Fibers, critical surface tension

Rapid Estimation of the Critical Surface Tension of Fibers... 

The measured normalized wetting rates for various test liquids (for cellulose fibers Fig. 23) can be transformed into the cosine of the contact angle (cos0) and plotted as a function of the liquid surface tension (Fig. 24). The resulting linear relationship cos 0 = 1 — b(y — jc) was established empirically by Zisman and Fox [120] and found to hold for solid with low surface tensions. The critical surface tension 7c corresponds to the surface tension of the liquid that will just spread over/wet completely the solid. The constant C reflects the capillary geometry of the porous solid and may change in a non-predictable manner during the penetration process of different test liquids. It was concluded from the experiments performed [112] that there is no need to determine the constant C in order to obtain solid-surface tensions, because the position of the maximum in the C-7iv cos 0 vs. 7iv plot, which is expected to... 

The repellency of fluorocarbon finishes depends on the structures of the fluorocarbon segment, the nonfluorinated segment of the molecule, the orientation of the fluorocarbon tail, the distribution and the amount of the fluorocarbon moiety on fibers, and the composition and geometry of the fabric [101]. The relationship between repellency and the structure of the fluorocarbon segment is in accord with the critical surface tension concept developed by Zisman and co-workers (see Chapter 11). Shafrin and Zisman [102] determined the wettabilities and critical surface tensions of -perfluoroalkyl substituted 77-heptadecanoic acids synthesized by Brace [103]. Once the seven outmost carbon atoms are fully fluorinated x = 7), the wettability of monolayers of the acids F(CF2)a(CH2)i6COOH approaches that of the perfluorocarboxylic acid F(CF2).vCOOH (Fig. 12.2). This suggests that a terminal perfluoroalkyl chain of seven carbon atoms is sufficiently... 

Surface Tension. Interfacial surface tension between fluid and filter media is considered to play a role in the adhesion of blood cells to synthetic fibers. Interfacial tension is a result of the interaction between the surface tension of the fluid and the filter media. Direct experimental evidence has shown that varying this interfacial tension influences the adhesion of blood cells to biomaterials. The viscosity of the blood product is important in the shear forces of the fluid to the attached cells viscosity of a red cell concentrate is at least 500 times that of a platelet concentrate. This has a considerable effect on the shear and flow rates through the filter. The surface stickiness plays a role in the critical shear force for detachment of adhered blood cells. 

Beaded nanofibers were produced from a water/PEO solution. Addition of ethanol to the water/PEO solution reduces the surface tension of the solution, and production of smooth PEO nanofibers can be obtained. High surface tension causes beaded fibers. On the other hand, smooth fibers without bead formation were seen in PVP/ ethanol solutions having a lower surface tension. Another way is to add a surfactant to the spinning solution. Surfactant contribution to the spinning solution decreases surface tension. An insoluble surfactant is also used to decrease the surface tension. In addition to solvents and surfactants, temperature is another factor for surface tension. In the pure liquid form, the surface tension of the liquid decrease with increasing temperature, as the equilibrium between the surface tension and the vapor pressure would decrease. At a certain (critical ] point, the interface between the liquid and the gas will disappear. 

The electrical potential causes the deformation of the fluid drop, and when the applied voltage develops enough force and balances with the fluid surface tension of the polymer solution, the drop is deformed under a cone shape with a semivertical angle of 30°. Beyond this critical value (Rayleigh limit], the electrostatic forces generated by the charge carriers overcome the surface tension and the deformed droplet undergoes a transition zone just before the fiber jet is initiated to the collector screen. By this way, fluid is... 

If this quantity is measured for a series of dilferent test liquids, the surface tension of the solid (fiber bundle, powder, or porous solid) can be determined. Because [m lt][rilp ] equals C-yiv cos 9 according to Eq. (53), C-yiv cos 9 can be plotted vs. the liquid-surface tension yiv of the test liquids used. This plot (see Fig. 23) will show a maximum that is analogous to Zisman s critical solid-vapor surface tension y of the investigated material [108,113]. 

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