Engineering topics

Xu, Cong

Publications and source records attributed to Xu, Cong.

7 records

DOE OSTI2023

Characterizing the Self-Assembly Properties of Monoolein Lipid Isosteres

Living cells feature lipid compartments which exhibit a variety of shapes and structures that assist essential cellular processes. Many natural cell compartments frequently adopt convoluted nonlamellar lipid architectures that facilitate specific biological reactions. Improved methods for controlling the structural organization of artificial model membranes would facilitate investigations into how membrane morphology affects biological functions. Monoolein (MO) is a single-chain amphiphile which forms nonlamellar lipid phases in aqueous solution and has wide applications in nanomaterial development, the food industry, drug delivery, and protein crystallization. However, even if MO has been extensively studied, simple isosteres of MO, while readily accessible, have seen limited characterization. An improved understanding of how relatively minor changes in lipid chemical structure affect self assembly and membrane topology could instruct the construction of artificial cells and organelles for modeling biological structures and facilitate nanomaterial-based applications. Here, we investigate the differences in self-assembly and large-scale organization between MO and two MO lipid isosteres. We show that replacing the ester linkage between the hydrophilic headgroup and hydrophobic hydrocarbon chain with a thioesther or amide functional group results in the assembly of lipid structures with different phases not resembling those formed by MO. Using light and cryo-electron microscopy, small-angle X-ray scattering, and infrared spectroscopy, we demonstrate differences in the molecular ordering and large-scale architectures of the self-assembled structures made from MO and its isosteric analogues. These results improve our understanding of the molecular underpinnings of lipid mesophase assembly and may facilitate the development of MO-based materials for biomedicine and as model lipid compartments.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2021

Change-point Detection and Image Segmentation for Time Series of Astrophysical Images

Many astrophysical phenomena are time-varying, in the sense that their intensity, energy spectrum, and/or the spatial distribution of the emission suddenly change. This paper develops a method for modeling a time series of images. Under the assumption that the arrival times of the photons follow a Poisson process, the data are binned into 4D grids of voxels (time, energy band, and x-y coordinates), and viewed as a time series of non-homogeneous Poisson images. The method assumes that at each time point, the corresponding multiband image stack is an unknown 3D piecewise constant function including Poisson noise. It also assumes that all image stacks between any two adjacent change points (in time domain) share the same unknown piecewise constant function. The proposed method is designed to estimate the number and the locations of all of the change points (in time domain), as well as all of the unknown piecewise constant functions between any pairs of the change points. The method applies the minimum description length principle to perform this task. A practical algorithm is also developed to solve the corresponding complicated optimization problem. Simulation experiments and applications to real data sets show that the proposed method enjoys very promising empirical properties. Applications to two real data sets, the XMM observation of a flaring star and an emerging solar coronal loop, illustrate the usage of the proposed method and the scientific insight gained from it.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2020

Adaptive multi-level checkpointing

In some examples, with respect to adaptive multi-level checkpointing, a transfer parameter associated with transfer of checkpoint data from a node-local storage to a parallel file system may be ascertained for the checkpoint data stored in the node-local storage. The transfer parameter may be compared to a specified transfer parameter threshold. A determination may be made, based on the comparison of the transfer parameter to the specified transfer parameter threshold, as to whether to transfer the checkpoint data from the node-local storage to the parallel file system.

97 MATHEMATICS AND COMPUTING↗

NASA NTRS2013

Profiling and Improving I/O Performance of a Large-Scale Climate Scientific Application

Exascale computing systems are soon to emerge, which will pose great challenges on the huge gap between computing and I/O performance. Many large-scale scientific applications play an important role in our daily life. The huge amounts of data generated by such applications require highly parallel and efficient I/O management policies. In this paper, we adopt a mission-critical scientific application, GEOS-5, as a case to profile and analyze the communication and I/O issues that are preventing applications from fully utilizing the underlying parallel storage systems. Through in-detail architectural and experimental characterization, we observe that current legacy I/O schemes incur significant network communication overheads and are unable to fully parallelize the data access, thus degrading applications' I/O performance and scalability. To address these inefficiencies, we redesign its I/O framework along with a set of parallel I/O techniques to achieve high scalability and performance. Evaluation results on the NASA discover cluster show that our optimization of GEOS-5 with ADIOS has led to significant performance improvements compared to the original GEOS-5 implementation.

Exascale↗

NASA NTRS1994

IRAS colors within M31: Evidence for deficiency of very small grains?

Significant differences are found in the IRAS color-color diagrams of small regions (2 min x 2 min, or 0.4 x 1.8 kpc) within the disk of M31 compared to Galactic cirrus, most noticeably demonstrated by a trend of low 60 to 100 micrometer surface brightness ratio and high 12 to 25 micrometer ratio. Based on physical arguments, we conclude that these color differences are best explained by assuming that 'very small grains' (VSG; but not polycylic aromatic hydrocarbons) are only half as abundant in M31 as they are in Galactic cirrus. We confirm this conclusion and test its detailed agreement with data by using the phenomenological model of Desert et al. (1990). In particular, we show that the data cannot be explained by postulating weaker UV heating in the disk of M31. We also show that the VSG-deficient model predicts correctly the correspondence between the IRAS colors and the 100 micrometer emissivity per H I atom in the outer disk of M31. 'Very small grains' are a leading candidate for the carrier of the 2175 A bump in the extinction curve. Our suggested VSG deficiency in M31 is thus consistent with recent Hubble Space Telescope (HST) observations which show evidence for a weaker and narrower 2175 A bump on the M31 extinction curve. Some speculation is offered as to possible links between very small grains and the low rate of current star formation in M31.

Xu, Cong↗

NASA NTRS1994

Far-Infrared sources and diffuse emission in M31

A study on the far-infrared (FIR) emission of M31 has been carried out with the High Resolution (HiRes) maps (approx. 1 min) derived from IRAS data. Sixty-eight FIR sources are detected in M31, which in general coincide with optical HII regions, and contribute 15, 23, 29, and 23 percent to the fluxes in 12, 25, 60, and 100 micron bands, respectively. The remaining diffuse emission, which dominates the FIR emission of M31, is studied using a dust heating model which utilizes the UV and optical photometry maps and the HI maps available in the literature. It is found that the global dust-to-gas ratio in M31 disk is 6.5 10(exp -3), very close to the dust-to-gas ratio in the solar neighborhood. There is a significant galactocentric gradient of the dust-to-HI-gas ratio, with an e-folding scale length of 9 kpc. The diffuse dust correlates tightly with the HI gas. The model indicates that the non-ionizing UV (913-4000A) radiation from massive and intermediate massive stars contributes only about 30 percent of the heating of the diffuse dust, while the optical-NIR (4000-9000A) radiation from the old stellar population is responsible for the most of the heating.

Xu, Cong↗

NASA NTRS1990

Quantifying the FIR interaction enhancement in paired galaxies

The Catalog of Isolated Pairs of Galaxies in the Northern Hemisphere, by Karachentsev (1972), was studied and a well-matched comparison sample taken from the Catalog of Isolated Galaxies, by Karachentseva (1973), in order to quantify the enhanced FIR emission properties of interacting galaxies.

Xu, Cong↗