Bihai Song

Determination of interfacial tension from the profile of a pendant drop using computer-aided image processing

The reliability of the method described in the preceding paper (B. Song and J. Springer, J. Colloid Interface Sci. 183, in press, 1996) for the determination of surface and interfacial tension (IFT) of liquid-fluid systems from the profile of a pendant drop has been studied experimentally. Influences on the resultant IFT values from factors such as the location of the drop profile and illumination conditions are considered. The reproducibility and accuracy of the measurement method were examined by measuring the time dependence of polyethylene glycol (PEG600)/air and water/air systems and by determining the temperature dependence of the polyethylene glycol (PEG6000)/N2 and LD (low-density) polyethylene/N2 systems. The effect of drop vibrations on the accuracy of the method is discussed.

Adhesion : Comparison between physico-chemical expected and measured adhesion of oxygen-plasma-treated carbon fibers and polycarbonate

Abstract The adhesive interaction between oxygen-plasma-treated, polyacrylonitrile-based, high-tensile-strength carbon fibers and a polycarbonate matrix has been studied. Several models have been used to predict the impact of the plasma treatment process on the strength of adhesion between both jointing partners. These approaches have been the thermodynamic work of adhesion which was calculated from the solid surface tensions, based on the results of contact angle measurements versus test liquids, the contact angle which was directly obtained via polycarbonate melt droplets on single carbon fibers and the zeta (ς)-potential data provided by streaming potential measurements. The results have been compared with the interfacial shear strength determined from the single-fiber fragmentation test. Additionally, the single-fiber tensile strength of the oxygen-plasma-treated carbon fibers was determined. We confirmed that any physico-chemical method on its own fails to describe exactly the measured adhesion. However, for the investigated system, the conscientious interpretation of the data obtained from wetting measurements, in conjunction with the thermodynamic approach, is sufficient to predict the success of a modification technique which has been applied to one component in order to improve adhesion.

Surface phenomena of liquid crystalline substances : Temperature-dependence of surface tension

Abstract The temperature dependence curve of surface tension has been measured using computer-aided pendant drop method for several low molecular liquid crystalline substances (LCs) near their phase transition temperatures. The results reveal anomalies (positive (∂γ/∂T)-values and discontinues change) in the surface tension vs. temperature curve for all the studied substances, which may indicate the development of highly ordered molecular state in the surface layer in the defined temperature ranges. Furthermore, a sharp decrease of surface tension has been observed for two LCs by approaching their crystallization points from higher temperatures, which seems to suggest that something like surface induced freezing occurs at the liquid surface of those substances by nearing their crystallization temperatures. The resulst are discussed on the basis of statistical thermodynamic models proposed in the literature.

Grafted Carbon Fibers and Their Physico-Chemical Properties. Part II. Grafting of Liquid Crystalline Methacrylic Monomers onto Carbon Fibers and the Influence of an Applied Voltage to the Carbon Fiber Embedded in a Liquid Crystalline Matrix

ABSTRACT: Results of grafting reactions of a liquid crystalline (lc) monomer ontocarbon fiber surfaces are presented and compared with results of noncovalently bondedlc polymer, which is proved reproducible by scanning electron microscopy. Electro-optical investigations under a polarizing microscope hints of the possibility of changinglc polymer and lc low molecular mass matrix molecules properties by applying a certainvoltage to the carbon fibers. Wetting and electrokinetic measurements were performedand correlated with grafting reaction parameters. Those investigations revealed anearly complete coverage of the carbon fiber surfaces by lc polymers. These measure-ments are suitable to characterize carbon fibers modified by grafted or coated polymers.

Surface Phenomena of Liquid Crystalline Substances - Temperature Dependence of Surface Tension of 4,4′-di-n-undecanoyloxybiphenyl

Abstract The surface tension of 4,4′-di-n-undecanoyloxybiphenyl has been determined as a function of temperature. A distinct deviation of its temperature dependence from the normal behavior was observed below a certain inversion temperature. The surface tension of the isotropic sample decreases sharply as the falling temperature approaches the underlying bulk phase transition point. The formation of some highly ordered structures at the liquid surface is revealed, which is consistent with the suggestion that the existence of an underlying highly ordered phase favors the pretransitional surface ordering of molecules at the free surface of an isotropic phase.