Free-Standing Filaments

The measurement of properties such as the resistivity or dielectric constant of PS requires some kind of contact with the PS layer. Evaporation of a metal onto the PS film-covered silicon sample produces a metal/PS/Si sandwich, which behaves like an MIS structure with an imperfect insulator. Such sandwich structures usually exhibit a rectifying behavior, which has to be taken into account when determining the resistivity [Si3, Bel4]. This can be circumvented by four-terminal measurements of free-standing PS films, but for such contacts the applied electric field has to be limited to rather small values to avoid undesirable heating effects. An electrolytic contact can also be used to probe PS films, but the interpretation of the results is more complicated, because it is difficult to distinguish between ionic and electronic contributions to the measured conductivity. The electrolyte in the porous matrix may short-circuit the silicon filaments, and wetting of PS in-... 

The formation of the TGBA phase from the isotropic liquid can also be observed in free standing films. The defects formed in this case tend to be filamentary in nature, see Plate 8. Such a texture is not normally seen in free standing films of other liquid crystals, and therefore the presence of this defect texture is diagnostic for the characterization of TGB phases. In addition to the observation of such filamentary textures in free standing films, filaments were also observed embedded in homeotropically aligned smectic A phases, see Plate 9. 

Figure 7.13 Temperature dependence of conductivity of free-standing biofilm formed by G. sulfurreducens strain CL-1 and pili filaments of strain KN400 measured with a four-probe method. Error bars represent SD. Data is a representative of several replicates (n = 4 for biofilm n = 3 biological replicates for pili). The difference in conductivity between pili and biofilm at 300 K is due to lower pili concentration required on smaller electrodes for four-probe measurements. Control experiments containing media buffer or with biofilm, but not bridging the gap, showed very low conductivity (<10 pS cm" at 300 K) that did not change with temperature. Inset Arrhenius fit for exponentially decreasing conductivity. Figure adapted from Ref. [4] with permission.

Recently Stannarius et al. formd that the geometrical structures of two phases of the same material can be practically identical, whereas filaments are stable only in the higher temperature mesophase. They also conclude that probably the spontaneous curvature of the layers, which might be caused by the out of plane polarization component of the polarization, favors the formation of the cylindrical fibers rather than the free-standing films. 

After the filaments leave the quench tank, they must be wiped or blown free of water in preparation for hot-drawing. Wet sections of the yarn will not be heated to the same temperature as the dry sections. The process for drawing monofilaments is very similar to ST. Roll stands and hot ovens, baths, or plates are used for the filaments. The yarns are usually annealed on heated rolls or in an air oven. A two-stage yarn may be used, with a total draw ratio commonly around 6 1. A finish must be applied to the filaments for lubrication and static control. The filaments are wound individually on tubes or flanged bobbins. 

The method of resonance vibrations (Section C above) has also been used, in the form of standing longitudinal wave measurements and flexural vibrations of short fiber segments. - In the latter case, the fiber cross-section shape and dimensions must be known with high accuracy the equations for calculating E and E" are similar to equations 12 and 13 (for circular cross-section) but with different numerical coefficients for one end clamped and one free. Despite the small flexural stiffness of thin fibers, this method has been employed on filaments as thin as 0.03 mm . 

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