Surface dynamics instability

The significance of LCVD is in the unique aspect of creating a new surface state that is bonded to the substrate material particularly polymeric material. The new surface state can be tailored to be surface dynamically stable. However, caution should be made that not all LCVD films fit in this category. Appropriately executed LCVD to lay down a type A plasma polymer layer creates surface dynamically stable surface state. In the domain, in which surface dynamic instability is a serious concern in the use of materials, a nanofilm by LCVD is quite effective in providing a surface dynamic stability, and other methods do not fare well in comparison to LCVD. 

The force loop for untreated samples are shown in Figure 26.14. The force loops for TMS plasma-treated and (TMS O2) plasma-treated surfaces are shown in Figure 26.15. Any sign of deviation from the parallelogram force loop is an indication of surface dynamic instability. Plasma polymerization coating of (TMS O2) seems to cause some degree of surface dynamical instability depending on the nature of substrate polymer, e.g., PTFE, UHMWPE, HDPE, and PMMA. 

The sessile droplet contact angle measurement is a simple and accurate method to obtain information pertinent to the surface energy of a sample. The Wilhelmy balance method, on the other hand, is a very useful method to investigate the surface dynamic aspect of a sample, which will be described in the following sections. The instability of some of plasma-treated polymer surface observed by the Wilhelmy balance method is also described in Chapter 30. 

Another complicated interfacial dynamic instability is observed when the surfactant covers the liquid surface in a frame, for example on a Langmuir trough. If the surface is subjected to a shear stress either produced by a one-dimensional flow in the liquid bulk or by an air flow the surface instability observed looks like a hair-needle-like flow (Linde Shuleva 1970, Linde Friese 1971, Schwartz et al. 1985). 

Instabilities in UTR polymer films are manifested in two main ways, namely, (i) defects resulting from the coating process, substrate nonuniformities, and conjoining pressure, and (ii) discontinuihes in the thermophysical properties of the hlms due to interfacial effects and polymer cooperative and surface dynamics. 

Instabilities in UTR films can also be manifested as discontinuities in the thermophysical properties of the films due to interfacial effects and polymer cooperative and surface dynamics. Polymer surfaces are regions of enhanced molecular mobility as compared to the bulk, given the decreased constraints on macromolecules at a free surface. The orientation of the surface groups is affected by the nature of the interfacing environment. This implies that polymeric surfaces can restrucmre (in terms of orientation of surface functionalities, concentration of surface groups, etc.) in response to a change in the interfacial phase, in order to adjust their surface properties to the properties of the interfacial medium. °... 

One alternative that has been explored [27] is to conduct an axisymmettic computation (with its associated efficiency) and use a linear stability analysis to fractionate annular ring-shaped ligaments shed from the jet periphery in this case. A Boundary Element Method (BEM) was employed to compute the local surface dynamics. See Chap. 15 for details regarding the computational methodology. While Fig. 27.6 indicates that 3-D instabilities occur prior to pinching of axisym-metric structures, the wavelengths of the azimuthal modes appear to be comparable to those of the axisymmetric waves. Nevertheless, the axisymmetric assumption, while providing drastic simplification, still provides much room for improvement as more computational power becomes available. 

Earlier, it was difficult to produce a clean surface and to characterize its surface structure. However, with the development of electronic industry, techniques have been developed to produce clean surface with well-defined properties. It has been possible to investigate catalytic oxidation on metal surface in depth. Example of dynamic instability at gas-liquid interface is provided by such studies. Studies on chemical oscillations during oxidation of CO over surface of platinum group metals have attracted considerable interest [62-68]. 

An interesting feature of the balance of forces which exists at a surface is demonstrated when a thin layer of a polymeric liquid or solution is placed in an electric field.The original theory considered the dynamic instability which is created when a dielectric media, polymer liquid, is sandwiched between a conductive liquid and a conductive substrate (Figure 10.3). 

The mass transfer between two fluid phases must go through the interface. Due to the existence of surface tension gradient on the interface and the density gradient between the interface and the main fluid, the fluid dynamic instability or bifurcation may appear under certain conditions, which can significantly influence the process efficiency. 

In the course of interfacial mass transfer, from molecular point of view, the process is stochastic, that means some local molecules may undergo the mass transfer in advance than the others, so that small concentration gradient (where i — x, y, z) is established at the interface. As the surface tension a is function of concentration, it follows that the surface tension gradient is also created at the interface. If is increased up to a critical point, the fluid dynamic instability will appear to induce the interfacial convection as well as the formation of orderly structure at the interface. At the same time, the rate of mass transfer may be enhanced or suppressed depending on the properties of the mass transfer system concerned such phenomena is generally regarded as interfacial effect. 

Since the drop volume method involves creation of surface, it is frequently used as a dynamic technique to study adsorption processes occurring over intervals of seconds to minutes. A commercial instrument delivers computer-controlled drops over intervals from 0.5 sec to several hours [38, 39]. Accurate determination of the surface tension is limited to drop times of a second or greater due to hydrodynamic instabilities on the liquid bridge between the detaching and residing drops [40],... 

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