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Seminare - Winter Semester 2017/2018

Seminar über "spezielle Probleme in der Theorie der Kolloide"

Ort: HHU Düsseldorf, Seminarraum 25.32.O2.51
Programm:
DatumSprecher/inThemaZeit
Fr 27.10.2017   S. Goh  Interfacial Shocks in the Dynamics of 1D Ferrogels  14:30 s.t.
Do 16.11.2017   C. Hoell  Dynamical density functional theory for microswimmers  14:30 s.t.
Mo 20.11.2017   B. Liebchen  Synchronization in Chiral Active Matter  14:30 s.t.
Fr 01.12.2017   A. Ider  Dynamics of microswimmers near complex interfaces  14:30 s.t.
Fr 08.12.2017   C. Scholz  Individuals and Chains of self-propelled robots  14:30 s.t.
Di 19.12.2017   S. Jahanshahi  From circle swimmers to circle flyers  14:30 s.t.
Fr 19.01.2018   S. Babel  Dynamical density functional theory for a nano-scale capacitor  14:30 s.t.
Fr 26.01.2018   N. Siboni  Diffusion and glassy dynamics in highly polydisperse systems  14:30 s.t.
Mi 31.01.2018   M. Eshraghi  Pre-melting of Hard-core Yukawa BCC crystal in vicinity of Charged-Hard walls  14:30 s.t.
gez.: Prof. Dr. H. Löwen

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Seminar über "Spezielle Probleme der Computersimulation weicher Materie"

Ort: HHU Düsseldorf, Seminarraum 25.32.O2.51
Programm:
DatumSprecher/inThemaZeit
Do 12.10.2017   Z. Guo  The stability of nanobubbles in the bulk  14:30 s.t.
Mo 20.11.2017   B. Liebchen  Synchronization in Chiral Active Matter  14:30 s.t.
Di 19.12.2017   S. Jahanshahi  From circle swimmers to circle flyers  14:30 s.t.
Do 25.01.2018   M. Golkia  Elastic properties of glasses  14:30 s.t.
Fr 26.01.2018   N. Siboni  Diffusion and glassy dynamics in highly polydisperse systems  14:30 s.t.
Mi 31.01.2018   M. Eshraghi  Pre-melting of Hard-core Yukawa BCC crystal in vicinity of Charged-Hard walls  14:30 s.t.
Do 01.02.2018   S. Ganguly  Free energy of grain boundaries in 2D and 3D  14:30 s.t.
gez.: Prof. Dr. J. Horbach

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Seminar “Soft Matter”

Ort: HHU Düsseldorf, Seminarraum 25.32.O2.51
Programm:
DatumSprecher/inInstitutZeit
Mo 29.01.2018   Peter Virnau  Universität Mainz  14:30 s.t.
Peter Virnau: „Critical Behavior of Active Brownian Particles / Knots in Polymer Melts and Chromosomes”
Universität Mainz - Seminar@HHUD: 29.1.2018 14:30 s.t., Seminarroom 25.32 O2.51

In this talk I will report on three very recent (and somewhat distinct) projects. In the first project, we study active Brownian particles as a paradigm for genuine non-equilibrium phase transitions. Access to the critical point in computer simulations is obstructed by the fact that the density is conserved. We propose a modification of sampling finite-size fluctuations and successfully test this method for the 2D Ising model. Using this model allows us to determine accurately the critical point of two dimensional active Brownian particles at Pecr=40, Φcr=0.597. Based on this estimate, we study the corresponding critical exponents β, γ/ν and ν. Our results are incompatible with the 2D-Ising exponents, thus raising the question whether there exists a corresponding non-equilibrium universality class.

