dr. Jan Anton Koster Zernike Institute, Universiteit Groningen

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1 Monday Feb , 11:00-12:00 dr. Jan Anton Koster Zernike Institute, Universiteit Groningen Charge transport and recombination in organic solar cells Two major loss processes in organic solar cells: bimolecular recombination and exciton decay. To enable a further improvement of organic solar cell efficiency a clear and thorough understanding of the fundamental processes that occur in organic solar cells is a necessity. One of the processes limiting the efficiency of organic solar cells is bimolecular recombination of photogenerated charge carriers and in the quest for new high-performance materials it is important to establish just how many carriers are lost through this process. A new and straightforward method to quantify what fraction of charge carriers recombine bimolecularly under operating conditions is presented. By fitting an analytical model to the experimental data of polythiophene/fullerene cells, we find that the bimolecular recombination constant is suppressed relative to the Langevin rate. The second part of this talk focuses on the transport efficiency of excitons towards the interface between both blend constituents. By imaging a polythiophene/zno blend with electron tomography (TEM) a three dimensional description of the morphology on the nanometre scale is obtained. As well as the efficiency, the morphology of this materials combination depends on spin coating conditions ranging from coarse to fine. The 3D morphology data are then used to calculate the efficiency of exciton quenching. Good agreement is found between solar cell efficiency, photo-induced absorption data and exciton quenching efficiency. Subsequently, a stochastic model has been developed which describes the morphological structure of these TEM images rather well. The model has been validated by comparing the efficiency of exciton quenching for real and simulated data. This model can be used to generate virtual photoactive layers of hybrid solar cells in order to identify morphologies with improved efficiency.

2 Monday Feb , 11:00-12:00 dr. Vincenzo Vitelli Instituut-Lorentz, Universiteit Leiden The fate of the sonic vacuum A sonic vacuum is an elastic medium where the speeds of sound can be made to vanish by tuning a parameter such as pressure in random packings of soft spheres or average connectivity in disordered fibrous networks. In a sonic vacuum, the propagation of mechanical disturbances, no matter how tiny, takes place via acoustic shocks, without any hint of linear sound waves. In this talk, we derive the fundamental equations governing the dynamics of shear and compressional acoustic shocks and discuss the fate of the sonic vacuum, as pressure or connectivity are increased away from the critical point. The role of disorder in attenuating the shocks is also determined as a function of applied load/connectivity and amplitude of the initial perturbation.

3 Monday Feb , 11:00-12:00 dr. Debabrata Panja Institute for Theoretical Physics, University of Amsterdam Dynamics of bacteriophage genome ejection in vitro and in vivo Bacteriophages, phages for short, are viruses of bacteria. Typically their capsids are about ~60 nm in size. In ~95% cases their genomic material is a double-stranded DNA, packed typically at a density of ~500 mg/ml. After a brief introduction to phages, their dimensions, structures and infection-initiation phenomenology, I will discuss the thermodynamics of mature phages. Using in vitro experimental data I will argue that the phages maintain a high osmotic pressure within their capsids. In vitro, this osmotic pressure allows them to use the physics and thermodynamics of water movement to eject their genomic material. The actual infection dynamics - in vivo - is even more curious. For that the phages use the osmotic gradient that the bacteria need to maintain across their cell membranes for growth: in effect, thanks to the phages, the mechanism that allows bacteria to grow is also their Achilles heel. In short, the phages are fantastic infection machines.

4 Monday Feb , 11:00-12:00 dr. Antoine Chateauminois ESPCI, Paris Mechanical rejuvenation and ageing of glassy polymer films within contacts The effects of repeated large strain shear cycles on the dynamics of a glassy acrylate polymer are investigated using an original contact method. It is based on the measurement of the shear properties of thin (about 50 µm) polymer films geometrically confined within contacts between elastic substrates. Under small amplitude (20nm-1µm) oscillating lateral displacements, friction at the contact interface can be neglected and the measurement of the contact lateral response thus provides information about the rheology of the sheared polymer film. Using this approach, the complex shear modulus of the polymer film can be measured both in the linear viscoelastic and in the non linear regimes. The investigations are focused on the changes in mechanical properties induced in a large strain regime where the polymer glass is cyclically sheared up to the yield point. During the application of large strain cycles, the mechanical response of the polymer glass slowly evolves toward a quasi stabilized state which is described from the measurement of an apparent - strain and pressure dependent - complex shear modulus. When the applied strain is increased by a tenfold factor, this apparent shear modulus decreases by about one decade. These changes are investigated in details from a consideration of the time dependent linear viscoelastic properties after the application of the large strain cycles. Both mechanical rejuvenation and recovery (ageing) effects are evidenced. Immediately after non linear cyclic deformation, enhanced molecular mobility is indicated by a decrease in the storage modulus and a corresponding increase in the loss modulus. As a result of increased segmental mobility, physical ageing is reactivated and the modulus slowly recovers toward its initial value. These observations can be qualitatively accounted for by a shift of the relaxation time spectra toward shorter times after the large strain stimulus.

