Fluttering-induced flow in a closed chamber

We study the emergence of fluid flow in a closed chamber that is driven by dynamical deformations of an elastic sheet. The sheet is compressed between the sidewalls of the chamber and partitions it into two separate parts, each of which is initially filled with an inviscid fluid. When fluid exchange is allowed between the two compartments of the chamber, the sheet becomes unstable, and its motion displaces the fluid from rest. We derive an analytical model that accounts for the coupled, two-way, fluid-sheet interaction. We show that the system depends on four dimensionless parameters: the normalized excess length of the sheet compared to the lateral dimension of the chamber, $\Delta$; the normalized vertical dimension of the chamber; the normalized initial volume difference between the two parts of the chamber, $v_{\text{du}}(0)$; and the structure-to-fluid mass ratio, $\lambda$. We investigate the dynamics at the early times of the system's evolution and then at moderate times. We obtain the growth rates and the frequency of vibrations around the second and the first buckling modes, respectively. Analytical solutions are derived for these linear stability characteristics within the limit of the small-amplitude approximation. At moderate times, we investigate how the sheet escapes from the second mode. Given the chamber's dimensions, we show that the initial energy of the sheet is mostly converted into hydrodynamic energy of the fluid if $\lambda\ll 1$, and into kinetic energy of the sheet if $\lambda\gg 1$. In both cases most of the initial energy is released at time $t_{\text{p}}\simeq \ln[c \Delta^{1/2}/v_{\text{du}}(0)]/\sigma$, where $\sigma$ is the growth rate and $c$ is a constant.Comment: 25 pages, 12 figure

Surface nanochemical studies of polymers and other organic surfaces by scanning force microscopy

The nanochemical characterization of organic/polymer surfaces is an increasing technological need that cannot be met by current ultrahigh vacuum, charged particle analytical approaches. The high spatial resolution of scanning force microscopy offers the possibility of making measurements on a nanometer scale relatively simply. In the case of species displaying different pKa values that are within a reasonable range, electrostatic contrast may be used in order to distinguish between surface species (such as COOH and CH3, for example), when the sample is examined by lateral force microscopy. In the case of most polymers, however, a more promising approach is to obtain contrast by measuring a combination of van der Waals forces and (for polar polymers) H-bonding and polar effects. This can be used to distinguish components of certain polymer blends, for example. In the case of distinguishing between non-polar polymers, the pull-off forces measured by AFM can be calculated by applying the appropriate contact mechanical model, provided that this series of polymers exhibits similar mechanical properties. Under this condition, the pull-off force is proportional to the work of adhesion, which can also be calculated from Israelachvili's approximation to the Lifshitz theory of van der Waals interactions, leading to the observation that pull-off force increases with refractive index of the polymer. Chemical differences between organic surfaces can frequently be distinguished by AFM approaches, provided that the situation is not dominated by the effects of mechanical properties.</p

High-Performance Polyethylene Fibers “Al Dente”: Improved Gel-Spinning of Ultrahigh Molecular Weight Polyethylene Using Vegetable Oils

We demonstrate that the major drawbacks of so-called gel spinning and solid-state processing of “virgin”, i.e. never molten or fully dissolved, ultrahigh molecular weight polyethylene (UHMW PE) to produce ultrahigh modulus and ultrahigh strength fibers and films, which are the unfavorably low polymer concentrations in highly flammable solvents typically employed in the former process and low production rates in the latter, can be largely avoided by employing relatively poor—as opposed to good—solvents, including, among others, fatty acids and natural oils omnipresent in, for example, fruits, nuts, and seeds, which have additional major recovery and environmental advantages.ISSN:1520-5835ISSN:0024-929

Oligo(ethylene glycol)-terminated monolayers under electrolyte solution studied with scanning force microscopy

Oligo(ethylene glycol) terminated molecules were adsorbed on gold. A repulsion with hydrophobically functionalized probes under electrolyte solution was observed. This repulsive force depends strongly on the ion concentration indicating an electrostatic origin. Short chain oligomers with 3-8 EG units adsorbed on gold carry some effective surface charge, while this vanishes for a chain with 45 units.</p

Surface forces, surface chemistry and tribology

Scanning probe methods have been applied to the investigation of tribological phenomena on the nanometre and nanonewton scale. The systems studied have included parallel investigation of identical tribosystems on the macro and nano scales, where the inherent differences in the AFM/LFM and flat-on-disk experiments have been compared; oxide-covered surfaces in contact under electrolytes, where the adhesion hysteresis and frictional behaviour was shown to be strongly dependent on the solution pH; and polymer surfaces, where advantage can be taken of variations in the interactions between the scanning tip and different polymers, to perform chemically sensitive, high-resolution surface imaging of polymer blends. (C) 1998 Elsevier Science Ltd. All rights reserved.</p

One-step creation of hierarchical fractal structures

Relatively recently, we advanced a route to create, in a controlled fashion, combined horizontal and vertical stratified structures by simple and energy-efficient processing operations employing static mixing elements. While in state-of-the-art static mixing the focus is on layer multiplication, here the aim is to create hierarchical fractal structures. Therefore, the main question addressed in this article is how structures, rather than layers, can be multiplied. The key aspect is the addition of layers on the sides or in the midplane of the flow during the process; every addition step increases the hierarchy by one level. This article derives the general formalism for forming fractal structures with controlled hierarchy, and we develop the language required to design and construct the dies. The main part of the article addresses this main topic and is based on the splitting serpentine static mixer geometry that can be easily made on the parting surfaces of a mold on both the micro- and the macroscale. The second part of the article addresses the strategy to minimize the number of mirroring steps, eventually avoiding mirroring completely, and is based on the rotation-free multiflux static mixer geometry. With the design language derived, complex hierarchical fractal structures can be generated simply by changing the number and sequence of operators within extrusion dies or molds, providing a one-step solution to produce material structures for potential use in diverse applications ranging from advanced mechanical systems to photovoltaic devices, where controlled assembly of dissimilar materials, and the realization of huge interfaces and genuine cocontinuity throughout the cross section, is critical.</p