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High Pressure Thermofluiddynamics and Rheology

Prof. Dr. rer. nat. habil. Andreas Wierschem

  • Job title: Professur für Hochdruckthermofluiddynamik und Rheologie
  • Phone number: +49 9131 85-29566
  • Email: andreas.wierschem@fau.de

Picture of the Narrow-Gap Rheometer

Narrow-Gap Rheometer with sensor inside

The group deals with a large variety of topics. We study the effect of pressure up to about 900 MPa on organic and biomaterials. Pressure levels of more than 100 MPa may yield pressure-induced solidification, protein denaturation, dramatic changes in enzyme activity and inactivation of micro-organisms. Hence, they are not only relevant for diesel injection but also for food preservation and processing, and are promising for the preparation vaccines. An important issue is the adaption of measurement techniques to detect how high pressure affects the systems.

Rheology studies the flow behavior and the deformation and of materials under mechanical load. The material behavior is often governed by a complex microstructure and its respective changes under load. Our main focus is on narrow-gap rheometry. We set up a narrow-gap rotational rheometers that allow measuring rheological properties of layers as thin as a few micrometers. This has many advantages: Only very few amount of sample is needed, viscosity and normal-stress differences can be studied at shear rates that are up to 2 decimal powers higher than in standard rheometers, average rheological quantities of cells may be determined in single runs…

Besides high pressures and complex material behavior, we also study general fluid dynamics. Here our focus is on particle-laden flows and on geometry-induced qualitative changes of flow regimes for passive flow control and to yield optimum flow regimes.

  • Second Newtonian region of polymer solutions
    (Third Party Funds Single)
    Term: 1. January 2016 - 31. March 2019
    Funding source: DFG-Einzelförderung / Sachbeihilfe (EIN-SBH)
                                      Polymer solutions are a class of fundamental rheological systems with countless applications. Their viscosity typically depends strongly on the shear load: At low shear rates, in the 1st Newtonian branch, the polymer solutions have a constant viscosity. Beyond this branch, the viscosity diminishes considerably. Finally, at very high shear rates another regime of constant viscosity, the 2nd Newtonian branch is expected. Apart from shear rate, the viscosity depends substantially on the type of polymer and its structure, the molecular weight and its distribution, the polymer concentration, and on the solvent. In the 1st Newtonian branch, different regimes of polymer solutions can be distinguished depending particularly on concentration and on molecular weight. While the 1st Newtonian branch and the onset of shear thinning are well studied, this is not the case for the 2nd Newtonian branch. Here, detailed knowledge about viscosity, normal-stress differences and the orientation of the polymers, their dependencies on the above-mentioned parameters as well as studies about the onset of the 2nd Newtonian branch are lacking. Proper knowledge about this branch, however, is not only necessary for a fundamental understanding of polymer solutions but is also needed in a number of industrial applications. Essential reason for the lack of knowledge about the 2nd Newtonian branch is the deficiency of adequate measuring technique: The shear-rate range is typically 1-2 decimal orders beyond the parameter range covered by commercial rheometers. Besides, very low viscosities need to be detected. We have improved the geometrical precision of rotational rheometers by about a factor of 30 to 100. This permits measurements at considerably narrower gaps that enable reaching the necessary high shear rates and precision. Furthermore, to the best of the authors knowledge, the improved rheometer is the only one that allows for detecting normal forces in polymer solutions and for carrying out rheo-optical studies at shear rates up to 105 s-1. It is aimed for further enhancing the precision within the project. Central aim of the project is to characterize the 2nd Newtonian branch of well-defined as well as industrially relevant polymer solutions. Simultaneously with the viscosity measurements, shear-induced normal forces due to the solution viscoelasticity and the orientation of the polymers are to be detected. The latter is to study whether the microstructure changes in the 2nd Newtonian branch. The main issue is to clarify if there are different regimes as a function of polymer concentration in the 2nd Newtonian branch similar to those in the 1st Newtonian branch. Furthermore, the impact of molecular weight, changes in polymer structure and solvent should be elucidated. To avoid shear-induced degradation, the study is planned to be carried out with semi-flexible polymers.

  • Substrate impact on flow induced particle motion in laminar shear flow
    (Third Party Funds Single)
    Term: 1. October 2015 - 15. October 2019
    Funding source: DFG-Einzelförderung / Sachbeihilfe (EIN-SBH)
    Flow induced detachment and motion of solid particles on a fixed substrate or granular bed is of fundamental relevance in numerous natural and industrial systems. As examples sediment and granular transport in filtration, natural watercourses and piping systems, particle transport in the respiratory system, and the cleaning of surfaces may be mentioned. In microfluidics, spherical particles are progressively used as information carrier or for actuators like valves or pumps. Therefore, an important precondition is the controlled positioning of particles. While the motion of single particles on smooth walls is quite well understood, this does not hold for structured surfaces and granular beds. Despite numerous studies concerning the onset of particle motion, the motion itself or rather the material transport, these technologically important processes are still not well described. This is due to the fact that particle removal has been studied exclusively for disordered sediment layers and has been tried to describe using not well-defined parameters in the existing models. Since the precise particle arrangement is ignored, the effects of substrate geometry, exposure to the flow and shading by neighboring particles remains unclear. Hence, the impact of substrate geometry and of neighboring particles on the onset of particle motion and the motion along the substrate itself remains unclear.Granular beds hinder particle motion in two ways: On the one hand, the uneven substrate poses an obstacle, which downstream contact angle has to be overcome, on the other hand, its shades the particle against the flow. In an analogue manner, neighboring particles serve as an additional hindrance. Starting from this observation, we plan to vary the relevant parameters. The aim of the project is to describe the onset of particle motion and the motion along the substrate itself in laminar shear flows quantitatively in a model without recourse to not well-defined parameters.

