Electromagnetic nanopositioner

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.Includes bibliographical references (p. 195-200).This thesis presents the analysis, design, and control of a new class of magnetic nanopositioner. Applications for this class of positioner include sample positioning for scanning microscopy and interferometry, nanofabrication, vibration cancellation, biological cell tracking/positioning, and beam focusing/steering. The nanometer-resolution positioning required in these applications is often provided using piezoelectric ceramic actuators. The drawbacks to using piezoelectric actuators include high hysteretic heating, lightly damped structural resonances, the need for preload on the actuator stack, as well as the requirement for a high voltage amplifier. This thesis demonstrates an electromagnetically driven nanopositioner that is suspended on rubber bearings as a promising, low cost alternative to the piezoelectric nanopositioners. Several key features of the electromagnetic nanopositioner are the flux-steering actuator that applies a force linear in both coil current and displacement, replacement of the conventional metal flexures with rubber bearings, as well as power and sense electronics that can be easily integrated into a compact package. A prototype of this class of nanopositioner with 100,pm of travel and a maximum force output of 460 N was built and tested.(cont.) A closed-loop bandwidth of 580 Hz was obtained using capacitance distance sensor feedback. The feasibility and procedure for casting rubber bearings was investigated. Several room-temperature vulcanizing (RTV) rubbers were considered for low volume, in-lab production of test specimens. A compression specimen was cast from a two-part RTV silicone rubber that was found to be suitable. A compression fixture that was previously used to test bonded rubber pads was modified to accept the cast rubber bearings. The cast rubber bearing was found to have the predicted DC stiffness and the stiffness increased with frequency as expected. Casting of rubber bearings was demonstrated as a feasible method for putting rubber bearings into devices such as nanopositioners.by David P. Cuff.S.M

Metastability of non-reversible mean-field Potts model with three spins

We examine a non-reversible, mean-field Potts model with three spins on a set with $N\uparrow\infty$ points. Without an external field, there are three critical temperatures and five different metastable regimes. The analysis can be extended by a perturbative argument to the case of small external fields. We illustrate the case of large external fields with some phenomena which are not present in the absence of external field.Comment: 34 pages, 12 figure

Structural and electronic properties of Pb1-xCdxTe and Pb1-xMnxTe ternary alloys

A systematic theoretical study of two PbTe-based ternary alloys, Pb1-xCdxTe and Pb1-xMnxTe, is reported. First, using ab initio methods we study the stability of the crystal structure of CdTe - PbTe solid solutions, to predict the composition for which rock-salt structure of PbTe changes into zinc-blende structure of CdTe. The dependence of the lattice parameter on Cd (Mn) content x in the mixed crystals is studied by the same methods. The obtained decrease of the lattice constant with x agrees with what is observed in both alloys. The band structures of PbTe-based ternary compounds are calculated within a tight-binding approach. To describe correctly the constituent materials new tight-binding parameterizations for PbTe and MnTe bulk crystals as well as a tight-binding description of rock-salt CdTe are proposed. For both studied ternary alloys, the calculated band gap in the L point increases with x, in qualitative agreement with photoluminescence measurements in the infrared. The results show also that in p-type Pb1-xCdxTe and Pb1-xMnxTe mixed crystals an enhancement of thermoelectrical power can be expected.Comment: 10 pages, 13 figures, submitted to Physical Review

Investigating the prevalence of reactive online searching in the COVID-19 pandemic

Background: The ongoing coronavirus disease (COVID-19) pandemic has placed an unprecedented strain on global society, healthcare, governments and mass media. Public dissemination of government policies, medical interventions and misinformation has been remarkably rapid and largely unregulated during the COVID-19 pandemic, resulting in increased misinterpretations, miscommunication, and public panic. Being the first full-scale global pandemic of the digital age, COVID-19 has presented novel challenges pertinent to government advice, the spread of news and misinformation, and the trade-off between the accessibility of science and the premature public use of unproven medical interventions. Objective: This study aims to assess the use of internet search terms relating to COVID-19 information and misinformation during the global pandemic, identify which were most used in six affected countries, investigate any temporal trends and the likely propagators of key search terms, and determine any correlation between the per capita cases and deaths with the adoption of these search terms in each of the six countries. Methods: This study uses relative search volume data extracted from Google Trends for search terms linked to the COVID-19 pandemic alongside per capita case and mortality data extracted from the European Open Data Portal, to identify the temporal dynamics of the spread of news and misinformation during the global pandemic in six affected countries (Australia, Germany, Italy, Spain, United Kingdom, United States of America). A correlation analysis was carried out to ascertain any correlation between the temporal trends of search term use and the rise of per capita mortality and disease cases. Results: Of the selected search terms, most were searched immediately following promotion by governments, public figures or viral circulation of unfounded claims, but also relating to the publication of scientific resources, which were sometimes misinterpreted before further dissemination. Strong correlations were identified between the volume of these COVID-19-related search terms, and per capita mortality and cases. Conclusions: These findings illustrate the increased rate and volume of public consumption of novel information during a global healthcare crisis. The strong positive correlation between mortality and online searching, particularly in countries with lower COVID-19 testing rates, may demonstrate the imperative to safeguard official communications and dispel misinformation in these countries. Online news, government briefings and social media provide a powerful tool for the dissemination of important information to the public during pandemics, but their misuse, and the presentation of misrepresented medical information, should be monitored, minimised and addressed to safeguard public safety. Ultimately, governments, public health authorities and scientists have a moral imperative to safeguard the truth and maintain an accessible discourse with the public to inhibit fear

Density-independent prey choice, taxonomy, life history, and web characteristics determine the diet and biocontrol potential of spiders (Linyphiidae and Lycosidae) in cereal crops

Spiders are among the dominant invertebrate predators in agricultural systems and are significant regulators of insect pests. The precise dynamics of biocontrol of pests in the field are, however, poorly understood. This study investigates how density-independent prey choice, taxonomy, life stage, sex, and web characteristics affect spider diet and biocontrol. We collected spiders in four genera of Linyphiidae (i.e., Bathyphantes, Erigone, Tenuiphantes, and Microlinyphia), and individuals from the Lycosidae genus Pardosa, and their proximate prey communities from barley fields in Wales, UK between April and September 2018. We analyzed the gut contents of 300 individual spiders using DNA metabarcoding. From the 300 spiders screened, 89 prey taxa were identified from 45 families, including a wide range of pests and predators. Thrips were the dominant prey, present in over a third of the spiders sampled, but a type IV functional response appears to reduce their predation at peak abundances. Spider diets significantly differed based on web characteristics, but this depended on the genus and sex of the spider and it was not the principal separating factor in the trophic niches of linyphiids and lycosids. Diets significantly differed between spider genera and life stages, reflected in different propensities for intraguild predation and pest predation. Adult spiders predated a greater diversity of other predators, and juveniles predated a greater diversity of pests. Overall, Tenuiphantes spp. and Bathyphantes spp. exhibited the greatest individual potential for biocontrol of the greatest diversity of pest genera. The greater trophic niche complementarity of Pardosa spp. and Erigone spp., however, suggests that their complementary predation of different pests might be of greater overall benefit to biocontrol. Sustainable agriculture should aim to optimize conditions throughout the cropping cycle for effective biocontrol, prioritizing provision for a diversity of spiders which predate a complementary diversity of pest species