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Title:Shaking, slamming, and vibrating yield-stress fluids: Inducing particle motion in rheologically-complex materials
Author(s):Koch, Jeremy Alexander
Director of Research:Ewoldt, Randy H.; Lange, David A.
Doctoral Committee Chair(s):Ewoldt, Randy H.
Doctoral Committee Member(s):Pearlstein, Arne J.; Smith, Kyle C.
Department / Program:Mechanical Sci & Engineering
Discipline:Theoretical & Applied Mechans
Degree Granting Institution:University of Illinois at Urbana-Champaign
Fluid mechanics
Yield stress
Abstract:A yield-stress fluid behaves effectively as a solid for stresses below a critical threshold. Examples include soap foam, peanut butter, aloe gel, and sand -- materials that can hold their shape under their own weight, but which can be reversibly reshaped by external forces. Notably, yield-stress fluids are capable of suspending macroscopic particles much larger than the size of a fluid structural element, preventing the particles from either rising with buoyancy or sinking due to their weight. Of course, these particles can be put in motion by stirring the fluid, but there are other means by which the particles can be displaced. We focus on two complex flow scenarios in which initially suspended density-mismatched particles are set in motion by external conditions. In the first, the motion of air bubbles in fresh concrete is considered. Simple model materials, a well-studied yield-stress fluid (Carbopol in water) and a granular medium (millimetric glass beads in silicone oil), are compared to fresh concrete in lab-scale vibration experiments and rheology measurements. The granular medium demonstrates the same fluidization phenomenon as the fresh concrete in response to vibration, suggesting granular force-chain dynamics can rationalize the effect of vibration on concrete. This understanding is used to explain the mechanism of air bubble motion during vibration. More fundamental questions are raised, however, since both Carbopol and the granular medium are jammed, repulsive systems characterized as soft glassy materials -- systems of disordered, metastable particles unified under a common rheological behavior. Two parameters are proposed to distinguish granular materials, and the use of these parameters is demonstrated with the model materials. In a second study, an isolated particle in a nongranular yield-stress fluid whose container is subject to abrupt accelerations is considered. Through careful choice of fluid properties and acceleration forcing function, counterintuitive behaviors are experimentally observed: sinking air bubbles and rising steel spheres. This phenomenon is rationalized with theory, but new considerations are necessary for a quantitative analysis. A modification to a suspension criterion found in the literature is proposed to account for rigid-body accelerations, and its viability is tested in a novel experiment. Additionally, the yield stress measurement for this scenario is reconsidered: Is the steady-state rheology necessarily relevant to a flow occurring on a timescale of hundredths of seconds? Using a new interpretation of the input conditions in strain-controlled rheological tests, transient rheology measurements are made that are more emblematic of the flow conditions seen in the sinking bubble phenomenon. It is demonstrated that the yield stress can be a function of the deformation timescale of the flow, and when this transient yield stress is factored into the modified suspension criterion, the critical value observed during the sinking bubble phenomenon is placed into agreement with the critical value for quasi-static flow.
Issue Date:2017-12-08
Rights Information:Copyright 2017 Jeremy A. Koch
Date Available in IDEALS:2018-03-13
Date Deposited:2017-12

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