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Title:Precision rotational rheometry
Author(s):Johnston, Michael
Advisor(s):Ewoldt, Randy H.
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
surface tension
Abstract:For rotational rheometers, alignment of rotating surfaces is of prime importance in conducting accurate, reproducible measurements. A custom glass bottom surface has been designed and fabricated that has < 1 µm of planar misalignment with respect to the instrument axis of rotation. This also enables visualization of samples from below and precise alignment of custom top plates that are attached to a standard geometry by a temperature sensitive adhesive. The glass bottom plate has enabled research in two areas: (i) surface tension effects on torque measurement and (ii) effects of microtextured surfaces on shear stress under gap controlled conditions. Surface tension results in a torque that should not occur in an ideal, rotationally-symmetric geometry. This work identifies and explains the effect, which is due to surface tension and contact line traction forces. The surface tension torque is reduced by maximizing rotational symmetry of the contact line, minimizing evaporation and the migration of the contact line, reducing the radial location of the contact line, and lowering the surface tension. Identifying and eliminating the surface tension torque is critical for low viscosities, intrinsic viscosities, soft materials, sub-dominant viscoelastic components, small gaps, and any circumstance where the low-torque limit is experimentally important. Microtextured surfaces, or deterministic placement of µm scale surface asperities, have been shown to reduce shear stress in hydrodynamic systems by reducing surface shear forces and by producing lift to support a hydrodynamic film. We have developed a triborheometric setup that enables precise control of the gap in order to explore the effect of microtextures on shear stress. We have also correctly non-dimensionalized the system and validated it with numerical results. Results show that the reduction in shear stress is larger than numerical results predict.
Issue Date:2014-01-16
Rights Information:Copyright 2013 Michael Thomas Johnston
Date Available in IDEALS:2014-01-16
Date Deposited:2013-12

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