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A New Star-Like SurfaceTexture for Enhanced Hydrodynamic Lubrication Performance

Uddin M. S., Liu Y. W., Shankar S.

Archives of Metallurgy and Materials

2017

Vol. 62, iss. 3

1863--1869

This paper presents a numerical modelling and optimization of a new ‘star-like’ geometric texture shape with an aim to improve tribological performance. Initial studies showed that the triangle effect is the most dominant in reducing the friction. Motivated with this, a ‘star-like’ texture shape consisting of a series of triangular spikes around the centre of the texture is proposed. It is hypothesised that by increasing the triangular effect on a texture shape, the converging micro-wedge effect is expected to increase, hence increasing the film pressure and reducing the friction. Using the well-known Reynolds boundary conditions, numerical modelling of surface texturing is implemented via finite difference method. Simulation results showed that the number of apex points of the new ‘star-like’ texture has a significant effect on the film pressure and the friction coefficient. A 6-pointed texture at a texture density of 0.4 is shown to be the optimum shape. The new optimum star-like texture reduces the friction coefficient by 80%, 64.39%, 19.32% and 16.14%, as compared to ellipse, chevron, triangle and circle, respectively. This indicates the potential benefit of the proposed new shape in further enhancing the hydrodynamic lubrication performance of slider bearing contacts.

Adsorption of natural surfactants present in sea waters at surfaces of minerals: contact angle measurements

Mazurek A., Pogorzelski S.J., Boniewicz-Szmyt K.

Oceanologia

2009

No. 51 (3)

377-403

The wetting properties of solid mineral samples (by contact angles) in original surfactant-containing sea water (Gulf of Gdańsk, Baltic) were characterised under laboratory conditions on a large set (31 samples) of well-classified stones of diverse hydrophobicity using the sessile drop (ADSA-P approach), captive bubble and inclined plate methods. An experimental relation between the static contact angle θ_eq and stone density ρ was obtained in the form θ_eq = B-ρ+ C, where B = 12.23 š 0.92, C = - (19.17 š 0.77), and r2 = 0.92. The histogram of ?eq distribution for polished stone plates exhibited a multimodal feature indicating that the most abundant solid materials (hydrophilic in nature) have contact angles θ_eq = 7.2, 10.7, 15.7 and 19.2^(o), which appear to be applicable to unspecified field stones as well. The contact angle, a pH-dependent quantity, appears to be a sensitive measure of stone grain size, e.g. granite. The captive bubble method gives reproducible results in studies of porous and highly hydrophilic surfaces such as stones and wood. The authors consider the adsorption of natural sea water surfactants on stone surfaces to be the process responsible for contact angle hysteresis. In the model, an equation was derived for determining the solid surface free energy from the liquid's surface tension γ_LV it also enabled the advancing θ_A and receding θ_R contact angles of this liquid to be calculated. Measurements of contact angle hysteresis Δθ(=θ_A - θ_R) with surfactant-containing sea water and distilled water (reference) on the same stone surfaces allowed the film pressure ΔΠ (1.22 to 8.80 mJ m-2), solid surface free energy ??S (-17.03 to -23.61 mJ m-2) and work done by spreading ΔWS (-1.23 to -11.52 mJ m-2) to be determined. The variability in these parameters is attributed to autophobing, an effect operative on a solid surface covered with an adsorptive layer of surfactants. The wetting behaviour of solid particles is of great importance in numerous technological processes including froth flotation, demulgation, anti-foaming procedures and the coal industries. It is believed that the approach presented here and the examples of its application to common sea water/solid mineral systems could be successfully adapted to optimise several surfactant-mediated adsorption processes (see below) of practical value in natural water ecology.