Dana Grecov

Professor

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Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2021)
Biomechanical modeling and hemorheological assessment of ascending thoracic aortic aneurysm, aortic heart valve, and blood clot (2019)

Cardiovascular diseases account for the most cause of death over the globe annually, summarized by the World Health Organization. An aortic aneurysm is one of the cardiovascular diseases with localized abnormal growth of a blood vessel with the primary risk of aneurysm rupture or aortic dissection. The precise pathological pathway for disease progression in aneurysm formation is not completely understood; however, biomechanically, disrupted blood flow from a diseased heart valve and thrombus formation potential in the dissection could contribute to the increased risk. The current ascending thoracic aortic aneurysm (ATAA) management rely heavily on ATAA diameter and blood pressure rather than biomechanical and hemodynamical parameters including arterial wall deformation or wall shear stress (WSS). Therefore, this thesis firstly evaluated the biomechanical contributions to ATAA progression under the influence of anatomy, hypertension, and hematocrit using fully coupled fluid-structure interaction (FSI) with arterial wall anisotropy to provide additional information in patient evaluations. The investigation was then extended to study the effect of blood rheology on the hemodynamics of a bileaflet mechanical heart valve with particle image velocimetry (PIV) validation. Finally, the rheological experimentations were conducted to analyze the coagulation process and the interactions between heparinized blood and the anticoagulation reversal agents. The ATAA analysis showed significant variations in the maximum WSS despite minimal differences in flow velocity between normotension and hypertension. The three different ATAA models identified different aortic expansions that were not uniform under pulsatile pressure and a geometry depended on elevated wall stress under hypertension. The investigation on the heart valve revealed the hematocrit influenced the shear stress distributions over a cardiac cycle. The structural stresses in the mechanical valve were affected by the shear stress distributions in the blood flow. Parameter dependencies study indicated that the hematocrit is influential when conducting patient-specific modeling of prosthetic heart valves. Finally, the use of small amplitude oscillatory shear (SAOS) rheometry for studying blood coagulation provided a comprehensive assessment with the combination of multiple rheological parameters for untreated and heparin neutralized blood. The coagulation characterization could be used towards the existing FSI models to account for potential blood clot formations in future studies.

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Numerical simulation of flow and microstructure in nematic liquid crystalline materials (2013)

Liquid crystals are known for their anisotropic characteristics, which lead to a preferred orientation of their molecules in the vicinity of solid surfaces. The ability of liquid crystalline materials to form ordered boundary layers with good load-carrying capacity and outstanding lubricating properties has been widely demonstrated. In order to study the advantages of implementing liquid crystals as lubricants, the steady state/time transient isothermal flow of thermotropic/lyotropic, nematic/chiral nematic liquid crystals between two concentric/eccentric cylinders and in planar Couette geometries were studied numerically. To consider the influence of the microstructure formation/evolution on the macro-scale attributes of the flow, the Leslie-Ericksen and Landau-de Gennes theories were employed. Simplicity of the Leslie-Ericksen theory in capturing the orientational alignment angle of the molecules makes it a viable candidate for modelling the flow of flow-aligning nematic liquid crystals. On the other hand, the Landau-de Gennes nematodynamics equations are well suited for predicting texture formation since defects and disclinations are non-singular solutions of the governing equations. The Landau-de Gennes theory for the liquid crystalline microstructure along with continuity and momentum equations were solved simultaneously using General PDE and Laminar Flow modules of COMSOL Multiphysics. The investigation of flow characteristics and orientation of liquid crystalline molecules for different rotational velocities/shear rates and anchoring angles at the boundaries were presented. Furthermore, nucleation and evolution of singularities in texture of the liquid crystalline materials were tracked over the simulation time. Moreover, alterations in the macro-scale attributes of the flow such as velocity profile, pressure distribution and first normal stress difference along with the evolution of defects were studied inside the liquid crystalline domain. The implementation of Landau-de Gennes nematodynamic governing equations for LCs flow simulations offered an insight in application of these materials as lubricants. It was shown the LCs could provide protection against the wearing mechanism by forming a shielding layer in the vicinity of solid surfaces. Three-dimensional simulations of a simplified prosthetic hip joint suggested that liquid crystalline materials should be considered as potential bio-lubricants.

