Fluorination of diamond — C4F9I and CF3I photochemistry on diamond (100)

The radiation-induced decomposition of C₄F₉I and CF₃I overlayers at 119 K on diamond (100) surfaces has been shown to be an efficient route to fluorination of the diamond surface. X-ray photoelectron spectroscopy has been used for photoactivation as well as for studying the photodecomposition of the fluoroalkyl iodide molecules, the attachment of the photofragments to the diamond surface, and the thermal decomposition of the fluoroalkyl ligands. Measured chemical shifts agree well with ab initio calculations of both C 1s and F 1s binding energies. It is found that chemisorbed CF₃ groups on diamond (100) decompose by 300 K whereas C₄F₉ groups decompose over the range 300 to 700 K and this reactivity difference is rationalized on steric grounds. Both of these thermal decomposition processes produce surface C---F bonds on the diamond. The surface C---F species thermally decompose over a wide temperature range extending up to 1500 K. Hydrogen passivation of the diamond surface is ineffective in preventing free radical attack from the photodissociated products of the fluoroalkyl iodides; I atoms produced photolytically abstract H from surface C---H bonds to yield hydrogen iodide at 119 K allowing diamond fluorination. The attachment of chemisorbed F species to the diamond (100) surface causes band bending as the surface states are occupied as a result of chemisorption. This results in a shift to higher binding energy of the diamond-related C 1s levels present in the surface and subsurface regions which are sampled by XPS on the diamond. The use of photoactivation of fluoroalkyl iodides for the fluorination of diamond surfaces provides a convenient route compared to other methods involving the action of atomic F, molecular F₂, XeF₂ and F-containing plasmas.     

Front roughening in three-dimensional imbibition

We investigate the structure and dynamics of the interface between two immiscible liquids in a three-dimensional disordered porous medium. We apply a phase-field model that includes explicitly disorder and discuss both spontaneous and forced imbibition. The structure of the interface is dominated by a length scale ξ_× which arises from liquid conservation. We further show that disorder in the capillary and permeability act on different length scales and give rise to different scalings and structures of the interface properties. We conclude with a range of applications.     

低噪音摩阻复台材料影响因素的探讨

采用动态粘弹谱仪（ＤＤＶ）、电子扫描显微镜（ＳＥＭ）和摩擦试验机等对影响摩阻复合材料制动噪音的因素进行了探讨。结果表明，摩阻材料基体的合理改性以提高其损耗角正切ｔａｎδ对减少振动、降低噪音行之有效。同时对传统解决噪音的方法，如添加固体润滑剂、橡胶共混等进行了分析比较。     

Friction coefficients of spheres having sticky or hairy surfaces

The theoretical equations for friction coefficients of spheres having sticky or hairy surfaces are presented. In the present treatment, the sticky surface is represented by a higher viscosity of fluid in the surface layer and the hairy surface is assumed to have a porous structure. The friction coefficients are given as functions of the thickness of the surface layer, the fluid viscosity in the surface layer and the segmental density of hairy chains.     

Intermittency as a transition to turbulence in pipes: A long tradition from Reynolds to the 21st century

Intermittencies are commonly observed in fluid mechanics, and particularly, in pipe flows. Initially observed by Reynolds (1883), it took one century for reaching a rather full understanding of this phenomenon whose irregular dynamics (apparently stochastic) puzzled hydrodynamicists for decades. In this brief (non-exhaustive) review, mostly focused on the experimental characterization of this transition between laminar and turbulent regimes, we present some key contributions for evidencing the two concomittant and antagonist processes that are involved in this complex transition and were suggested by Reynolds. It is also shown that a clear explicative model was provided, based on the nonlinear dynamical systems theory, the experimental observations in fluid mechanics only providing an applied example, due to its obvious generic nature.     

