251 research outputs found
Imbibition in mesoporous silica: rheological concepts and experiments on water and a liquid crystal
We present, along with some fundamental concepts regarding imbibition of
liquids in porous hosts, an experimental, gravimetric study on the
capillarity-driven invasion dynamics of water and of the rod-like liquid
crystal octyloxycyanobiphenyl (8OCB) in networks of pores a few nanometers
across in monolithic silica glass (Vycor). We observe, in agreement with
theoretical predictions, square root of time invasion dynamics and a sticky
velocity boundary condition for both liquids investigated.
Temperature-dependent spontaneous imbibition experiments on 8OCB reveal the
existence of a paranematic phase due to the molecular alignment induced by the
pore walls even at temperatures well beyond the clearing point. The ever
present velocity gradient in the pores is likely to further enhance this
ordering phenomenon and prevent any layering in molecular stacks, eventually
resulting in a suppression of the smectic phase in favor of the nematic phase.Comment: 18 pages, 8 figure
A newborn with ankyloblepharon filiforme adnatum: a case report
Ankyloblepharon filiforme adnatum is a rare congenital anomaly. We report a case of ankyloblepharon filiforme adnatum in a newborn Caucasian male whose paediatric examination was otherwise unremarkable. Ankyloblepharon filiforme adnatum can present as an isolated finding, in association with other anomalies, or as part of a well-defined syndrome
Morphologic Parameters for Successful Lunar Landing Sites
The Moon, with its abundant resources, intriguing science questions, and vast unexplored surface area, is the most attainable and useful near-term target for future human exploration. In recognition of this fact, Presidential Space Policy Directive 1 (PSPD-1) has directed the United States to return to the Moon for long-term exploration and utilization, beginning with the 7th American human lunar landing by 2024 and building to sustainable surface presence by 2028
Bostonia: The Boston University Alumni Magazine. Volume 33
Founded in 1900, Bostonia magazine is Boston University's main alumni publication, which covers alumni and student life, as well as university activities, events, and programs
Pseudogap and charge density waves in two dimensions
An interaction between electrons and lattice vibrations (phonons) results in
two fundamental quantum phenomena in solids: in three dimensions it can turn a
metal into a superconductor whereas in one dimension it can turn a metal into
an insulator. In two dimensions (2D) both superconductivity and charge-density
waves (CDW) are believed to be anomalous. In superconducting cuprates, critical
transition temperatures are unusually high and the energy gap may stay unclosed
even above these temperatures (pseudogap). In CDW-bearing dichalcogenides the
resistivity below the transition can decrease with temperature even faster than
in the normal phase and a basic prerequisite for the CDW, the favourable
nesting conditions (when some sections of the Fermi surface appear shifted by
the same vector), seems to be absent. Notwithstanding the existence of
alternatives to conventional theories, both phenomena in 2D still remain the
most fascinating puzzles in condensed matter physics. Using the latest
developments in high-resolution angle-resolved photoemission spectroscopy
(ARPES) here we show that the normal-state pseudogap also exists in one of the
most studied 2D examples, dichalcogenide 2H-TaSe2, and the formation of CDW is
driven by a conventional nesting instability, which is masked by the pseudogap.
Our findings reconcile and explain a number of unusual, as previously believed,
experimental responses as well as disprove many alternative theoretical
approaches. The magnitude, character and anisotropy of the 2D-CDW pseudogap are
intriguingly similar to those seen in superconducting cuprates.Comment: 14 pages including figures and supplementary informatio
Polar Volatiles Exploration in Peary Crater Enabled by NASA's Kilopower Project
For more than 50 years, scientists have discussed the possibility of the existence of water ice and other frozen volatiles at the lunar poles [1]. However, it was not until the 1990s when the polar orbiting spacecraft Clementine and Lunar Prospector collected data supporting these hypotheses [2]. Subsequent missions, including the Lunar Reconnaissance Orbiter (LRO) mission [3], and the Lunar Crater Observation and Sensing Satellite (LCROSS) mission [4], provided further evidence that supports the existence of water ice deposits at the lunar poles. During NASA's Constellation Program, several areas at both lunar poles polar were included in 50 Regions of Interest (ROI) for intensive study by the Lunar Reconnaissance Orbiter Camera (LROC) [5]. These polar ROI focused on peaks and craters rims that received high amounts of solar illumination, assuming initial missions back to the lunar surface would utilize solar arrays to generate electricity. Recently, the successful demonstration of NASA's Kilopower Project at the National Nuclear Security Administration (NNSA) Nevada National Security Site makes it possible to consider lunar polar missions at locations other than highly illuminated regions. The Kilopower Project was initiated in 2015 to demonstrate subsystem-level technology readiness of a small space fission power system [6]. This abstract describes the science objectives and operations for a mission concept developed at NASA Glenn Research Center's COMPASS Concurrent Engineering Team for a 1-year exploration of Peary Crater focused on prospecting for lunar polar volatiles
Fermi surface nesting in several transition metal dichalcogenides
By means of high-resolution angle resolved photoelectron spectroscopy (ARPES)
we have studied the fermiology of 2H transition metal dichalcogenide polytypes
TaSe2, NbSe2, and Cu0.2NbS2. The tight-binding model of the electronic
structure, extracted from ARPES spectra for all three compounds, was used to
calculate the Lindhard function (bare spin susceptibility), which reflects the
propensity to charge density wave (CDW) instabilities observed in TaSe2 and
NbSe2. We show that though the Fermi surfaces of all three compounds possess an
incommensurate nesting vector in the close vicinity of the CDW wave vector, the
nesting and ordering wave vectors do not exactly coincide, and there is no
direct relationship between the magnitude of the susceptibility at the nesting
vector and the CDW transition temperature. The nesting vector persists across
the incommensurate CDW transition in TaSe2 as a function of temperature despite
the observable variations of the Fermi surface geometry in this temperature
range. In Cu0.2NbS2 the nesting vector is present despite different doping
level, which lets us expect a possible enhancement of the CDW instability with
Cu-intercalation in the CuxNbS2 family of materials.Comment: Accepted to New J. Phy
Gas damping force noise on a macroscopic test body in an infinite gas reservoir
We present a simple analysis of the force noise associated with the
mechanical damping of the motion of a test body surrounded by a large volume of
rarefied gas. The calculation is performed considering the momentum imparted by
inelastic collisions against the sides of a cubic test mass, and for other
geometries for which the force noise could be an experimental limitation. In
addition to arriving at an accurated estimate, by two alternative methods, we
discuss the limits of the applicability of this analysis to realistic
experimental configurations in which a test body is surrounded by residual gas
inside an enclosure that is only slightly larger than the test body itself.Comment: 8 pages. updated with correct translational damping coefficient for
cylinder on axis. added cylinder orthogonal to symmetry axis, force and
torque. slightly edited throughou
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