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Mathematical Modeling of Interaction of Wet Particles and Application to Fluidized Beds (2011)

In many industrial operations, such as fluidized bed granulators, coaters, and fluid cokers, a binding or reacting liquid is introduced into the system. Due to the effects of liquid, the multi-phase transport phenomena of these systems are more complicated compared to conventional gas-solid fluidization systems. In this thesis, mathematical modeling is used to study the interaction of wet particles. First, a coalescence model is developed to describe the binary collision of wet particles. The model is in the form of a wet coefficient of restitution and is used to determine the critical velocity—the boundary between coalescence or rebound outcomes—for a range of capillary numbers. Model predictions are compared with the available experimental data and good agreement is found. The model accounts for both liquid viscosity and surface tension effects and is used to investigate the boundary between collisions with dominant capillary and respective viscous effects. Then, by incorporating time- and temperature-dependant variations of the viscosity and thickness of the liquid coating, the model is used to determine the agglomeration tendency of bitumen-coated coke particles in fluid cokers. A simplified mathematical model and numerical solution of the Navier-Stokes equations are used to study the rupture of stretching liquid bridges between two solid spherical particles. The simplified model considers the geometry of the problem in which the gas-liquid interface is represented with a parabola. The numerical simulations of the Navier Stokes equations are performed with FLUENT and are used to investigate the viscous, surface tension, inertial, gravitational, and contact angle effects on the rupture distance and liquid distribution. Finally, the interaction of multiple wet particles is addressed by implementing the wet coefficient of restitution proposed in this thesis, using MFIX, an open-source Discrete Element Method (DEM) tool. DEM simulations of a fluidized bed consisting of mono-sized solid spherical particles pre-coated with identical liquid coatings are performed, and the effect of coating viscosity and thickness on the fluidization behaviour is investigated. Snapshots of the instantaneous particle positions are presented, and time-averaged values of the bed centroid in the y-direction, wet coefficient of restitution, and relative normal collision velocity are analyzed.

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Master's Student Supervision (2010 - 2020)
Numerical modelling of liquid crystal flows in confined domains (2019)

Liquid crystals are anisotropic, viscoelastic materials with properties intermediate of solids and liquids. They are useful structural and functional materials and due to their ability to form ordered layers close to the bounding surfaces they are used as lubricants. Under the application of a hydrodynamic field, based on the type of velocity profile, values of non-dimensional numbers and anchoring angles, different orientation profiles are observed. Leslie-Ericksen and Landau-de Gennes theories are used to understand the evolution of the microstructure. Leslie-Ericksen theory, due to its simplicity can be used to obtain the behavior of flow aligning nematic liquid crystals. Landau-de Gennes theory is a mesoscopic model and apart from its ability to capture singular solutions can be employed in a study of lubrication using liquid crystals. This research work contains studies of liquid crystals in different flow conditions, such as the Couette flow and the Channel flow. In the study of Couette flow of Graphene oxide suspensions in water, the Leslie-Ericksen theory was used to obtain the orientation and viscosity profiles at different shear rates, up until flow alignment was observed. In the numerical study of pressure-driven channel flows, a Marker and Cell based solution methodology was implemented to solve the Leslie-Ericksen hydrodynamic theory. The expected flow alignment of liquid crystals was obtained which validated the solver. A preliminary study combining the moving wall and pressure driven flow showed that the orientation profile obtained depends on the local direction of shear and the direction of shear gradient. The scaling analysis was applied to Landau-de Gennes theory to derive the simplified equations of a planar lubrication theory. The theory was then validated by comparison with the solution obtained from numerically solving the full set of equations using the Couette flow profile. The parametric studies conducted showed that the solution was in the Elasticity-driven steady state. A discussion of the physical conditions showed its applicability for films of thickness lesser than 1 mm for a 1 m long domain.

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Rheo-PTV analysis of complex fluids (2019)

Complex fluids are ubiquitous in the everyday life. Their study enables a better perception of the surrounding environment as it can be used to model a variety of industrial, biological and geophysical applications. This study presents a novel optical capillary rheometer that benefits from the advantages of both capillary rheometry and flow visualization.The experimental setup developed in this research was applied to three different case studies, each highlighting an aspect of the rheological behavior of complex fluids. In the first case study, the slip mechanism of various concentrations of carbopol gel, as a simple yield stress fluid, was systematically investigated with respect to its material properties. Presence of a fully plugged flow and a thin layer of a Newtonian solvent, lubricating the unyielded gel at wall stresses below the yield stress was confirmed. The wall slip behavior of the carbopol gels at wall stresses above the yield stress was studied as well. It was shown that the slip velocities have a power-law correlation with the wall shear stress below the yielding point, and a linear correlation after the yielding point. Furthermore, the sliding threshold below which the gel sticks to the wall, was found to have a linear relationship with its yield stress and the fluidity of its solvent. The start-up flow of laponite as a thixotropic yield stress fluid was examined in the second case study. The time dependent behavior of the fluid was studied in terms of the variations of shear stress at a constant flow rate. Finally, the behavior of synovial fluid at high shear rates was investigated as the third case study. Synovial fluid of the knee joint undergoes extensive range of shear rates during high physical activities. Rheological behavior of synovial fluid was studied at shear rates as high as 7000 s-1, which is seven times greater than the maximum shear rate typically measurable by a rotational rheometer. The method developed in this research enables to further study the complex behavior of fluids that are challenging using the conventional rheometry methods.

