Index of content:
Volume 92, Issue 2, 15 July 2002
- INTERDISCIPLINARY AND GENERAL PHYSICS (PACS 1-41, 43-47, 79, 81-84, 89-99)
92(2002); http://dx.doi.org/10.1063/1.1481772View Description Hide Description
The transmission of light through ionic aqueous ferrofluids under different magnetic fields is measured. The results show that the transmission intensity increases quickly as soon as the magnetic field is applied, and then weakens with time. Both the initial increase and following decrease of the transmission intensity are proportional to the volume fraction of particles in the ferrofluid and the magnetic field strength B. Once the magnetic field is removed, the transmission intensity rises rapidly to a stable value, which is inversely proportional to the and B, and is less than the value before the magnetic field was applied. These results indicate that there exists a dynamical process that the chains of magnetic particles in the ferrofluid form, join, and then break.
High temperature oxidation of Si(100) by neutral oxygen cluster beam: Coexistence of active and passive oxidation areas92(2002); http://dx.doi.org/10.1063/1.1480111View Description Hide Description
We report the study of Si(100) oxidation by oxygen clusters with an average size of 2000 molecules at substrate temperatures ranging from 850 to 1100 °C. It has been found that at two areas corresponding to passive and active oxidation of silicon coexist within the same impact spot on the surface. In the central part of the impact spot, no surfaceetching occurs due to the formation of a protective oxide layer. In the area surrounding the central part, a circular groove with a steep inner and slightly sloping outer walls is formed as a result of surfaceetching via the reaction The reactive sticking coefficient of oxygen obtained from the etch rate in active oxidation area is significantly higher than that for oxidation by molecular oxygen. The coexistence of the two areas is attributed to a quasi-Gaussian flux density distribution in the beam cross section. The observed abrupt drop of the etch rate at the inner wall of the groove marks the transition from active to passive oxidation.Siliconoxidation by an oxygen cluster beam shows specific features that have not been observed in previous works with molecular oxygen. The critical flux density corresponding to the transition does not, for instance show an Arrhenius behavior. Besides, this study shows the crucial role of the beam spatial parameters when supersonic sources are used to study surface reactions.
92(2002); http://dx.doi.org/10.1063/1.1486052View Description Hide Description
The axial heat flow within an aligned fiber composite containing matrix cracks bridged by the fibers is analyzed. A unit cell is defined and an exact expression for the constriction resistance of the unit cell is found. For values of fiber volume fraction and crack spacing that occur in actual composites, the thermal interaction between cracks is found to be almost negligible when the fiber/matrix interface is perfect. The asymptotic behavior under conditions of severe interface debonding is found.
92(2002); http://dx.doi.org/10.1063/1.1487454View Description Hide Description
We examine the effect of a dynamic stress on the reduction of flow in porous media using polymergels formed in situ. To develop the theory for the response of the gel, we consider three dominant factors: (a) compressive (elastic)deformation of the gel and porous medium, (b) microscopic flow in this system, and (c) gel displacement. The latter occurs when the stress p is larger than a certain critical value satisfying (R=effective pore radius), where the constant is an increasing function of elastic modulus of the gel and its cross-linking energy. The expulsion of the gel above is reminiscent of growing Saffman-Taylor instabilities. To derive analytic expressions for the macroscopic saturation profiles we use the formalism for fully miscible two-phase flow. The equation of evolution of the pressure, established by mass balance arguments, was solved analytically. For the pressure obeys an exponential saturation function while for it first increases, reaches a maximum value, and then decreases towards an asymptotic value. These theoretical predictions are supported by our experiments consisting of injecting (salt) water at a constant flow rate in porous samples containing a organically cross-linked polymergel (co-polymer of acrylamide and t-butyl-acrylate cross-linked using poly-ethylene-imine). The data confirms further that the product is constant and prove that both and the maximum pressures increase with intrinsic gel strength.
92(2002); http://dx.doi.org/10.1063/1.1468251View Description Hide Description
Polarized confocal Raman mapping is used to characterize the degree of molecular alignment in spin coated conjugated polymer thin films. To this end, the polarization selection rules of a specific Raman active mode are employed to infer indirectly the local molecular alignment. The film can be monitored and characterized with a diffraction limited spatial resolution fixed by the microscope optics. In addition, we demonstrate examples of characterization of molecular alignment in highly oriented samples by monitoring the angular dependence of the polarized Raman signal. Finally, we show the use of temperature dependent Raman scattering to monitor thermal phase transitions in bulk conjugated polymer samples. These procedures can be used as in situ characterization methods in a wide variety of experimental situations and offer a sensitive probe of processing protocols used in the fabrication of polymer optoelectronic devices.