6,839 research outputs found
Precise algorithms to compute surface correlation functions of two-phase heterogeneous media and their applications
The quantitative characterization of the microstructure of random
heterogeneous media in -dimensional Euclidean space via a
variety of -point correlation functions is of great importance, since the
respective infinite set determines the effective physical properties of the
media. In particular, surface-surface and surface-void
correlation functions (obtainable from radiation scattering experiments)
contain crucial interfacial information that enables one to estimate transport
properties of the media (e.g., the mean survival time and fluid permeability)
and complements the information content of the conventional two-point
correlation function. However, the current technical difficulty involved in
sampling surface correlation functions has been a stumbling block in their
widespread use. We first present a concise derivation of the small-
behaviors of these functions, which are linked to the \textit{mean curvature}
of the system. Then we demonstrate that one can reduce the computational
complexity of the problem by extracting the necessary interfacial information
from a cut of the system with an infinitely long line. Accordingly, we devise
algorithms based on this idea and test them for two-phase media in continuous
and discrete spaces. Specifically for the exact benchmark model of overlapping
spheres, we find excellent agreement between numerical and exact results. We
compute surface correlation functions and corresponding local surface-area
variances for a variety of other model microstructures, including hard spheres
in equilibrium, decorated "stealthy" patterns, as well as snapshots of evolving
pattern formation processes (e.g., spinodal decomposition). It is demonstrated
that the precise determination of surface correlation functions provides a
powerful means to characterize a wide class of complex multiphase
microstructures
Hyperuniformity of generalized random organization models
Studies of random organization models of monodisperse spherical particles
have shown that a hyperuniform state is achievable when the system goes through
an absorbing phase transition to a critical state. Here we investigate to what
extent hyperuniformity is preserved when the model is generalized to particles
with a size distribution and/or nonspherical shapes. We begin by examining
binary disks in two dimensions and demonstrate that their critical states are
hyperuniform as two-phase media, but not hyperuniform nor multihyperuniform as
point patterns formed by the particle centroids. We further confirm the
generality of our findings by studying particles with a continuous size
distribution. Finally, to study the effect of rotational degrees of freedom, we
extend our model to noncircular particles, namely, hard rectangles with various
aspect ratios, including the hard-needle limit. Although these systems exhibit
only short-range orientational order, hyperuniformity is still preserved. Our
analysis reveals that the redistribution of the "mass" of the particles rather
than the particle centroids is central to this dynamical process. The
consideration of the "active volume fraction" of generalized random
organization models may help to resolve which universality class they belong to
and hence may lead to a deeper theoretical understanding of absorbing-state
models. Our results suggest that general particle systems subject to random
organization can be a robust way to fabricate a wide class of hyperuniform
states of matter by tuning the structures via different particle-size and
-shape distributions. This in turn potentially enables the creation of
multifunctional hyperuniform materials with desirable optical, transport, and
mechanical properties
Random Scalar Fields and Hyperuniformity
Disordered many-particle hyperuniform systems are exotic amorphous states of
matter that lie between crystals and liquids. Hyperuniform systems have
attracted recent attention because they are endowed with novel transport and
optical properties. Recently, the hyperuniformity concept has been generalized
to characterize scalar fields, two-phase media and random vector fields. In
this paper, we devise methods to explicitly construct hyperuniform scalar
fields. We investigate explicitly spatial patterns generated from Gaussian
random fields, which have been used to model the microwave background radiation
and heterogeneous materials, the Cahn-Hilliard equation for spinodal
decomposition, and Swift-Hohenberg equations that have been used to model
emergent pattern formation, including Rayleigh-B{\' e}nard convection. We show
that the Gaussian random scalar fields can be constructed to be hyperuniform.
We also numerically study the time evolution of spinodal decomposition patterns
and demonstrate that these patterns are hyperuniform in the scaling regime.
Moreover, we find that labyrinth-like patterns generated by the Swift-Hohenberg
equation are effectively hyperuniform. We show that thresholding a hyperuniform
Gaussian random field to produce a two-phase random medium tends to destroy the
hyperuniformity of the progenitor scalar field. We then propose guidelines to
achieve effectively hyperuniform two-phase media derived from thresholded
non-Gaussian fields. Our investigation paves the way for new research
directions to characterize the large-structure spatial patterns that arise in
physics, chemistry, biology and ecology. Moreover, our theoretical results are
expected to guide experimentalists to synthesize new classes of hyperuniform
materials with novel physical properties via coarsening processes and using
state-of-the-art techniques, such as stereolithography and 3D printing.Comment: 16 pages, 18 figure
Optimized Large Hyperuniform Binary Colloidal Suspensions in Two Dimensions
The creation of disordered hyperuniform materials with potentially
extraordinary optical properties requires a capacity to synthesize large
samples that are effectively hyperuniform down to the nanoscale. Motivated by
this challenge, we propose a fabrication protocol using binary
superparamagnetic colloidal particles confined in a 2D plane. The strong and
long-ranged dipolar interaction induced by a tunable magnetic field is free
from screening effects that attenuates long-ranged electrostatic interactions
in charged colloidal systems. Specifically, we find a family of optimal size
ratios that makes the two-phase system effectively hyperuniform. We show that
hyperuniformity is a general consequence of low isothermal compressibilities,
which makes our protocol suitable to systems with other long-ranged soft
interactions, dimensionalities and/or polydispersity. Our methodology paves the
way to synthesize large photonic hyperuniform materials that function in the
visible to infrared range and hence may accelerate the discovery of novel
photonic materials
Predicting permeability via statistical learning on higher-order microstructural information
Quantitative structure-property relationships are crucial for the
understanding and prediction of the physical properties of complex materials.
