Microscale structural element

Due to the small characteristic dimension, the flow in microchemical systems is laminar. As a result, mixing relies only on molecular diffusion instead of the more efficient turbulence that large-scale systems typically exhibit. At the same time, the diffusion time scale is much shorter due to the small size of a microscale device. However, structural elements that play the role of static micromixers may be necessary to spread fast flows, enhance fluid-solid contact, increase mixing of incoming gases, etc. One such example is the post-micromixer discussed in Ref. [5]. 

Microfluidics is also a cross-disciplinary subject that uses the methods and principles of microelectronics to construct very small analogs or models of such macroscopic fluidic elements as wind tunnels, valves, or fluidic amplifiers. The natural question that comes to mind is at what dimensional scale does fluid motion depart from the extremely well understood and well established laws of fluid dynamics There is no definitive answer to that question yet since the study of fluid motion in microscale and nanoscale structures is still at an early stage. 

Since the assumption of uniformity in continuum mechanics may not hold at the microscale level, micromechanics methods are used to express the continuum quantities associated with an infinitesimal material element in terms of structure and properties of the micro constituents. Thus, a central theme of micromechanics models is the development of a representative volume element (RVE) to statistically represent the local continuum properties. The RVE is constracted to ensure that the length scale is consistent with the smallest constituent that has a first-order effect on the macroscopic behavior. The RVE is then used in a repeating or periodic nature in the full-scale model. The micromechanics method can account for interfaces between constituents, discontinuities, and coupled mechanical and non-mechanical properties. Their purpose is to review the micromechanics methods used for polymer nanocomposites. Thus, we only discuss here some important concepts of micromechanics as well as the Halpin-Tsai model and Mori-Tanaka model. 

As the properties of materials are cJosely related to their structures, the realization of a material with certain properties can be achieved through stmcture control. The stmcture of ceramic materials, however, consists of many types of microstructural elements sucdi as particJes, grains, pores, defects, fibers, layers, and interfaces. These microstmctural elements can be classified by size into four scale levels (i) atomic-molecular scale (ii) nanoscale (iii) microscale, and (iv) macroscale, as shown in Figure 8.1. Besides their physical and chemical nature, other features of these elements including their morphology, configuration, distribution, and orientation are also important. It is obvious, therefore, that there are many factors which can control the structure of materials - not only the types of the elements, but also their features. 

The water in the neighborhood of the smectitic clay surface is structured due to the hydrogen bond, and the viscosity varies inversely with distance from the surface. In this case we can apply the finite element method to solve the microscale equations (8.20) and (8.21), as described previously. 

Several advanced PyMS configurations have been described. Boon et al. [712] have presented a multi-purpose external ion source FTICR mass spectrometer for rapid microscale analysis of complex mixtures. External source DT-FTlCR-MS allows obtaining nominal mass spectra, temperature windows, HRMS data and exact elemental composition and MS/MS data on selected ions. For more detailed structural analysis of the more volatile part of the pyrolysate PyGC-MS and PyGC-HRMS are frequently applied. Laser pyrolysis experiments benefit... 

In order to proper, characterize the macroscopic properties of the fibrous structure the effective properties of the nanofiber on microscale must be prior determined. The effective properties of the nanofiber can be determined by homogenization procedure using representative volume element (RVE). A concentric composite cylinder embedded with a caped carbon nanotube represents RVE as shown by Figure 2. A carbon nanotube with a length 2, radii a is embedded at the center of matrix materials with a radii R and length 2L. 

The remarkable properties of electrospun CNTs nanocomposites continue to draw attention in the development of multifunctional properties of nanostructures for many applications.. Multiscale model for calculation macroscopic mechanical properties for fibrous sheet is developed. Effective properties of the fiber at microscale determined by homogenization using modified shear-lag model, while on the second stage the point-bonded stochastic fibrous network at macroscale replaced by multilevel finite beam element net. Elastic modulus and Poisson s ratio dependence on CNT volume concentration are calculated. Effective properties fibrous sheet as random stochastic network determined numerically. We conclude that an addition of CNTs into the polymer solution results in significant improvement of rheological and structural properties. 

---