825 research outputs found
Uncovering the effects of interface-induced ordering of liquid on crystal growth using machine learning
The process of crystallization is often understood in terms of the
fundamental microstructural elements of the crystallite being formed, such as
surface orientation or the presence of defects. Considerably less is known
about the role of the liquid structure on the kinetics of crystal growth. Here
atomistic simulations and machine learning methods are employed together to
demonstrate that the liquid adjacent to solid-liquid interfaces presents
significant structural ordering, which effectively reduces the mobility of
atoms and slows down the crystallization kinetics. Through detailed studies of
silicon and copper we discover that the extent to which liquid mobility is
affected by interface-induced ordering (IIO) varies greatly with the degree of
ordering and nature of the adjacent interface. Physical mechanisms behind the
IIO anisotropy are explained and it is demonstrated that incorporation of this
effect on a physically-motivated crystal growth model enables the quantitative
prediction of the growth rate temperature dependence
Supply Shock Versus Demand Shock: The Local Effects of New Housing in Low-Income Areas
We study the local effects of new market-rate housing in low-income areas using microdata on large apartment buildings, rents, and migration. New buildings decrease nearby rents by 5 to 7 percent relative to locations slightly farther away or developed later, and they increase in-migration from low-income areas. Results are driven by a large supply effectâwe show that new buildings absorb many high-income householdsâthat overwhelms any offsetting endogenous amenity effect. The latter may be small because most new buildings go into already-changing areas. Contrary to common concerns, new buildings slow local rent increases rather than initiate or accelerate them
Optical, electronic, and dynamical phenomena in the shock compression of condensed matter
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2003.Includes bibliographical references (leaves 109-113).This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Despite the study of shock wave compression of condensed matter for over 100 years, scant progress has been made in understanding the microscopic details. This thesis explores microscopic phenomena in shock compression of condensed matter including electronic excitations at the shock front, a new dynamical formulation of shock waves that links the microscopic scale to the macroscopic scale, and basic questions regarding the role of crystallinity in the propagation of electromagnetic radiation in a shocked material. In Chapter 2, the nature of electronic excitations in crystalline solid nitromethane are examined under conditions of shock compression. Density functional theory calculations are used to determine the crystal bandgap under hydrostatic stress, uniaxial strain, and shear strain for pure and defective materials. In all cases, the bandgap is not lowered enough to produce a significant population of excited states. In Chapter 3, a new multi-scale simulation method is formulated for the study of shocked materials. The method allows the molecular dynamics simulation of the system under dynamical shock conditions for orders of magnitude longer time periods than is possible using the popular non-equilibrium molecular dynamics (NEMD) approach. An example calculation is given for a model potential for silicon in which a computational speedup of 10â” is demonstrated. Results of these simulations are consistent with some recent experimental observations. Chapters 4 and 5 present unexpected new physical phenomena that result when light interacts with a shock wave propagating through a photonic crystal.(cont.) These new phenomena include the capture of light at the shock wave front and re-emission at a tunable pulse rate and carrier frequency across the bandgap, and bandwidth narrowing of an arbitrary signal as opposed to the ubiquitous bandwidth broadening. Reversed and anomalous Doppler shifts are also predicted in light reflected from the shock front.by Evan J. Reed.Ph.D
Variable domain N-linked glycosylation and negative surface charge are key features of monoclonal ACPA: implications for B-cell selection
Autoreactive B cells have a central role in the pathogenesis of rheumatoid
arthritis (RA), and recent findings have proposed that anti-citrullinated
protein autoantibodies (ACPA) may be directly pathogenic. Herein, we
demonstrate the frequency of variable-region glycosylation in single-cell
cloned mAbs. A total of 14 ACPA mAbs were evaluated for predicted N-linked
glycosylation motifs in silico and compared to 452 highly-mutated mAbs from RA
patients and controls. Variable region N-linked motifs (N-X-S/T) were
strikingly prevalent within ACPA (100%) compared to somatically hypermutated
(SHM) RA bone marrow plasma cells (21%), and synovial plasma cells from
seropositive (39%) and seronegative RA (7%). When normalized for SHM, ACPA
still had significantly higher frequency of N-linked motifs compared to all
studied mAbs including highly-mutated HIV broadly-neutralizing and
malaria-associated mAbs. The Fab glycans of ACPA-mAbs were highly sialylated,
contributed to altered charge, but did not influence antigen binding. The
analysis revealed evidence of unusual B-cell selection pressure and
SHM-mediated decreased in surface charge and isoelectric point in ACPA. It is
still unknown how these distinct features of anti-citrulline immunity may have
an impact on pathogenesis. However, it is evident that they offer selective
advantages for ACPA+ B cells, possibly also through non-antigen driven
mechanisms
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