In the second project, we investigate the occurrence of knots in polymer melts. In polymer physics it is typically assumed that chains in melts can be described by effective random walks without excluded volume interactions. We show that this paradigm is problematic as the latter severely overrate the occurrence of knots. Interestingly, we find that the structure of a chain in a melt is very similar to the structure of a single chain undergoing a collapse transition at the Theta-point, which in turn are not well-represented by random walks either. Finally, I will report on very recent developments on the structure of DNA in the cell nucleus. A few months ago, three-dimensional structures of individual chromosomes and genomes in nuclei of single haploid mouse embryonic stem (ES) cells have been determined for the first time based on Hi-C chromosome conformation contact data.  In this project, we further analyze these structures and provide first evidence that G1 phase chromosomes are knotted, consistent with the fact that plots of contact probability vs sequence separation show a power law dependence that is intermediate between that of a fractal globule and an equilibrium structure.

Critical Behavior of Active Brownian Particles / Knots in Polymer Melts and Chromosomes
Mi 10.01.2018   Vladimir Khodygo  Aberystywth University, United Kingdom  14:30 s.t.
Vladimir Khodygo: „Heterogeneous populations of active rods confined within two dimensional channels”
Aberystywth University, United Kingdom - Seminar@HHUD: 10.1.18 14:30 s.t., Seminarroom 25.32 O2.51

Active swarms, consisting of of individual agents which consume energy to move, are known to produce a diverse range of collective behavior. Many examples of active swarms are biological in nature (e.g. fish shoals and bird flocks) and have been modeled extensively by numerical simulations. Numerical simulations of swarms assume that the swarm is homogeneous: that is that every agent has exactly the same dynamical properties. However, real biological swarms are often heterogeneous, such as in the case of a microbial population in the gut or in the soil (where many different species of bacteria may be present simultaneously). Here we conduct molecular dynamics simulations of active rods confined within a two-dimensional rectangular channel. Such simulations serve as a simple model for the dynamics of a swarm of motile rod-like bacteria in the absence of hydrodynamic effects. We explore the case of mixed swarms in which self-propelled rods have a range of motilities and steric interactions. Our results show that the confining boundaries play a strong role in driving the segregation of such mixed populations.

Heterogeneous populations of active rods confined within two dimensional channels
Mo 18.12.2017   Nick Koumakis  School of Physics and Astronomy, University of Edinburgh, United Kingdom  14:30 s.t.
Nick Koumakis: „Studies of model bacteria as "living active colloids"”
School of Physics and Astronomy, University of Edinburgh, United Kingdom - Seminar@HHUD: 18.12.2017 14:30 s.t., Seminarroom 25.32 O2.51

Active matter, containing self-propelling constituents, has recently sparked the interest of the soft matter scientific community. Active components may be biological or even chemically driven particles. The ability of active matter to provide energy through motion at a microscopic level, produces interesting phenomena, such as effective interactions, swarming, internally generated flows and activity induced phase transitions. Here, we use Escherichia coli bacteria as "living active colloids" to touch on a few exciting examples of active matter systems. We will initially discuss the interactions between swimmers and passive particles in an application that uses activity to transport particles to target locations. We will then move on to an example of self-assembly of light-driven bacteria which become active only when illuminated. Finally we will discuss how swimming bacteria produce of trails in a complex fluid near criticality.

Studies of model bacteria as "living active colloids"
Fr 15.12.2017   Marco Heinen  Universidad de Guanajuato, León, Mexico  14:30 s.t.
Marco Heinen: „Structure and Diffusion of a Fractal Hard Sphere Fluid”
Universidad de Guanajuato, León, Mexico - Seminar@HHUD: 15.12.17 14:30 s.t., Seminarroom 25.32 O2.51

Porous media can host complex fluids in a non-integer (fractal) dimensional configuration space. An example of high application relevance is reservoir rock containing oil, water, natural gas, or multiphase mixtures thereof. The fractal structure of porous media, and the diffusion of single particles through the pores of such media are well-studied problems. However, dense fluid phases of strongly interacting particles in fractal confinement have not yet received the same attention and remain mostly unexplored until today. In recent work [1] we have introduced statistical mechanical methods to compute the equilibrium correlations among dense, disordered phases of mesoscopic particles in 1:68(2)-dimensional fractal confinement by Monte Carlo simulations. Our simulation results are in good agreement with the predictions of the Percus-Yevick integral equation, analytically continued from integer to non-integer dimension and solved numerically by a spectral method. The anomalous diffusive dynamics of fractal hard spheres in thermodynamic equilibrium is probed by fractal Dynamic Monte Carlo simulations, and reveals a number of peculiarities that are not observed for point particles in fractal dimension, nor for hard spheres in integer dimension: Extended hard spheres in fractal dimension exhibit anomalous diffusion with two different (short-time and long-time) exponents of the mean squared displacement as a function of time, even in case of non-interacting particles that form an ideal gas. For dense fluids of interacting, non-overlapping fractal hard spheres, we observe a superposition of anomalous diffusion, due to the fractal configuration space, and subdiffusion due to the particle interactions at intermediate time scales.

