Microswimmers – From Single Particle Motion to Collective Behavior

Locomotion of autonomous microswimmers is a fascinating field at the cutting edge of science. It combines the biophysics of self-propulsion via motor proteins, artificial propulsion mechanisms, swimming strategies at low Reynolds numbers, the hydrodynamic interaction of swimmers, and the collective motion and synchronisation of large numbers of agents. The articles of this Special Issue are based on the lecture notes of an international summer school, which was organized by the DFG Priority Programme 1726 “Microswimmers – From Single Particle Motion to Collective Behaviour” in the fall of 2015. The minireviews provide a broad overview of the field, covering both elementary and advanced material, as well as selected areas from current research.     

Shock-wave velocity of a femtosecond-laser-produced plasma

Fluorescence measurements have been used to characterize the velocity of atoms in a femtosecond-laser-produced plasma. Nanogram amounts of a copper sample were ablated by the focused radiation (λ=775 nm) of an all-solid-state laser. The laser was operated at a pulse rate of 10 Hz with an energy of 200μJ per pulse. The microplasma expanded into a defined argon atmosphere of pressures between 0.02 and 850 mbar. Atomic fluorescence was excited in the laser plume by a dye-laser pulse with the wavelength set to the line Cu I 282.4 nm. The narrowed beam of the dye-laser was directed into the plasma at different heights above the sample surface. The fluorescence radiation was measured with an échelle-spectrometer, equipped with an intensified-charge-coupled device as the detector. The velocity depends strongly on the pressure of the ambient atmosphere and the distance from the sample surface. The highest velocity found at an argon pressure of 0.02 mbar was 1.0×10⁶ cm s⁻¹.     

In situ characterization of semiconductor saturable absorber mirrors in an operating Yb:KGW mode-locked laser

We report in situ characterization of a semiconductor saturable absorber mirror (SESAM) in an operating Yb:KGW mode-locked laser. The technique may be described as a pump-probe experiment in which the intracavity beam acts as a pump beam while the output of the same laser is used as a test beam for the SESAM reflectivity. At zero delay, the probe pulse overlaps in time with the subsequent intracavity pulse. The method is an alternative to standard pump-probe measurements in situations where the intracavity parameters such as energy fluence onto the SESAM, pulse length, and center wavelength cannot be achieved simultaneously with available lasers.     

Bose-Einstein condensation in semiconductors: the key role of dark excitons

Bose-Einstein condensation in semiconductors is controlled by the nonelementary-boson nature of excitons. Pauli exclusion between the fermionic components of composite excitons produces dramatic exchange couplings between bright and dark states. In microcavities, where bright excitons and photons form polaritons, they force the condensate to be linearly polarized, as observed. In bulk, they also force linear polarization, but of dark states, due to interband Coulomb scatterings. To evidence this dark condensate, indirect processes are thus needed.     

Anomalous polymer sedimentation far from equilibrium

A single flexible polymer in strong sedimentation fields is investigated using hydrodynamic simulations and scaling arguments. For short chains and small fields compaction is observed. For elevated fields or long chains the chain stretches and the sedimentation coefficient decreases, in agreement with ultracentrifuge experiments on linear as well as circular DNA. For very large fields a tadpole forms consisting of a compact leading head and a trailing stretched tail.     

Centrality dependence of charged-hadron transverse-momentum spectra in d+Au collisions at sqrt[s(NN)]=200 GeV

We have measured transverse momentum distributions of charged hadrons produced in d+Au collisions at sqrt[s(NN)]=200 GeV. The spectra were obtained for transverse momenta 0.25相似文献