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A Narrow-Gap Rotational Rheometer for High Shear Rates and Biorheological Studies

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A Narrow-Gap Rotational Rheometer for High Shear Rates and Biorheological Studies A narrow-gap rotational rheometer for high shear rates and biorheological studies Ein Dünnspaltrotationsrheometer für hohe Scherraten und biorheologische Untersuchungen Der Technischen Fakultät der Friedrich-Alexander-Universität Erlangen-Nürnberg zur Erlangung des Doktorgrades Dr.-Ing. vorgelegt von Haider Mohammed Ali Dakhil, M.Sc. aus Bagdad-Irak Als Dissertation genehmigt von der Technischen Fakultät der Universität Erlangen-Nürnberg Tag der mündlichen Prüfung: 12.02.2016 Vorsitzender des Promotionsorgans: Prof. Dr.-Ing. habil. Peter Greil Gutachter: Prof. Dr.rer.nat. Andreas Wierschem Prof. Dr.rer.nat. Rainer Buchhloz Acknowledgement I am very grateful to Prof. Dr. rer. nat. Andreas Wierschem for his supervision and astute insight during the course of my Ph.D degree at the Institute of Fluid Mechanics (Lehrstuhl für Strömungsmechanik, LSTM). My sincere gratitude also goes to Prof. Dr.-Ing. habil. Antonio Delgado who gave me the opportunity to be a part of this project and has been a constant support throughout. I would like to acknowledge the Iraqi Ministry of Higher Education and Scientific Research (MoHESR) and the German Academic Exchange Service (Deutscher Akademischer Austauschdienst, DAAD) for their financial support during my stay in Germany. My appreciation and gratitude to my present and former colleagues in the research group (High Pressure Thermofluiddynamics and Rheology Research), M.Sc.Monika Kostrzewa, and M.Sc. José Alberto Rodriguez Agudo. At this juncture, I would express my special thanks to Dr.-Ing. Holger Hübner and Mrs. Anette Amtmann for their valuable advice and support. This work would have been more difficult without their cooperation. I owe a lot to the technical assistance provided by Mr. Jürgen Heubeck and Mr. Horst Weber in the mechanical and electrical workshop of the institute. I would like to thank the administrative staff of the LSTM, Rita Scheffler-Kohler, Martina Montel-Kandy and Sonja Hupfer at this juncture for keeping things at ease. Thanks to the The IT department; Thorsten Bielke and Sebastian Röhl for their support and motivation throughout. I owe my highest gratitude to my family, who have always stood beside me. They have inspired and oriented the major part of my life morally and spiritually. I would like say thank you to my brothers Ahmed and Hassan. My wife Zaniab Al-Mimar and my daughters Fatimah, Aya and Sarah to whom I am in debt, for being there for me. To my family members I present this work. Blank page Abstract I Abstract In this thesis, a commercial rotational rheometer has been modified to facilitate rheological measurements at a gap width of a few micrometers. Working at narrow gaps presents many advantages: It enables to extend the range of applicability of the rheometer to measure, for instance, low viscosities, high shear rates and normal stresses at the smallest amount of samples and to carry out experiments directly at biological cells. Although the parallel-disk geometry allows for easy adjustment of the gap width, in commercial rheometers it suffers from errors that restrict the measurement gap width such as squeeze flow of air during zero-gap error setting due to misalignment of the parallel-disks and insufficient flatness of the plates while zeroing the device plates. To minimize the zero-gap error, a procedure has been developed to align the parallel-disks to each other within an uncertainty of ±0.7 µm gap width, i.e. the zero-gap precision is improved by a factor of 30 and more. In oscillation, the precision can be enhanced to about ±0.2 µm. Compared to other narrow- gap devices, our setup offer the advantage to not only allow for oscillatory studies but also to enable unidirectional studies at a narrow-gap width, at the same time, taking full advantages of the versatility of commercial rheometers. In the first part of this work, a commercial rotational rheometer in the parallel-disk configuration is modified so that the disks are aligned perpendicular to the axis of rotation with a precision in parallelism of about 1 µm independent of the rheometer reading. It enables samples to be studied at gap widths well below the absolute error of commercial rheometers, which is typically in the range between 25 µm to 75 µm. At gap widths of 20 µm, this modification allows the measurement range for shear rate to be extended from typically 103 s-1 up to 105 s-1. It also enables measurement of low viscosities such as that of solvents or water. As an application example, the viscosity functions of the polymer solutions are studied at high shear rates up to 105 s-1 with the modified parallel-disk geometry in the second part of this work. Shear-rate