Nucleated dewetting in supported ultra-thin liquid films with hydrodynamic slip

This study reveals the influence of the surface energy and solid/liquid boundary condition on the breakup mechanism of dewetting ultra-thin polymer films. Using silane self-assembled monolayers, SiO$_2$ substrates are rendered hydrophobic and provide a strong slip rather than a no-slip solid/liquid boundary condition. On undergoing these changes, the thin-film breakup morphology changes dramatically -- from a spinodal mechanism to a breakup which is governed by nucleation and growth. The experiments reveal a dependence of the hole density on film thickness and temperature. The combination of lowered surface energy and hydrodynamic slip brings the studied system closer to the conditions encountered in bursting unsupported films. As for unsupported polymer films, a critical nucleus size is inferred from a free energy model. This critical nucleus size is supported by the film breakup observed in the experiments using high speed \emph{in situ} atomic force microscopy.Comment: 8 pages, 9 figures, including supplementary materia

Capillary leveling of stepped films with inhomogeneous molecular mobility

A homogeneous thin polymer film with a stepped height profile levels due to the presence of Laplace pressure gradients. Here we report on studies of polymeric samples with precisely controlled, spatially inhomogeneous molecular weight distributions. The viscosity of a polymer melt strongly depends on the chain length distribution; thus, we learn about thin-film hydrodynamics with viscosity gradients. These gradients are achieved by stacking two films with different molecular weights atop one another. After a sufficient time these samples can be well described as having one dimensional viscosity gradients in the plane of the film, with a uniform viscosity normal to the film. We develop a hydrodynamic model that accurately predicts the shape of the experimentally observed self-similar profiles. The model allows for the extraction of a capillary velocity, the ratio of the surface tension and the viscosity, in the system. The results are in excellent agreement with capillary velocity measurements of uniform mono- and bi-disperse stepped films and are consistent with bulk polymer rheology.Comment: Accepted for publication in Soft Matter, Themed Issue on "The Geometry and Topology of Soft Materials

Self-Similarity and Energy Dissipation in Stepped Polymer Films

The surface of a thin liquid film with nonconstant curvature is unstable, as the Laplace pressure drives a flow mediated by viscosity. We present the results of experiments on one of the simplest variable curvature surfaces: a stepped polymer film. Height profiles are measured as a function of time for a variety of molecular weights. The evolution of the profiles is shown to be self-similar. This self-similarity offers a precise measurement of the capillary velocity by comparison with numerical solutions of the thin film equation. We also derive a master expression for the time dependence of the excess free energy as a function of the material properties and film geometry. The experiment and theory are in excellent agreement and indicate the effectiveness of stepped polymer films to elucidate nanoscale rheological properties.Comment: 5 pages, 4 figures, article accepted for publication in Physical Review Letter

Capillary-driven flow induced by a stepped perturbation atop a viscous film

Thin viscous liquid films driven by capillarity are well described in the lubrication theory through the thin film equation. In this article, we present an analytical solution of this equation for a particular initial profile: a stepped perturbation. This initial condition allows a linearization of the problem making it amenable to Fourier analysis. The solution is obtained and characterized. As for a temperature step in the heat equation, self-similarity of the first kind of the full evolution is demonstrated and a long-term expression for the excess free energy is derived. In addition, hydrodynamical fields are described. The solution is then compared to experimental profiles from a model system: a polystyrene nanostep above the glass transition temperature which flows due to capillarity. The excellent agreement enables a precise measurement of the capillary velocity for this polymeric liquid, without involving any numerical simulation. More generally, as these results hold for any viscous system driven by capillarity, the present solution may provide a useful tool in hydrodynamics of thin viscous films.Comment: Accepted for publication in Physics of Fluid

What's working memory to do with it? A case study on teenagers

Effective teachers recognise that as their students grow, the way in which their students learn changes. This is related to different developmental stages of the brain that occur as a child becomes an adult. This article discusses the concept of working memory and explores how working memory changes during adolescence. The research presented here used an approach to measuring working memory using electroencephalography (EEG) to examine differences in the capacity for using working memory between older and younger adolescent students at a school in Western Australia. The differences in the neurological processes related to working memory in adolescents of different ages were examined with implications for teachers in secondary schools

What's working memory to do with it? A case study on teenagers

Effective teachers recognise that as their students grow, the way in which their students learn changes. This is related to different developmental stages of the brain that occur as a child becomes an adult. This article discusses the concept of working memory and explores how working memory changes during adolescence. The research presented here used an approach to measuring working memory using electroencephalography (EEG) to examine differences in the capacity for using working memory between older and younger adolescent students at a school in Western Australia. The differences in the neurological processes related to working memory in adolescents of different ages were examined with implications for teachers in secondary schools

Academic Perspectives on Agribusiness: An International Survey

The IFAMR is published by (IFAMA) the International Food and Agribusiness Management Review. www.ifama.orgpromotion and tenure, agribusiness, teaching, grantsmanship, research, Agribusiness, Institutional and Behavioral Economics, Productivity Analysis, Teaching/Communication/Extension/Profession, Q130,