IMFUFA seminarer
- Se tidligere afholdte seminarer her
Program for foråret 2010
Seminarerne afholdes (hvor andet ikke er oplyst) hver torsdag i bygn 27.1, lokale I og varer fra kl. 13.00 til kl. 15.00.
Arrangør: Carsten Lunde Petersen
04.02. Bjarne Toft (IMADA, SDU)
Piet Hein og John Nash - beautiful minds - og spillet HEX
Brætspillet HEX har meget simple regler og kan derfor straks spilles af alle. Men det er meget svært at spille godt, og selv om man let kan vise, at der er en vindende strategi for første spiller, så kender man ikke engang et vindende første træk for første spiller (men man har gode gæt).
Det er simpeltheden i spillets regler, kombineret med vanskelighederne ved godt spil, og de mange uløste problemer, som gør HEX matematisk interessant.
HEX blev opdaget af Piet Hein i 1942, og uafhængigt af Piet Hein blev det opdaget igen af John Nash i slutningen af 1940'erne i Princeton.
Foredraget er en bred-billed fortælling om de to "beautiful minds" Piet Hein og John Nash (med mest fokus på Hein), krydret med matematiske resultater og uløste problemer om spillet HEX.
11.02. Niels Larsen (Head of Middle Atmosphere Research Division, DMI)
Polar stratospheric clouds and ozone depletion: Formation of solid particles in the upper troposphere and lower stratosphere.
Polar stratospheric clouds (PSC) play a mantadory role for chemical depletion of stratospheric ozone and the development of the Antarctic ozone hole. On the surfaces of clouds particles, reservoir halogen species are converted through heterogeneous reactions to potentially ozone depleting radicals. At the same time solid type PSC particles are responsible for denitrification of the polar stratosphere which prolongs the ozone depletion. Different types of PSCs excist, consisting both of supercooled liquid and solid phase particles, composed of nitric acid, sulphuric acid, and water, with different properties with respect to ozone depletion.
The talk will give a brief overview of those atmospheric chemical and microphysical processes involved in stratospheric ozone depletion. In particular uncertainties exsist how solid type cloud particles are nucleated from highly supercooled ternary solution particles. Examples of microphysical modelling and simulation of the formation of PSCs will be presented.
Nucleation and freezing of background liquid sulphate aerosols also have implications for the formation of cirrus clouds in the upper troposphere. The talk will briefly touch upon the formation of cirrus clouds in the tropical tropopause layer and the implications for transport of water vapour from the troposphere into the stratosphere where the water vapour acts as a strong greenhouse gas.
18.02. Henrik Schlichtkrull (Inst. f. Mat. Fag, KU)
Fourier analyse på kompakte symmetriske rum.
I den klassiske teori for Fourier rækker udvikles en 2pi-periodisk funktion i Fourierrækken f(θ) = ån cn einθ
Denne teori kan generaliseres, idet man betragter funktioner som er defineret på et homogent rum X=U/K af en kompakt gruppe U. Funktionerne udvikles efter en ortonormal basis som bestemmes ud fra repræsentationerne af U. I det klassiske tilfælde er U cirkelgruppen og K triviel.
Et eksempel som også er velkendt, er tilfældet hvor X er kuglefladen S2, realiseret som det homogene rum SO(3)/SO(2). I dette tilfælde drejer det sig om udvikling i kuglefunktioner (spherical harmonics).
Hvis U er en Lie gruppe er dens repræsentationer parametriseret ved højeste vægte af Cartan og Weyl. Hvis ydermere U/K er et symmetrisk rum fremkommer en meget eksplicit Fourier analyse. Jeg afslutter foredraget med at præsentere et nyt resultat fra denne teori, som Gestur Olafsson og jeg har fundet. Det drejer sig om at karakterisere støtten for en funktion på X, koncentreret nær origo, ved hjælp af dens Fourier koefficienter. I det klassiske tilfælde gælder det altså spørgsmålet:
Hvordan kan man aflæse af Fourier koefficienterne cn, at f har støtte i et interval [-ε,ε]?
