A Piezoelectric Minirheometer for Measuring the Viscosity of Polymer Microsamples

This paper describes the electromechanical design, operating principles and performance of a rheometer able to characterize the rheological behavior of microsamples of viscoelastic materials, such as polymer solutions, melt, and rubbers. It was developed with a view to portability, robustness, and ease of operation for very small samples. The rheometer operates by subjecting the samples to small-amplitude sinusoidal strain rates via an inverse piezoelectric actuator and detecting the stress response of the material via a direct piezoelectric sensor. The device operates under frequency-sweep mode in a very wide range of frequencies. Required sample sizes are typically three orders of magnitude smaller than for conventional rheometers. Owing to its lack of moving parts, the rheometer has an extremely simple design and is insensitive to vibration. Measurements on pressure-sensitive adhesives and other polymeric systems are presented and validated against a standard cone-and-plate rheometer

An applied investigation of viscosity–density fluid sensors based on torsional resonators

Real-time viscosity and density measurements give insight into the status of many chemical and biochemical processes and allow for automated controls. In many applications, sensors that enable the real-time measurements of fluid properties use resonant elements. Such sensors measure induced changes in the element’s resonance frequency and damping that can be related to the fluid properties. These sensors have been widely researched, though they are not yet commonly used in industrial processes. This study investigates two resonant elements to measure the viscosity and density of Newtonian fluids. The first is a probe-style viscosity-density sensor, and the second is a non-intrusive tubular viscosity sensor. These two sensors were investigated using analytical, numerical, and experimental methods. In the analytical method, the sensors’ resonance frequencies and bandwidths were predicted based on reduced-order models for both structure and fluid. In the numerical method, the interaction of the resonant element with the fluid was investigated by means of computational fluid dynamics (CFD). Experiments were conducted for validation, to evaluate the sensors’ capabilities, and understand cross-sensitivity effects between viscosity and density. This work successfully modeled and validated the two different torsional resonant element sensors, namely the probe-style viscosity-density sensor and the tubular viscosity sensor against experiments. There are two key output parameters, i.e., resonance frequency and bandwidth. Using these parameters, it is possible to predict fluid viscosity and density. Overall, this work demonstrates the potential of numerical modeling for the development of torsional resonance sensors. These findings directly affect the development of the future generation of fluid viscosity and density sensors

Characterization of Industrial and Biological Complex Fluids Using Confocal Microscopy.

Complex fluids are materials that respond to applied stresses in a way that is intermediate between that of a purely viscous fluid or a fully elastic solid. The characterization of material properties depends upon the ability to both apply variable stresses or strains to the complex fluid and to measure the resulting response. Often the macroscopic response of these materials depends upon the microscale structure. In this dissertation, confocal laser scanning microscopy is used to analyze microscopic changes in complex fluids. We developed methods to apply the stress or strain that matches the application or environment of the material to be characterized. For instance, confocal images of the emulsion structure of fountain solution in ink exposed to oscillatory shear flow on model substrates were acquired. We found that surfactants inhibit aqueous droplet wetting on hydrophobic substrates. Without surfactants, surface coverage of the aqueous fountain solution on the hydrophobic substrate became quite high. This result is relevant to defects in lithographic printing. To characterize the material properties of biofilms a tunable small-scale device was needed. We developed a flexible microfluidic rheometer to apply a compressive force of ~200 pL volume. We used confocal microscopy to detect the deformation of a membrane in contact with a test material when compressive stresses were applied. This measurement allowed us to characterize material properties including elastic modulus and relaxation time of soft viscoelastic solids, biofilms in particular. We report evidence of strain hardening in biofilms; a result that could have implications for the understanding of clearance of biofilms in industrial and physiological environments. To understand the source of this phenomenon we applied confocal microscopy to characterize the structure of bacteria aggregated in a polysaccharide matrix. Interestingly, while aggregated bacteria and bacteria in biofilms are held together by ostensibly the same material we find their inter-bacterial distance to be quite different. Once this tool was developed the origin of strain-hardening in biofilms could be addressed. The strain-induced trajectories of individual bacteria found using image processing of confocal micrographs were analyzed to show that strain hardening occurs without an increase in volume fraction.Ph.D.Chemical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/61778/1/dnhohne_1.pd