Definition of Microfluidics

Chemical reaction and mass transfer are two unique phenomena that help define chemical engineering. Chapter 8 described problems involving chemical reaction and mass transfer in a porous catalyst, and how to model chemical reactors when the flow was well defined, as in a plug-flow reactor. Those models, however, did not account for the complicated flow situations sometimes seen in practice, where flow equations must be solved along with the transport equation. Microfluidics is the chemical analog to microelectro-mechanical systems (MEMS), which are small devices with tiny gears, valves, and pumps. The generally accepted definition of microfluidics is flow in channels of size 1 mm or less, and it is essential to include both distributed flow and mass transfer in such devices. 

A fundamental question initially arises about the definition of microfluidics. The basic meaning of this terminology is the flow at small scales. The primary advantage is the utilization of breakdown phenomena in scaling laws for new effects and better performance. Hence, the importance is not the size of surrounding instrumentation and the material of the device but the space where the fluid is processed. The minimization of the entire system may be beneficial but is not a requirement of a microfluidic system. The key issue of microfluidics is the microscopic quantity of fluid in which small-scale causes change in fluid behavior. 

There are different points of view regarding device size and fluid quantity for the definition of microfluidic device. The microelectromechanical systems (MEMS) terminology indicates that the device size must be smaller than 1 mm. Electrical and mechanical engineers are interested to work on microfluidics because of their fabrication capabilities using microtechnology. Their idea is to shrink the device size and thus define microfluidics in terms of size to take advantage of the new effects and better performance. The objective is to shrink down the pathway of the chemicals. Another preferred way to define microfluidics is based on fluid quantities. Figure 1.1 (a,b) shows fhe size and volume characteristics of different microsystems. 

The definition of secondary and follow-up assays is not straightforward. It really depends on the quality and economics of the assays. For example, functional ion channel assays are mostly used as a second line during the early phase of drug discovery. However, technological advances, such as the introduction of microfluidics and improved detection technologies make them more and more suitable for first-line profiling. 

This review contains examples of microfluidic platforms for lab-on-a-chip applications which were selected as fitting to our platform definition and no comprehensiveness is claimed. The review should, however, provide the reader with some orientation in the field and the ability to select platforms with appropriate characteristics on the basis of application-specific requirements. 

THE NEED FOR THE MICROFLUIDIC PLATFORM APPROACH Definition of a Microfluidic Platform ... 

Segmented Flow Microfluidics Definition of segmented flow microfluidics ... 

Laminar flow is the definitive characteristic of microfluidics. Fluids flowing in channels with dimensions on the order of 50mm and at readily achievable flow speeds are characterized by low Reynolds number. Re, defined as... 

Li and Harrison carried out the first cell assay in microchannels [2]. This seminal work made use of electrokinetically driven flow (electroosmosis and electrophoresis) to transport bacteria, yeast, and mammalian cells in channels and to implement low-volume chemical lysis (cell death). This theme of microfluidics-based cell transport, sorting, and lysis has continued to be a popular application, as well as related work in using microfluidics to culture cells and to pattern them into structures. The utility of these methods is acknowledged (and that they are featured in several good reviews [1] and other entries in the encyclopedia) but focuses here on describing microfluidics-based cell assays that fit the definition described above - application of a stimulus and measurement of a response. 

Extension to 2D and 3D Systems In the majority of microfluidic cases where 1/k is much smaller than the channel height, the Helmholtz-Smoluchowski equation provides a reasonable estimate of the flow velocity at the edge of the double layer field. As such when modeling two- and three-dimensional flow systems, it is common to apply this equation as a slip boundary condition on the bulk flow field. Since beyond the double layer by definition... 

We will briefly introduce examples of applications of microfluidic devices which enable rapid analysis of small volumes of dynamic samples with mass spectrometric detection. Please note that some of the discussed approaches are covered by the broad definition of time-resolved mass spectrometry (TRMS) (cf. Section 1.3), while others lay the foundations for possible TRMS measurements following further technical developments. 

M 14] [P 14] Splitting of droplets can only occur at aspect ratios <0.2 (for definition see Effect of aspect ratio) [29]. Since this is an important microfluidic action, it was worth finding suitable novel mixing strategies to overcome this limitation. 

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