SEM study of worn surface morphology of an indigenous ‘EPDM’ rubber

Ethylene Propylene Diene Monomer (EPDM) rubber emerges as a dominant elastomer for major engineering applications like automobiles, constructions, electric and electronic industries and many more. The major engineering properties of EPDM are its outstanding heat, ozone and weather resistance ability. The resistance to polar substances and steam is also good. EPDM rubber has a common use as seals in automobiles.In the present work friction and sliding wear behaviors of ethylene propylene diene monomer rubbers (EPDM) of different hardness have been studied against steel counterpart under dry working condition. Different hardness of EPDM have been achieved by adding different proportion (parts per hundred) of carbon black (CB) content with the main ingredients of EPDM. Tribo-testing has been carried out in a multi tribo-tester (Ducom, India). EPDM rubber of different hardness like 55 Å, 70 Å and 85 Å has been slid against EN-8 stainless steel roller of the tester. Experiments have been conducted with different rotational speeds of the wheel at a constant load of 25N for a constant duration of 900 s. The coefficient of friction (COF), mass loss and wear of EPDM rubbers have been determined from the test data. The worn surface morphology has also been studied using scanning electron microscope (SEM) and concluded accordingly.Present experimental work attempts to highlight some important tribo-characteristics of an indigenous EPDM rubber as well as to shed light on various possible areas of further research works.     

Effect of thickness and boundary conditions on the behavior of viscoelastic layers in sliding contact with wavy profiles

In this work, the sliding contact of viscoelastic layers of finite thickness on rigid sinusoidal substrates is investigated within the framework of Green's functions approach. The periodic Green's functions are determined by means of a novel formalism, which can be applied, in general, to either 2D and 3D viscoelastic periodic contacts, regardless of the contact geometry and boundary conditions.Specifically, two different configurations are considered here: a free layer with a uniform pressure applied on the top, and a layer rigidly confined on the upper boundary. It is shown that the thickness affects the contact behavior differently, depending on the boundary conditions. In particular, the confined layer exhibits increasing contact stiffness when the thickness is reduced, leading to higher loads for complete contact to occur. The free layer, instead, becomes more and more compliant as thickness is reduced.We find that, in partial contact, the layer thickness and the boundary conditions significantly affect the frictional behavior. In fact, at low contact penetrations, the confined layer shows higher friction coefficients compared to the free layer case; whereas, the scenario is reversed at large contact penetrations. Furthermore, for confined layers, the sliding speed related to the friction coefficient peak is shifted as the contact penetration increases. However, once full contact is established, the friction coefficient shows a unique behavior regardless of the layer thickness and boundary conditions.     

SURFACE MODIFICATION WITH HYDROGELS VIA MACROINITIATORS FOR ENHANCED FRICTION PROPERTIES OF BIOMATERIALS

ABSTRACT

A general method of surface modification is described which is based on dip-coating of a substrate with a macroinitiator and subsequent free radical polymerization of functional monomers. Using this method, it is possible to fix poly(acrylic acid) hydrogels on polymer surfaces, e.g. on catheters, which drastically reduces the friction of these materials. Similarly, other biological relevant properties, especially reduced protein or bacteria adsorption can be achieved by choosing appropriate monomers.

The substrate was first homogeneously dip-coated with e.g. the water-insoluble macroinitiator poly(octadecene-co-maleic anhydride), partially reacted to the tert.-butyl perester. Homogeneity, thickness, and reactivity of the macroinitiator layer was characterized in detail. After a temper step, surface homo- and copolymerizations of ionic monomers were carried out in water directly from the modified surface. The consistency of the hydrogel coating could be well controlled by the reaction conditions and the monomer composition. The correlation between the experimental parameters, the composition of the surface coating, and the friction properties was established. A relatively thick, slightly crosslinked poly(acrylic acid) hydrogel coating reduces the friction coefficient by 95% compared to that of uncoated surfaces.     

Pressure Drop and Performance Characteristics of Water-Based Al2O3 and CuO Nanofluids in a Triangular Duct

An experimental study is performed to determine the pressure drop and performance characteristics of Al₂O₃/water and CuO/water nanofluids in a triangular duct under constant heat flux where the flow is laminar. The effects of adding nanoparticles to the base fluid on the pressure drop and friction factor are investigated at different Reynolds numbers. The results show that at a specified Reynolds number, using the nanofluids can lead to an increase in the pressure drop by 35%. It is also found that with increases in the Reynolds number, the rate of increase in the friction factor with the volume fraction of nanoparticles is reduced. Finally, the performance characteristics of the two nanofluids are investigated using the data of pressure drop and convective heat transfer coefficient. The results show that the use of Al₂O₃/water nanofluid with volume fractions of 1.5% and 2% is not helpful in the triangular duct. It is also concluded that at the same volume fraction of nanoparticles, using Al₂O₃ nanoparticles is more beneficial than CuO nanoparticles based on the performance index.