109 research outputs found
The effect of realistic geometries on the susceptibility-weighted MR signal in white matter
Purpose: To investigate the effect of realistic microstructural geometry on
the susceptibility-weighted magnetic resonance (MR) signal in white matter
(WM), with application to demyelination.
Methods: Previous work has modeled susceptibility-weighted signals under the
assumption that axons are cylindrical. In this work, we explore the
implications of this assumption by considering the effect of more realistic
geometries. A three-compartment WM model incorporating relevant properties
based on literature was used to predict the MR signal. Myelinated axons were
modeled with several cross-sectional geometries of increasing realism: nested
circles, warped/elliptical circles and measured axonal geometries from electron
micrographs. Signal simulations from the different microstructural geometries
were compared to measured signals from a Cuprizone mouse model with varying
degrees of demyelination.
Results: Results from simulation suggest that axonal geometry affects the MR
signal. Predictions with realistic models were significantly different compared
to circular models under the same microstructural tissue properties, for
simulations with and without diffusion.
Conclusion: The geometry of axons affects the MR signal significantly.
Literature estimates of myelin susceptibility, which are based on fitting
biophysical models to the MR signal, are likely to be biased by the assumed
geometry, as will any derived microstructural properties.Comment: Accepted March 4 2017, in publication at Magnetic Resonance in
Medicin
Breakup of particle-laden droplets in airflow
The atomisation of suspension containing liquid and dispersed particles is
prevalent in many applications. Previous studies of droplet breakup mainly
focused on homogeneous fluids, and the heterogeneous effect of particles on the
breakup progress is unclear. In this study, the breakup of particle-laden
droplets in airflow is investigated experimentally. Combining synchronised
high-speed images from the side view and the 45 view, we compare the
morphology of particle-laden droplets with that of homogeneous fluids in
different breakup modes. The results show that the higher effective viscosity
of particle-laden droplets affects the initial deformation, and the
heterogeneous effect of particles appears in the later breakup stage. To
evaluate the heterogeneous effect of particles quantitatively, we eliminate the
effect of the higher effective viscosity of particle-laden droplets by
comparing cases corresponding to the same inviscid Weber number. The
quantitative comparison reveals that the heterogeneous effect of particles
accelerates the fragmentation of liquid film and promotes localised rapid
piercing. A correlation length that depends on the particle diameter and the
volume fraction is proposed to characterise the length scale of the
concentration fluctuation under the combined effect of the initial flattening
and later stretching during the droplet breakup process. Based on this
correlation length, the fragment size distributions are analysed, and the
scaling results agree well with the experimental data.Comment: 29 pages, 19 figure
Learning from Interventions using Hierarchical Policies for Safe Learning
Learning from Demonstrations (LfD) via Behavior Cloning (BC) works well on
multiple complex tasks. However, a limitation of the typical LfD approach is
that it requires expert demonstrations for all scenarios, including those in
which the algorithm is already well-trained. The recently proposed Learning
from Interventions (LfI) overcomes this limitation by using an expert overseer.
The expert overseer only intervenes when it suspects that an unsafe action is
about to be taken. Although LfI significantly improves over LfD, the
state-of-the-art LfI fails to account for delay caused by the expert's reaction
time and only learns short-term behavior. We address these limitations by 1)
interpolating the expert's interventions back in time, and 2) by splitting the
policy into two hierarchical levels, one that generates sub-goals for the
future and another that generates actions to reach those desired sub-goals.
This sub-goal prediction forces the algorithm to learn long-term behavior while
also being robust to the expert's reaction time. Our experiments show that LfI
using sub-goals in a hierarchical policy framework trains faster and achieves
better asymptotic performance than typical LfD.Comment: Accepted for publication at the Thirty-Fourth AAAI Conference on
Artificial Intelligence (AAAI-20
Coalescence of immiscible sessile droplets on a partial wetting surface
Droplet coalescence is a common phenomenon and plays an important role in
multi-disciplinary applications. Previous studies mainly consider the
coalescence of miscible liquid, even though the coalescence of immiscible
droplets on a solid surface is a common process. In this study, we explore the
coalescence of two immiscible droplets on a partial wetting surface
experimentally and theoretically. We find that the coalescence process can be
divided into three stages based on the timescales and force interactions
involved, namely (I) the growth of the liquid bridge, (II) the oscillation of
the coalescing sessile droplet, and (III) the formation of a partially-engulfed
compound sessile droplet and the subsequent retraction. In stage I, the
immiscible interface is found not to affect the scaling of the temporal
evolution of the liquid bridge, which follows the same 2/3 power law as that of
miscible droplets. In Stage II, by developing a new capillary timescale
considering both surface and interfacial tensions, we show that the interfacial
tension between the two immiscible liquids functions as a nonnegligible
resistance to the oscillation which decreases the oscillation periods. In Stage
III, a modified Ohnesorge number is developed to characterize the
visco-capillary and inertia-capillary timescales involved during the
displacement of water by oil; a new model based on energy balance is proposed
to analyze the maximum retraction velocity, highlighting that the viscous
resistance is concentrated in a region close to the contact line.Comment: 20 pages, 9 figure
Numerical simulation of secondary breakup of shear-thinning droplets
The breakup of non-Newtonian droplets is ubiquitous in numerous applications.
