A Technique For Determining The Rheology Of Lubricants At High Pressures And High Shear Rates

Author : Jianyi Du (S.M.)
Publisher :
ISBN 13 :
Total Pages : 153 pages
Book Rating : 4.:/5 (15 download)

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Download or read book Rheological Characterization of Lubricants in Automotive Industries Through Development of a Novel Capillary Breakup Extensional Rheometer written by Jianyi Du (S.M.) and published by . This book was released on 2018 with total page 153 pages. Available in PDF, EPUB and Kindle. Book excerpt: Extensional rheology describes the response of fluids to elongational deformations, which is important to everyday phenomena such as jetting, drop impact and fragmentation. This thesis describes the development of an improved device to study the extensional rheology of complex fluids such as paints, motor oils, metalworking lubricants and waxes frequently used in the automotive industry. Key processes in the automotive industry can be potentially optimized with a better understanding of the rheology of these materials, including paint spray, auto-body coating, engine lubrication, and other processes involving high deformation rates. The experimental technique of capillary thinning is utilized for characterizing the fluids and an improved design of the Capillary Breakup Extensional Rheometer (CaBER) is introduced. The new instrument features higher resolution and faster sampling of the laser micrometer measurements, better controlled motor actuation, a novel environmental temperature control system as well as customized fixtures to facilitate the experiments. In this experimental setup, a liquid sample is rapidly stretched between two coaxial plates, which leads to the formation of a liquid bridge in the middle connecting hemispherical liquid reservoirs at each end. The temporal evolution of the radius of the resulting liquid bridge is controlled by inertial, viscous, elastic and capillary effects. By measuring the evolution in the mid-plane radius and the shape of the filament using a laser micrometer as well as a high-speed camera with a well-established image processing algorithm, we can probe the underlying fluid properties given specific constitutive models. The new extensional rheometer is validated using a number of different fluid systems, including a rheologically well-studied Newtonian calibration fluid of glycerol and a series of aqueous food thickeners. Exploratory tests also show the capabilities and limitations of the device through a series of calibration oils with distinct fluid properties. Three different materials used in automotive industries are tested. First, two commercially available motor oils with the same viscosity index (1OW-30) appearing identical in shear flow, as well as having similar surface tensions are selected. Capillary thinning measurements show that these fluids can be differentiated by the onset of strain-rate-thickening close to breakup, corresponding to the dimensionless Weissenberg number exceeding Wi ~\~ 0.5. This weakly elastic response for each lubricant can be modeled and understood using a simple second order fluid model, which is then numerically fit with the experimental data. Compared with the model parameters obtained from fitting the Newtonian and Hookean dumbbell models, consistency is observed in material properties including the extensional viscosities and the relaxation time, thus showing a smooth transition of the rheological characteristics as the filament profile evolves and the strain rate in the sample diverges. Second, a commercial waxy lubricant (DC-290) for the metal stamping process is selected. This material is first characterized through surface tension measurements, differential scanning calorimetry and shear rheometry to determine the onset of wax precipitation (at a wax appearance temperature of 47°C). Below this wax appearance temperature, the material shows a series of non-Newtonian behaviors, including a non-trivial yield stress, shear-thinning and thixotropic effects with dependence on temperature, shear rate and shear-history. These characterizations are further connected to the capillary thinning profiles inspected at different temperatures, demonstrating a transition from a stable liquid bridge that never breaks up due to the existence of a yield stress, to the inertio-capillary thinning profile resulting in liquid bridge corrugations and droplet formation. For a specific temperature of 40°C, the experimental diameter thinning at the mid-plane shows better correspondence to the prediction obtained from considering inertia-visco-capillary interactions than that obtained from only considering visco-capillary interactions or potential flow limit. Some discrepancies between the experimental results and the inertia-visco-capillary prediction still exist and relate to the yield stress. Finally, another commercial liquid lubricant (FC-6130), which was first independently characterized to be a low viscosity Newtonian fluid through shear rheometry, is examined using capillary thinning to determine the limits of the instrument. A variant of the capillary thinning technique, known as the slow retraction method (SRM) was applied using a smaller plate geometry in an attempt to minimize inertial as well as gravitational effects on the liquid bridge which adversely affect the accuracy of measurements for low viscosity materials. The resulting thinning profile close to breakup shows good correspondence to the prediction from visco-capillary interactions, indicating a purely viscous Newtonian response. The results from these experiments show the capabilities of the new capillary thinning system, and also reveal new strategies to characterize the extensional rheological properties of viscoelastic or weakly viscoelastic materials of relevance in the automotive industries. A deeper understanding of these materials will help in optimizing the industrial processes in which they are applied.