Nature of self-diffusion in two-dimensional fluids

Self-diffusion in a two-dimensional simple fluid is investigated by both analytical and numerical means. We investigate the anomalous aspects of self-diffusion in two-dimensional fluids with regards to the mean square displacement, the time-dependent diffusion coefficient, and the velocity autocorrelation function using a consistency equation relating these quantities. We numerically confirm the consistency equation by extensive molecular dynamics simulations for finite systems, corroborate earlier results indicating that the kinematic viscosity approaches a finite, non-vanishing value in the thermodynamic limit, and establish the finite size behavior of the diffusion coefficient. We obtain the exact solution of the consistency equation in the thermodynamic limit and use this solution to determine the large time asymptotics of the mean square displacement, the diffusion coefficient, and the velocity autocorrelation function. An asymptotic decay law of the velocity autocorrelation function resembles the previously known self-consistent form, $1/(t\sqrt{\ln t})$, however with a rescaled time.Comment: 10 pages, to appear in New Journal of Physic

A Survey on Password Guessing

Text password has served as the most popular method for user authentication so far, and is not likely to be totally replaced in foreseeable future. Password authentication offers several desirable properties (e.g., low-cost, highly available, easy-to-implement, reusable). However, it suffers from a critical security issue mainly caused by the inability to memorize complicated strings of humans. Users tend to choose easy-to-remember passwords which are not uniformly distributed in the key space. Thus, user-selected passwords are susceptible to guessing attacks. In order to encourage and support users to use strong passwords, it is necessary to simulate automated password guessing methods to determine the passwords' strength and identify weak passwords. A large number of password guessing models have been proposed in the literature. However, little attention was paid to the task of providing a systematic survey which is necessary to review the state-of-the-art approaches, identify gaps, and avoid duplicate studies. Motivated by that, we conduct a comprehensive survey on all password guessing studies presented in the literature from 1979 to 2022. We propose a generic methodology map to present an overview of existing methods. Then, we explain each representative approach in detail. The experimental procedures and available datasets used to evaluate password guessing models are summarized, and the reported performances of representative studies are compared. Finally, the current limitations and the open problems as future research directions are discussed. We believe that this survey is helpful to both experts and newcomers who are interested in password securityComment: 35 pages, 5 figures, 5 table

Enhancement of pool boiling heat transfer using nanostructured surfaces on aluminum and copper

Enhanced pool boiling critical heat fluxes (CHF) at reduced wall superheat on nanostructured substrates are reported. Nanostructured surfaces were realized using a low temperature process, microreactor-assisted-nanomaterial-deposition. Using this technique we deposited ZnO nanostructures on Al and Cu substrates. We observed pool boiling CHF of 82.5 W/cm2 with water as fluid for ZnO on Al versus a CHF of 23.2 W/cm2 on bare Al surface with a wall superheat reduction of 25-38 C. These CHF values on ZnO surfaces correspond to a heat transfer coefficient of ~ 23000 W/m2K. We discuss our data and compare the behavior with conventional boiling theory.Keywords: boiling enhancement, electronic cooling, nanoscale heat transferKeywords: boiling enhancement, electronic cooling, nanoscale heat transfe

Spermidine-induced recovery of human dermal structure and barrier function by skin microbiome.

An unbalanced microbial ecosystem on the human skin is closely related to skin diseases and has been associated with inflammation and immune responses. However, little is known about the role of the skin microbiome on skin aging. Here, we report that the Streptococcus species improved the skin structure and barrier function, thereby contributing to anti-aging. Metagenomic analyses showed the abundance of Streptococcus in younger individuals or those having more elastic skin. Particularly, we isolated Streptococcus pneumoniae, Streptococcus infantis, and Streptococcus thermophilus from face of young individuals. Treatment with secretions of S. pneumoniae and S. infantis induced the expression of genes associated with the formation of skin structure and the skin barrier function in human skin cells. The application of culture supernatant including Streptococcal secretions on human skin showed marked improvements on skin phenotypes such as elasticity, hydration, and desquamation. Gene Ontology analysis revealed overlaps in spermidine biosynthetic and glycogen biosynthetic processes. Streptococcus-secreted spermidine contributed to the recovery of skin structure and barrier function through the upregulation of collagen and lipid synthesis in aged cells. Overall, our data suggest the role of skin microbiome into anti-aging and clinical applications

