Condensed Matter Physics Seminar
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2012-06-01 (Friday)
Sali Duża Teoretyczna (229), Hoża 69 at 12:15 
Anna Myłyk (IPPT PAN)Dynamika układów wielocząstkowych opadających grawitacyjnie w lepkim płynie
Na seminarium przedstawie wyniki mojej pracy doktorskiej,gdzie został przebadany wpływ płaskiej pionowej ściany naewolucję kropli zawiesiny opadającej pod wpływem siłygrawitacyjnej w lepkim płynie dla liczb Reynoldsa znaczniemniejszych od jedności. Płyn wewnątrz i na zewnątrzopadającej kropli był identyczny, a cząstki początkowobyły przypadkowo rozmieszczone wewnątrz sferycznejobjętości. Wykonano eksperymenty i symulacje numeryczne.W doświadczeniach badana była ewolucja kropel złożonych zcząstek szklanych w glicerynie. Do obliczeń numerycznychzastosowano model cząstek punktowych. Najważniejszymwynikiem jest stwierdzenie, że w obecności pionowej ścianyczas i długość destabilizacji kropli ulegają skróceniu.
I will present results from my Ph.D. thesis. In this work theimpact of a flat vertical wall on the evolution of a suspensiondrop has been investigated. The drop settles under theinfluence of the gravitational force in a viscous fluid at theReynolds number much smaller than unity. The fluid insideand outside the falling drop is identical, and particles areinitially randomly distributed inside a spherical volume.Experiments and numerical simulations have been performed.In the experiment, the evolution of droplets composed of glassparticles in glycerin has been studied. In the numericalcalculations the point-particle model has been used. Themain conclusion is that in the presence of a vertical wall,the drop destabilization time and length are shorter thanin an unbounded fluid.
2012-05-18 (Friday)
Sali Duża Teoretyczna (229), Hoża 69 at 12:15
Paweł Potasz (Politechnika Wrocławska i NRC Canada)Electronic properties of gated triangular graphene quantum dots: Magnetism, correlations, and geometrical effects
2012-04-20 (Friday)
Sali Duża Teoretyczna (229), Hoża 69 at 12:15
dr Jan Guzowski (IChF PAN)Effective interactions between colloidal particles trapped at the surface of liquid droplets
If colloidal particles get trapped at a fluid-fluid interface they interacteffectively via deformations of the interface. These so-called capillary in-teractions can easily be tuned by changing external fields and they dependsensitively on the shape of the particles. This makes them good candidatesfor designing self-assembling systems and provides a convenient experimental playground for studying basic issues of statistical mechanics in two dimensions in the presence of long-ranged interactions [1]. On the other hand, recent experiments [2] show that capillary forces can have important consequences for stabilizing so-called Pickering emulsions, which are formed by particle-covered droplets (e.g., oil) in a solvent (e.g., water). Whereas considerable theoretical progress has been made in understanding capillary interactions at flat interfaces, basic issues such as the balance of forces acting on the interface and the influence of the incompressibility of the liquid enclosed by spherical interfaces have not yet been fully resolved. In the seminar I will present analytical and numerical results of the detailed calculations of the effective interaction potentials between particles trapped at the surface of free [3] or sessile [4-5] liquid droplets and discuss their consequences for formation of interfacial colloidal structures.[1] A. Dominguez, M. Oettel and S. Dietrich, Phys. Rev. E 82, 011402 (2010)[2] B. Madivala, J. Fransaer and J. Vermant, Langmuir 25, 2718 (2009)[3] J. Guzowski, M. Tasinkevych, S. Dietrich, Phys. Rev. E 84, 031401 (2011)[4] J. Guzowski, M. Tasinkevych, S. Dietrich, Eur. Phys. J. E 33, 219 (2010)[5] J. Guzowski, M. Tasinkevych, S. Dietrich, Soft Matter 7, 4189 (2011)
2012-04-13 (Friday)
Sali Duża Teoretyczna (229), Hoża 69 at 12:15
Dr Filip Dutka (IChF PAN)Mesoscopic analysis of Gibbs' criterion for sessile nanodroplets on trapezoidal substrates
By taking into account precursor films accompanying nanodroplets ontrapezoidal substrates we show that on a mesoscopic level ofdescription one does not observe the phenomenon ofliquid-gas-substrate contact line pinning at substrate edges. Thisphenomenon is present in a macroscopic description and leads tonon-unique contact angles which can take values within a rangedetermined by the so-called Gibbs' criterion. Upon increasing thevolume of the nanodroplet the apparent contact angle evaluated withinthe mesoscopic approach changes continuously between two limitingvalues fulfilling Gibbs' criterion while the contact line movessmoothly across the edge of the trapezoidal substrate. The spatialextent of the range of positions of the contact line, corresponding tothe variations of the contact angle between the values given by Gibbs'criterion, is of the order of ten fluid particle diameters.
