Electron Injection By Means of a Ballistic Source

The need for more robust micro electro-mechanical systems (MEMS) devices is greatly increasing. But, along with new technologies come problems, which need to be addressed. One of the more important problems is stiction. Stiction is the strong interfacial adhesion present between contacting crystalline microstructure surfaces. This often causes many MEMS devices to fail. A new method of overcoming stiction has been disclosed. Electrons are stored at the interface between silicon dioxide and silicon nitride to create a force when an external applied electric field is applied to a MEMS component containing the stored charge. The purpose of this project was to devise a method of putting electrons into the oxide-nitride interface by ballistic injection of electrons through the use of a scanning electron microscope (SEM), a Manufacturing Electron Beam Exposure System (MEBES), and an electron flood gun. The amount of charge is measured by the degree of shift of the flatband voltage of the structure. Although initial data shows the order of magnitude of electrons stored at the oxide-nitride interface via ballistic injection is less than that using high field conditions the experimentation provides a proof of concept

Global bifurcation of solitary waves for the Whitham equation

The Whitham equation is a nonlocal shallow water-wave model which combines the quadratic nonlinearity of the KdV equation with the linear dispersion of the full water wave problem. Whitham conjectured the existence of a highest, cusped, traveling-wave solution, and his conjecture was recently verified in the periodic case by Ehrnstr\"om and Wahl\'en. In the present paper we prove it for solitary waves. Like in the periodic case, the proof is based on global bifurcation theory but with several new challenges. In particular, the small-amplitude limit is singular and cannot be handled using regular bifurcation theory. Instead we use an approach based on a nonlocal version of the center manifold theorem. In the large-amplitude theory a new challenge is a possible loss of compactness, which we rule out using qualitative properties of the equation. The highest wave is found as a limit point of the global bifurcation curve.Comment: 38 pages, 3 figure

The Mass Dependance of Satellite Quenching in Milky Way-like Halos

Using the Sloan Digital Sky Survey, we examine the quenching of satellite galaxies around isolated Milky Way-like hosts in the local Universe. We find that the efficiency of satellite quenching around isolated galaxies is low and roughly constant over two orders of magnitude in satellite stellar mass ($M_{*}$ = $10^{8.5}-10^{10.5} \, M_{\odot}$), with only $\sim~20\%$ of systems quenched as a result of environmental processes. While largely independent of satellite stellar mass, satellite quenching does exhibit clear dependence on the properties of the host. We show that satellites of passive hosts are substantially more likely to be quenched than those of star-forming hosts, and we present evidence that more massive halos quench their satellites more efficiently. These results extend trends seen previously in more massive host halos and for higher satellite masses. Taken together, it appears that galaxies with stellar masses larger than about $10^{8}~M_{\odot}$ are uniformly resistant to environmental quenching, with the relative harshness of the host environment likely serving as the primary driver of satellite quenching. At lower stellar masses ($< 10^{8}~M_{\odot}$), however, observations of the Local Group suggest that the vast majority of satellite galaxies are quenched, potentially pointing towards a characteristic satellite mass scale below which quenching efficiency increases dramatically.Comment: 14 pages, 8 figure

A Dichotomy in Satellite Quenching Around L* Galaxies

We examine the star formation properties of bright (~0.1 L*) satellites around isolated ~L* hosts in the local Universe using spectroscopically confirmed systems in the Sloan Digital Sky Survey DR7. Our selection method is carefully designed with the aid of N-body simulations to avoid groups and clusters. We find that satellites are significantly more likely to be quenched than a stellar mass-matched sample of isolated galaxies. Remarkably, this quenching occurs only for satellites of hosts that are themselves quenched: while star formation is unaffected in the satellites of star-forming hosts, satellites around quiescent hosts are more than twice as likely to be quenched than stellar-mass matched field samples. One implication of this is that whatever shuts down star formation in isolated, passive L* galaxies also plays at least an indirect role in quenching star formation in their bright satellites. The previously-reported tendency for "galactic conformity" in color/morphology may be a by-product of this host-specific quenching dichotomy. The S\'ersic indices of quenched satellites are statistically identical to those of field galaxies with the same specific star formation rates, suggesting that environmental and secular quenching give rise to the same morphological structure. By studying the distribution of pairwise velocities between the hosts and satellites, we find dynamical evidence that passive host galaxies reside in dark matter halos that are ~45% more massive than those of star-forming host galaxies of the same stellar mass. We emphasize that even around passive hosts, the mere fact that galaxies become satellites does not typically result in star formation quenching: we find that only ~30% of ~0.1 L* galaxies that fall in from the field are quenched around passive hosts, compared with ~0% around star forming hosts.Comment: 14 pages, 9 figure

