Ionic conductivity of solid polyelectrolyte complexes with varying water content: application of the dynamic structure model.

We present a systematic study of the dc conductivity of solid polyelectrolyte complexes (PEC) of type xPSS PSS·(1 - xPSS) PDADMA as a function of temperature, and of PSS and water content, respectively. PSS stands for poly(styrenesulfonate) and PDADMA for poly(diallyldimethylammonium). Apart from these polyions, small ions like Na+ and Cl- can be incorporated into the complexes. The amount of small ions and its type (Na+ or Cl-) depend on the PEC composition. We show that, in contrast to dried polyelectrolyte complexes where the chloride ions are macroscopically immobile, Cl- ions become mobile if water is absorbed into the PEC. In PEC with an excess of PDADMA (xPSS < 0.5), hydrated Cl- ions govern the ionic conductivity. On the other hand, in PEC with an excess of PSS (xPSS > 0.5), the conductivity is determined by the sodium ions. For the first time we show that the dependence of the conductivity on composition can be described by power-laws as derived within the framework of the dynamic structure model originally developed for glassy ion conductors by Bunde, Ingram and Maass (J. Non-Cryst. Solids, 1994, 172-174, 1222). This power-law behavior is found in PEC with an excess of Cl- ions as well as in those with an excess of Na+ ions. Also the model predictions concerning the temperature dependence of the power-law exponents on the one hand and the composition dependence of the activation enthalpy on the other hand, are found to be valid. These findings indicate that in polyelectrolyte complexes ions travel via pathways of ion specific sites through the polyelectrolyte matrix. The results on hydrated PEC are compared to those of dry PEC where the dynamic structure model is only applicable for PEC with an excess of PSS (xPSS > 0.5).

Characterization of azimuthal and radial velocity fields induced by rotors in flows with a low Reynolds number.

We theoretically and experimentally investigate the flow field that emerges from a rodlike microrotor rotating about its center in a nonaxisymmetric manner. A simple theoretical model is proposed that uses a superposition of two rotlets as a fundamental solution to the Stokes equation. The predictions of this model are compared to measurements of the azimuthal and radial microfluidic velocity field components that are induced by a rotor composed of fused microscopic spheres. The rotor is driven magnetically and the fluid flow is measured with the help of a probe particle fixed by an optical tweezer. We find considerable deviations of the mere azimuthal flow pattern induced by a single rotating sphere as it has been reported by Di Leonardo et al. [Phys. Rev. Lett. 96, 134502 (2006)]. Notably, the presence of a radial velocity component that manifests itself by an oscillation of the probe particle with twice the rotor frequency is observed. These findings open up a way to discuss possible radial transport in microfluidic devices.

Sympathetic cooling of complex molecular ions to millikelvin temperatures.

Gas-phase singly protonated organic molecules of mass 410 Da (Alexa Fluor 350) have been cooled from ambient temperature to the hundred millikelvin range by Coulomb interaction with laser-cooled barium ions. The molecules were generated by an electrospray ionization source, transferred to and stored in a radio-frequency trap together with the atomic ions. Observations are well described by molecular dynamics simulations, which are used to determine the spatial distribution and thermal energy of the molecules. In one example, an ensemble of 830 laser-cooled 138Ba+ ions cooled 200 molecular ions to less than 115 mK. The demonstrated technique should allow a large variety of protonated molecules to be sympathetically cooled, including molecules of much higher mass, such as proteins.

Femtosecond laser-induced two-photon polymerization of inorganic-organic hybrid materials for applications in photonics.

Investigations of two-photon polymerization of inorganic-organic hybrid materials initiated by femtosecond Ti:sapphire laser pulses are performed. First applications of this technique for the fabrication of three-dimensional microstructures and photonic crystals in inorganic-organic hybrid polymers with a structure size down to 200 nm and a periodicity of 450 nm are discussed.

Sub-diffraction limited structuring of solid targets with femtosecond laser pulses.

Possibilities to produce sub-diffraction limited structures in thin metal films and bulk dielectric materials using femtosecond laser pulses are investigated. The physics of ultrashort pulse laser ablation of solids is outlined. Results on the fabrication of sub-micrometer structures in 100-200 nm chrome-coated surfaces by direct ablative writing are reported. Polarization maintaining optical waveguides produced by femtosecond laser pulses inside crystalline quartz are demonstrated.