922 research outputs found

    Behavior of Complex Knots in Single DNA Molecules

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    We used optical tweezers to tie individual DNA molecules in knots. Although these knots become highly localized under tension, they remain surprisingly mobile and undergo thermal diffusion with classical random walk statistics. The diffusion constants of knots with different complexities correlate with theoretical calculations of knot sizes. We show that this correlation can be explained by a simple hydrodynamical model of "self-reptation" of the knot along a polymer

    Flash Flood Simulation Using Geomorphic Unit Hydrograph Method: Case Study Of Headwater Catchment Of Xiapu River Basin, China

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    The flash flood refers to flood produced by heavy local rainfalls and often occurs in mountainous areas. It is characterized by a quick rise of water level causing a great threat to the lives of those exposed. Many countries and regions face the threat of flash floods. However, some traditional hydrological models can hardly simulate the flash flood process well due to the lack of hydrological data and the insufficient understanding of complicated runoff mechanism in mountainous and hilly areas. According to this condition, a new hydrological model based on the framework of Xinanjiang model, widely used in humid and semi-humid regions in China, is presented to simulate flash flood. The highlight of new model is using the geomorphic unit hydrograph (GUH) method to simulate the overland flow process. This method has clear physical concept and can easily provide unit hydrographs of various time intervals only based on DEM data. This feature makes the method extremely valuable in ungauged catchment. The new presented hydrological model is used in the headwater catchment of Xiapu River basin and the results demonstrate that the computed data generally agrees well with the measured data and it can be treated as a useful tool for flash flood hazard assessment in mountainous catchment

    The three-dimensional simulation of vortex on the bottom of a particle in gas-liquid two- phase flow

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    The sophisticated industrial applications demand deep knowledge of local flow hydrodynamics on a particle surface in fixed bed reactors in order to improve process efficiency, particularly micro- (or particle-) scale of gas-liquid two-phase flow, which the study of vortex on the bottom of a particle is much more meaningful. The three-dimensional numerical simulations of gas-liquid two phase flow passing a spherical particle were investigated using Computational Fluid Dynamics (CFD) methodology with the volume of fluid (VOF) method. The effects of gas velocity, liquid velocity, liquid-solid contact angle, surface tension coefficient, and liquid viscosity on the interface status of fluid flow were presented in this paper. The simulation conditions were particle diameters of 10mm, water and air as liquid and gas respectively, droplet size 4mm, and an atmospheric pressure. Results show that the droplet movement and gas streamlines are greatly dependent on the air flow conditions and the liquid physical conditions. To better observe the flow field around the liquid droplet, the 3D plot is drawn for the contact angle of 50° with the gas velocity of 1.0m/s at the time instant of 0.015s as shown in Figure 1. Vortices are produced on the bottom of a particle known as Von Kármán vortex street, which have the opposite direction of rotation and the double row arrangement vortex. In addition, vortices are found to be more obvious and farer from the particle at higher gas and/or liquid velocities, and the droplet moves faster with the increasing gas velocity (0.2m/s-2.0m/s) and liquid velocity (0.24m/s-0.465m/s). Moreover, the generated vortices can be clearly observed at downstream of particle, and the droplet shape varies with the flow time. Vortex radius changes from 0.5mm to 2.5mm with the time going at the gas velocity 0.5m/s and liquid velocity 0.24m/s. The liquid-solid contact angle between 40° and 80° mainly affects particle surface wettability, which results in the different droplet shape and flow gas streamlines. At the beginning, the shape of the droplet is spherical and thereafter it changes to appropriate shape according to the surface properties and the contact angle. When the liquid-solid contact angle is 50°, vortex is comparatively density which radius is only 0.5mm initially, and then changes to 3.5mm approximately with an increase of the flow time. Meanwhile, the surface tension can affect the contractile properties and the gas streamline Please click Additional Files below to see the full abstract

    On-chip Spectrometer Formed by a Multi-stage Structure

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    With apparent size and weight advantages, on-chip spectrometer could be a good choice for the spectrum analysis application which has been widely used in numerous areas such as optical network performance monitoring, materials analysis and medical research. In order to realize the broadband and the high resolution simultaneously, we propose a new on-chip spectrometer structure, which is a two-stage structure. The coarse wavelength division is realized by the cascaded Mach-Zehnder interferometers, which is the first stage of the spectrometer. The output of the Mach-Zehnder interferometers are further dispersed by the second stage structure, which can be realized either by arrayed waveguide gratings or by digital Fourier transform spectrometer structure. We further implemented the thermo-optic modulation for the arrayed waveguide gratings to achieve a higher spectral resolution. The output channel wavelengths of the spectrometer are modulated by the embedded heater to obtain the first order derivative spectra of the input optical signal to obtain a 2nm resolution. With respect to the computer simulation and device characterization results, the 400nm spectral range and the nanoscale resolution have been demonstrated

