Elastic flow instabilities of non-Newtonian fluids in shear flows

The present work deals with the investigation of flow phenomena of non-Newtonian, viscoelastic fluids and particularly with the emergence of elastic flow instabilities and their underlying mechanism. It is mainly focused on the experimental and theoretical investigation of the viscoelastic Taylor-Couette flow for which previous approaches exhibited some unanswered questions and apparent discrepancies to theoretical predictions. The present study is based on experiments in different Taylor-Couette cells using various polymer solutions with diverse rheological properties. Here, the various cells differ from each other just by their characteristic curvature. The objective is the experimental verification of the universal criterion which predicts the onset of elastic instability of a viscoelastic flow with curved streamlines. Based on a customized, constitutive model which is able to describe the explicit shear rheology of the used polymer solutions, linear stability analysis is performed with aiming at an interpretation of the experimental findings. A supplementary study is presented which addresses the possible degradation of the polymer solutions during the Taylor-Couette measurements. Its results are essential to provide a consistent interpretation of the Taylor-Couette measurements.Die vorliegende Arbeit befasst sich mit verschiedenen Strömungsphänomenen nicht-Newtonscher, viskoelastischer Flüssigkeiten, insbesondere mit elastischen Strömungsinstabilitäten und deren zugrundeliegendem Mechanismus. Der Fokus liegt hierbei auf der experimentellen wie theoretischen Untersuchung des viskoelastischen Taylor-Couette-Flusses, dessen bisherige Behandlung offene Fragen und Unstimmigkeiten gegenüber theoretischen Vorhersagen aufweist. Grundlage unserer Studie bilden Experimente mit mehreren Polymerflüssigkeiten unterschiedlicher rheologischer Eigenschaften in verschiedenen Taylor-Couette-Geometrien, die sich in ihrem Krümmungsparameter unterscheiden. Zielsetzung ist insbesondere die experimentelle Verifizierung eines universellen Kriteriums, das die lineare elastische Instabilität einer viskoelastischen Strömung mit gekrümmten Stromlinien vorhersagt. Basierend auf einem angepassten, konstitutiven Flüssigkeitsmodell, welches die Scherrheologie der verwendeten Flüssigkeiten zu erfassen vermag, wird mit Hilfe einer linearen Stabilitätsanalyse versucht, die experimentellen Ergebnisse nachzuvollziehen. Ergänzend dazu werden die Ergebnisse einer Studie vorgestellt, die die Degradation der Polymerflüssigkeiten im Verlauf der Taylor-Couette Messungen zum Gegenstand hat. Erst diese lassen eine stimmige Interpretation der vorherigen Resultate zu

Elasto-inertial turbulence

Turbulence is ubiquitous in nature yet even for the case of ordinary Newtonian fluids like water our understanding of this phenomenon is limited. Many liquids of practical importance however are more complicated (e.g. blood, polymer melts or paints), they exhibit elastic as well as viscous characteristics and the relation between stress and strain is nonlinear. We here demonstrate for a model system of such complex fluids that at high shear rates turbulence is not simply modified as previously believed but it is suppressed and replaced by a new type of disordered motion, elasto-inertial turbulence (EIT). EIT is found to occur at much lower Reynolds numbers than Newtonian turbulence and the dynamical properties differ significantly. In particular the drag is strongly reduced and the observed friction scaling resolves a longstanding puzzle in non-Newtonian fluid mechanics regarding the nature of the so-called maximum drag reduction asymptote. Theoretical considerations imply that EIT will arise in complex fluids if the extensional viscosity is sufficiently large

Serpentine channels: micro -- rheometers for fluid relaxation times

We propose a novel device capable of measuring the relaxation time of viscoelastic fluids as small as 1\,ms. In contrast to most rheometers, which by their very nature are concerned with producing viscometric or nearly-viscometric flows, here we make use of an elastic instability which occurs in the flow of viscoelastic fluids with curved streamlines. To calibrate the rheometer we combine simple scaling arguments with relaxation times obtained from first normal-stress difference data measured in a classical shear rheometer. As an additional check we also compare these relaxation times to those obtained from Zimm theory and good agreement is observed. Once calibrated, we show how the serpentine rheometer can be used to access smaller polymer concentrations and lower solvent viscosities where classical measurements become difficult or impossible to use due to inertial and/or resolution limitations. In the absence of calibration the serpentine channel can still be a very useful comparative or index device.Comment: accepted for for publication in Lab on a chi

Enhanced radiation sensitivity and radiation recall dermatitis (RRD) after hypericin therapy – case report and review of literature

BACKGROUND: Modern radiotherapy (RT) reduces the side effects at organ at risk. However, skin toxicity is still a major problem in many entities, especially head and neck cancer. Some substances like chemotherapy provide a risk of increased side effects or can induce a "recall phenomenon" imitating acute RT-reactions months after RT. Moreover, some phototoxic drugs seem to enhance side effects of radiotherapy while others do not. We report a case of "radiation recall dermatitis" (RRD) one year after RT as a result of taking hypericin (St. John's wort). CASE REPORT: A 65 year old man with completely resected squamous cell carcinoma of the epiglottis received an adjuvant locoregional RT up to a dose of 64.8 Gy. The patient took hypericin during and months after RT without informing the physician. During radiotherapy the patient developed unusual intensive skin reactions. Five months after RT the skin was completely bland at the first follow up. However, half a year later the patient presented erythema, but only within the area of previously irradiated skin. After local application of a steroid cream the symptoms diminished but returned after the end of steroid therapy. The anamnesis disclosed that the patient took hypericin because of depressive mood. We recommended to discontinue hypericin and the symptoms disappeared afterward. CONCLUSION: Several drugs are able to enhance skin toxicity of RT. Furthermore, the effect of RRD is well known especially for chemotherapy agents such as taxans. However, the underlying mechanisms are not known in detail so far. Moreover, it is unknown whether photosensitising drugs can also be considered to increase radiation sensitivity and whether a recall phenomenon is possible. The first report of a hypericin induced RRD and review of the literature are presented. In clinical practise many interactions between drugs and radiotherapy were not noticed and if registered not published. We recommend to ask especially for complementary or alternative drugs because patients tend to conceal such medication as harmless

Application to light-induced control of open quantum systems

In linear control, balanced truncation is known as a powerful technique to reduce the state-space dimension of a system. Its basic principle is to identify a subspace of jointly easily controllable and observable states and then to restrict the dynamics to this subspace without changing the overall response of the system. This work deals with a first application of balanced truncation to the control of open quantum systems which are modeled by the Liouville-von Neumann equation within the Lindblad formalism. Generalization of the linear theory has been proposed to cope with the bilinear terms arising from the coupling between the control field and the quantum system. As an example we choose the dissipative quantum dynamics of a particle in an asymmetric double well potential driven by an external control field, monitoring population transfer between the potential wells as a control target. The accuracy of dimension reduction is investigated by comparing the populations obtained for the truncated system versus those for the original system. The dimension of the model system can be reduced very efficiently where the degree of reduction depends on temperature and relaxation rate

Nucleosynthesis and the variation of fundamental couplings

We determine the influence of a variation of the fundamental ``constants'' on the predicted helium abundance in Big Bang Nucleosynthesis. The analytic estimate is performed in two parts: the first step determines the dependence of the helium abundance on the nuclear physics parameters, while the second step relates those parameters to the fundamental couplings of particle physics. This procedure can incorporate in a flexible way the time variation of several couplings within a grand unified theory while keeping the nuclear physics computation separate from any model-dependent assumptions.Comment: 8 pages, no figure