  • Einfluss der Partikellagerung auf den strömungsinduzierten Einsatz der Partikelbewegung
    (Third Party Funds Single)
    Term: 1. July 2010 - 30. June 2012
    Funding source: DFG-Einzelförderung / Sachbeihilfe (EIN-SBH)
    Das strömungsinduzierte Lösen und Entfernen von Feststoffpartikeln von einem festen Untergrund ist von grundlegender Bedeutung in einer Vielzahl natürlicher wie industrieller Systeme. Beispielhaft seien der Sediment- bzw. Granulattransport in Fließgewässern sowie in der Lebensmittel- und Pharmaindustrie, die Reinigung von Oberflächen z.B. in Produktionsanlagen und von Wafern, Filtration, Mikrofluidik oder auch der Partikeltransport in Atemwegen genannt. Aufgrund ihrer großen Bedeutung werden sowohl das Einsetzen des Partikeltransports als auch das Entfernen der Partikel von der Oberfläche sowohl theoretisch als auch experimentell intensiv untersucht.Während die Bewegung einzelner Partikel auf ebenen Wänden als recht gut verstanden gelten kann, gilt dies nicht für Partikelanhäufungen und Sedimentschichten. Dies resultiert zum einen daraus, dass das Entfernen der Partikel vor allem bei turbulenten Strömungen untersucht wurde, zum anderen wurde bisher ausschließlich der Einsatz der Bewegung irregulär angeordneter Partikel auf ebenen Unterlagen oder in eingeebneten aber willkürlich angeordneten Sedimentschichten untersucht. In der Beschreibung bleibt die genaue Partikelanordnung unberücksichtigt. So bleiben auch die genauen Mechanismen wie Einfluss von Druck und Schubspannung, die genaue Lagerung der Partikel, Abschattung durch Nachbarpartikel oder durch das Ruhen der Partikel in Vertiefungen unklar. Zudem sind auch die kritischen Zeitdauern für den Einsatz der Partikelbewegung nicht bekannt.Im Rahmen des beantragten Projekts soll der Einfluss der Partikellagerung auf den strömungsinduzierten Einsatz der Partikelbewegung experimentell untersucht werden. Zu diesem Zweck soll auf einem regelmäßig angeordneten Substrat aus identischen Partikeln systematisch die Anzahl der Kontaktpunkte, die Orientierung des Substrats bzgl. der Strömungsrichtung und der Gitterabstand des Substrats variiert werden. Neben der Messung der kritischen Kennzahlen für den Einsatz der Bewegung unter stationärer Scherströmung sollen die kritischen Zeitdauern für den Einsatz der Bewegung im überkritischen Parameterbereich bestimmt werden. Ausgehend von Messungen an Einzelpartikeln auf dem Substrat sollen die Untersuchungen auf Partikelgruppen auf dem Substrat und schließlich auf eine geschlossene Partikelschicht ausgeweitet werden.

The research group is subdivided into 3 areas:

  • High pressure
    Studies group around the question how product properties and production processes can be improved by application of pressure of up to several thousand bars. Therefore, it is necessary to know the material parameters at these high pressures and to develop adequate in situ-measurement methods under high pressure.
  • Fluiddynamics
    Focus is on multiphase flows. Particular attention is on the impact of substrates on the dynamics of film flows, on the flow-induced particle motion and on the development of adequate measurement technique.
  • Rheology
    Rheology studies the flow and deformation behavior of matter. The group’s focus is on narrow-gap rheometry, which allows measurements with minimal sample amounts (order of magnitude 10 µL) at shear rates of more than 105 s-1 but also of mechanical properties of cells and tissue, and of their load and adhesion limits.

 

Actual or recent topics:

  • High pressure processes
    • Pressure-induced phase transitions
    • Determination of material parameters under high pressures
    • Measurement techniques under high pressures
  • Fluiddynamics
    • Flow-induced particle motion
    • Film flows
  • Rheology
    • Narrow-gap rheometry
    • Biorheology
    • Rheology at high shear rates
    • Polymer solutions

A number of different measuring devices is available at the chair. The list can be found at https://www.lstm.tf.fau.eimgu/research/equipment/

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