http://nova.newcastle.edu.au/vital/access/services/Feed ${session.getAttribute("locale")} 5 http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:10021 Triboelectric charge accumulation both poses problems and offers opportunities for dry particulate processing. It generates hazards in many industrial systems, but is exploited in several important applications, including electrophotographic toner charging and triboelectric separation. However, the charging of continuous particulate flows on solid surfaces is poorly understood, and design of devices to optimise triboelectric behaviour is often qualitative or based on trial and error. This study attempts to identify the main charging mechanisms for a continuous particulate flow on a flat surface. The proposed charging models incorporate contact time, velocity and mode of contact. A model incorporating contact area effects arising from sliding and rolling contact is consistent with experimental data, as long as a charge limit is imposed on bouncing particles. A mechanism for this charge limit, involving separation discharge, is proposed. The effect of delocalised image charge on the system is estimated and found to be negligible. Some general design principles are proposed for triboelectric optimisation of particle processing devices. 2012-02-10T01:50:07.061Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:9918 Triboelectrification is a ubiquitous feature of dry particulate processes. It generates hazards in many industrial systems, but is exploited in several other important applications, including electrophotographic toner charging and triboelectric separation. Unfortunately, the charging of continuous particulate flows during interaction with solid surfaces is poorly understood, and design of devices to maximise or minimize triboelectrification is usually qualitative or based on trial and error. This paper presents the results of an experimental case study of the relationship between the accumulated charge and the mechanical attributes of a particulate flow, including contact time, velocity, and the mode of contact. Silica particles of ~ 1 mm diameter and a stainless steel slide tribocharger were used for this case study. 2012-02-08T01:40:03.760Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:5139 Sliding contact usually seems to promote triboelectric charge transfer between solids, but the reasons for this are not always clear. Existing theories propose a number of explanations, including an increase in contact area due to interfacial deformation, frictional heating and direct material transfer. In the absence of these mechanisms, sliding may still promote charging by occasioning a changing contact pattern. This changing contact results in a greater cumulative surface area available for charging than static contact. A theoretical structure is introduced to characterize the effect of changing contact on perfectly insulating surfaces with instantaneous contact charging. This is extended to include non-instantaneous charging by means of a simple charging-time model. 2011-09-13T00:40:14.776Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:8078 Wall slip of foams is important in several practical contexts. If the surface is not perfectly wetted by the interstitial liquid, the foam–wall interface may exhibit a yield stress. This paper represents the first quantitative study of this phenomenon. A simple model of the interface between disorganized foam and a solid surface is presented. This model treats the interface as a network of soap film contact lines. Experimental data are presented on the tension at soap film contact lines, and the stress at the foam–solid interface, at very slow translation speeds and for several surfaces. These agree broadly with theoretical predictions up to parity of the interface yield stress with the bulk yield stress of the foam. The experimental data include the yield tension for a soap film contact line pulled in an oblique direction with respect to the contact line normal. These data are also broadly consistent with theoretical predictions. 2011-07-05T05:50:35.442Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:8046 The cumulative triboelectrification of solids by repeated contact and separation is not completely understood. For nominally identical contact, the transferred charge often requires multiple cycles to saturate, and in some cases does not saturate at all. Several explanations have been proposed for this behaviour, but quantitative validation is complicated by the potentially dominant role of separation discharge. This paper presents a new method for controlling or suppressing the discharge, without affecting the initial transferred charge. The phenomenon of separation discharge is described, and its effect on charge accumulation speculated upon. The proposed charge measurement technique is then discussed quantitatively. Lastly, the design and construction of a prototype experimental apparatus are described. 2011-07-04T06:20:14.909Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:7894 Steady-state rising foam is an ideal medium for studying foam coalescence phenomena. The coalescence process in this type of foam is characterized analytically. The spatial distribution of bubble size depends on the effect of individual coalescence events on the bubble size distribution, the rate at which they occur, and the rise rate of the bubbles. The film rupture rate is assumed to depend on the film area and the film drainage time, and is calculated using several simple models. This analysis agrees well with measurements conducted in a laboratory foam cell. However, the time-scale of the experiments indicated that film drainage was occurring predominantly in the hydrodynamic regime, and that non-equilibrium rupture mechanisms were primarily responsible for the observed coalescence. 2011-06-20T00:00:14.979Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:6229 Details of the design and construction of a laboratory scale copper cyclone tribocharger and a preliminary study on the triboelectric properties of sulfide minerals are presented. The charge attained by galena, chalcopyrite and silica sand was noted to increase with increasing applied air flowrate. This occurred irrespective of particle size or density for the limited samples in the preliminary study and may indicate better mechanical contact of particles with the cyclone wall at higher air flowrates. 2010-05-11T04:30:18.223Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:3505 When a gas is sparged into an aqueous solution of surfactant in a column, a froth layer is formed above the layer of liquid. If water is injected into the rising column of froth at a particular position, two distinct layers are formed in the froth: a wet layer below the injection plane, and a dry layer above it. In this study, a phase coexistence analysis was performed on the three-layer system. Predictions from the governing drift-flux equation were compared, using two different representations of the drainage terms, which had one and two adjustable parameters, respectively. The analysis was used to predict liquid fractions and superficial liquid velocities for the wet and dry froth zones. These were compared to experimental measurements performed with a laboratory froth column. Both versions of the analysis were in good agreement with experiment. That with the single-parameter drainage term (derived from the Richardson–Zaki equation) was marginally superior. Theoretical predictions of the liquid fraction and overflow rate from the dry froth layer are supported by the data. Considerable convective motions were observed in the wet froth layer, driven by differences in density between the entering wash liquid and the surrounding froth. 2010-04-27T05:15:50.310Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:5568 Contact lines between soap films and solid surfaces have been studied comparatively little. This is surprising, as a network of these contact lines constitutes the contact between an aqueous foam and an imperfectly wetted surface, and is critical for understanding foam slip on this type of surface. Data on the tension at a 'creeping' soap film contact line are presented. Surprisingly, given that viscous interactions were shown to be unimportant at these low velocities, this was substantially less than the sum of the 'creeping' tension for one wetting and one de-wetting contact line on the same surfaces. A possible explanation for this observation is presented, involving a free energy analysis of the deformed lamellae. 2010-04-27T04:39:44.682Z ]]>