References

  1. M. Heinen, S.K. Schnyder, J.F. Brady & H. Löwen, PRL 115, 052301 (2015)
Structure and Diffusion of a Fractal Hard Sphere Fluid
Mo 11.12.2017   Lorenz Baumgarten  TU Dortmund  14:30 s.t.
Buckling elastischer Kapseln bei thermischen Fluktuationen
Do 23.11.2017   Hendrik Bartsch  Max Planck Institute for Intelligent Systems, Stuttgart  14:30 s.t.
Hendrik Bartsch: „Insights into the diversity of smectic phases in ionic liquid crystals”
Max Planck Institute for Intelligent Systems, Stuttgart - Seminar@HHUD: 23.11.17 14:30 s.t., Seminarroom 25.32 O2.51

Ionic liquid crystals (ILCs) are anisotropic mesogenic molecules which carry charges and therefore combine properties of liquid crystals, e.g., the formation of mesophases, and of ionic liquids, such as low melting temperatures and tiny triple-point pressures. Previous density functional calculations have revealed that the phase behavior of ILCs is strongly affected by their molecular properties, i.e., their aspect ratio, the loci of the charges, and their interaction strengths. Here, we report new findings concerning the phase behavior of ILCs as obtained by density functional theory and Monte Carlo simulations. The most important result is the occurrence of a novel, wide smectic-A phase, at low temperature, the layer spacing of which is larger than that of the ordinary high-temperature smectic-A phase.

Insights into the diversity of smectic phases in ionic liquid crystals
Mo 13.11.2017   Yuanjian Zheng  École polytechnique fédérale de Lausanne EPFL, Schweiz  14:30 s.t.
Yuanjian Zheng: „Statistical Physics of Disordered Systems”
École polytechnique fédérale de Lausanne EPFL, Schweiz - Seminar@HHUD: 13.11.2017 14:30 s.t., Seminarroom 25.32 O2.51

The presence of disorder in physical systems is a ubiquitous phenomenon that in many instances lead to interesting consequences beyond what their ordered counterparts may suggest. However, it is often neglected in favor of simpler models or treatments that are more tractable in nature. In this talk, we briefly examine the role of disorder in a disparate variety of classical and quantum systems and elucidate its non-trivial consequences. In particular, we discuss the role of disorder in work extraction at the quantum level, design and architecture of adaptive immune systems and the mechanical behavior of soft biological tissues.

Statistical Physics of Disordered Systems
Do 14.09.2017   Jaydeb Chakrabarti  S. N. Bose National Centre for Basic Sciences, Kolkata, India  15:30 s.t.
Jaydeb Chakrabarti: „Heterogeneous Dynamics in a Driven System”
S. N. Bose National Centre for Basic Sciences, Kolkata, India - Seminar@HHUD: 14.9.17 15:30 s.t., Seminarroom 25.32 O2.51

We consider a system consisting of a binary mixture of oppositely charged colloidal particles driven by a constant electric field via Brownian Dynamics simulations. We observe crossover in dynamics: from an initial fast relaxation in the homogeneous state to a slowed-down lane state via a pre-lane state with anomalous dynamical responses in terms of a non-Fickian exponential tail in self-van Hove functions and a stretched exponential relaxation in both distinct van Hove functions and self-overlap functions. The anomaly results from distribution of particle diffusion which is also reflected via the broadening in the dynamical susceptibility. In contrast, in the lane state, the dynamical susceptibility show distinct peaks due to enhanced separation of timescales due to distinct responses in the coexisting slow and fast particles. Furthermore, we investigate how the growth of structural heterogeneity in the system induces the dynamic heterogeneity and the associated slowing down in the steady states. In order to probe this, the time evolution of the dynamical and structural quantities is monitored after the system is suddenly exposed to the field. The ‘prelane’ state show signatures of aging while the lane state forms via rapid decrease in diffusion as time progresses.