Fabrication and characterization of nanostructured surfaces for enhanced heat transfer

This objective of this study is to investigate the capability of nanostructured surfaces on dissipating heat flux by performing pool boiling and convective flow boiling. The generation of ultra-high heat flux from high performance electric devices has motivated a number of investigations related to advanced heat transfer especially in two-phase boiling performance. It has been reported by a number of researchers that nanostructured surfaces can result in much enhanced boiling performance, compared to the conventional methods by creating desire conditions for heat transfer. In this thesis, various nanostructured surfaces having different morphology were prepared on several engineering relevant substrates and were characterized for their pool boiling performance. Microreactor-assisted-nanomaterial-deposition, MAND™ was used to fabricate a variety of different ZnO nanostructured surfaces by careful adjustment of the processing parameters. ZSM-5 zeolite was synthesized using hydrothermal reaction. ZnO nanostructures in minichannel were also successfully deposited via a flow cell for the application of flow boiling experiment. Scanning electron microscopy (SEM) and Atomic Force Microscopy (AFM) were carried out to characterize the micro- and nanostructures. Contact angle measurement was conducted to evaluate wettability and X-ray Diffraction (XRD) was used to determine the crystalline structures. The most significant enhancement of critical heat flux (CHF) and heat transfer coefficient (HTC) was observed in the flower like ZnO nanostructured surface. We observed pool boiling CHF of 80-82.5 W/cm² for nanostructured ZnO on Al surfaces versus a CHF of 23.2 W/cm² on a bare Al surface with a wall superheat reduction of 25-38°C. This new CHF values on nanostructured surfaces corresponds to a boiling heat transfer coefficient as high as ~ 23000 W/m²K. This represents an increase of almost 4X in CHF on nano-textured surfaces, which is the highest enhancement factor reported today

A Feasibility Analysis of an Application-Based Partial Initialization (API) Protocol for Underwater Wireless Acoustic Sensor Networks

Initialization methods for underwater wireless acoustic sensor networks (UWASNs) have been proposed as a subset of other network protocols under the simple assumption that all the nodes in the network can be initialized at once. However, it is generally time- and energy-intensive to initialize all nodes in a UWASN due to unstable underwater channel conditions. To improve network efficiency, we propose the Application-based Partial Initialization (API) protocol, which initializes only the same number of nodes as the number of activated nodes required to run a specific application. Reducing the number of active nodes is also particularly advantageous underwater since the replacement of batteries is costly. To the best of our knowledge, the API is the first approach that initializes nodes partially according to applications. Thus, we investigate the feasibility of the API for a UWASN by analyzing its performance via simulations. From the results, it is shown that the API provides similar data statistics compared with the conventional full initialization that initializes all nodes. Moreover, the API outperforms the full initialization in terms of the initialization time and message overhead performances

A Study of Standardizing Frequencies Using Channel Raster for Underwater Wireless Acoustic Sensor Networks

In this paper, we propose the method to standardize acoustic frequencies for underwater wireless acoustic sensor networks (UWASNs) by applying the channel raster used in the terrestrial mobile communications. The standardization process includes: (1) Setting the available acoustic frequency band where a channel raster is employed via the frequency specification analysis of the state-of-the art underwater acoustic communication modems. (2) Defining the center frequencies and the channel numbers as a function of channel raster, and the upper limit of the value of channel raster. (3) Determining the value of the channel raster suitable for the available acoustic frequency band via simulations. To set the value, three performance metrics are considered: the collision rate, the idle spectrum rate, and the receiver computational complexity. The simulation results show that the collision rate and the idle spectrum rate according to the value of channel raster have a trade-off relationship, but the influence of channel raster on the two performance metrics is insignificant. However, the receiver computational complexity is enhanced remarkably as the value of channel raster increases. Therefore, setting the value of channel raster close to its upper limit is the most adequate in respect of mitigating the occurrence of a collision and enhancing the reception performance. The standardized frequencies based on channel raster can guarantee the frequency compatibility required for the emerging technologies like the Internet of Underwater Things (IoUT) or the underwater cognitive radio, but also improves the network performance by avoiding the arbitrary use of frequencies