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Understanding Shear Thinning Using Brownian Dynamics Simulation

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Understanding Shear Thinning Using Brownian Dynamics Simulation University of Tennessee at Chattanooga UTC Scholar Student Research, Creative Works, and Honors Theses Publications 5-2021 Understanding shear thinning using Brownian dynamics simulation Mackenzie Nicole Wall University of Tennessee at Chattanooga, [email protected] Follow this and additional works at: https://scholar.utc.edu/honors-theses Part of the Physical Chemistry Commons Recommended Citation Wall, Mackenzie Nicole, "Understanding shear thinning using Brownian dynamics simulation" (2021). Honors Theses. This Theses is brought to you for free and open access by the Student Research, Creative Works, and Publications at UTC Scholar. It has been accepted for inclusion in Honors Theses by an authorized administrator of UTC Scholar. For more information, please contact [email protected]. 1 Understanding Shear Thinning Using Brownian Dynamics Simulation Mackenzie Nicole Wall Departmental Honors Thesis University of Tennessee at Chattanooga Department of Chemistry and Physics Examination Date: 23 March 2021 Dr. Luis Sanchez-Diaz Dr. John Lee Lecturer of Physics Associate Professor of Chemistry Project Director Department Examiner 2 Abstract: In this work, we study the changes in structure during the shear thinning regime using Brownian Dynamics with a simple steady-state shear flow of binary charged colloidal suspension. Previous research has analyzed the viscosity, radial distribution, elasticity and plasticity of materials with rheo-SANS experimentation; however, less research has been conducted to replicate the experiment through computer simulations. With Brownian Dynamic Simulation, this study was able to reproduce the results obtained in a recent rheo-SANS experiment and it also explored the viscosity, radial distribution, elastic and plastic behavior of a system under different parameters. The comparison of simulated data with experimental data revealed the computer simulation to successfully generate results indicative of shear thinning behavior in both the viscosity versus shear data as well as the radial distribution data. The simulated system was also able to successfully generate systems which exhibited plastic behavior and elastic behavior. 3 Table of Contents Cover Sheet ……………………………………………………………………………………... 1 Abstract …………………………………………………………………………………………. 2 Chapter 1: Introduction ……………………………………………………………….…… 4-14 1.1 Introduction to Colloids and BDS ………………………………….……………...... 5 1.2 Viscosity………………………………………………………...………....……….... 6 1.3 Newtonian and Non-Newtonian Fluids ……………………………………………... 7 1.4 Shear Thickening ……………………………………………………………………. 8 1.5 Shear Thinning ………………………………………………………………………. 9 1.6 Rheology ………………………………………...…………………………………. 10 1.7 Elasticity and Plasticity …………………………………………………………..… 11 Chapter 2: Methodology ……………………………………………………...………….. 15- 18 2.1 The Yukawa Potential …..………………………………………………………….. 16 2.2 Radial Distribution Function G(r) ………………………………………………….. 16 2.3 Brownian Dynamics …………………………………………………………………17 2.4 Viscosity as a function of Shear ……………………………………………………. 18 2.5 Elasticity and Plasticity …………………………………………………………….. 19 Chapter 3: Results and Discussion ……………………………………………………… 20- 34 3.1 Comparisons with Experimental Data …………………………………………..…. 21 3.2 Radial Distribution Results ………………………………………………………… 22 3.3 Viscosity vs Shear Results …………………………………………………………. 30 3.4 Elasticity vs Plasticity Results ……………………………………………………... 31 Conclusion …………………………………………………………………………..…….. 35- 36 Supporting Materials: The Code …………………………………………………………….. 37 References……………………………………………………………………………………… 48 4 Chapter 1: Introduction 5 1.1 Introduction to Colloids, BDS, and our Project A colloid is a mixture that has particles ranging between 1 and 1000 nanometers in diameter, yet are still able to remain evenly distributed throughout the solution. These mixtures are homogeneous, and the particles remain dispersed meaning that they will settle in the bottom of a container. Examples of colloids include whipped cream, mayonnaise, milk, butter, gelatin and some biological systems like blood. Such examples can begin to explain why studying colloids is necessary. Colloidal studies can benefit businesses by helping to improve product quality to then improve sales. Colloidal studies can be beneficial for safety purposes, such as studying the behavior of oil for the sake of employee safety on oil rigs. And such studies on blood can lend obvious contributions to the health sciences. Thus, many industries benefit from this colloidal and fluid focused research, such industries in food, cosmetics, pharmaceuticals and medicine [1,2]. One type of research used to study colloids is BD simulations which are used to simulate such colloidal