Heterogeneous Dynamics in a Driven System
Mo 04.09.2017   Simone Dussi  Utrecht University, The Netherlands  14:30 s.t.
Simone Dussi: „When shape is enough”
Utrecht University, The Netherlands - Seminar@HHUD: 4.9.2017 14:30 s.t., Seminarroom 25.32 O2.51

I will give an overview of my PhD, in which I studied entropy-driven phase transitions in colloidal systems [1-7]. By using computer simulations and theory, we have shown that the particle shape is enough for the formation of several thermodynamic phases.

In this talk, I will focus on chiral and biaxial liquid crystal phases. In particular, I will highlight the problem of predicting the macroscopic chiral behaviour of liquid crystals from the microscopic chirality of the particles. By introducing a novel chiral particle model, namely particles with a twisted polyhedral shape, we obtain a stable fully-entropy-driven cholesteric phase by computer simulations [4]. By slightly modifying the triangular base of the particle, we are able to switch from a left-handed prolate to a right-handed oblate cholesteric using the same right-handed twisted particle model. Furthermore, we find qualitative agreement with the theoretical prediction based on a second-virial theory, that we previously applied to hard helices [2,3]. Our results unveil how the competition between particle biaxiality and chirality is directly transmitted at a higher level into the nematic phases.

This will contribute to identify the design rules based on particle shape needed to guide the synthesis and the self-assembly of the future colloidal building blocks.

References

  1. B. de Nijs*, S. Dussi*, F. Smallenburg, J. Meeldijk, D. Groenendijk, L. Filion, A. Imhof, A. van Blaaderen, M. Dijkstra - Nature Materials 14, 56 (2015)
  2. S. Belli, S. Dussi, M. Dijkstra,R. van Roij - Phys. Rev. E 90, 020503(R) (2014)
  3. S. Dussi, S. Belli, R. van Roij, M. Dijkstra - J. Chem. Phys. 142, 074905 (2015)
  4. S. Dussi, M. Dijkstra - Nature Communications 7,11175 (2016)
  5. HE Bakker, S Dussi, et al. - Soft Matter 12, 9238 (2016)
  6. M. Marechal, S. Dussi , M. Dijkstra - J. Chem. Phys. 146, 124905 (2017)
  7. S. Dussi, N. Tasios, M. Dijkstra, "Recipes for biaxial colloidal liquid crystals", in preparation
When shape is enough
Mo 31.07.2017   Pinaki Chaudhuri  The Institute of Mathematical Sciences, Chennai, India  14:30 s.t.
Long-range dynamic correlations during aging of gels
Fr 28.07.2017   Suvendu Mandal  Universität Innsbruck, Austria  14:30 s.t.
Suvendu Mandal: „The dynamics of driven and active Brownian particles”
Universität Innsbruck, Austria - Seminar@HHUD: 28.7.17 14:30 s.t., Seminarroom 25.32 O2.51

While the linear response is well-characterized in terms of the fluctuation-dissipation theorem, few exact results are available for strong driving. Here we study the time-dependent velocity of a colloidal particle immersed in a dilute suspension of hard spheres in response to a step force switched on at time zero using Brownian dynamics simulations. Our main quantity of interest is the time-dependent mobility and its approach to the stationary state mobility. A stationary state solution for the mobility exact in first order of the packing fraction has been established earlier in terms of a power-series expansion with respect to the force on the tracer particle [1]. We extend this result to the case of arbitrarily strong driving including the complete time-dependence of the response upon switching on the force. We show that in the stationary state, our analytic solution recovers the anticipated limit for strong driving [1] and captures the response in first order of the packing fraction for any strength of the force.

