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This work experimentally examines the detachment of liquid droplets from both oleophilic and oleophobic fibres, using an atomic force microscope. The droplet detachment force was found to increase with increasing fibre diameter and forces were higher for philic fibres than phobic fibres. We also considered the detachment of droplets situated on the intersection of two fibres and arrays of fibres (such as found in fibrous mats or filters) and found that the required detachment forces were higher than for similarly sized droplets on a single fibre, though not as high as expected based on theory. A model was developed to predict the detachment force, from single fibres, which agreed well with experimental results. It was found that the entire dataset (single and multiple fibres) could be best described by power law relationships.

The evolution of pressure drop, drainage rate, saturation, and efficiency of combined philic, and phobic oil mist filters in real-time are examined. The experiments used four different filter configurations, with a combination of oleophobic and oleophilic fibrous filter media, and one oleophilic only reference. The effect of separating filter materials of differing wettability, with a mesh, was also explored. It was found that inclusion of a mesh between layers promoted increased drainage rates and resulted in a significantly lower pressure drop across the filter. The overall mass-based filtration efficiency was also slightly higher for the configurations containing the mesh. Conversely, re-entrainment of droplets from the rear face of the filter was only observed in filter configurations without the central mesh. Filters with oleophobic initial layers did not display a classical “depth filtration” pressure drop curve. The oleophobic media was found to possess lower steady-state saturation than oleophilic media. Additionally, the steady-state saturation of the oleophilic filter media, when placed at the rear of the filter, was lower when the central mesh was present. The saturation values were compared with recently published theory.

Context: Mice are commonly used in studies investigating the effects of diesel exhaust exposure on respiratory health. A plethora of studies in this field has resulted in a range of exposure protocols, from inhalation of diesel exhaust, to the administration (via various routes) of diesel exhaust particles in solution. Objective: In this study, we compared the physiological consequences of short-term exposure to diesel exhaust via inhalation to those due to exposure to the same diesel exhaust particles suspended in solution and delivered intranasally. Materials and methods: Adult BALB/c mice were exposed to diesel exhaust via inhalation for 2hours per day for 8 days. A representative, simultaneous sample of particles was collected and a second group of mice then exposed to them suspended in saline. A low and a high-dose were studied, with these matched based on respiratory parameters. Six and twenty-four hours after the last exposure we measured bronchoalveolar inflammation, lung volume, lung function and the amount of elemental carbon in alveolar macrophages. Results: Exposure via either route elicited pulmonary inflammation and changes in lung function. We identified significant differences in response between the two routes of exposure, with mice exposed via inhalation generally displaying more realistic dose-response relationships. Mice exposed via intranasal instillation responded more variably, with little influence of dose. Conclusions: Our results suggest that selection of the route of exposure is of critical importance in studies such as this. Further, inhalation exposure, while more methodologically difficult, resulted in responses more akin to those seen in humans.

Fibrous filters and mist eliminators are widely used in a range of manufacturing and process industries worldwide to remove liquid mists from gas streams, thereby coalescing them to recover the bulk liquidand prevent emissions. The range of applications include compressed gas cleaning, engine crankcase ventilation, (liquefied) natural gas, propane (LPG) and hydrocarbon production and processing, cooling towers, machining and cutting processes and a range of other process engineering applications. Although research on mist filtration has been increasing since the late 1950s, and some of the fundamental physico-chemical processes were described as early as 1870, the scientific questions in the field are far from resolved. This is largely due to the complex kinetics of gas and fluid phases, and the wide range of possible filter geometric properties and surface energy. However, over the last 15 years, significant progress has been made in describing and modelling mist filter systems. Most models developed to date, are however empirically derived, and therefore only applicable to a narrow range of filter media and operating conditions. This paper reviews the state of current research, comparing currently available models for capture, saturation and pressure drop and discussing their applicability. Key gaps in the literature and necessary directions for future research are also given. The work also reviews published experimental data to-date, showing the range of media studied and summarising relationships which can be identified by collating results from all studies which provide sufficient data.

This work has examined the physico-chemical properties of coalescing filters on a micro-scale, considering them as either a capillary system or a fibre-droplet system. This work has developed a validated theoretical model for droplet-fibe forces in both phillic and phobic systems and a computational fluid dynamics solver for droplet-fibre systems. This work will significantly improve the design and optimisation of coalescing filter systems.

This work details the results of a study into the relationship between pressure drop and liquid saturation in mist (or coalescing) filters. Liquid saturation (clogging) in mist filters is of critical importance as it is directly related to filter efficiency and flow resistance. Experiments were conducted to determine steady-state saturation and pressure drop values in commonly used oleophillic fibrous filter media, using a range of different combinations of face velocity and number of layers of media within the filter element. Several empirical relationships for saturation and pressure drop were derived based on the relationships found. In addition, a capillary-based saturation model has been described and fitted to the experimental data. A good agreement between the model and data was obtained when an empirically fitted term was added. Equations were developed which allow such variables to be determined from known parameters.

The well-known phenomena of Plateau–Rayleigh instability has been simulated using computational fluid dynamics (CFD). The breakup of a liquid film into an array of droplets on a cylindrical element was simulated using a volume-of-fluid (VOF) solver and compared to experimental observations and existing theory. It is demonstrated that the VOF method can correctly predict the breakup of thins films into an array of either axisymmetric droplets or clam-shell droplets, depending on the surface energy. The existence of unrealistically large films is precluded. Droplet spacing was found to show reasonable agreement with theory. Droplet motion and displacement under fluid flow was also examined and compared to that in previous studies. It was found that the presence of air flow around the droplet does not influence the stable film thickness; however, it reduces the time required for droplet formation. Novel relationships for droplet displacement were derived from the results.