25.02. Nicoletta Gnan (DNRF Centre ’Glass and Time’ IMFUFA, NSM.)
What can we learn from off-equilibrium systems' behavior?
Condensed matter is frequently found in out-of-equilibrium states. For example, for systems like supercooled liquids, dense colloids, spin systems, etc., the (off-equilibrium) glass state occurs naturally after cooling or compression from a state of thermal equilibrium. Although off-equilibrium processes have been widely studied, a thermodynamic description as for the non-equilibrium states is still lacking. An effort in this direction is the introduction of the concept of an effective temperature connected to the idea of a heat-flow from the glass to the bath. However experimental and simulation results have shown different and sometimes controversial results in the attempt to connect the temperature measured to the one of an extended off-equilibrium thermodynamic approach showing that a deeper understanding is still needed.
In the seminar I will recall the definition of this temperature using the fluctuation-dissipation relation. Then I will show some results from simulations from different systems and I will discuss whether or not it is always meaningful to refer to this quantity as a temperature with a thermodynamic meaning.
04.03. Louise Dalgaard (NSM, RUC)
How to study pancreatic β-cells? Applications in models of type 2 diabetes.
Diabetes mellitus is a disease of permanently increased blood glucose levels (hyperglycemia). Patients with diabetes have an absolute or relative lack of the hormone insulin, whose main roles are to increase cellular uptake of glucose, decrease glucose production of the liver and decrease release of fatty acids from adipose tissue. Insulin is produced in the pancreas by β-cells residing in small clusters of hormone producing cells: The Islets of Langerhans. Insulin secretion by β-cells is always affected in diabetes mellitus. In type 1 diabetes (juvenile diabetes) β-cells are destroyed in auto-immune attacks, and since the human body cannot differentiate new β-cells a life long supplement of exogenous insulin is necessary. In type 2 diabetes or prediabetic states β-cells suffer from increased blood glucose levels and changes in the blood lipid profile, which leads to a decline in β-cell function and survival that further increases blood glucose levels.
In this seminar, I’ll describe a number of the different methods and models used by islet and β-cell biologists to study β-cell function, survival and proliferation by various stimuli that imitate the state of the β-cell in and during development of type 2 diabetes.
11.03. Dan Charly Christensen (Historie/Kultur og Identitet, RUC)
Hvad karakteriserer en stor videnskabsmand? - Fællestræk mellem Ørsted og Bohr.
Det er kotyme blandt videnskabshistorikere at skelne mellem interne og externe faktorer, når en berømt videnskabsmand (-kvinde) skal karakteriseres. Mens Bohrs opvækst giver ham gode kort på hånden, er Ørsteds og hans søskendes både en bekræftelse og et dementi af tesen om de gode gener. Den externe modstand de mødte rummer klare fællestræk og løsninger. Jeg vil give en række eksempler på deres, men navnlig Ørsteds forcer som universitetslærer og kulturpersonlighed, og - hvis der bliver tid til det - pege på nogle af de kendetegn ved hans videnskabssyn, som strider imod tidens forskningspolitik, f.eks. hans opfattelse af forholdet mellem fysik og matematik og mellem naturvidenskab og teknologi.
18.03. Modeldag (program følger)
Matematiske modeller i vandmiljø
- Ph.d. Karen Timmermann (DMU)
- Seniorforsker Ken Haste Andersen, DTU Aqua
- Lektor Mogens Flint (SDU)
- Andy Visser (DTU Aqua)
25.03. Claudio Maggi (DNRF Centre ’Glass and Time’, IMFUFA, NSM)
Experimental Studies of Supercooled and Glassy Systems.
Several symmetries of the liquid state are lost when a fluid system undergoes the glass transition[1].
Approaching this ultra-viscous regime, via fast cooling or compression, the dynamics of (almost) all liquids slows-down dramatically often in a deeply non-Arrhenius way. Many standard experimental techniques are used to investigate this universal behavior: dielectric spectroscopy, calorimetry, particles or radiation scattering techniques (neutrons, X-rays, visible light), pump-probe spectroscopy (Kerr-effect, thermally stimulated scattering), NMR, etc.