2012-03-30 (Friday)
Sali Duża Teoretyczna (229), Hoża 69 at 12:15
Prof. Marek Napiórkowski (IFT UW)Siły Casimira w obszarze kondensacji Bosego-Einsteina
2012-03-23 (Friday)
Sali Duża Teoretyczna (229), Hoża 69 at 12:15
Dr Rafał Kotyński (Zakład Optyki Informacyjnej WF, UW)Obrazowanie nadrozdzielcze przy użyciu struktur plazmonicznych
Tematem seminarium będzie obrazowanie w widzialnym zakresie widmowym z rozdzielczością lepszą niż wynika to z kryterium dyfrakcyjnego Rayleigha.Taką możliwość daje plazmonika, czyli optyka struktur subfalowychzłożonych z dielektryków oraz metali i wykorzystująca plazmonypowierzchniowe polarytony do przenoszenia sygnału optycznego.Rozdzielczość rzędu jednej dziesiątej długości fali można uzyskać w warstwowych periodycznych strukturach metaliczno-dielektrycznych, czyli tzw. soczewkach płaskich. Bezdyfrakcyjna transmisja światła przez strukturę przypomina zjawisko superkolimacji uzyskiwanej w kryształach fotonicznych, ale ma ponadto charakter nadrozdzielczy i ograniczona jest do polaryzacji TM pozwalającej na powstawanie sprzężonych modów plazmonowych na granicach warstw. Na granicach tych obserwuje się ponadtotzw. ujemne załamanie światła, co pozwala określić taki materiał warstwowy mianem metamateriału optycznego. Efektywna głębokość penetracjiw głąb struktury jest o ok. 2 rzędy wielkości wyższa niż głębokośćnaskórkowa metalu (srebra) przy zawartości metalu w strukturze dochodzącejdo 50%, co pozwala określać strukturę mianem przezroczystego metalu.
2012-03-09 (Friday)
Sali Duża Teoretyczna (229), Hoża 69 at 12:15
dr Jan Chwedeńczuk (IFT UW)Jak, znając gęstość, zostać beneficjentem korelacji
Granica szumu śrutowego, czyli skalowanie niepewności estymacji parametrujak 1/\sqrt{N}, gdzie N jest liczbą cząstek w układzie, jestcharakterystyczna dla układów nieskorelowanych. Celem seminarium jestzaprezentowanie, jak znając jednociałową gęstość układu i nie posiadającżadnej wiedzy o korelacjach wielociałowych, dokonać estymacji parametru zprecyzją poniżej granicy szumu śrutowego. Zostanie podany przykład, gdyparametrem tym jest względna faza pomiędzy dwoma interferującymikondensatami Bosego-Einsteina, a precyzja estymacji skaluje się jakN^{-2/3}.
