Shear Friction Force Research Articles

Effects of surfactant CTAB on performance of flat-plate CLPHP based on PEMFC cooling

In this study, passive flat-plate closed-loop pulsating heat pipe (CLPHP) cooling is proposed to address the structural characteristics and cooling requirements of proton exchange membrane fuel cells (PEMFCs). Through visual images, non-visualized temperature and pressure measurements, and theoretical analysis, the effect of surfactant cetyltrimethylammonium bromide (CTAB) on the performance of a binary non-azeotropic mixed working medium flat-plate CLPHP with a 40% filling ratio (FR) and 5:1 mass ratio of methanol and deionized water was studied. It was found that the addition of surfactant CTAB reduced the surface tension and increased the shear friction force of the working fluid. The optimal CTAB concentration was 20 ppm, at which the start-up critical heat flux of the flat-plate CLPHP was reduced. At a heating power of 400 W, the heat transfer resistances reduced by 29.9%, which improved the heat transfer power of the flat-plate CLPHP. The backflow of the liquid-phase working medium at the bottom of the elbow in the evaporation section was promoted, dry-out phenomenon was avoided, temperature fluctuation of the flat-plate CLPHP was reduced, and temperature stability of the PEMFC was improved.

Effects of Silicone Oil on Electrowetting to Actuate a Digital Microfluidic Drop on Paper.

The effects of an immiscible, lubricating polydimethylsiloxane fluid, referred to as silicone oil, on the static deformation and on the dynamic motion of a water drop on paper induced by electrowetting were investigated. The deformation of a drop on a hydrophobic film of amorphous fluoropolymers top-coated with less hydrophobic silicone oil was much more predictable, reversible and reproducible than on the uncoated surface. In the dynamic tribological experiment for a sliding drop along an inclined surface, a significant decrease in the friction coefficient, with an unexpected dependency of the contact area, was observed. Based on the curve fitting analysis, the shear stress and the net friction force were estimated quantitatively. Because of the tribological effect and the reduced shear friction force of the oil film, the static and the dynamic electrowetting states of the water drop were enhanced.

Simulation of nanopowder compaction in terms of granular dynamics

The uniaxial compaction of nanopowders is simulated using the granular dynamics in the 2D geometry. The initial arrangement of particles is represented by (i) a layer of particles executing Brownian motion (isotropic structures) and (ii) particles falling in the gravity field (anisotropic structures). The influence of size effects and the size of a model cell on the properties of the structures are studied. The compaction of the model cell is simulated with regard to Hertz elastic forces between particles, Cattaneo-Mindlin-Deresiewicz shear friction forces, and van der Waals-Hamaker dispersion forces of attraction. Computation is performed for monodisperse powders with particle sizes ranging from 10 to 400 nm and for “cohesionless” powder, in which attractive forces are absent. It is shown that taking into account dispersion forces makes it possible to simulate the size effect in the nanopowder compaction: the compressibility of the nanopowder drops as the particles get finer. The mean coordination number and the axial and lateral pressures in the powder systems are found, and the effect of the density and isotropy of the initial structure on the compressibility is analyzed. The applicability of well-known Rumpf’s formula for the size effect is discussed.

Wear of crosslinked polyethylene under different tribological conditions

Ultra high molecular weight polyethylene (UHMWPE) wear debris has been shown to be a major cause of long term failure of total joint replacements. Recently, crosslinking has been extensively introduced to reduce the wear of UHMWPE. In this study the wear of non-crosslinked and crosslinked UHMWPE were compared under a range of conditions. The materials examined were UHMWPE GUR 1050, non-crosslinked, moderately crosslinked--5MRad, and highly crosslinked--10MRad. The wear was examined on a multidirectional pin on plate rig. The effect of counterface roughness on wear under different kinematics was examined. The results from the different counterface conditions showed that highly crosslinked UHMWPE had significantly lower wear against both smooth and scratched counterfaces. However the reduction in wear for crosslinked polyethylene was less for scratched counterfaces. The second part of the study showed that all the UHMWPE's produced lower wear rates under lower multidirectionality because of reduced cross shear frictional forces and work. These findings are relevant to the consideration of the use of crosslinked polyethylene in the knee, where the kinematics have lower levels of cross shear and in the hip and knee against roughened metallic counterfaces.

Particle damping for passive vibration suppression: numerical modelling and experimental investigation

As a simple and passive means, particle damping provides vibration suppression with granular particles embedded within their containing holes in a vibrating structure. Unlike in traditional damping materials, mechanisms of energy dissipation of particle damping are primarily related to friction and impact phenomena which are highly non-linear. In the research work reported in the paper we investigate elastic beam and plate structures with drilled longitudinal holes filled with damping particles. Our focus is on the form of damping due to shear friction induced by strain gradient along the length of the structure. We present physical models to take into account of the shear frictional forces between particle layers and impacts of the particles with the containing holes. A numerical procedure is presented to predict the damping effect. Experimental tests of the beam and plate structures for various different damping treatments are also conducted. Model predictions are validated by experimental results. The particle damping is found to be remarkably strong for a broadband range. Moreover, the shear friction is determined to be the major contributing mechanism of damping, especially at a high volumetric packing ratio. The numerical and experimental findings suggest that the best damping effect might be achieved by using a design of multiple particle chambers involving an appropriate combination of the impact, friction and shear mechanisms, in contrast to a transverse-type particle or single-mass damper.