Browsing by Subject "Surface texture"
Now showing 1 - 5 of 5
Results Per Page
Sort Options
Item Open Access The effect of fluid viscoelasticity in lubricated contacts in the presence of cavitation(Elsevier, 2021-03-27) Gamaniel, Samuel Shari; Dini, D.; Biancofiore, LucaIn this work we study the influence of fluid viscoelasticity on the performance of lubricated contacts in the presence of cavitation. Several studies of viscoelastic lubricants have been carried out, but none of them have considered the possibility of the presence of cavitation. To describe the effect of viscoelasticity, we use the Oldroyd-B model. By assuming that the product between ϵ, i.e. the ratio between vertical and horizontal length scales, and the Weissenberg number (Wi), i.e. the ratio between polymer relaxation time and flow time scale, is small, we can linearise the viscoelastic thin film equations, following the approach pioneered by "Tichy, J., 1996, Non-Newtonian lubrication with the convected Maxwell model." Consequently, the zeroth-order in ϵWi corresponds to a Reynolds equation modified to describe also the film cavitation through the mass-conserving Elrod-Adams model. We consider the flow of viscoelastic lubricants using: (i) a cosine profile representing a journal bearing unwrapped geometry, and (ii) a pocketed profile to model a textured surface in lubricated contacts. The introduction of viscoelasticity decreases the length of cavitated region in the cosine profile due to the increasing pressure distribution within the film. Consequently, the load carrying capacity increases with Wi by up to 50% in the most favorable condition, confirming the beneficial influence of the polymers in bearings. On the other hand for the pocketed profile, results show that the load can increase or decrease at higher Wi depending on the texture position in the contact. The squeeze flow problem between two plates is also modeled for viscoelastic lubricants considering an oscillating top surface. For this configuration a load reduction is observed with increasing Wi due to the additional time needed to reform the film at high Wi. Furthermore, if viscoelastic effects increase, the cavitation region widens until reaching a value of Wi for which a full-film reformation does not occur after the initial film rupture.Item Open Access Interplay between wall slip and cavitation: a complementary variable approach(Elsevier, 2019) Biancofiore, Luca; Giacopini, M.; Dini, D.In this work a stable and reliable numerical model based on complementary variables is developed to study lubricated contacts characterised by slip at one or both surfaces and in the presence of cavitation. This model can be used to predict surface behaviour when cavitation induced by e.g. the presence of surface texture, slip, or a combination of the two is encountered, with varying surface parameters. For this purpose, two different algorithms are coupled to predict the formation of cavitation, through a mass-conserving formulation, and the presence of slip at the wall. The possible slippage is described by a limiting shear criterion formulated using a Tresca model. To show the flexibility of our model, several bearing geometries have been analysed, such as a twin parabolic slider, a cosine profile used to mimic a bearing, and a pocketed slider bearing employed to study the effect of surface texture. We observe that the lubrication performance (i.e. low friction coefficient) can be improved by using materials that promote slippage at the moving wall. The location of the slippage region can be optimised to find the lowest value of friction coefficient. Our theoretical developments and numerical implementation are shown to produce useful guidelines to improve and optimise the design of textured superoleophobic surfaces in the presence of lubricated contacts.Item Open Access Oil droplet manipulation on superomniphobic textured surfaces(2020-07) Yelekli, EcemMicrofluidic systems are mostly composed of closed microchannels in which flow is generated by syringe or pressure pumps. The flow in these channels can be droplet-based however access to each droplet individually in these systems is not possible. As an alternative approach to these channel-based devices, droplets can also be manipulated on surfaces by generating surface energy gradients. Since in these systems droplets can be handled individually and samples can be carried in small packages, these systems can perform more controlled operations. For instance, the concentration and volume of the samples can be adjusted more precisely. These systems can be very useful for biological analysis as well as chemical synthesis. Until now, transport of water droplets by using surface energy gradients has been demonstrated in literature. On the other hand, controlled transport of oil droplets on surfaces remained as a challenging task because of their low surface tension. In addition, in the literature, most of the work about oil droplet transportation was carried out in an aqueous environment, and therefore it restricts its potential for applications. This work demonstrates the transportation of microliter sized oil droplets by utilizing textured superomniphobic surfaces in a controlled way for the first time. By applying vertical vibration to the surface, oil droplets overcome hysteresis and move by following the textured