University of Houston

About: University of Houston is a education organization based out in Houston, Texas, United States. It is known for research contribution in the topics: Population & Poison control. The organization has 23074 authors who have published 53903 publications receiving 1641968 citations.

Mechanics of hard-magnetic soft materials

Abstract: Soft materials that can undergo rapid and large deformation through the remote and wireless action of external stimuli offer a range of tantalizing applications such as soft robots, flexible electronics, and biomedical devices. A natural and simple embodiment of such materials is to embed magnetic particles in soft polymers. Unfortunately, existing magnetically responsive soft materials such as magnetorheological elastomers and ferrogels typically use magnetically-soft particles such as iron and iron oxides, which are characterized by the low coercivity and hence lack the capability to retain remnant magnetism. Accordingly, their deformation is limited to simple elongation or shortening, rendering these materials substantially unsuited for the complex transformations required in many applications. To introduce shape-programmability, magnetically-hard particles with high coercivity have been incorporated in mechanically soft materials. In addition, recent works aimed at ameliorating this situation have developed fabrication techniques and facile routes to engineer rapid and complex transformations in a programmable manner by introducing intricate patterns of magnetic polarities in soft materials. The resulting structures, when properly designed, have been shown to exhibit a diverse and rich array of actuation behavior. In this work, we develop a suitable theoretical framework to analyze these so-called hard-magnetic soft materials to facilitate the rational design of magnetically activated functional structures and devices based on a quantitative prediction of complex shape changes. We adopt a nonlinear field theory to describe the finite deformation coupled with magnetic fields and argue that the macroscopic behavior of the fabricated materials requires a new constitutive classification — ideal hard-magnetic soft material — which assumes that (i) the material has a residual magnetic flux density, and (ii) the induced magnetic flux density exhibits a linear relation with the applied actuating magnetic field. We implement the theory and constitutive law in a finite-element framework and find remarkable agreement between the simulation and experimental results on various deformation modes of hard-magnetic soft materials. Using the developed (and validated) model, we present a set of illustrative examples to highlight the use of our model-based simulation to guide the design of experimentally realizable complex shape-morphing structures based on hard-magnetic soft materials.

Marketplace, Reseller, or Hybrid: A Strategic Analysis of Emerging E-Commerce Model

Abstract: Traditionally, online retailers have primarily been resellers where they purchase the products from the manufacturers and resell them to the consumers. Recently, an emerging e-commerce model, online marketplace has reshaped the traditional e-commerce market, where now online retailers can serve as a platform to connect manufacturers and buyers directly. Common wisdom suggests that this new format will mitigate the double-marginalization effect and benefit both the intermediary and manufacturers due to the revenue sharing scheme. However, we find that the upstream competition between the manufacturers will alter this result. Essentially, in the reseller mode, the intermediary can function as the moderator to alleviate the fierce price competition which happens in the online marketplace. Further, we also identify that the order-fulfillment cost will interplay with the intensity of the upstream competition governing the mode choice. In specific, when the order-fulfillment cost is relatively small and the competition intensity is low, the pure platform mode will become the equilibrium mode; when the order-fulfillment cost is relatively big or the competition intensity is high enough, then the pure reseller mode will arise as the equilibrium mode; further when the order-fulfillment cost and competition intensity are both at moderate level, then the hybrid mode will evolve as the equilibrium mode. The intuition hinges on the trade-off between transfer of the pricing right and the responsibility for order fulfillment accompanied by the mode change. Our findings not only complement the emerging online marketplace literature but also provide testable empirical questions on the relationship and magnitude of different factors steering the mode choice.

Reconfigurable Intelligent Surfaces Assisted Communications With Limited Phase Shifts: How Many Phase Shifts Are Enough?

Abstract: Reconfigurable intelligent surface (RIS) has drawn a great attention worldwide as it can create favorable propagation conditions by controlling the phase shifts of the reflected signals at the surface to enhance the communication quality. However, the practical RIS only has limited phase shifts, which will lead to the performance degradation. In this paper, we evaluate the performance of an uplink RIS assisted communication system by giving an approximation of the achievable data rate, and investigate the effect of limited phase shifts on the data rate. In particular, we derive the required number of phase shifts under a data rate degradation constraint. Numerical results verify our analysis.

Beam energy dependence of moments of the net-charge multiplicity distributions in Au+Au collisions at RHIC

Abstract: We report the first measurements of the moments-mean (M), variance (sigma(2)), skewness (S), and kurtosis (kappa)-of the net-charge multiplicity distributions at midrapidity in Au + Au collisions at seven energies, ranging from root s(NN) = 7.7 to 200 GeV, as a part of the Beam Energy Scan program at RHIC. The moments are related to the thermodynamic susceptibilities of net charge, and are sensitive to the location of the QCD critical point. We compare the products of the moments, sigma(2)/M, S sigma, and kappa sigma(2), with the expectations from Poisson and negative binomial distributions (NBDs). The S sigma values deviate from the Poisson baseline and are close to the NBD baseline, while the kappa sigma(2) values tend to lie between the two. Within the present uncertainties, our data do not show nonmonotonic behavior as a function of collision energy. These measurements provide a valuable tool to extract the freeze-out parameters in heavy-ion collisions by comparing with theoretical models.

Antibacterial and Antifungal Activities of Nine Medicinal Plants from Zaire

Abstract: Zairean medicinal plants were extracted and screened for in vitro antibacterial and antifungal activities. Of nine plants investigated, extracts of six plants (Alchornea cordifolia, Bridelia ferruginea, Eucalyptus citriodora, Hymenocardia acida, Maprounea africana and Sida rhombifolia) demonstrated a marked antibacterial activity against the microorganisms Klebsiella pneumoniae, Staphylococcus aureus and Streptococcus mutans; and extracts of four plants (A. cordifolia, E. citriodora, M. africana and S. rhombifolia) exhibited significant antifungal activity against Aspergillus niger, Candida albicans and Microsporum gypseum. Extracts from A. cordifolia and M. africana were the most active with a MIC value of 62.5 μg/ml. Fractionation of the crude methanol extracts revealed that the bactericidal and fungicidal constituents of the active plants were mainly distributed in their ethyl acetate and aqueous fractions. These results may provide a basis for the isolation of compounds of biological interest ...