Isolating the hydrodynamic triggers of the dive response in eastern oyster larvae

© The Author(s), 2015. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Limnology and Oceanography 60 (2015): 1332–1343, doi:10.1002/lno.10098.Understanding the behavior of larval invertebrates during planktonic and settlement phases remains an open and intriguing problem in larval ecology. Larvae modify their vertical swimming behavior in response to water column cues to feed, avoid predators, and search for settlement sites. The larval eastern oyster (Crassostrea virginica) can descend in the water column via active downward swimming, sinking, or “diving,” which is a flick and retraction of the ciliated velum to propel a transient downward acceleration. Diving may play an important role in active settlement, as diving larvae move rapidly downward in the water column and may regulate their proximity to suitable settlement sites. Alternatively, it may function as a predator-avoidance escape mechanism. We examined potential hydrodynamic triggers to this behavior by observing larval oysters in a grid-stirred turbulence tank. Larval swimming was recorded for two turbulence intensities and flow properties around each larva were measured using particle image velocimetry. The statistics of flow properties likely to be sensed by larvae (fluid acceleration, deformation, vorticity, and angular acceleration) were compared between diving and non-diving larvae. Our analyses showed that diving larvae experienced high average flow accelerations in short time intervals (approximately 1–2 s) prior to dive onset, while accelerations experienced by non-diving larvae were significantly lower. Further, the probability that larvae dove increased with the fluid acceleration they experienced. These results indicate that oyster larvae actively respond to hydrodynamic signals in the local flow field, which has ecological implications for settlement and predator avoidance.This work was supported by NSF grant OCE-0850419, NOAA Sea Grant NA14OAR4170074, grants from the WHOI Coastal Ocean Institute, discretionary WHOI funds, a WHOI Ocean Life Fellowship to LM, and a Grove City College Swezey Fellowship to EA

Supramolecular peptide composite assemblies: Mimicking biological form and function in synthetic systems

Microtubules (MTs) are dynamic, multifunctional biomaterials that facilitate a range of complex biological process in cells ranging from regulation of cell morphology to separation of chromosomes during cell division to directing the intracellular transport of molecular cargo.1 The remarkable precision, versatility, and dynamic nature of these non-equilibrium structures has motivated our desire to mimic their structure and function in synthetic materials. Here, I will identify a number of the key attributes responsible for MT form and function, and describe our efforts to merge computation and experiment to design, synthesize, and study a family of self-assembling peptides intended to mimic MTs. MTs are self-assembled biological filaments assembled from tightly bound heterodimers of α and β tubulin. These dimers assemble head-to-tail into protofilaments that associate laterally into closed sheets forming the characteristic tubular morphology of the MTs. These tubules are approximately 25 nm in diameter and can be many micrometers long, though the length of the MTs is subject to their dynamic assembly and disassembly within a cell (dynamic instability). Ultimately, both the initial assembly and dynamic instability of MTs are governed by complex electrostatic and hydrogen bonding interactions between tubulin heterodimers and other functional biomolecules within the cell. These interactions allow biology to effectively program MT form and function to meet the dynamic and evolving needs of a cell. From a synthetic materials perspective, we aim to create simplified peptide or composite peptide molecules capable of similar programmable functional assembly that could similarly be used to facilitate dynamic or adaptable organization of nanomaterials. To guide the design and facilitate understanding of these peptide systems, we utilize a combination of density functional theory (DFT) and self-consistent field theory (SCFT) that can reveal simplified or distilled molecular characteristics needed in an artificial MT scheme. These computational studies have provided insight into the necessary molecular geometries, peptide compositions, and even targeted intermolecular interactions built into our MT-mimetic designs. In particular here, I will describe a collection of simulation-inspired peptides in which we demonstrate that molecular shape, electrostatic interactions, hydrogen bonding, and solvent interactions influence peptide self assembly into sheets, fibers, ribbons, vesicles and tubules (Figure 1).2,3 Moreover, we show that by creating hybrid or composite compositions containing multiple functionalities, it is possible to control molecular self-assembly through interactions with secondary molecules. For example, select bola-peptide compositions are shown to undergo unique self-assembly in collaboration with the surfactant sodium dodecylsulfate, creating a composite structure that is resistant to enzymatic degradation. In another example, molecules comprising self assembling peptides, such as diphenylalanine, and boronic acid form ribbon-structures whose reversible self-assembly is mediated by binding of polysaccharides to the boronic acids. Just as in the natural MT system, the self-assembly (and disassembly) in these hybrid systems is regulated by molecular shape, electrostatic and hydrogen bonding interactions, and the programmable response of these molecules to chemical stimuli. Continued development of these hybrid, composite peptide systems is aimed at developing a new class of biomimetic molecular materials which mimic not only the form, but also the underlying function of some of Nature’s most compelling supramolecular creations