Collection and deposition of particles in the upper airway and lungs is of considerable importance – for example, when studying chronic diseases, or when determining the efficacy of aerosol drug delivery. Modelling of particle deposition usually assumes either constant flow (typically at maximum inspiration), or oscillating flow – ignoring any effects of the lung’s motion. This paper presents a preliminary examination of the effects of ignoring mesh motion when modelling the lungs. Initially, an idealised lung model was created, corresponding to generations 0 to 3 of Weibel’s morphology[14]. Simulations were then made using this geometry for steady flow, oscillating flow, and flow developed by expanding the lung. The expansion of the lung was modelled using a mesh motion library developed by the authors. This model allowed the expansion of the lung to be prescribed. Results from the simulations show significant differences between the three modelling options – relating to both the predicted flow field, and particle deposition sites. Robustness of the moving mesh modelling technique is demonstrated on a high-resolution geometry created from CT scans of a Sprague-Dawley rat model lung.

Plateau-Rayleigh Instability (PRI) is the well known phenomena of the breakup of a liquid column or cylinder. Such a process is integral to the operation of a range of natural and anthropogenic systems, such as gas-liquid and liquid-liquid separators, fuel cells, the accumulation of dewdrops on spider webs, and many more. Volume Of Fluid (VOF) methods, such as available in OpenFOAM, should be able to accurately resolve PRI in such systems. One such system, in which PRI is integral, is the filtration of oil or water aerosol mists using fibrous filters. In many cases, entrainment (or carryover) of liquid from fibers occurs. The mechanisms behind such entrainment are poorly understood. This work will validate the OpenFOAM VOF against classical PRI theory, both with and without a secondary fluid phase flowing through the system (e.g. air). Furthermore, the work will utilise the validated two-phase VOF solver to examine the phenomena of liquid reentrainment from mist filters.

Phobic droplet–fiber systems possess complex geometries, which have made full characterization of such systems difficult. This work has used atomic force microscopy (AFM) to measure droplet–fiber forces for oil droplets on oleophobic fibers over a range of fiber diameters. The work adapted a previous method and a theoretical model developed by the authors for philic droplet–fiber systems. A Bayesian statistical model was also used to account for the influence of surface roughness on the droplet–fiber force. In general, it has been found that the force required to move a liquid droplet along an oleophobic filter fiber will be less than that required to move a droplet along an oleophilic fiber. However, because of the effects of pinning and/or wetting behavior, this difference may be less than would otherwise be expected. Droplets with a greater contact angle ( 110°) were observed to roll along the fiber, whereas droplets with a lesser contact angle (<90°) would slide.

This paper presents the results of an extensive series of measurements of droplet-fibre forces in mist (coalescing) filters. A method was developed which allowed droplet-fibre forces to be measured using an Atomic Force Microscope (AFM). The AFM was also be utilised to measure the surface roughness of filter fibres, which was found to significantly affect the results. A theoretical model was also developed to predict the droplet fibre forces, based on wetting theory. Measurements of forces on polymer fibres are presented here, along with the same fibres after coating with a fluorinated copolymer coating to render them oleophobic. The oloephillic results show good agreement with the theoretical model. The phobic results differ significantly from the phillic, and a model is currently being developed for this case. Such measurements are vital for validating CFD models such as the one being developed by the authors.

Droplet-fibre interactions are found in many natural and anthropogenic systems. A common industrial example is fibrous filtration - used to capture liquid (e.g. oil) mists. The filters used consist mostly of highly porous arrays of randomly layered fibres. Given the random (complex) nature of these filters, the existing models describing their behaviour are mainly empirical in nature and thus only applicable over a narrow range of parameters and operating conditions. Therefore simulation of these filters using computational fluid dynamics offers a viable alternative to the existing models. In this work we will detail the development of a solver that couples the Lagrangian tracking of particles with a volume-of-fluid (VOF) solver. This solver is built on the existing open-source OpenFOAM CFD libraries, which have been modified to allow the physically accurate modelling of small particles. The solver also models the collection of these particles, where there is a transition from the discrete treatment (as in the Lagrangian tracking of the particles) to the volume-of-fluid treatment. The solver allows the simulation of the motion of small liquid droplets, the capture of these droplets by filter fibres, the coalescence of these captured droplets, into films and the subsequent break up of these films into droplet arrays by Plateau-Rayleigh instability. Also simulated is the movement of these coalesced droplets within the filter, leading to the drainage of oil from the filter.A validation of the fundamental physical mechanisms in the filter was performed, by comparing the simulated conformation of liquid droplets and films on the fibre to Plateau-Rayleigh instability theory. The model showed general agreement with both theory and observations. The simulated capture efficiency was also compared to capture efficiencies predicted by the single fibre efficiency (SFE) theory. A good agreement between the two was found.

This work presents a theoretical model describing the force required to move a coalesced liquid droplet along an oleophilic filter fiber. Measurements have been made using the atomic force microscope (AFM) to examine these forces over a range of fiber and droplet diameters as well as oil properties. Good agreement between measured and modeled forces was found. The influence of droplet displacement perpendicular to the fiber on the force required to move the droplet has also been determined experimentally and theoretically. It was found that fiber surface inhomogeneities are likely to influence results. This work has also established empirical relationships that can be used to predict the force, based on a known droplet volume, for the liquid types used.