Resonant steps and spatiotemporal dynamics in the damped dc-driven Frenkel-Kontorova chain

Kink dynamics of the damped Frenkel-Kontorova (discrete sine-Gordon) chain driven by a constant external force are investigated. Resonant steplike transitions of the average velocity occur due to the competitions between the moving kinks and their radiated phasonlike modes. A mean-field consideration is introduced to give a precise prediction of the resonant steps. Slip-stick motion and spatiotemporal dynamics on those resonant steps are discussed. Our results can be applied to studies of the fluxon dynamics of 1D Josephson-junction arrays and ladders, dislocations, tribology and other fields.Comment: 20 Plain Latex pages, 10 Eps figures, to appear in Phys. Rev.

Electrical Conductivity of Electrospun Polyaniline and Polyaniline-Blend Fibers and Mats

Submicrometer fibers of polyaniline (PAni) doped with (+)-camphor-10-sulfonic acid (HCSA) and blended with poly(methyl methacrylate) (PMMA) or poly(ethylene oxide) were electrospun over a range of compositions. Continuous, pure PAni fibers doped with HCSA were also produced by coaxial electrospinning and subsequent removal of the PMMA shell polymer. The electrical conductivities of both the fibers and the mats were characterized. The electrical conductivities of the fibers were found to increase exponentially with the weight percent of doped PAni in the fibers, with values as high as 50 ± 30 S/cm for as-electrospun fibers of 100% doped PAni and as high as 130 ± 40 S/cm upon further solid state drawing. These high electrical conductivities are attributed to the enhanced molecular orientation arising from extensional deformation in the electrospinning process and afterward during solid state drawing. A model is proposed that permits the calculation of mat conductivity as a function of fiber conductivity, mat porosity, and fiber orientation distribution; the results agree quantitatively with the independently measured mat conductivities.United States. Army Research Office (Institute for Soldier Nanotechnologies, Contract ARO W911NF-07-D- 0004

Wettability Switching Techniques on Superhydrophobic Surfaces

The wetting properties of superhydrophobic surfaces have generated worldwide research interest. A water drop on these surfaces forms a nearly perfect spherical pearl. Superhydrophobic materials hold considerable promise for potential applications ranging from self cleaning surfaces, completely water impermeable textiles to low cost energy displacement of liquids in lab-on-chip devices. However, the dynamic modification of the liquid droplets behavior and in particular of their wetting properties on these surfaces is still a challenging issue. In this review, after a brief overview on superhydrophobic states definition, the techniques leading to the modification of wettability behavior on superhydrophobic surfaces under specific conditions: optical, magnetic, mechanical, chemical, thermal are discussed. Finally, a focus on electrowetting is made from historical phenomenon pointed out some decades ago on classical planar hydrophobic surfaces to recent breakthrough obtained on superhydrophobic surfaces

Preventing the Cassie-Wenzel Transition Using Surfaces with Noncommunicating Roughness Elements

Control and switching of liquid droplet states on artificially structured surfaces have significant applications in the field of microfluidics. The present work introduces the concept of using structured surfaces consisting of noncommunicating roughness elements to prevent the transition of a droplet from the Cassie to the Wenzel state. The use of noncommunicating roughness elements leads to a confinement of the medium under the droplet in its Cassie state. Transition to the Wenzel state on such surfaces requires expulsion of this confined medium, which offers significantly increased resistance to the Wenzel transition unlike surfaces consisting of communicating roughness elements. This enhances the robustness of the Cassie state and significantly minimizes the possibility of the Cassie-Wenzel transition. In the present work, the resistance of a surface to the Wenzel transition is measured in terms of the electrowetting (EW) voltage required to trigger this transition. It is seen that surfaces with noncommunicating roughness elements (cratered surfaces) require significantly higher voltages to trigger the Wenzel transition than corresponding surfaces with communicating roughness elements. The findings from the present work also indicate that EW-induced droplet morphology control characteristics show a strong dependence on the nature of the roughness elements (communicating versus noncommunicating). Different aspects of droplet morphology and EW-induced state transition control on surfaces with noncommunicating toughness elements are analyzed; it is seen that such surfaces offer significant possibilities for the development of robust superhydrophobic surfaces