Micro structures description

Steels and other structural transition-metal alloys are hardened by various extrinsic factors. The compositions and internal micro-structures of these materials are very complex. Therefore, simple descriptions and/or interpretations of their behaviors cannot be given, so they will not be discussed here. 

Micro structured reactors will not only increase reactor compactness and reduce the size of sometimes expensive samples but will also allow a thorough fluidic description of the flow in the reactor. The mechanism, which assists here, is the laminar-flow regime, which develops owing to the small reactor dimensions. 

The reaction is also influenced by the heat of reaction that develops during the conversion of the reactants, a problem in tubular screening reactors. In micro structures, the heat transport through the walls of the channels is facilitated owing to their small dimensions. The catalysts are deposited on the walls of these micro structures and will thus have the appropriate environment for exothermic reactions by enabling fast quenching of the reaction with near isothermal conditions. Hence also the heat and mass balance in the reactor will be decoupled, which permits the analytical description of the flow in the screening reactor. 

In the remainder of this section, I discuss the significance of these future trends in fluid mechanics and transport phenomena, beginning with the evolution of new theoretical tools and concluding with a more detailed description of future objectives, challenges, and opportunities for fluid mechanics research at the microscale. Some of the latter discussion is drawn from a more comprehensive report, The Mechanics of Fluids with Micro-structure written in 1986 by Professor R. A. Brown (MIT) and myself (with substantial input from the chemical engineering research community) as part of a general NSF-sponsored study of The Future of Fluid Mechanics Research. 

The theory starts with a description of the micro structure by 11 parameters. The sediment grains are characterized by their grain density (p ) and bulk modulus (Kp, the pore fluid by its density (p, ), bulk modulus (K, ) and viscosity (r ). The porosity (([)) quantifies the amount of pore space. Its shape and distribution are specified by the permeability (k), a pore size parameter a=dy3 (j)/(l-(j)), d = mean grain size (Hovem and Ingram 1979 Courtney and Mayer 1993), and stracture factor a (0 < r < 1) indicating... 

In summary, while the relatively simple continuum models may provide a good explanation of the overall mechanical characteristics of living cells, a fuller description of subcellular deformations and interactions within the cell requires more complex microstructure-based models. In spite of many foreseen challenges, integration of continuum and micro-structural approaches into a hybrid method would allow the development of comprehensive mechanical models not only for the whole cell but also for the subcellular regions and intracellular components. Finally, it is important to point out that proper choice of theoretical models to interpret the experimental data is crucial as it can strongly affect and influence the derived cell mechanical properties. This of course requires the researcher to be fully aware of the limitations of theoretical models. 

The article point outs overhead pores is the leading reasons for loess collapsibihty, through the processing and analysis of the pictures of natural loess and Silicified loess, quantify description of the micro-stmcture characteristics of collapsible loess. Although natural loess and Silicified loess are both granular overhead structure, silicified loess is cemented micro-structure, thereby it can eliminate collapsibility. 

Abstract The polymer electrolyte fuel cell (PEFC) consists of disparate porous media microstructures, e.g. catalyst layer, microporous layer, gas diffusion layer, as the key components for achieving the desired performance attributes. The microstmcture-transport interactions are of paramount importance to the performance and durability of the PEFC. In this chapter, a systematic description of the stochastic micro structure reconstmction techniques along with the numerical methods to estimate effective transport properties and to study the influence of the porous structures on the underlying transport behavior is presented. 

Standard multi-scale methodologies as presented in this review Chapter have developed significantly over the last 15 years. The method is able to describe the large deformation response of inelastic media with complex micro-structure, but certain key limitations exist. One such limitation is the description of shells or plates with complex micro-structure which cannot be captured in a layered-wise composite shell approach. Clearly a large munber of fiber-reinforced composite products fall into this category. This limitation exists as conventional first-order approximations can t pass second-order information, such as macroscopic deformation gradients (e.g. in bending), to the RVE boundaries. This information is required in shell theory. 

Despite recent promising strategies, the principle of micro-process engineering is still not widely used in combinatorial catalysis. One drawback certainly is the increasing distance from industrial applications with decreasing dimensions. However, the small structures possess laminar flow conditions that are fully accessible by analytical as well as numerical macroscopic descriptions. This offers the chance to describe thoroughly the fluidic, diffusive and reactive phenomena in catalysis to find intrinsic kinetics on using, for example, non-porous sputtered catalysts. 

Computational fluid dynamics enables us to investigate the time-dependent behavior of what happens inside a reactor with spatial resolution from the micro to the reactor scale. That is to say, CFD in itself allows a multi-scale description of chemical reactors. To this end, for single-phase flow, the space resolution of the CFD model should go down to the scales of the smallest dissipative eddies (Kolmogorov scales) (Pope, 2000), which is inversely proportional to Re-3/4 and of the orders of magnitude of microns to millimeters for typical reactors. On such scales, the Navier-Stokes (NS) equations can be expected to apply directly to predict the hydrodynamics of well-defined system, resolving all the meso-scale structures. That is the merit of the so-called DNS. 

In fact, extremum tendencies expressing the dominant mechanisms in systems like turbulent pipe flow (Li et al, 1999), gas-liquid-solid flow (Liu et al, 2001), granular flow, emulsions, foam drainages, and multiphase micro-/nanoflows also follow similar scenarios of compromising as in gas-solid and gas-liquid systems (Ge et al., 2007), and therefore, stability conditions established on this basis also lead to reasonable descriptions of the meso-scale structures in these systems. We believe that such an EMMS-based methodology accords with the structure of the problems being solved, and hence realize the similarity of the structures between the physical model and the problems. That is the fundamental reason why the EMMS-based multi-scale CFD improves the... 

Abstract A simplified quintuple model for the description of freezing and thawing processes in gas and liquid saturated porous materials is investigated by using a continuum mechanical approach based on the Theory of Porous Media (TPM). The porous solid consists of two phases, namely a granular or structured porous matrix and an ice phase. The liquid phase is divided in bulk water in the macro pores and gel water in the micro pores. In contrast to the bulk water the gel water is substantially affected by the surface of the solid. This phenomenon is already apparent by the fact that this water is frozen by homogeneous nucleation. 

Abstract. Surface pressure/area isotherms of monolayers of micro- and nanoparticles at fluid/liquid interfaces can be used to obtain information about particle properties (dimensions, interfacial contact angles), the structure of interfacial particle layers, interparticle interactions as well as relaxation processes within layers. Such information is important for understanding the stabilisation/destabilisation effects of particles for emulsions and foams. For a correct description of II-A isotherms of nanoparticle monolayers, the significant differences in particle size and solvent molecule size should be taken into account. The corresponding equations are derived by using the thermodynamic model of a two-dimensional solution. The equations not only provide satisfactory agreement with experimental data for the surface pressure of monolayers in a wide range of particle sizes from 75 pm to 7.5 nm, but also predict the areas per particle and per solvent molecule close to the experimental values. Similar equations can also be applied to protein molecule monolayers at liquid interfaces. 

We note from all this prior work that structure at the macro, micro, nano and molecular levels of organisation will all be important. Secondly, the properties of the composite food product will not be related simply to a list of its components (the recipe), since different structural forms can be assembled from the same components by different processes. Emphasis on structure and its origin discriminates food materials science from the former descriptive approach of formulation/process empiricism embodied in most recipes. 

---