Peeling from a patterned thin elastic film

Inspired by the observation that many naturally occurring adhesives arise as textured thin films, we consider the displacement controlled peeling of a flexible plate from an incision-patterned thin adhesive elastic layer. We find that crack initiation from an incision on the film occurs at a load much higher than that required to propagate it on a smooth adhesive surface; multiple incisions thus cause the crack to propagate intermittently. Microscopically, this mode of crack initiation and propagation in geometrically confined thin adhesive films is related to the nucleation of cavitation bubbles behind the incision which must grow and coalesce before a viable crack propagates. Our theoretical analysis allows us to rationalize these experimental observations qualitatively and quantitatively and suggests a simple design criterion for increasing the interfacial fracture toughness of adhesive films.Comment: 8 pages, To appear in Proceedings of Royal Society London, Ser.

From melt flow index to rheogram

A knowledge of the complete flow curve or rheogram of a polymeric melt depicting the variation of the melt viscosity over industrially relevant range of shear rate and temperature is essential in the design of polymer processing equipment, process optimization and trouble-shooting. These data are generated on sophisticated rheometers that are beyond the financial and technical means of most plastics processors. The only flow parameter available to the processor is the melt flow index of the material. In the present work, a method has been proposed to estimate the rheograms of a melt at temperatures relevant to its processing conditions with the use of a master curve, knowing the melt flow index and glass transition temperature of the material. Master curves that coalesce rheograms of different grades at various temperatures have been generated and presented for low density polyethylene, high density polyethylene, polypropylene, polystyrene and styrene-acrylonitrile copolymer

Dynamic of a non homogeneously coarse grained system

To study materials phenomena simultaneously at various length scales, descriptions in which matter can be coarse grained to arbitrary levels, are necessary. Attempts to do this in the static regime (i.e. zero temperature) have already been developed. In this letter, we present an approach that leads to a dynamics for such coarse-grained models. This allows us to obtain temperature-dependent and transport properties. Renormalization group theory is used to create new local potentials model between nodes, within the approximation of local thermodynamical equilibrium. Assuming that these potentials give an averaged description of node dynamics, we calculate thermal and mechanical properties. If this method can be sufficiently generalized it may form the basis of a Molecular Dynamics method with time and spatial coarse-graining.Comment: 4 pages, 4 figure

Rheograms for asphalt from single viscosity measurement

Asphalt materials are used in a variety of applications such as road paving, waterproofing, roofing membranes, adhesive binders, rust proofing and water resistant coatings. There are available in a number of grades distinguished in terms of their softening point and flow resistance. The selection of the proper grade of asphalt for a particular application is governed by the desired flow behaviour. A knowledge of the complete flow curve depicting the variation of melt viscosity with shear rate at the relevant temperatures is necessary not only for proper grade selection, but also for specifying processing conditions for aggregate mixing and spraying. The rheological data are also useful in assessing end use performance. The scientific techniques for generating the rheological data involve the use of expensive, sophisticated instruments. Generation of the necessary flow data using these instruments is beyond the financial and technical means of most processors of asphalt materials. The engineering techniques involving the use of inexpensive vacuum viscometers are relatively easy, but provide a single point viscosity measurement at low shear rate. In the present work, a method is proposed for unifying the viscosity versus shear rate a data at various temperatures for a number of asphalt grades. A master curve has been generated that is independent of the grade of asphalt and the temperature of viscosity measurement. The master curve can be used to generate rheograms at desired temperatures for the asphalt grade of interest, knowing its zero-shear viscosity at that temperature

Melt rheology of polymer blends from melt flow index

Polymer blends have received a lot of attention in recent years due to the possibility of getting compounds with novel and/or different properties through proper marriage of the properties of the respective mono-components. The flow behaviour of blends when subjected to stresses is complex and is often found not to vary monotonically with composition. Mixing rules and mixture theories have been used for estimating melt viscosity of a blend at zero shear rate. However a knowledge of the entire rheogram is desirable for process optimization, process design and trouble shooting. In the present paper a method proposed earlier to estimate the rheograms of polymer melts through the use of the melt flow index has been extended to polyblends. A method for obtaining the melt flow index of the polymer blends at various compositions from the melt flow index of the individual components and the blend ratio has been suggested based on the altered free volume state model. Curves that are coalesced using the melt flow index of the blend at different blend ratios have been presented for a polypropylene-high density polyethylene blend, a high density polyethylene-polymethyl methacrylate blend, a polystyrene-polymethyl-methacrylate blend, a polystyrene-polyacetal blend and a polymethylmethacrylate-polyacetal blend

