Instability, surface dynamical

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. 

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. 

Surface Patterning Approaches Based on Surface Instabilities or Dynamic Process Control... 

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],... 

The studies on adhesion are mostly concerned on predictions and measurements of adhesion forces, but this section is written from a different standpoint. The author intends to present a dynamic analysis of adhesion which has been recently published [7], with the emphasis on the mechanism of energy dissipation. When two solids are brought into contact, or inversely separated apart by applied forces, the process will never go smoothly enough—the surfaces will always jump into and out of contact, no matter how slowly the forces are applied. We will show later that this is originated from the inherent mechanical instability of the system in which two solid bodies of certain stiffness interact through a distance dependent on potential energy. 

For solid surfaces interacting in air, the adhesion forces mainly result from van der Waals interaction and capillary force, but the effects of electrostatic forces due to the formation of an electrical double-layer have to be included for analyzing adhesion in solutions. Besides, adhesion has to be studied as a dynamic process in which the approach and separation of two surfaces are always accompanied by unstable motions, jump in and out, attributing to the instability of sliding system. 

The role of instabilities involving confined impurity atoms has been investigated by Mtiser using a model in which two one-dimensional (1-D) or 2-D surfaces were separated by a very low concentration of confined atoms and slid past one another.25 The motion of the confined atoms was simulated with Langevin dynamics where the interactions between these atoms were neglected and the atom-wall interactions were described by... 

The occurrence of kinetic instabilities as well as oscillatory and even chaotic temporal behavior of a catalytic reaction under steady-state flow conditions can be traced back to the nonlinear character of the differential equations describing the kinetics coupled to transport processes (diffusion and heat conductance). Studies with single crystal surfaces revealed the formation of a large wealth of concentration patterns of the adsorbates on mesoscopic (say pm) length scales which can be studied experimentally by suitable tools and theoretically within the framework of nonlinear dynamics. [31]... 

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