2012-03-02 (Friday)
Sali Duża Teoretyczna (229), Hoża 69 at 12:15
Prof. Mai Suan Li (IF PAN)Fibril Formation of Amyloid Peptides: from Lattice to All-Atom Simulations
Alzheimer's, Parkinson's, Huntington's, type II diabetes, Mad Cow diseaseetc are associated with protein misfolding. In the case of Alzheimer'sdisease, common toxic species leading to this pathology are the extensivedeposits of fibrillar aggregation of amyloid peptides. Understanding thenature and mechanism of formation of amyloid fibrils plays an importantrole in finding proper treatments. Our study reveals that the fibrilgrowth is governed by the two-stage dock-lock mechanism. In the initialstage a monomer docks onto the underlying fibril, and the locking of amonomer requires a much longer time scale. After the lock stage themonomer adopts the conformation that is commensurate with the underlying lattice. Using lattice and all-atom models, we have obtained the main factors that control the fibril formation process. Namely, in agreementwith experiments, we have shown that fibril formation times of polypeptidechains are strongly correlated with hydrophobicity, Coulomb interactionand the population of the fibril-prone conformation in the monomer state.The higher this population the faster is the fibril growth process andthis dependence may be described by a single exponential function. Ourresult opens a new way to understand the fibrillogenesis of bio moleculesat the monomer level. The role of peptide inhibitors of amyloidaggregation will be briefly discussed.1. P.H. Nguyen, Mai Suan Li, G. Stock, J. E. Straub, and D. Thirumalai, Proc. Natl. Acad. Sci. USA 104, 111-116 (2007)2. Mai Suan Li, D.K. Klimov, J. E. Straub, and D. Thirumalai, J. Chem. Phys. 129, 175101 (2008).3. H. B. Nam, M. Kouza, H. Zung, and Mai Suan Li, J. Chem. Phys. 132, 165104 (2010)4. Mai Suan Li, N.T. Co, G. Reddy, C-K. Hu, and D. Thirumalai, Phys. Rev. Lett. 105, 218101 (2010).5. M.H. Viet, S.T. Ngo, N.S. Lam , and Mai Suan Li, J. Phys. Chem. B 115, 7433 (2011)
2012-01-20 (Friday)
Sali Duża Teoretyczna (229), Hoża 69 at 12:15
Dr Mykola Tasinkevych (MPI for Intelligent Systems, Stuttgart)Catalytic Janus Micromotors
Synthetic micro-motors which can move cargo in a well controlled waythrough a liquid environment are of signi cant interest for applicationssuch as targeted drug delivery, biosensing, or shuttle-transport of livingcells. One promising approach is to use catalytically active Januscolloidal particles as model micro-motors. Due to an asymmetric decorationof their surface with a catalyst, which promotes a speci c chemicalreaction in the surrounding liquid, concentration gradients of thereaction products develop along the surface of the particle. Depending onthe systems, various self-propulsion mechanisms emerge, such as bubblepropulsion, self-electrophoresis, or self-di usiophoresis. In this talk weconsider only the last mechanism, i.e. self-propulsion due toself-generated electrically neutral solute gradients. First, we discussthe self-di usiophoretic motion of a spheroidal particle, which is coveredby a catalyst over a cap-like region centered at one of the poles of theparticle. We describe how theself-phoretic velocity depends on the aspect ratio of the polar and theequatorial diameters of the particle and on the fraction of the particlesurface contributing to the chemical reaction. Next we show that suchparticles can be used as micro-carriers. As a simple model for acarrier-cargo system we consider a catalytically active particle connectedby a thin rigid rod to a catalytically inert cargo particle. We show thatthe velocity of the composite strongly depends on the relative orientationof the carrier-cargo link. Accordingly, there is an optimal con gurationfor the linkage. The subtlety of such carriers is underscored by theobservation that a spherical particle completely covered by catalyst,which is motionless when isolated, acts as a carrier once attached to acargo.
2012-01-13 (Friday)
Sali Duża Teoretyczna (229), Hoża 69 at 12:15
Dr Tiziana Di Matteo (King's College London, UK)Econophysics: from markets to networks
In this talk I will briefly give a broad overview of the state of the art in Econophysics: a discipline that has already a rich history and even controversial trends [1]. In particular, I will show results concerning the characterization and visualization of correlations in financial systems by means of network theory and I will introduce a new tool to filter relevant information in these systems and to extract the hierarchical structure of the market [2]. I will discuss the economic meaning of the financial market hierarchical structure and its dynamical evolution [3,4]. I will investigate the statistical signatures of the ‘credit crunch’ financial crisis that unfolded between 2008 and 2009 discussing results that show an intriguing trend that highlights a consistently decreasing centrality of the financial sector over the last 10 years [5]. I will also introduce a graph-theoretic approach to extract clusters and hierarchies in complex data- sets in an unsupervised and deterministic manner, without the use of any prior information. Applications to financial data-sets show that industrial sectors and specific activities can be extracted and meaningfully identified from the analysis of the collective fluctuations of prices in an equity market [6]. [1] "Topical Issue: Trends in Econophysics" in EPJB, Vol. 55, No. 2 (2007). [2] M. Tumminello, T. Aste, T. Di Matteo, R. N. Mantegna, PNAS 102, n. 30 (2005) 1 10421. [3] F. Pozzi, T. Di Matteo and T. Aste, Advances in Complex Systems 11 (2008) 927. [4] T. Di Matteo, F. Pozzi, T. Aste, The European Physical Journal B 73 (2010) 3-11. [5] T. Aste, W. Shaw, T. Di Matteo, New J. Phys. 12 (2010) 085009. [6] Won-Min Song, T. Di Matteo and Tomaso Aste, submitted (2011).