Micro basic concepts

For applications in the field of micro reaction engineering, the conclusion may be drawn that the Navier-Stokes equation and other continuum models are valid in many cases, as Knudsen numbers greater than 10 are rarely obtained. However, it might be necessary to use slip boimdaty conditions. The first theoretical investigations on slip flow of gases were carried out in the 19th century by Maxwell and von Smoluchowski. The basic concept relies on a so-called slip length L, which relates the local shear strain to the relative flow velocity at the wall ... 

This chapter introduces the basic concepts that are needed to understand the quantum phenomena observed in molecular nanomagnets. All tunneling studies presented here were performed by magnetization measurements on single crystals using an array of micro-SQUIDs [4]. 

The basic concept of a H2 PSA process is relatively simple. The impurities from the H2-containing feed gas mixture are selectively adsorbed on a micro- and meso-porous solid adsorbent (zeolites, activated carbons, silica and alumina gels) at a relatively high pressure by contacting the feed gas with the solid in a packed column of... 

We now introduce some recently reported and interesting research that reinforces the basic concepts of LLE. Cantwell and eo-workers have introduced the concept of a true LLE that has been downsized to a micro extraction scale (17). In the past, the eoneept of a miero-LLE, abbreviated /xLLE, as introdueed by the EPA and promulgated through their 500 series of methods, was designed to conduct TEQA on samples from sources of drinking water. Method 508 required that 35 mL of groundwater or tap water be placed in a 40 mL vial and extraeted with exaetly 2 mL of hexane. Organochlorine pesticides such as aldrin, alachlor, dieldrin, heptachlor, and so forth are easily partitioned into the hexane. A l-/xL aliquot is then injected either manually or via autosampler into a GC-ECD to achieve... 

Valves are one of the basic elements in a micro-TAS concept and should be designed with the operation of a chemical system in mind. Therefore, system aspects such as minimal dead volume, leakage and siniplicity play a m or role in the design of the valves. The valves can woilr individually or two valves can be coupled together wnth a membrane to form a pump. Two valves can also be placed next to each other with one common outlet channel and in this way form a switching valve as shown in figure 2. 

The constant growth of entropy postulated in the second law is closely related to the following principle By the irreversible process the system is spontaneously directed towards the most frequently occurring set of micro states which corresponds to the same macro state of the system. The macroscopic state of equilibrium - where entropy is at its maximum - is the state which is linked to the largest number of different micro states. This basic concept is the basis of the Boltzmann relation that defines the entropy function S in statistical thermodynamics. 

With these basic concepts in mind, the chapters in this book focus on biomaterials consisting of different materials such as metallic, ceramic, polymeric, and composite. The impact of recent advances in the area of nano- and micro technology on biomaterial design is highlighted in Chapter 46. 

Despite this much-discussed theoretical background, the number of concrete studies comparing historical ideas and students conceptions is fairly low in chemistiy education. Furthermore, most studies cited deal with isolated topics. Systematic overviews concerning basic ideas like micro-macro thinking, chemical reaction, the particulate nature of matter, energy conversions etc. are mostly not available, except for van Driel et al. (1998) and the case of the chemical equilibrium. 

The long-term goal in the science of thermochemical conversion of a solid fuel is to develop comprehensive computer codes, herein referred to as a bed model or CFSD (computational fluid-solid dynamics). Firstly, this CFSD code must be able to simulate basic conversion concepts, with respect to the mode, movement, composition and configuration of the fuel bed. The conversion concept has a great effect on the behaviour of the thermochemical conversion process variables, such as the molecular composition and mass flow of conversion gas. Secondly, the bed model must also consider the fuel-bed structure on both micro- and macro-scale. This classification refers to three structures, namely interstitial gas phase, intraparticle gas phase, and intraparticle solid phase. Commonly, a packed bed is referred to as a two-phase system. 

The original Danckwerts model has later been extended to relate the renewal rate s to many flow parameters, and to account for the existence of the micro scale flow of the fluid within the individual eddies. Further modifications relate to the fact that not all the penetrating eddies reach the whole way to the interface. Many model extensions have thus been developed based on the basic surface-renewal concept. A review of these models is given by Sideman and Pinczewski [135]. The various extensions are motivated by the inherent assumptions regarding, the governing equations, the boundary conditions, the age distribution function and/or the mean contact time. 

In the course of this development, avenues for micro-pattern production based on microelectronics opened, perhaps not in detail, but in die basic underlying concepts. 

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