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Cellulose nanocrystals aqueous suspensions as water-based lubricants (2018)

Lubrication is an effective means of controlling wear and reducing friction. Friction and wear are the major cause of material wastage and loss of mechanical performance. To reduce the friction, most of the mechanical devices are lubricated by oils or in some cases by water. To enhance the properties of lubricants a chemical component or blend is added to improve their performance. In this research, we have used Cellulose Nanocrystals (CNC) as additives in water-based lubricants. CNC is synthesized from native cellulose which is one of the most abundant biopolymer resource available. It has many advantages such as renewable, biodegradable and non-toxic.Tribological tests were performed on a pin on cylinder tribometer to investigate the application of CNC as water-based lubricants additives. The coefficient of friction and wear between a stainless-steel shaft and a chrome steel ball were measured in the presence of the CNC lubricant with different concentrations.One of the applications were water is used as a lubricant is in gland sealed slurry pumps. Gland seals prevent pumped fluid from leaking into the environment. The gland seal packing material is tested with CNC lubricant to study the behavior of the new lubricant as a possible alternative of water in industrial applications. Effect of normal force, rotational speed and shaft diameter on the coefficient of friction and wear were studied as well. It was found that adding 2 wt.% of CNC in water improved lubrication and provided a very low friction coefficient of approximately 0.09. It reduces the wear depth and width by more than 50%. The improvement of the coefficient of friction and wear is mainly due to the high strength of CNC rods and alignment of CNC nanoparticles.

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Rheological characterization of fumed silica lubricating greases (2018)

Lubricating greases have been widely used for rail lubrication systems. For an efficient grease pump design, it is important to study grease shear viscosity and it is also crucial to analyze grease yielding behavior to determine its consistency on rail surface. Among all rheological properties measured through experiments, yield stress is an ill-defined property, which investigation of a reproducible method for its determination can be invaluable. As the flow properties of a material will be usually influenced by the changes in environment temperature, studying the effects of temperature on the rheological properties of grease are important. In this study, different rheological measurements and visualization techniques, previously developed to study a wide range of materials, have been performed to characterize fumed silica based lubricating greases manufactured by L.B. Foster Rail Technologies Corp. Using commercial rheometers and different approaches to determine the yield points of these materials, it was revealed that the values obtained by curve fitting on steady-state flow curves, creep, amplitude sweep crossover and stress ramp-up were roughly similar. The microstructure of this grease was analyzed using Scanning Electron Microscope (SEM) on Cryo and non-Cryo modes. Besides visualizing a new thickener microstructure, it was shown that the heterogeneous structures developed by small fumed silica agglomerates lead to the formation of greases with higher shear viscosities. Finally, thermo-rheological analysis of these samples revealed that these materials follow neither Arrhenius equation nor time-temperature superposition principle.

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A rheological study of treatments for osteoarthritis (2016)

Osteoarthritis is a common joint disease causing pain and inflammation that limits mobility and functionality. Osteoarthritis is a widespread disease, and despite it's prevalence there is no cure. The disease progresses by degrading joint cartilage and synovial fluid. The current work aims to contribute to existing knowledge regarding osteoarthritis through three research projects including a rheological study of novel anti-inflammatory hyaluronic acid derivatives, a case study on the effects of oral glucosamine supplementation on synovial fluid viscosity, and development of a microfluidic rheometer that may be used to study low viscosity fluids such as hyaluronic acid and synovial fluid at high shear rates. The anti-inflammatory hyaluronic acid derivative demonstrated shear thinning and viscoelastic behaviour as expected. This behaviour is common among viscosupplements and is therefore promising for it's potential use as a viscosupplement, however, the viscosity and viscoelasticity were significantly lower than commercial viscosupplements. It is recommended to investigate modulating viscosity and viscoelasticity with techniques such as cross-linking and increasing hyaluronic acid concentration. In a patient discontinuing oral glucosamine supplementation the viscosity was found to be greater prior to stopping treatment at low shear rates and greater after stopping treatment at high shear rates. In a second patient no significant change was observed in viscosity. Future study should investigate the effect of glucosamine on viscoelastic behaviour, and to study the effect of glucosamine on synovial fluid viscosity over an extended period of time. A PDMS microfluidic rheometer was developed using a soft-lithography process. The rheometer was validated with water at room temperature and was found to predict fluid viscosity with a maximum of 6% error at shear rates as high as 30,000 s-¹. It is recommended to investigate possible channel deformation as a cause for decreased accuracy at higher shear rates and resultant operating pressures.