suspensions under various parameters. In fact, some of the earlier studies regarding colloids in light of altering parameters such as particle size, shear rate, viscosity etc. were due to interest from industry. Industrial companies were curious to better understand why their procedure of coating paper, which increased the coating’s viscosity in the process, was resulting in torn papers and ruined machinery [3]. Similar investigations have been conducted since then. For example, studies to observe how the viscosity changes as a function of an external force (shear) are more common [4,1]. Regardless of the subject of the colloidal studies, a major component analyzed is viscosity. The focus of this particular research aims to recreate rheological data in a simulated manner for the purposes of comparing with experimental data. Other research has revealed that various colloids are studied under rheometers and with small angle neutron scattering with the 6 intent to observe shear thinning behavior such as in Lanotte et. al’s rheometric study on red blood cell morphology, but few have chosen opted to study the behavior with computer simulations [5]. 1.2 Viscosity When examining these colloidal systems, viscosity is a key subject analyzed. Viscosity is a fluid’s resistance to flow. This tendency is modeled by Newton’s Law of Viscosity which describes the relationship between shear stress and shear rate in a fluid [6]. Viscosity is given as 휏 휂 = Equation 1 훾 Where 휏is shear stress,훾is shear rate, and viscosity is determined in Pascal seconds (푃푎 ⋅ 푠) [7,8]. Trends of viscosity can be observed as a function of shear rate as seen in Figure 1. Figure 1: Graph of viscosity as a function of shear rate Shearing occurs when internal friction of a fluid increases and the amount of force needed to overcome the friction increases [8]. This illustrates why more viscous fluids require more force to flow. Figure 2 depicts the process of shearing and is the model that was used to establish Newton’s Law of Viscosity (Equation 1) [8]. 7 Figure 2: Diagram of shearing Shear is the force applied to the fluid system in a specific direction. As increasing shear is applied to the fluid, the response in viscosity separates fluids into two categories: Newtonian fluids and Non-Newtonian Fluids [9]. 1.3 Newtonian and Non-Newtonian Fluids In nature, we can classify liquids in two categories. Newtonian and non-Newtonian. If the viscosity does not change as shear increases, then it is a Newtonian fluid as seen in Figure 3 [10]. Fluids such as water and alcohol fall into this category. Figure 3: Graph of Newtonian fluid 8 In comparison, fluids whose viscosities change are Non-Newtonian fluids as seen in Figure 4 [11]. Figure 4: Graph of Non-Newtonian fluids compared to Newtonian fluids There are two kinds of Non-Newtonian fluids: shear thinning fluids and shear thickening fluids. 1.4 Shear Thickening If viscosity increases as shear increases, the fluid experiences shear thickening [3]. Many materials incorporate such materials into their products of their ability to dampen and absorb shock by dispersing the initial force applied [3]. Other common materials that incorporate shear thickening fluids are Kevlar vests. Shear thickening fluids help disperse the force of bullets and increase overall rigidity of the vests more efficiently [3]. This material is even incorporated into products as simple as sporting equipment. Similar to a Kevlar vest, a tennis racket aims to redirect an incoming ball, but the strings of the racket must maintain their rigidity while still dispersing the vibrations from the impact [3]. 9 1.5 Shear Thinning The other type of Non-Newtonian fluid is shear thinning fluids. Shear thinning fluids are the opposite of shear thickening fluids. These fluids will decrease in viscosity as shear force increases [12,2]. Ketchup is a prime example of shear thinning behavior. When turned upside down, ketchup will not readily flow from its container, so the bottle must be hit on its bottom or squeezed and once the action ceases so too does the flow. The force exerted is decreasing the viscosity of the ketchup allowing it to flow more easily, and without the added force, the ketchup resumes its initial viscosity. Other types of shear thinning materials include paints and blood [5]. Paints desire shear thinning for practical reasons. Paints will maintain a higher viscosity under no shear while they are in their containers which prevents valuable products from accidentally running out of its container. But when a product is needed, applying small shear in the form of a squeeze to the bottle for example will decrease the product’s viscosity allowing it to be easily dispensed from its bottle. On a more serious topic, typical shear thinning fluids can be observed in biological systems in the form of blood. Specifically, shear thinning
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