Although the non-Newtonian property can significantly change the droplet
breakup process, most previous studies consider Newtonian droplets, and the
effects of the non-Newtonian properties on the breakup process are still
unclear. This study focuses on the secondary breakup of shear-thinning droplets
by numerical simulation. The volume of fluid method is used to capture
interface dynamics on adaptive grids. To compare shear-thinning droplets and
Newtonian droplets, a new definition of the Ohnesorge number is proposed by
considering the characteristic shear rate in the droplet induced by the
airflow. The results show that compared with the Newtonian fluid, the
shear-thinning properties can change the effective viscosity distribution
inside the droplet, alter the local deformation, change the droplet morphology,
and affect the transition in the droplet breakup regime.Comment: 14 pages, 15 figure
Domain-domain interactions in Filamin A (16-23) impose a hierarchy of unfolding forces
The quaternary structure of Filamin A (FLNa) 16-23 was recently shown to
exhibit multiple domain-domain interactions that lead to a propeller-like
construction. Here we present single molecule force spectroscopy experiments to
show a wide variety of mechanical responses of this molecule and compare it
with its linear counterpart FLNa 1-8. The compact structure of FLNa 16-23 leads
to a broad distribution of rupture forces and end-to-end lengths in the
force-extension mode and multiple unraveling timescales in the force-clamp
mode. Moreover, a subset of force-extension trajectories reveals a mechanical
hierarchy in which the rupture of domain-domain interactions at high forces
(200 pN) liberates the unfolding of individual domains at low forces (100 pN).
This mechanism may also explain the order of magnitude difference in the rates
of the biexponential fits to the distribution of unfolding dwell times under
force-clamp. Overall, FLNa 16-23 under a force of 100 pN is more compliant than
the linear FLNa 1-8. Since a physiological role of FLNa is to crosslink actin
filaments, this range of responses allows it to accommodate a broad spectrum of
forces exerted by the cell and its environment
Quorum sensing: cell-to-cell communication in Saccharomyces cerevisiae
Quorum sensing (QS) is one of the most well-studied cell-to-cell communication mechanisms in microorganisms. This intercellular communication process in Saccharomyces cerevisiae began to attract more and more attention for researchers since 2006, and phenylethanol, tryptophol, and tyrosol have been proven to be the main quorum sensing molecules (QSMs) of S. cerevisiae. In this paper, the research history and hotspots of QS in S. cerevisiae are reviewed, in particular, the QS system of S. cerevisiae is introduced from the aspects of regulation mechanism of QSMs synthesis, influencing factors of QSMs production, and response mechanism of QSMs. Finally, the employment of QS in adaptation to stress, fermentation products increasing, and food preservation in S. cerevisiae was reviewed. This review will be useful for investigating the microbial interactions of S. cerevisiae, will be helpful for the fermentation process in which yeast participates, and will provide an important reference for future research on S. cerevisiae QS
Biomimetic Z-scheme photocatalyst with a tandem solid-state electron flow catalyzing H_2 evolution
Similar to natural photosynthetic systems, artificial photosynthetic systems require synergistic cooperation between light harvesting, charge separation and redox catalysis. Herein, a three-dimensional (3D) hierarchical photocatalyst is designed with a novel Z-scheme two-photon excitation, defined by the complementary absorption of higher energy and lower energy photons by cadmium sulfide nanowires (CdS NWs) and cobalt–benzimidazole (Co-bIm) coordination polymers (CBPs), respectively. Without any noble-metal co-catalyst, the microscopically integrated CdS–CBP photocatalysts demonstrated dramatically enhanced photocatalytic activities of H_2 evolution, which were up to 10.6 folds higher than those of pristine CdS NWs. Structurally, the intimate interfacial contact between the 3D CdS NW scaffold and the discrete CBP microstructures benefits their strong electronic interaction and efficient charge separation. Upon simultaneous light excitation, a tandem solid-state electron flow from CdS to CBP and then from metal (Co) to ligand (bIm) precisely catalyzes the reduction of pre-activated H atoms on the bIm ligands for efficient H_2 evolution
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