In our group we study the dynamics of these supercooled liquids with rather unconventional Techniques. Our unique experimental tools can measure the mechanical responses of these systems in wide temperature-frequency range[2]. We can measure, for example, the shear-modulus of a liquid that is the resistance that the material opposes when subject to a volume-preserving deformation. Recently we have reported the shear-modulus measured[3, 4] for a number of liquids close to the glass transition in a dynamic range spanning from 1 mHz to 10 kHz. The access to this quantity allow us to relate the dramatic slow-down of the relaxation of these systems to their elastic properties.
An additional typical feature of these liquids in the ultraviscous phase is that their molecules move in a strongly non-homogeneous way. Recent works have focused on how to estimate a dynamic correlation length from the temperature evolution of the dynamics i.e.: in an indirect way[5]. We combine meachanical and dielectric measuerements to understand the probe-dependence of the dynamic correlation length. We are curretly developing a diffraction-limited dynamic light scattering experiment that is capable to measure the dynamic heterogeneity directly in colloidal glassy systems.
Moreover, when these liquids are supercooled in the ultra-viscous phase, they easily fall out of the equilibrium. When such a system is a little cooled or densified the time required to re-establish the equilibrium grows so much that we can appreciate the aging of he relaxation processes. In the nonequilibrium regime the fluctuation-dissipation theorem (FDT), a fundamental result of equilibrium many-body theory, ceases to be valid. A generalization of the FDT has been proposed for glassy systems in general (i.e. disordered spin-glasses, molecular glasses and colloidal glasses). Despite the strong effort put in testing the general form of the theorem the experiments performed so far have given conflicting results. We combine the measurements of the spontaneous correlation function of the rotational dynamics obtained via dynamic light scattering[6], and measurements of the birefringence[7] response to study the generalized theorem finding evidence of its reakdown[8].
[1] Jeppe C. Dyre, Rev. Mod. Phys. 78, 953 (2006).
[2] T. Christensen and N. B. Olsen, Rev. Sci. Instrum. 66, 5019 (1995).
[3] C. Maggi, B. Jakobsen et al., J. Phys. Chem. B (in press), Supercooled Liquid Dynamics Studied via Shear-Mechanical Spectroscopy. arXiv:0806.1400v2 [cond-mat.soft]
[4] B. Jakobsen, C. Maggi, et al., J. Chem. Phys. 129, 184502 (2008), Investigation of the shear-mechanical and dielectric relaxation processes in two mono-alcohols close to the glass transition. arXiv:0805.4519v2 [cond-mat.soft]
01.04 Skærtorsdag
Intet seminar
08.04. Robert Krarup Feidenhans'l (Niels Bohr Instituttet, KU)
Molecular Movies - a new possibility with time resolved x-ray scattering?
X-ray based science is currently undergoing a dramatic evolution. Powerful X-ray laser sources are being developed at the major accelerator laboratories around the world promising extremely high x-ray photon fluxes with a time structure less than 100 femto seconds. This will enable the recording of what is dubbed 'Molecular Movies'. In April 2009 the first x-ray laser was turned on, the SLAC in Stanford outperforming its original specifications on day one. This opened the door to a new world of x-ray science.
X-ray diffraction has for many decades been a widespread used technique for the determination of static atomic and molecular structures. Atoms move typically on timescales of about 100 femto second. Hence, the recording of time resolved movies requires a time resolution of at least the same order of magnitude. In the talk I will discuss the development of the determination of static structures and introduce the pump probe technique. The talk will include a review of current femtosecond x-rays sources and examples from the dynamic of solid thin films and from reactions in liquids will be given. The talk will end by an outlook of the European X-free electron laser currently being built in
15.04. Henrik Kragh Sørensen (Institut for Videnskabsstudier, Aarhus Universitet)
Den Romantiske Matematiks Ironi: Helte, Ligninger og Begrebs-Centreret Matematik
I begyndelsen af 1800-tallet gennemgik matematikken en række vigtige erkendelsesmæssige og institutionelle forandringer. I dette foredrag ønsker jeg at illustrere og kontekstualisere nogle af disse forandringer ved at diskutere en række eksempler fra nogle af den romantiske periodes aktive matematikere, herunder NIELS HENRIK ABEL (1802–1829), CARL FRIEDRICH GAUSS (1777–1855) og NIKOLAI IVANOVICH LOBACHEVSKY (1792–1856).