5 Monday Mar , 11:00-12:00 prof. dr. Gerard Barkema Institute for Theoretical Physics, Utrecht University Anomalous Dynamics of Polymers The transport properties of atoms or molecules can often be characterized as a combination of drift and diffusion, in which the linear and squared displacement increase linearly with time, respectively. In polymeric systems, however, the transport properties often shows a much richer diversity, including cases where the squared displacement increases as t α with α<1; such transport is known as anomalous diffusion. The talk will show how such anomalous dynamics arises in simple models of phantom and self-avoiding polymers, and what its key signatures are. Next, from the perspective of anomalous diffusion, we will study polymer translocation through a nanopore, and the dynamics of melts. We will also discuss the role of anomalous dynamics in nucleation, as well as ongoing research on the self-diffusion in vitreous silica (glass) near the glass transition. We will also discuss the techniques of large-scale computing, required for this research

6 Monday April 4, 2011, 11:00-12:00 TU/e Applied Physics, room Spectrum 2.03 dr. Brian Tighe Institute-Lorentz for Theoretical Physics, Leiden University Oscillatory rheology in soft sphere packings Athermal packings of soft repulsive spheres are a model for foams, emulsions and other soft materials that undergo a jamming transition. Packings are jammed -- resist shear like a solid -- when the mean number of contacts between particles is high. When the contact number drops to a critical value they unjam, or flow, in response to any amount of shear stress. I will describe the response of jammed packings to oscillatory shear at finite frequency, a common rheological masurement. The frequency dependent shear modulus is comprised of two parts, the storage (in phase) and loss (out of phase) moduli, and both display critical scaling governed by the number of contacts in excess of the critical value. I will show how this behavior is caused by an anomalous excess of slow relaxation modes that appear in packings near unjamming and explain all of the observed rheology with asymptotic scaling analysis.

7 Monday May 23, 2011, 11:00-12:00 TU/e Applied Physics, room Spectrum 2.03 Dr. Tadaaki Ikoma Graduate School of Science and Technology, Niigata University, Japan Magnetic Field Effect on the Spin Dynamics of Geminate Electron-hole Pairs in the Photoconduction of a Polymer Film Magnetoconductance (MC), the change in electric conductance under the influence of an external magnetic field, of molecular semiconductor is one important property in molecular spintronic devices. Although numerous attempts have been made to study the MC effect in molecular semiconductors recently, the mechanism of MC effect is still controversial. The importance of the hyperfine interaction in carrier pair has been pointed out and the MC effects in low magnetic field attract much attention from the researchers in molecular spintronics. In order to clarify the mechanism of low field MC effect, we have studied the spin dynamics of electronhole (e-h) pair in the photoconduction of poly(n-vinylcarbazole) (PVCz) film doped with various electron acceptors. The photocarrier generation can be regarded as geminate e-h pair dynamics. The carrier dynamics beginning from the e-h pair makes the interpretation of the MC effect simpler than the case of dark conduction. We have succeeded to observe the MC effects below ca. 1 mt, of which phase was opposite to that at high fields. The observed low field effects prove the coherent spin dynamics in the e-h pair due to the hyperfine interactions.

8 Monday July 18, 2011, 11:00-12:00 TU/e Applied Physics, room Spectrum 2.03 Prof. Dr. Holger Stark TU Berlin, Institut für Theoretische Physik The strange world of low Reynolds numbers: Fluid dynamics in the microscopic world On the micron scale, the dynamics of colloidal suspensions but also the locomotion of microorganisms is governed by low Reynolds numbers where fluid flow is laminar and inertia does not play any role. The talk introduces this regime with a special emphasis on its biological implications. Then I present some of our own work to demonstrate how interesting physics arises when the laminar flow is coupled to additional degrees of freedom. In particular, I will first show how hydrodynamic interactions between driven oscillators, mediated solely by flow fields, lead to the formation of metachronal waves. Microorganisms such as paramecium are covered by a carpet of beating filaments called cilia and use these waves to move forward. Secondly, bacteria swim with the help of a rotating bundel of helical filaments. We model these filaments and their actuation by a rotary motor with the help of Kirchhoff's theory of elastic rods and observe interesting buckling instabilities. Finally, I discuss a model swimmer called squirmer, which uses a prescribed velocity field at its surface to propel itself. We simulate this squirmer in a cylindrical microchannel with the help of a technique called multi-particle collision dynamics. In particular, I explain how a microswimmer can move upstream in a Poiseuille flow.