Showing 2071 - 2080 of 5708 Items
Nonlinear vibrational-state excitation and piezoelectric energy conversion in harmonically driven granular chains
Date: 2016-05-05
Creator: C. Chong, E. Kim, E. G. Charalampidis, H. Kim, F., Li, P. G. Kevrekidis
Access: Open access
- This article explores the excitation of different vibrational states in a spatially extended dynamical system through theory and experiment. As a prototypical example, we consider a one-dimensional packing of spherical particles (a so-called granular chain) that is subject to harmonic boundary excitation. The combination of the multimodal nature of the system and the strong coupling between the particles due to the nonlinear Hertzian contact force leads to broad regions in frequency where different vibrational states are possible. In certain parametric regions, we demonstrate that the nonlinear Schrödinger equation predicts the corresponding modes fairly well. The electromechanical model we apply predicts accurately the conversion from the obtained mechanical energy to the electrical energy observed in experiments.
Unidirectional Transition Waves in Bistable Lattices
Date: 2016-06-13
Creator: Neel Nadkarni, Andres F. Arrieta, Christopher Chong, Dennis M. Kochmann, Chiara, Daraio
Access: Open access
- We present a model system for strongly nonlinear transition waves generated in a periodic lattice of bistable members connected by magnetic links. The asymmetry of the on-site energy wells created by the bistable members produces a mechanical diode that supports only unidirectional transition wave propagation with constant wave velocity. We theoretically justify the cause of the unidirectionality of the transition wave and confirm these predictions by experiments and simulations. We further identify how the wave velocity and profile are uniquely linked to the double-well energy landscape, which serves as a blueprint for transition wave control.
Walker Sisters and Collecting in Victorian Boston
Date: 2007-01-01
Creator: Laura Fecych Sprague
Access: Open access
- Catalog of an exhibition held at the Bowdoin College Museum of Art Oct. 14, 2007 to August 24, 2008 to celebrate the Walker Art Building's renovation and expansion by architects Machado and Silvetti, Boston.
A tale of two exponentiations in N=8 supergravity
Date: 2019-11-10
Creator: Paolo Di Vecchia, Andrés Luna, Stephen G. Naculich, Rodolfo Russo, Gabriele, Veneziano, Chris D. White
Access: Open access
- The structure of scattering amplitudes in supergravity theories continues to be of interest. Recently, the amplitude for 2→2 scattering in N=8 supergravity was presented at three-loop order for the first time. The result can be written in terms of an exponentiated one-loop contribution, modulo a remainder function which is free of infrared singularities, but contains leading terms in the high energy Regge limit. We explain the origin of these terms from a well-known, unitarity-restoring exponentiation of the high-energy gravitational S-matrix in impact-parameter space. Furthermore, we predict the existence of similar terms in the remainder function at all higher loop orders. Our results provide a non-trivial cross-check of the recent three-loop calculation, and a necessary consistency constraint for any future calculation at higher loops.
Climate change and dissolved organic carbon export to the Gulf of Maine
Date: 2016-10-01
Creator: Thomas G. Huntington, William M. Balch, George R. Aiken, Justin Sheffield, Lifeng, Luo, Collin S. Roesler, Philip Camill
Access: Open access
- Ongoing climate change is affecting the concentration, export (flux), and timing of dissolved organic carbon (DOC) exported to the Gulf of Maine (GoM) through changes in hydrologic regime. DOC export was calculated for water years 1950 through 2013 for 20 rivers and for water years 1930 through 2013 for 14 rivers draining to the GoM. DOC export was also estimated for the 21st century based on climate and hydrologic modeling in a previously published study. DOC export was calculated by using the regression model LOADEST to fit seasonally adjusted concentration discharge (C-Q) relations. Our results are an analysis of the sensitivity of DOC export to changes in hydrologic conditions over time since land cover and vegetation were held constant over time. Despite large interannual variability, all rivers had increasing DOC export during winter and these trends were significant (p < 0.05) in 10 out of 20 rivers for 1950 to 2013 and in 13 out of 14 rivers for 1930 to 2013. All rivers also had increasing annual export of DOC although fewer trends were statistically significant than for winter export. Projections for DOC export during the 21st century were variable depending on the climate model and greenhouse gas emission scenario that affected future river discharge through effects on precipitation and evapotranspiration. The most consistent result was a significant increase in DOC export in winter in all model-by-emission scenarios. DOC export was projected to decrease during the summer in all model-by-emission scenarios, with statistically significant decreases in half of the scenarios.
Interview with Angus King by Andrea L’Hommedieu
Date: 2009-11-03
Creator: Angus S King
Access: Audio recording permanently restricted
Biographial Note
Angus Stanley King, Jr. was born March 31, 1944, in Virginia. His father, Stanley King, was a lawyer in Alexandria and worked as U.S. commissioner and federal magistrate. Angus was active in the civil rights movement, especially in regard to schooling. He was graduated from Dartmouth in 1966 and received his law degree from the University of Virginia in 1969. He worked as a legislative assistant to Senator William D. Hathaway on education, labor, and transportation policy before entering private law practice in Brunswick, Maine, from 1975 to 1983. He hosted the public broadcasting television program Maine Watch, a public affairs talk show. In 1989, he started Northeast Energy Management to run electrical energy conservation projects throughout Maine. As an Independent, he served as governor of Maine from 1994 to 2002; and at the time of this interview he was a lecturer at Bowdoin College in addition to pursuing business interests.