Mange af de meste berømte produktive matematikere fra begyndelsen af 1800-tallet var prototypiske romantiske helte—miskendte genier, som døde unge efter at have lidt i den materielle verden for at studere de evige matematiske ideer. Men den romantiske indflydelse på tidens matematik går videre end det rent biografiske. Især inden for den tyske romantik blev matematikken indlejret i en kulturel kontekst, som gør det muligt for os at diskutere perspektiver på litterær romantisk ironi fra et matematikbaseret synspunkt.
I det 19. århundredes første del udviklede matematikken sig væk fra eksplicitte formelmanipulationer og til i stigende grad at omhandle begreber. Som en del af denne udvikling begyndte matematikere at stille fundamentalt nye typer spørgsmål, som førte til nye typer svar. Således gik matematikere fra at eftersøge eksplicitte formler som resultater til at spørge hvorvidt sådanne formler overhovedet var mulige. På samme tid ledte andre opdagelser (heriblandt ikke-euklidisk geometri) til, at matematikere distancerede deres undersøgelser fra udforskningen af naturen. Dermed begyndte matematikken at blive opfattet som en autonom disciplin, som beskæftigede sig med en immateriel matematisk verden.
Siden det 15. århundrede havde matematikere ledt efter generelle formler, som tillod den algebraiske løsning af ligninger af alle grader. Men omkring 1830—og sammenfaldende med den sen-romantiske periode—ledte den nye begrebs-centrerede tilgang nogle unge matematikere som ABEL og EVARISTE GALOIS (1811–1832) til at reformulere spørgsmålet i termer af “løsbarhed” nærmere end “løsning”. Derved flyttede de fokus til at undersøge repræsentérbarhed inden for visse begrænsede formelle systemer, og dette førte til uforudsete resultater.
22.04. Program følger
29.04. Lærke Bang Jacobsen (IMFUFA, NSM)
Hvorfor skal man lave fysikeksperimenter i gymnasiet?
Laboratoriearbejde i fysikundervisningen i gymnasiet fylder meget, men de læringsmæssige argumenter er ikke altid tydelige hverken ved læsning af læreplaner, ved samtaler med lærere og elever eller gennem forskningsarbejder fra de seneste årtier. I dette seminar vil jeg klarlægge hvilke argumenter, der findes i litteraturen for laboratoriearbejde, samt give en fortolkning af disse i forhold til de specifikke eksperimenter, der typisk benyttes i gymnasiets fysikundervisning. Min tese er, at ved at læreren tydeligt afklarer det specifikke argument for hver laboratoriearbejde, vil det eksperimentelle arbejde dels give bedre mening for eleverne, og ikke mindst i højere grad give anledning til at nå de læringsmæssige intentioner. Jeg vil fremlægge resultaterne af mine case-studier, der netop afklarer betydningen af en artikulering af de læringsmæssige intentioner i relation
til elevernes diskussioner under laboratoriearbejdet samt i deres rapportering.
Kort sagt, så ønsker jeg her at fremlægge et værktøj for lærere til at opfinde eller genopfinde argumenterne for specifikke laboratoriearbejder, samt retfærdiggøre at en artikulering gør en positiv forskel.
06.05. Poul Norby (Risø DTU)
In situ diffraction studies of materials for energy technology
Obtaining detailed information about materials under real operating conditions is a challenge, but very important in order to understand relations between structure and properties. Likewise, studies of materials during synthesis, reaction or transformation may give information about nucleation and growth mechanisms. Using high intensity synchrotron X-ray radiation it is possible to follow even fast reactions and study materials in their working environment, e.g. inside a fuel cell, battery or catalytic reactor. Examples of in situ studies of e.g. catalysts and lithium batteries will be given.