tracks. Superoleophobicity is required to decrease the affinity of oil on the surface so that the motion of droplets can be achieved. This system has advantages such as the ability to control droplet motion individually by using a single input (vertical vibration) as well as mixing droplets in precise ratios, preventing clogging in channels and cross contamination as well as eliminating the usage of syringe pumps. In this project, initial focus was on examining the topography effect on superoleophobicity and fabricating superomniphobic surfaces. Surfaces were fabricated on silicon wafers by using conventional lithography technique. In this stage, two different microstructure profile was used on the surfaces: mushroom microstructure and straight sided microstructure. It was observed that mushroom microstructures were required to maintain superoleophobicity. Also, the effect of side length of microstructures, the distance between the microstructures and TiO2 coating on wettability were investigated. In order to achieve oil droplet transportation, superomniphobic textured surfaces were developed and these surfaces were tested by applying vertical vibration. As a final aim of this project, these surfaces were used for the nanoparticle synthesis.Item Embargo Textured surfaces for oil droplet transport(Elsevier BV, 2023-08-22) Yelekli Kirici, Ecem; Naji, Mayssam; Çanakçı, Ahmet Selim; Erdem, Emine YegânDroplet-based microfluidic systems bring the advantage of handling samples in discrete forms and these systems can be built as a surface-based platform, eliminating the need of a carrier fluid. Droplet transport on surfaces can be achieved by creating local energy gradients by using electrowetting, thermocapillary forces, chemical gradients and surface texture. These methods are developed for aqueous droplets; while controlled transportation of oil droplets remains as a challenge due to their low surface tension. One of the major challenges in achieving controlled oil droplet manipulation is to obtain oil repellent -oleophobic- surfaces. In this work, pillar arrays with mushroom shaped profiles were designed, fabricated, and tested to study oil wetting on textured surfaces. Contact angles higher than 170° were achieved for hexadecane droplets in Fakir state. Based on these findings, surface texture ratchet tracks that are composed of an array of arc shaped pillars were built to generate local energy gradients to manipulate oil droplets in a continuous fashion. Motion of hexadecane droplets at a speed of 7 mm/s was achieved. The results of this study are crucial for applications of oil droplet transport such as in biochemistry, smart surface development, wearable device design as well as microsystem packaging.Item Open Access Viscoelastic effects in lubricated contacts in the presence of cavitation(2020-11) Gamaniel, Samuel ShariA model is proposed to study the influence of fluid viscoelasticity on the performance of lubricated contacts in the presence of cavitation. Previous studies on viscoelastic lubricants did not consider the presence of cavitation, rather reported negative pressures in regions where cavitation was expected to occur. The proposed model uses the Oldroyd-B constitutive model to describe the presence of cavitation and assumes that the Deborah number (De), the ratio between polymer relaxation time and flow time scale, is small. In doing so, the viscoelastic thin film equations can be linearised in a similar approach to what was pioneered by ”Tichy, J., 1996, Non-Newtonian lubrication with the convected Maxwell model.” The zeroth order solution in De corresponds to the Reynolds equation and has been modified to describe also the film cavitation through the mass-conserving Elrod-Adams model. We model several bearing configurations for the flow of viscoelastic lubricants using (i) a cosine/parabolic profile representing a journal bearing unwrapped geometry, and (ii) a pocketed profile to model a textured surface in lubricated contacts. Introducing viscoelasticity to the cavitating journal bearing decreases the length of the non-active (cavitation) region due to an increasing pressure distribution in the lubricant film. This results in an increase to the load carrying capacity with increasing De corroborating the beneficial influence of the polymers in fluid film bearings. The pocket profile is shown to either increase or decrease the load carrying capacity with increasing viscoelastic effects, depending on the location of surface texturing at the contact. An oscillating squeeze flow problem is modeled for viscoelastic lubricants between two flat plates with motion only at the top surface. A reduction in the load carrying capacity at larger values of De is observed as film reformation is seen to be retarded with increasing viscoelastic effects. As viscoelastic effects become stronger, the nonactive region is grows continuously until reaching a value of De beyond which a full film reformation does not occur upon the inception of cavitation. The study is extended to a direct numerical simulations using the openFoam toolbox. A model that couples a solver for incompressible, isothermal, two phase flow with interaction between the phases and a solver for viscoelastic fluids is proposed. However, DNS are only valid for lower values of De as instabilities occur as a result of the non-linear coupling.