We also investigate active colloidal hard-sphere glasses using Brownian dynamics simulations. While the properties of passive colloidal glasses are fairly understood over the years, not much is known for active colloidal glasses. In this talk we discuss the decoupling between active speed and rotational diffusion in colloidal glasses. These findings agree with theoretical predictions obtained by the mode-coupling theory of the glass transition.

References

  1. T. M. Squires and J. F. Brady, Physics of Fluids 17, 0731
The dynamics of driven and active Brownian particles
Mi 26.07.2017   Vladimir S. Filinov  Russian Academy of Sciences, Moscow, Russia  14:30 s.t.
Vladimir S. Filinov: „Peculiarities of momentum distribution functions of strongly correlated charged fermions”
Russian Academy of Sciences, Moscow, Russia - Seminar@HHUD: 26.7.17 14:30 s.t., Seminarroom 25.32 O2.51

The main difficulty for path integral Monte Carlo studies of Fermi systems results from the requirement of antisymmetrization of the density matrix [1] and is known in literature as the ′sign problem′ [2-6] . To overcome this issue the new numerical version of the Wigner approach to quantum mechanics for treatment thermodynamic properties of degenerate systems of fermions has been developed. The new path integral representation of quantum Wigner function in the phase space has been obtained for canonical ensemble. Explicit analytical expression of the Wigner function accounting for Fermi statistical effects by effective pair pseudopotential has been presented. Derived pseudopotential depends on coordinates, momenta and degeneracy parameter of fermions and takes into account coordinate – momentum principle uncertainly.

The new quantum Monte-Carlo method for calculations of average values of arbitrary quantum operators has been proposed.

To test the developed approach calculations of the momentum distribution function of the degenerate ideal system of Fermi particles has been carried out in a good agreement with analytical Fermi distributions.

On other hand the first results on influence of interparticle interaction on momentum distribution functions show appearance of quantum "tails" in the Fermi distributions.

References

  1. Feynman R P and Hibbs A R (1965) Quantum Mechanics and Path Integrals (New York: McGraw-Hill)
  2. Filinov V S (2014) High Temperature, 52, 615
  3. Filinov V (2001) J. Phys. A: Math. Gen. 34, 1665
  4. McMahon J M, Morales M A, Pierleoni C, Ceperley D (2012) Rev. Mod. Phys. 84 1607
  5. Ceperley D (1991) J.Stat. Phys. 63 1237; (1992) Phys. Rev. Let. 69 331
  6. Militzer B and Pollock R (2000) Phys. Rev. E 61, 3470
  7. Galitskii V M and Yakimets V V (1967) Zh. Eksp. Teor. Fiz. 51 957; 1967 Sov. Phys. JETP 24 637
  8. Eletskii A V, Starostin A N and Taran M D (2005) Physics - Uspekhi 48 281
Peculiarities of momentum distribution functions of strongly correlated charged fermions
Di 25.07.2017   Surajit Sengupta  TIFR, Hyderabad, India  15:30 s.t.
Do thermodynamically stable rigid crystals exist?
Mo 10.07.2017   Smarajit Karmakar  TIFR, Hyderabad, India  15:30 s.t.
Growth of Amorphous Order and its role in the Dynamics of Supercooled Liquids
gez.: Prof. Dr. Egelhaaf, Prof. Dr. Horbach, Prof. Dr. Löwen

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poroSys-Vorträge

Datei: poroSys Talkslist

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Kolloquien

Ort: HHU Düsseldorf, Hörsaal 5J (Gebäude 25.31 Ebene 00)
Programm:
DatumSprecher/inInstitutZeit
Mo 29.01.2018   Prof. Nicolas Vogel  Friedrich-Alexander-Universität Erlangen-Nürnberg  17:00 s.t.
Prof. Nicolas Vogel: „Colloid clusters from confined self-assembly”
Friedrich-Alexander-Universität Erlangen-Nürnberg - Seminar@HHUD: 29.1.18 17:00 s.t., Lecture hall 26.11 6A

