A prototypical model for tensional wrinkling in thin sheets

The buckling and wrinkling of thin films has recently seen a surge of interest among physicists, biologists, mathematicians and engineers. This has been triggered by the growing interest in developing technologies at ever decreasing scales and the resulting necessity to control the mechanics of tiny structures, as well as by the realization that morphogenetic processes, such as the tissue-shaping instabilities occurring in animal epithelia or plant leaves, often emerge from mechanical instabilities of cell sheets. While the most basic buckling instability of uniaxially compressed plates was understood by Euler more than 200 years ago, recent experiments on nanometrically thin (ultrathin) films have shown significant deviations from predictions of standard buckling theory. Motivated by this puzzle, we introduce here a theoretical model that allows for a systematic analysis of wrinkling in sheets far from their instability threshold. We focus on the simplest extension of Euler buckling that exhibits wrinkles of finite length - a sheet under axisymmetric tensile loads. This geometry, whose first study is attributed to Lam´e, allows us to construct\ud a phase diagram that demonstrates the dramatic variation of wrinkling patterns from near-threshold to far-from-threshold conditions. Theoretical arguments and comparison to experiments show that for thin sheets the far-from-threshold regime is expected to emerge under extremely small compressive loads, emphasizing the relevance of our analysis for nanomechanics applications

Hip function characterization in the sagittal plane with varying gait speed

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2008.Includes bibliographical references (p. 22).The function of the human hip joint during the stance phase of walking can be characterized with a configuration of simple mechanical elements. This combination of elements is capable of providing general hip behavior in the sagittal plane. Data was collected from two healthy, young subjects who walked at slow, normal and fast gait speeds. The hip can be modeled with a torque actuator and two independent, linear torsional springs, which are activated at different times during the stance phase of gait. The activation times consistently identify gait cycle events across all three gait speeds. The first spring operates during the single limb stance of the gait cycle. The second spring is actuated during second double support, in the pre-swing phase. The springs effectively reduce the amount of work an unaccompanied torque actuator would have to exert in order to reproduce the hip gait pattern.by Erika R. Cerda.S.B

Optical realization of a quantum beam splitter

We show how the quantum process of splitting light may be modelled in classical optics. A second result is the possibility to engineer specific forms of a classical field.Comment: 5 page

Capillary deformations of bendable films

We address the partial wetting of liquid drops on ultrathin solid sheets resting on a deformable foundation. Considering the membrane limit of sheets that can relax compression through wrinkling at negligible energetic cost, we revisit the classical theory for the contact of liquid drops on solids. Our calculations and experiments show that the liquid-solid-vapor contact angle is modified from the Young angle, even though the elastic bulk modulus (E) of the sheet is so large that the ratio between the surface tension γ and E is of molecular size. This finding establishes a new type of “soft capillarity” that stems from the bendability of thin elastic bodies rather than from material softness. We also show that the size of the wrinkle pattern that emerges in the sheet is fully predictable, thus resolving a puzzle noticed in several previous attempts to model “drop-on-a-floating-sheet” experiments, and enabling a reliable usage of this setup for the metrology of ultrathin films

Improving Performance of QUIC in WiFi

QUIC is a new transport protocol under standardization since 2016. Initially developed by Google as an experiment, the protocol is already deployed in large-scale, thanks to its support in Chromium and Google's servers. In this paper we experimentally analyze the performance of QUIC in WiFi networks. We perform experiments using both a controlled WiFi testbed and a production WiFi mesh network. In particular, we study how QUIC interplays with MAC layer features such as IEEE 802.11 frame aggregation. We show that the current implementation of QUIC in Chromium achieves sub-optimal throughput in wireless networks. Indeed, burstiness in modern WiFi standards may improve network performance, and we show that a Bursty QUIC (BQUIC), i.e., a customized version of QUIC that is targeted to increase its burstiness, can achieve better performance in WiFi. BQUIC outperforms the current version of QUIC in WiFi, with throughput gains ranging between 20% to 30%

La nulidad del acto de expropiación. Análisis de sus fundamentos

72 p.La nulidad del acto de expropiación deriva ya sea del incumplimiento de su fin o de la desviación de su cometido específico. Por lo que primeramente para una mejor realización del tema en cuestión, expondremos las nociones básicas en relación con la institución de la expropiación, haciendo énfasis en su evolución en Chile y en los elementos que la caracterizan. Seguidamente ahondaremos en el acto expropiatorio y su correspondencia necesaria con la Ley expropiatoria, para llevar a cabo su fin, de no cumplirse éste el acto podrá ser declarado nulo, con lo que se hace obligatorio el reconocimiento a la Nulidad de Derecho Público, la que acarrea sus consecuencias y efectos particulares al acto viciado, dando origen a debates tanto doctrinarios como jurisprudenciales. Nos parece meritorio indicar el tratamiento que se le da a la nulidad del acto en el derecho comparado, derivándose en instituciones no comprendidas en nuestra legislación

Extended Huckel theory for bandstructure, chemistry, and transport. II. Silicon

In this second paper, we develop transferable semi-empirical parameters for the technologically important material, silicon, using Extended Huckel Theory (EHT) to calculate its electronic structure. The EHT-parameters areoptimized to experimental target values of the band dispersion of bulk-silicon. We obtain a very good quantitative match to the bandstructure characteristics such as bandedges and effective masses, which are competitive with the values obtained within an $sp^3 d^5 s^*$ orthogonal-tight binding model for silicon. The transferability of the parameters is investigated applying them to different physical and chemical environments by calculating the bandstructure of two reconstructed surfaces with different orientations: Si(100) (2x1) and Si(111) (2x1). The reproduced $\pi$- and $\pi^*$-surface bands agree in part quantitatively with DFT-GW calculations and PES/IPES experiments demonstrating their robustness to environmental changes. We further apply the silicon parameters to describe the 1D band dispersion of a unrelaxed rectangular silicon nanowire (SiNW) and demonstrate the EHT-approach of surface passivation using hydrogen. Our EHT-parameters thus provide a quantitative model of bulk-silicon and silicon-based materials such as contacts and surfaces, which are essential ingredients towards a quantitative quantum transport simulation through silicon-based heterostructures.Comment: 9 pages, 9 figure

Prospección de mercado de hortalizas bajo producción integrada en cuatro ciudades de la Octava Región de Chile

Published by Asociación de Economistas Agrarios de Chilewillingness to purchase, vegetables, integrated production, Agribusiness, Consumer/Household Economics, Marketing,

Effect of interactions on vortices in a nonequilibrium polariton condensate

We demonstrate the creation of vortices in a macroscopically occupied polariton state formed in a semiconductor microcavity. A weak external laser beam carrying orbital angular momentum (OAM) is used to imprint a vortex on the condensate arising from the polariton optical parametric oscillator (OPO). The vortex core radius is found to decrease with increasing pump power, and is determined by polariton-polariton interactions. As a result of OAM conservation in the parametric scattering process, the excitation consists of a vortex in the signal and a corresponding antivortex in the idler of the OPO. The experimental results are in good agreement with a theoretical model of a vortex in the polariton OPO