Ontogenetic changes in larval swimming and orientation of pre-competent sea urchin Arbacia punctulata in turbulence

© The Author(s), 2016. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Journal of Experimental Biology 219 (2016): 1303-1310, doi:10.1242/jeb.129502.Many marine organisms have complex life histories, having sessile adults and relying on the planktonic larvae for dispersal. Larvae swim and disperse in a complex fluid environment and the effect of ambient flow on larval behavior could in turn impact their survival and transport. However, to date, most studies on larvae–flow interactions have focused on competent larvae near settlement. We examined the importance of flow on early larval stages by studying how local flow and ontogeny influence swimming behavior in pre-competent larval sea urchins, Arbacia punctulata. We exposed larval urchins to grid-stirred turbulence and recorded their behavior at two stages (4- and 6-armed plutei) in three turbulence regimes. Using particle image velocimetry to quantify and subtract local flow, we tested the hypothesis that larvae respond to turbulence by increasing swimming speed, and that the increase varies with ontogeny. Swimming speed increased with turbulence for both 4- and 6-armed larvae, but their responses differed in terms of vertical swimming velocity. 4-Armed larvae swam most strongly upward in the unforced flow regime, while 6-armed larvae swam most strongly upward in weakly forced flow. Increased turbulence intensity also decreased the relative time that larvae spent in their typical upright orientation. 6-Armed larvae were tilted more frequently in turbulence compared with 4-armed larvae. This observation suggests that as larvae increase in size and add pairs of arms, they are more likely to be passively re-oriented by moving water, rather than being stabilized (by mechanisms associated with increased mass), potentially leading to differential transport. The positive relationship between swimming speed and larval orientation angle suggests that there was also an active response to tilting in turbulence. Our results highlight the importance of turbulence to planktonic larvae, not just during settlement but also in earlier stages through morphology–flow interactions.This work was supported by the National Science Foundation [OCE-0850419] and the National Oceanic and Atmospheric Administration Sea Grant [NA14OAR4170074]. K.Y.K.C. was supported by the Postdoctoral Scholar Program at the Woods Hole Oceanographic Institution (WHOI), with funding provided by the Coastal Ocean Institute, the Croucher Foundation and the Royal Swedish Academy of Sciences. K.Y.K.C. is currently funded by the Croucher Foundation. Additional funding was provided to L.S.M. through the WHOI Ocean Life Fellowship and discretionary WHOI funds, and to E.J.A. through the faculty sabbatical program at Grove City College

Regenerated sciatic nerve axons stimulated through a chronically implanted macro-sieve electrode

Sieve electrodes provide a chronic interface for stimulating peripheral nerve axons. Yet, successful utilization requires robust axonal regeneration through the implanted electrode. The present study determined the effect of large transit zones in enhancing axonal regeneration and revealed an intimate neural interface with an implanted sieve electrode. Fabrication of the polyimide sieve electrodes employed sacrificial photolithography. The manufactured macro-sieve electrode (MSE) contained nine large transit zones with areas of ~0.285 mm2 surrounded by eight Pt-Ir metallized electrode sites. Prior to implantation, saline or glial derived neurotropic factor (GDNF) was injected into nerve guidance silicone-conduits with or without a MSE. The MSE assembly or a nerve guidance conduit was implanted between transected ends of the sciatic nerve in adult male Lewis rats. At 3 months’ post-operation, fiber counts were similar through both implant types. Likewise, stimulation of nerves regenerated through a MSE or an open silicone conduit evoked comparable muscle forces. These results showed that nerve regeneration was comparable through MSE transit zones and an open conduit. GDNF had a minimal positive effect on the quality and morphology of fibers regenerating through the MSE; thus, the MSE may reduce reliance on GDNF to augment axonal regeneration. Selective stimulation of several individual muscles was achieved through monopolar stimulation of individual electrodes sites suggesting that the MSE might be an optimal platform for functional neuromuscular stimulation

Traveling water waves — the ebb and flow of two centuries

This survey covers the mathematical theory of steady water waves with an emphasis on topics that are at the forefront of current research. These areas include: variational characterizations of traveling water waves; analytical and numerical studies of periodic waves with critical layers that may overhang; existence, nonexistence, and qualitative theory of solitary waves and fronts; traveling waves with localized vorticity or density stratification; and waves in three dimensions