    Novel Insights into the Role of the Cytoskeleton in Cancer

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    The cytoskeleton is a complex network of highly ordered intracellular filaments that plays a central role in controlling cell shape, division, functions, and interactions in human organs and tissues, but dysregulation of this network can contribute to numerous human diseases, including cancer. To clarify the various functions of the cytoskeleton and its role in cancer progression, in this chapter, we will discuss the microfilament (actin cytoskeleton), microtubule (╬▓ÔÇÉtubulin), and intermediate filament (keratins, cytokeratins, vimentin, and lamins) cytoskeletal structures; analyze the physiological functions of the cytoskeleton and its regulation of cell differentiation; and investigate the roles of the cytoskeleton in cancer progression, the epithelialÔÇÉmesenchymal transition process (EMT), and the mechanisms of multidrug resistance (MDR) in relation to the cytoskeleton. Importantly, the cytoskeleton, as a key regulator of the transcription and expression of many genes, is known to be involved in various physiological and pathological processes, which makes the cytoskeleton a novel and highly effective target for assessing the response to antitumor therapy in cancer

    From perfect secrecy to perfect safety & security: Cryptography-based analysis of endogenous security

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    In this paper, we propose a conjecture that endogenous security without any prior knowledge is similar to perfect secrecy without any prior knowledge. To prove the conjecture, we first establish a cryptography model of instinct function security to transform the security problem in the network domain into an encryption problem in the cryptographic domain. Then, we inherit and apply the established ideas and means of Perfect Secrecy, and propose the concept, definition and corollaries of the perfect instinct function security (PIFS) corresponding to Perfect Secrecy. Furthermore, we take the DHR system as a concrete implementation of PIFS and propose the DHR Perfect Security Theorem corresponding to ShannonÔÇÖs Perfect Secrecy Theorem. Finally, we prove that the DHR satisfying the ÔÇťOne-Time ReconstructionÔÇŁ constraint is the sufficient and necessary condition to achieve perfect security. This means that the existence of PIFS is also proven. The analysis shows that any reconfigurable system can be encrypted by its construct and that the PIFS converts the one-way transparent superiority of the attacker into a double-blind problem for both the attacker and the defender, which leads to that the attacker is impossible to obtain useful construction information from the attacks and unable to find a better way than blind trial-and-error or brute-force attacks. Since the attackers are required to have the new powerful ability to crack the structure cryptogram, the threshold of cyber security is raised to at least the same level as cryptogram deciphering, thereafter the ubiquitous cyber threats are destined to be significantly reduced

    Determination of four sulfonamides residues in prawn by ultrasound-assisted matrix solid phase dispersive extraction combined with pre-column derivation high performance liquid chromatography-fluorescence

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    Objective To establish a method for the simultaneous determination of four sulfonamides residues in prawn, including sulfadiazine, sulfathiazole, sulfamerazine and sulfamethazine, by ultrasound-assisted matrix solid phase dispersion extraction coupled with pre - column derivation high performance liquid chromatography(HPLC). Methods Through the optimization of extraction conditions, ethyl acetate was selected as the extraction agent and florisil as the solid dispersion agent, and the sulfonamides in prawn were extracted with the method ultrasound-assisted matrix solid phase dispersion. The sulfonamides were pre-column derived by fluorescamine and detected by HPLC-fluorescence method. Results All sulfonamides showed good linearity in the concentration range of 2-100╬╝g/L, with the correlation coefficient>0.999. The limit of detection and limit of quantification was 0.5 and 2╬╝g/kg, respectively. The spike recoveries of blank prawn samples were 84.4%-93.9% at two levels of 2 and 20╬╝g/kg, with the relative standard deviation(n =3)less than 7.7%. Conclusion The method is simple, time-consuming and high precision, which meets the requirements of residue analysis

    Effects of Altitude on Fire Smoke Diffusion in Semi-Lateral Smoke Exhaust Highway Tunnels

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    This paper aims to study the effects of altitude and the size of smoke outlet on fire smoke diffusion and discharge in semi-lateral smoke exhaust highway tunnels. At first, simulations of semi-lateral smoke exhaust highway tunnels were carried out in FDS (Fire Dynamics Simulator), then the distribution laws of temperature, CO concentration, smoke mass flow, and visibility in the tunnel under the conditions of different altitudes and smoke outlet areas were analyzed to figure out the effects of altitude and size of smoke outlet on fire smoke diffusion and discharge in the said tunnels. The results suggest that, in case of the same fire source power, the velocity of smoke diffusion increases with the altitude; for curves of different altitudes, the tunnel roof temperature features are basically the same, that is, the higher the altitude, the higher the temperature at the tunnel roof. When the fire source power is 20 MW, the smoke mass flow at the smoke outlet decreases with the increase of altitude, but the CO concentration grows with it, indicating that the smoke exhaust efficiency is higher in high-altitude areas. When the altitude reaches 4200 m and the fire source power is 20 MW, with the increase of smoke outlet area, the smoke discharge effect of the tunnel shows an upward trend, taking both the smoke discharge effect and economy into consideration; the smoke outlet should take a size of 4 x 3 m
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