Summary
Interview includes discussion of: family and educational background; working at Pine Tree Legal in Skowhegan, Maine; King’s parents and their political leanings; King’s recollections of the Civil Rights movement; working for Senator Hathaway; U.S. Senate campaign (1972); Senator Hathaway’s relationship with Senator Muskie; meeting George Mitchell while he was involved in public broadcasting; Mitchell and the first Gulf War; King’s work on energy management; King’s decision to run as an Independent; Maine gubernatorial campaign (1994); and Mitchell’s legacy.
Does the neuropeptide GYS modulate stretch feedback pathways in the lobster cardiac neuromuscular system?
Date: 2014-08-01
Creator: Tricia Hartley
Access: Open access
- In many animals, there are groups of neurons, known as central pattern generators (CPGs), which are capable of controlling major everyday life functions. CPGs are responsible for functions that require patterned rhythmic activity, such as the heartbeat, digestion and locomotion. A CPG called the cardiac ganglion, consisting of only nine neurons, controls the rhythmic beating of the heart of the American lobster, Homarus americanus, by stimulating the muscle cells of the heart.My summer consisted of two separate projects in Patsy Dickinson’s neurophysiology lab, both studying the interaction of the cardiac ganglion with neuropeptides. These neuropeptides, GYSDRNYLRFamide (GYS) and SGRNFLRFamide (SGRN) are released hormonally into the cardiac neuromuscular system. The overarching goal of both projects was to determine the role of these neuropeptides in the lobster’s cardiac neuromuscular system.For my first project, I studied the interaction of the neuropeptide GYS with the stretch receptors of the lobster heart. Previous research has found these stretch receptors to be a form of excitatory feedback from the lobster heart to the cardiac ganglion, as heartbeat amplitude and frequency increase as heart is stretched. Further, the dendrites along the cardiac ganglion have been found to be stretch-sensitive, meaning when these dendrites were cut, this excitatory response is no longer observed. By stretching the heart with the dendrites intact and with GYS and next when the dendrites were cut and with GYS, the goal of this project was to determine if GYS would alter the feedback of the stretch receptors back to the cardiac ganglion to change heartbeat frequency and amplitude. Unfortunately, the intricacy involved in being able to cut the dendrites while allowing the heart to continue to beat proved very difficult and I moved on to my next project.The goal of my next project was to examine the interactions of the neuropeptides GYS and SGRN with the decreased and increased presence of nitric oxide, the second form of feedback from the heart muscle to the cardiac ganglion. Previous research shows nitric oxide as having an inhibitory effect, decreasing heartbeat amplitude and frequency. By applying both GYS and SGRN to both the isolated cardiac ganglion and the whole heart in the presence of both a nitric oxide inhibitor and donor, the hope is to be able to determine the interaction of these peptides with and without the presence of the feedback of nitric oxide. Because I started this project later in the summer, with the assistance of Sophie Janes’ data, I have been able to look at the effects of GYS on the whole heart, in addition to the combination of GYS with L-NA, a nitric oxide inhibitor. So far, the data has shown that the combination of GYS with L-NA causes less of a decrease in heartbeat frequency than GYS alone, which shows a significant decrease. We predict this is because GYS enhances the nitric oxide pathway, while L-NA is blocking the nitric oxide pathway, thus giving insight into the role of GYS within the lobster’s cardiac neuromuscular system. For my senior independent study I hope to continue this research and be able to continue to compile data for both SGRN and GYS on the isolated cardiac ganglion as well as on the whole heart, with a nitric oxide inhibitor and donor. Final Report of research funded by a Doherty Coastal Studies Research Fellowship.
A mutation in Tac1p, a transcription factor regulating CDR1 and CDR2, is coupled with loss of heterozygosity at chromosome 5 to mediate antifungal resistance in Candida albicans
Date: 2006-04-01
Creator: Alix Coste, Vincent Turner, Françoise Ischer, Joachim Morschhäuser, Anja, Forche, Anna Selmecki, Judith Berman, Jacques Bille, Dominique Sanglard
Access: Open access
- TAC1, a Candida albicans transcription factor situated near the mating-type locus on chromosome 5, is necessary for the upregulation of the ABC-transporter genes CDR1 and CDR2, which mediate azole resistance. We showed previously the existence of both wild-type and hyperactive TAC1 alleles. Wild-type alleles mediate upregulation of CDR1 and CDR2 upon exposure to inducers such as fluphenazine, while hyperactive alleles result in constitutive high expression of CDR1 and CDR2. Here we recovered TAC1 alleles from two pairs of matched azole-susceptible (DSY294; FH1: heterozygous at mating-type locus) and azole-resistant isolates (DSY296; FH3: homozygous at mating-type locus). Two different TAC1 wild-type alleles were recovered from DSY294 (TAC1-3 and TAC1-4) while a single hyperactive allele (TAC1-5) was isolated from DSY296. A single amino acid (aa) difference between TAC1-4 and TAC1-5 (Asn977 to Asp or N977D) was observed in a region corresponding to the predicted activation domain of Tac1p. Two TAC1 alleles were recovered from FH1 (TAC1-6 and TAC1-7) and a single hyperactive allele (TAC1-7) was recovered from FH3. The N977D change was seen in TAC1-7 in addition to several other aa differences. The importance of N977D in conferring hyperactivity to TAC1 was confirmed by site-directed mutagenesis. Both hyperactive alleles TAC1-5 and TAC1-7 were codominant with wild-type alleles and conferred hyperactive phenotypes only when homozygous. The mechanisms by which hyperactive alleles become homozygous was addressed by comparative genome hybridization and single nucleotide polymorphism arrays and indicated that loss of TAC1 heterozygosity can occur by recombination between portions of chromosome 5 or by chromosome 5 duplication. Copyright © 2006 by the Genetics Society of America.