range enables the study of the second Newtonian branch. Here we found that there is a crossover from disentangled to entangled solutions. It happens, however, at much higher concentrations than for the zero-viscosity. Furthermore, it is possible to measure the normal-stress differences and carrying out birefringence measurements to investigate changes in polymer orientation at these high shear rates. Abstract II Finally, within a few micrometers gap width, we determined the average rheological quantities of a biological cells. While cell-to-cell variation is typically very large, the average cell properties within the monolayer can be determined with much higher precision. This method enables quantification of the impact of biochemical treatment on the rheological properties of the cells and may be used as a diagnostic tool to identify variations in the rheological cell behaviour due to diseases. Depending on the chosen gap width, the cells may be compressed or elongated. Finally, the impact of pre-stress on the shear properties of the cells can be studied. While the dynamic moduli strongly increase with compression, the power-law exponent that describes the frequency-dependence of the moduli increases with the gap width. Zusammenfassung III Zusammenfassung In der vorliegenden Dissertation wird ein kommerzielles Rotationsrheometer modifiziert, um rheologische Untersuchungen bei Spaltweiten von wenigen Mikrometern zu ermöglichen. Messungen bei geringen Spaltweiten bieten eine Reihe an Vorteilen. So kann der Anwendungsbereich der Rheometer kann deutlich erweitert werden: Es ermöglicht beispielsweise die Messung niedriger Viskositäten, eine deutliche Erweiterung des Messbereichs von Viskosität und Normalspannungsdifferenzen hin zu hohen Scherraten, die Durchführung von rheologischen Messungen an geringen Probenmengen und direkt an biologischen Zellen. Zwar erlaubt die Platte- Platte-Konfiguration ein einfaches Einstellen der Spaltweite, jedoch wird die minimale Spaltweite in kommerziellen Rotationsrheometern durch diverse Unzulänglichkeiten wie Unebenheiten der Platten und Plattenneigung beschränkt. Ebenso verursacht die Quetschströmung der Luft eine fehlerhafte Festlegung des Nullpunkts. Um die Ungenauigkeiten bei der Einstellung des Messspalts zu minimieren, wurde eine Vorgehensweise einwickelt, die es erlaubt die Platten mit einer Ungenauigkeit von ±0,7 µm auszurichten. Dies entspricht einer Verbesserung der Präzision um einen Faktor 30 und mehr. Bei Oszillationsversuchen kann die Präzision bis auf etwa ±0,2 µm verbessert werden. Gegenüber anderen Dünschichtrheometern hat der vorgestellte Aufbau den Vorteil, nicht nur oszillatorisch sondern auch unidirektionale Scherversuche bei niedrigen Spaltbreiten zu ermöglichen und zugleich die komplette Vielseitigkeit kommerzieller Rheometer zu nutzen. Im ersten Teil der Arbeit wird die Modifikation der Platte-Platte-Konfiguration kommerzieller Rotationsrheometer vorgestellt. Es wird gezeigt, wie die Platten sukzessive senkrecht zur Drehachse ausgerichtet werden, um einen Genauigkeit in der Spaltweite von etwa 1 µm zu erreichen. Dabei wird die Spaltweite unabhängig vom der Bestimmung mit dem Rheometer bestimmt. Dadurch werden Messungen bei Spaltweiten deutlich unterhalb des Absolutfehlers kommerzieller Rotationsrheometer ermöglicht. Letzterer liegt typischerweise im Bereich zwischen 25 µm und 75 µm. So kann z.B. bei Spaltweiten von 20 µm der Messbereich für Scherraten von typischerweise 103 s-1 auf 105 s-1 erweitert werden. Es ermöglicht darüber hinaus die Messung niedriger Viskositäten wie die von Wasser oder von Lösungsmitteln. Als Anwendungsbeispiel werden im zweiten Teil der Arbeit Polymerlösungen bei Scherraten von 105 s-1 mit der modifizierten Platte-Platte-Konfiguration untersucht. Diese hohen Scherraten ermöglichen es, den zweiten newtonschen Bereich der Lösungen zu studieren. Es wird gezeigt, dass es im zweiten newtonschen Bereich einen Übergang von unverschlauften zu verschlauften Lösungen gibt. Der Übergang findet allerdings erst bei wesentlich höheren Konzentrationen statt. Darüber Zusammenfassung IV hinaus ist es mit der Modifikation möglich, auch Normalspannungsdifferenzen und optische Eigenschaften bei den hohen Scherraten zu untersuchen. Im dritten Teil der Arbeit werden mit dem modifizierten Rheometer die rheologischen Eigenschaften biologischer Zellen bei einer Spaltweite von einigen Mikrometern quantifiziert. Während die Variation zwischen den Zellen typischerweise sehr hoch ist, ermöglichen die Untersuchungen an Zellmonolagen die Bestimmung der durchschnittlich Zelleigenschaften mit hoher Genauigkeit. Dadurch können z.B. biochemische Einflüsse auf die Zellen nachgewiesen und quantifiziert werden. Dies lässt den Einsatz der Methode als Diagnosewerkzeug zu. Mit dem modifizierten Rheometer können die Zellen zudem komprimiert oder auseinandergezogen werden und so
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