broken symmetry, where complex interactions result in atomic-scale lines of charge and spin density. Although phonon anomalies and periodic distortions attest the importance of electron-phonon coupling in the formation of stripe phases, a direct time-domain view of vibrational symmetry breaking is lacking. We report experiments that track the transient multi-THz response of the model stripe compound La$_{1.75}$Sr$_{0.25}$NiO$_{4}$, yielding novel insight into its electronic and structural dynamics following an ultrafast optical quench. We find that although electronic carriers are immediately delocalized, the crystal symmetry remains initially frozen - as witnessed by time-delayed suppression of zone-folded Ni-O bending modes acting as a fingerprint of lattice symmetry. Longitudinal and transverse vibrations react with different speeds, indicating a strong directionality and an important role of polar interactions. The hidden complexity of electronic and structural coupling during stripe melting and formation, captured here within a single terahertz spectrum, opens new paths to understanding symmetry breaking dynamics in solids.Comment: 21 pages, 4 figures; updated version with journal re

MULTI-SCALE MECHANICS OF COMPOSITE SANDWICH STRUCTURES WITH BIOLOGICALLY INSPIRED FIBER REINFORCED FOAM CORES: A POTENTIAL TEMPLATE FOR DEVELOPING MULTIFUNCTIONAL STRUCTURES

This dissertation describes a novel multi-scale characterization and modeling approach for developing bio-inspired composite sandwich structures. The source of bio-inspiration was chosen to be Palmetto wood, a naturally occurring porous composite material with macrofiber reinforcement. Characterization of Palmetto Wood at multiple length scales revealed that the mechanical behavior is dominated by the stronger and stiffer macrofibers, while the porous cellulose matrix controls load transfer and failure between macrofibers. Shear dominated debonding and pore collapse mechanisms have been identified as the leading modes of failure mechanism. The role of macrofiber volume fraction and strain rate on macroscale response and damage evolution has been evaluated through experiments. It is seen that increase in macrofiber concentration increases the stiffness of the Palmetto wood, leading to a higher concentration of macrofiber in the outer region of the wood by evolution. A damage model has been developed to decouple the effect of the plastic strain and pore collapse on damage evolution. Using Palmetto wood as a template, prototype bioinspired sandwich composite structures have been fabricated using carbon fiber reinforcement in the foam core to translate the mechanics principles of Palmetto wood. The sandwich composite structures with bioinspired foam core and standard foam core have been characterized under quasi-static and dynamic three-point bending load. The model developed to study damage evolution in Palmetto wood has been applied to the behavior of bioinspired sandwich to quantify the parameters. The enhancement in mechanical behavior has been achieved by reinforcement of the carbon rods in the core like the macrofibers in the Palmetto wood. An increase in macroscale reinforcement in the core led to the behavior that tunes the material response to a better combination the flexural stiffness, energy absorbance and damage evolution characteristics. A Finite Element Analysis (FEA) model has been developed to numerically study the effects of reinforcement in the foam core on its flexural behavior as observed in the experimental characterization. The simulations performed using homogenized, isotropic properties from simulations of the bioinspired core affirm the experimental observations. The viability of developing multifunctional sandwich structures from the multiscale characterization and modeling of the bioinspired foam cores has also been investigated. Prototype sandwich battery structures were fabricated using copper coated fiberglass and Zn plate facesheet, a carbon foam core, and an adhesive of NH4Cl and ZnCl2 bound by HTPB and epoxy polymers. Very low power generation was demonstrated using the prototype batteries, however it was determined that the mechanical strength and energy absorbing capability were compromised, as expected from the model, indicating that the use of macrofiber reinforcement could potentially enhance multifunctional behavior

Capillary deformations of bendable films

We address the partial wetting of liquid drops on ultrathin solid sheets resting on a deformable foundation. Considering the membrane limit of sheets that can relax compression through wrinkling at negligible energetic cost, we revisit the classical theory for the contact of liquid drops on solids. Our calculations and experiments show that the liquid-solid-vapor contact angle is modified from the Young angle, even though the elastic bulk modulus (E) of the sheet is so large that the ratio between the surface tension γ and E is of molecular size. This finding establishes a new type of “soft capillarity” that stems from the bendability of thin elastic bodies rather than from material softness. We also show that the size of the wrinkle pattern that emerges in the sheet is fully predictable, thus resolving a puzzle noticed in several previous attempts to model “drop-on-a-floating-sheet” experiments, and enabling a reliable usage of this setup for the metrology of ultrathin films

Switch on, switch off: stiction in nanoelectromechanical switches

We present a theoretical investigation of stiction in nanoscale electromechanical contact switches. We develop a mathematical model to describe the deflection of a cantilever beam in response to both electrostatic and van der Waals forces. Particular focus is given to the question of whether adhesive van der Waals forces cause the cantilever to remain in the ‘ON’ state even when the electrostatic forces are removed. In contrast to previous studies, our theory accounts for deflections with large slopes (i.e. geometrically nonlinear). We solve the resulting equations numerically to study how a cantilever beam adheres to a rigid electrode: transitions between free, ‘pinned’ and ‘clamped’ states are shown to be discontinuous and to exhibit significant hysteresis. Our findings are compared to previous results from linearized models and the implications for nanoelectromechanical cantilever switch design are discussed

An analysis of the boundary layer in the 1D surface Cauchy–Born model

The surface Cauchy–Born (SCB) method is a computational multi-scale method for the simulation of surface-dominated crystalline materials. We present an error analysis of the SCB method, focused on the role of surface relaxation. In a linearized 1D model we show that the error committed by the SCB method is O(1) in the mesh size; however, we are able to identify an alternative "approximation parameter" – the stiffness of the interaction potential – with respect to which the relative error in the mean strain is exponentially small. Our analysis naturally suggests an improvement of the SCB model by enforcing atomistic mesh spacing in the normal direction at the free boundary. In this case we even obtain pointwise error estimates for the strain