Rheology of nylon 6 containing metal halides

Addition of metal halides to nylons has been shown to be advantageous in a number of ways. The decrease in melting temperature, increase in glass transition temperature and melt viscosity by such additions have allowed more convenient processing of low molecular weight polymers and thermally unstable polymers. Rheological data depicting the variation of melt viscosity with shear rate at temperatures relevant to processing are necessary in optimizing and trouble-shooting plastics processing operations. In the present paper, a method has been proposed to estimate, complete flow curves or rheograms of nylon-metal halide systems with the use of a master curve knowing the melt flow index and glass transition temperature of the system. The validity of the approach has been verified for the nylon 6-lithium chloride system and shown to hold good for any nylon-metal halide combination

Rheograms for engineering thermoplastics from melt flow index

A method proposed earlier has been extended to estimate complete flow curves or rheograms of engineering plastics. Master curves that are independent of the grade and temperature have been generated and presented for acrylics, polyacetal, nylons, polyethylene terephthalate, polycarbonate and polysulfone. The influence of the various molecular parameters on the viscosity behaviour of polymer melts have been explained rationally. More specifically, the effects of chain branching and of chain rigidity on the master curve of a resin type have been elucidated with reference to polyacetal and polysulfone, respectively. The method presented here can be used effectively by processors of engineering plastics

Natural convection of non-Newtonian power-law fluid in a square cavity with a heat-generating element

Development of modern technology in microelectronics and power engineering necessitates the creation of effective cooling systems. This is made possible by the use of the special fins technology within the cavity or special heat transfer liquids in order to intensify the heat removal from the heat-generating elements. The present work is devoted to the mathematical modeling of thermogravitational convection of a non-Newtonian fluid in a closed square cavity with a local source of internal volumetric heat generation. The behavior of the fluid is described by the Ostwald-de Waele power law model. The defining Navier–Stokes equations written using the dimensionless stream function, vorticity and temperature are solved using the finite difference method. The effects of the Rayleigh number, power-law index, and thermal conductivity ratio on heat transfer and the flow structure are studied. The obtained results are presented in the form of isolines of the stream function and temperature, as well as the dependences of the average Nusselt number and average temperature on the governing parameters

Neural Mechanisms for Heading and Structure-from Motion Perception

Two of the most important perceptual functions of the visual motion system are to compute our direction of heading as we move through the environment, and to deduce the three-dimensional structure of objects and the environment from motion cues. Below, we review experiments that provide insights into how these perceptual phenomena are constructed by the brain. Understanding how the motion system performs these analyses will likely have general applicability to other perceptual functions, both within and outside the motion pathway. For instance, understanding how motion signals are perceived as spatially constant despite eye movements, an important prerequisite for determining heading direction, may lead to a general understanding of spatial-perceptual constancy. Likewise, understanding how three-dimensional form is processed from motion cues in the dorsal visual pathway may provide important suggestions as to how form is derived from other visual cues in the ventral visual pathway

Neural Mechanisms for Heading and Structure-from Motion Perception

Two of the most important perceptual functions of the visual motion system are to compute our direction of heading as we move through the environment, and to deduce the three-dimensional structure of objects and the environment from motion cues. Below, we review experiments that provide insights into how these perceptual phenomena are constructed by the brain. Understanding how the motion system performs these analyses will likely have general applicability to other perceptual functions, both within and outside the motion pathway. For instance, understanding how motion signals are perceived as spatially constant despite eye movements, an important prerequisite for determining heading direction, may lead to a general understanding of spatial-perceptual constancy. Likewise, understanding how three-dimensional form is processed from motion cues in the dorsal visual pathway may provide important suggestions as to how form is derived from other visual cues in the ventral visual pathway