Colloids assemble into crystals upon increase of volume fraction. Complex structures can be achieved by varying composition, interaction, or shape of the constituent colloidal particles. Another method to modify the crystallization process is to utilize interfacial effects from confinement. Recently it has been shown that entropy favors icosahedral symmetry for colloids assembling in spherical confinement. Here we use droplet-based microfluidics to create homogeneous emulsion droplets as sources for defined spherical confinement. This allows systematic investigation of the assembly behavior of clusters containing between 100 and 10000 near- monodisperse colloidal spheres. We observe a discrete series of magic colloid clusters with icosahedral symmetry, borrowing the concept from atomic magic clusters. A two-step simulation scheme is proposed to reproduce accurately the structures observed in experiments. To understand and explain the formation of the magic colloid clusters, we propose a geometric model of Mackay and anti-Mackay shells and extract extremal principles. Our results underline possibilities to create complex structures from simple building blocks. The colloid clusters may find use as templates, photonic materials and building blocks for hierarchical assemblies.

References

  • De Nijs, Bart, et al. "Entropy-driven formation of large icosahedral colloidal clusters by spherical confinement." Nature materials 14.1 (2015): 56-60.
  • Vogel, Nicolas, et al. "Color from hierarchy: Diverse optical properties of micron-sized spherical colloidal assemblies." Proceedings of the National Academy of Sciences 112.35 (2015): 10845-10850.
  • Damasceno, Pablo F., Michael Engel, and Sharon C. Glotzer. "Predictive self-assembly of polyhedra into complex structures." Science 337.6093 (2012): 453-457.
Colloid clusters from confined self-assembly
Wichtiger Hinweis - geänderter Veranstaltungsort - Hörsaal 26.11 6A
Do 18.01.2018   Prof. Dr. Paul van der Schoot  Universiteit Eindhoven  16:30 s.t.
Geometric percolation in dispersions of rod-like colloids - impact of particle tortuosity and spontaneous symmetry breaking
Do 11.01.2018   Prof. Dr. Jörg Baschnagel  Université de Strasbourg  16:30 s.t.
Shear modulus and shear stress relaxation in glass-forming and polymer systems
Do 20.07.2017   Prof. Dr. Tanja Schilling  Universität Freiburg  16:30 s.t.
Prof. Dr. Tanja Schilling: „Playful Physics”
Universität Freiburg - Seminar@HHUD: 20.07.2017 16:30 s.t., Lecture hall 25.31 O0.5J

Chemotactic motion is the motion of living organisms in response to chemical signals as e.g. motion of bacteria towards sources of food. We discuss chemotaxis in a porous medium using as a model a biased (6quot;hungry") random walk on a percolating cluster. Incidentally, the model closely resembles the 1980s arcade game Pac-Man. We observe that, on the percolating cluster, the hungry random walker's mean-squared displacement shows anomalous dynamics that follow a power law with a dynamical exponent different from both that of a self avoiding random walk as well as that of an unbiased random walk. The change in dynamics with the propensity to move towards food is well described by a dynamical exponent that depends continuously on this propensity.

In the second part we apply a similar, physics-based approach to a more complex game, the game of chess. The complexity of a game is usually estimated in terms of the size of its state space (i.e. the number of possible configurations) and the size of its game tree (i.e. the number of distinct possible games). For chess these have been estimated to be on the order of its game tree (i.e. the number of distinct possible games). For chess these have been estimated to be on the order of 1042 resp 10120 . A chess player's experience however, shows that many possible configurations never occur in real play. The connectivity of state-space seems to matter significantly for the complexity. Using transition path sampling we show that the state space of chess consists of ca. 1020 pockets that are only weakly connected. The pockets are distinguished mainly by their pawn structure. Real games take place only in a few of these pockets. That chess is still highly complex can be attributed to the fact that 1022 - although considerably smaller than the entire set of states - is still a very large number.

Playful Physics
Do 29.06.2017   Prof. Dr. Holger Stark  Institut für Theoretische Physik, Technische Universität Berlin  16:30 s.t.
Microswimmers: From design principles to their emergent collective behavior
Für die Dozenten der Physik

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Seminar über Examensarbeiten aus dem Institut für Theoretische Physik, Lehrstuhl II

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Link: Physikalisches Kolloquium
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