For fluid flow in porous materials, characterizing the geometry of the pore
microstructure facilitates prediction of permeability, a key property that has
been extensively studied in material science, geophysics and chemical
engineering. In this work, we study the predictability of different structural
descriptors via both linear regressions and neural networks. A large data set
of 30,000 virtual, porous microstructures of different types is created for
this end. We compute permeabilities of these structures using the lattice
Boltzmann method, and characterize the pore space geometry using one-point
correlation functions (porosity, specific surface), two-point surface-surface,
surface-void, and void-void correlation functions, as well as the geodesic
tortuosity as an implicit descriptor. Then, we study the prediction of the
permeability using different combinations of these descriptors. We obtain
significant improvements of performance when compared to a Kozeny-Carman
regression with only lowest-order descriptors (porosity and specific surface).
We find that combining all three two-point correlation functions and tortuosity
provides the best prediction of permeability, with the void-void correlation
function being the most informative individual descriptor. Moreover, the
combination of porosity, specific surface, and geodesic tortuosity provides
very good predictive performance. This shows that higher-order correlation
functions are extremely useful for forming a general model for predicting
physical properties of complex materials. Additionally, our results suggest
that neural networks are superior to the more conventional regression methods
for establishing quantitative structure-property relationships
Lipid Desaturation Is a Metabolic Marker and Therapeutic Target of Ovarian Cancer Stem Cells
Lack of sensitive single-cell analysis tools has limited the characterization of metabolic activity in cancer stem cells. By hyperspectral stimulated Raman scattering imaging of single living cells and mass spectrometry analysis of extracted lipids, we report here significantly increased levels of unsaturated lipids in ovarian cancer stem cells (CSCs) as compared to non-CSCs. Higher lipid unsaturation levels were also detected in CSC-enriched spheroids compared to monolayer cultures of ovarian cancer cell lines or primary cells. Inhibition of lipid desaturases effectively eliminated CSCs, suppressed sphere formation in vitro, and blocked tumor initiation capacity in vivo. Mechanistically, we demonstrate that NF-κB directly regulates the expression levels of lipid desaturases and that inhibition of desaturases blocks NF-κB signaling. Collectively, our findings reveal that increased lipid unsaturation is a metabolic marker for ovarian CSCs and a target for CSC-specific therapy.
Interobserver and intraobserver variability in pH-impedance analysis between 10 experts and automated analysis.
Neuroactive steroid effects on autophagy in a human embryonic kidney 293 (HEK) cell model
Neuropsychiatric and neurodegenerative disorders are correlated with cellular stress. Macroautophagy (autophagy) may represent an important protective pathway to maintain cellular homeostasis and functionality, as it targets cytoplasmic components to lysosomes for degradation and recycling. Given recent evidence that some novel psychiatric treatments, such as the neuroactive steroid (NAS) allopregnanolone (AlloP, brexanolone), may induce autophagy, we stably transfected human embryonic kidney 293 (HEK) cells with a ratiometric fluorescent probe to assay NAS effects on autophagy. We hypothesized that NAS may modulate autophagy in part by the ability of uncharged NAS to readily permeate membranes. Microscopy revealed a weak effect of AlloP on autophagic flux compared with the positive control treatment of Torin1. In high-throughput microplate experiments, we found that autophagy induction was more robust in early passages of HEK cells. Despite limiting studies to early passages for maximum sensitivity, a range of NAS structures failed to reliably induce autophagy or interact with Torin1 or starvation effects. To probe NAS in a system where AlloP effects have been shown previously, we surveyed astrocytes and again saw minimal autophagy induction by AlloP. Combined with other published results, our results suggest that NAS may modulate autophagy in a cell-specific or context-specific manner. Although there is merit to cell lines as a screening tool, future studies may require assaying NAS in cells from brain regions involved in neuropsychiatric disorders
Predicting progression of mild cognitive impairment to dementia using neuropsychological data: a supervised learning approach using time windows
Background: Predicting progression from a stage of Mild Cognitive Impairment to dementia is a major pursuit in current research. It is broadly accepted that cognition declines with a continuum between MCI and dementia. As such, cohorts of MCI patients are usually heterogeneous, containing patients at different stages of the neurodegenerative process. This hampers the prognostic task. Nevertheless, when learning prognostic models, most studies use the entire cohort of MCI patients regardless of their disease stages. In this paper, we propose a Time Windows approach to predict conversion to dementia, learning with patients stratified using time windows, thus fine-tuning the prognosis regarding the time to conversion. Methods: In the proposed Time Windows approach, we grouped patients based on the clinical information of whether they converted (converter MCI) or remained MCI (stable MCI) within a specific time window. We tested time windows of 2, 3, 4 and 5 years. We developed a prognostic model for each time window using clinical and neuropsychological data and compared this approach with the commonly used in the literature, where all patients are used to learn the models, named as First Last approach. This enables to move from the traditional question "Will a MCI patient convert to dementia somewhere in the future" to the question "Will a MCI patient convert to dementia in a specific time window". Results: The proposed Time Windows approach outperformed the First Last approach. The results showed that we can predict conversion to dementia as early as 5 years before the event with an AUC of 0.88 in the cross-validation set and 0.76 in an independent validation set. Conclusions: Prognostic models using time windows have higher performance when predicting progression from MCI to dementia, when compared to the prognostic approach commonly used in the literature. Furthermore, the proposed Time Windows approach is more relevant from a clinical point of view, predicting conversion within a temporal interval rather than sometime in the future and allowing clinicians to timely adjust treatments and clinical appointments.FCT under the Neuroclinomics2 project [PTDC/EEI-SII/1937/2014, SFRH/BD/95846/2013]; INESC-ID plurianual [UID/CEC/50021/2013]; LASIGE Research Unit [UID/CEC/00408/2013
- …