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Computational analysis of fluid structure interaction in artificial heart valves (2013)

The development of heart valve stenosis and sclerosis can lead to the development of fatal complications such as congestive heart failure. Therefore, severe valve stenosis requires a surgical operation with artificial heart valve replacement. Given that the geometrical differences between artificial valves would significantly influence hemodynamic performance around the implanted valve, additional knowledge for the interactions between blood flow and the artificial valve is necessary. Therefore, in order to proceed, this study proposes an advanced computational fluid dynamics (CFD) simulation using a fluid-structure interaction (FSI) technique to investigate artificial valve leaflet motion under different physiological conditions. Among various FSI technique, it is proposed to simulate the motion of the artificial heart valve with a fully-coupled algorithm and arbitrary Lagrangian-Eulerian formulation (ALE) using a monolithic solver. Models are constructed using a realistic aortic root for both the bileaflet and bioprosthetic valves with additional modifications and considerations for the flexible arterial wall. Normal physiological blood pressure and conditions are used to simulate healthy scenarios, which are compared with experiments. Validation is conducted by analysing particle image velocimetry (PIV) experimental data from ViVitro Lab. Hemodynamic performance analyses are conducted and found that both velocity and maximum von Mises stress are higher if calculated using a rigid wall model. The leaflet dynamics, on the other hand, is relatively the same for rigid or flexible wall model. Clinically relevant scenarios are also simulated for both mechanical and bioprosthetic valves. The clinical focus for the mechanical valve is on the malfunction of the valve due to leaflet restrictions. In addition, the clinical focus for the bioprosthetic valve is on the systolic deficiency due to different tissue properties.

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Industrial bio-lubricants performance and characterization (2013)

The general trend towards the use of high performance lubricants and environmentally friendly products supports the design of new industrial lubricants. Therefore, there are good practical reasons to extend the research related to lubrication. Bio-oils, as promising growing substitutes for mineral oils, need more research to deal with new and inherited problems. Meanwhile, there is no complete understanding of the lubrication phenomenon, nor a complete rheological characterization of oil lubricants. This research is an effort to study industrial bio-lubricants and to develop a more comprehensive approach, at the same time correlating their rheological and tribological behavior. Different commercial canola oil based lubricants were studied using different techniques. For validation and comparison, engine oil, silicone oil and mineral hydraulic oil were tested. Bio-lubricants exhibited constant viscosity at both moderate and high shear rates and shear thinning at low shear rates and temperatures below 30 degrees Celsius. Frequency sweep tests revealed a significant viscoelasticity of bio-lubricant which developed over time.Time dependence, structure recovery, gap size effect, surfactant behavior, and geometry’s material influence were all investigated. A high pressure cell and a polarized light microscope coupled with the rheometer were used to investigate the bio-lubricants. Thermal analysis was conducted using a differential scanning calorimeter. Several transition points were identified in the range of temperatures from -30 to 100 degrees Celsius, and the results have been connected to the viscoelastic behavior. Different tribological tests were used to investigate the lubricity of lubricants and bio-lubricants added by liquid crystals. The coefficient of friction, at tested temperatures, and the wear rate were observed over time. Adding two percent of ionic liquid crystals improved the wear resistance of the oil, but the bio-lubricant had the lowest coefficient of friction. This research could be considered as pioneer work. An attempt was made to achieve profound perspective matching between rheometry, tribology and thermal analysis. Some assumptions explaining the rheological and tribological behavior were hypothesized and associated with arguments and discussions. Based on, Imaginary scenario of bio-hydraulic oil behavior within a small gap was visualized.

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Osteoarthritic synovial fluid rheology and correlation with protein concentration (2013)

Synovial fluid (SF) is a lubricant for articulating joints. The study of SF rheological properties has gained significance due to SF viscoelastic properties, and SF’s ability to sustain a considerable load. The rheological performance of SF is linked to the joint’s condition. A joint disease such as osteoarthritis (OA) reduces SF rheological properties. This study is aimed at investigating the shear and extensional rheological properties of osteoarthritic synovial fluid (OA SF). Additionally, this study is aimed at correlating SF rheological properties with its protein concentration.Shear rheological properties of 35 OA SF samples were investigated at a physiological temperature (37 °C) using cone-and-plate shear rheometer. Furthermore, the effects of the temperature, the centrifugation, and the storage at -20 °C for two weeks were also studied on some samples. Additionally, the time-dependent rheological properties were investigated by rotation and oscillation tests. Extensional rheological properties were studied using a capillary breakup extensional rheometer (CaBER). First, the effects of different CaBER configurations on the extensional rheological measurements were investigated in order to determine the optimal configuration. Then, the extensional rheological properties of 21 OA SF samples were studied. The protein concentrations of SF were determined using a bicinchoninic acid (BCA) protein assay kit. I also investigate the correlations between rheological properties and protein concentration.The understanding of SF rheological properties will lead to a better understanding of its lubrication properties, and to the development of a rheological analogue to SF or to a periprosthetic fluid.

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