Hydrodynamic microfluidic systems

Miniaturisation of various devices and systems has become a popular trend in many areas of modern nanotechnology such as microelectronics, optics, etc. In particular, this is very important in creating chemical or electrochemical sensors where the amount of sample required for the analysis is a critical parameter and must be minimized. In this work we will focus on a micrometric channel flow system. We will call such miniaturised flow cells microfluidic systems , i.e. cells with one or more dimensions being of the order of a few microns. Such microfluidic channels have kinetic and analytical properties which can be finely tuned as a function of the hydrodynamic flow. However, presently, there is no simple and direct method to monitor the corresponding flows in. situ. 

The measuring spot density on several subsequently commercialized biosensors has greatly increased, allowing arrays to be probed and generating parallel interaction data. One recent development involves microfluidics systems with hydrodynamic addressing (HA) of the solutions (Fig. 19). By the... 

Recent developments in microelectrome-chanical systems (MEMS) have enabled the integration and fabrication of numerous micro components such as pumps, valves, and nozzles into complex high-speed microfluidic machines. These systems possess geometrical dimensions in the range 1-1,000 pm, which are 10-10 -10-10" times less than conventimial machines, and operate at liquid flow speeds up to 300 m/s. It has been confirmed that microfluidic systems, like their large-scale counterparts, are susceptible to the deleterious effects of cavitation when appropriate hydrodynamic conditions develop. Cavitation damage in micro-orifices has been reported by Mishra and Peles [2], Small pits on the silicon surface have been detected after only 7-8 h of operation under cavitating flow. 

Utilizing the interplay of interface generated forces with hydrodynamic forces and geometrical constrictions even complex tasks like closed-loop control cycles can be implemented in microfluidic systems. These opportunities have been applied for the development of a smart operation unit for droplet merging with automatically outbalancing of timing errors for the droplets arrival. 

Fig. 2 Due to low Re numbers in microfluidic systems, it is possible to accomplish (a) controlled and predictable diffusion, (b) laminar flows, and (c) hydrodynamic focusing of streams of fluids...

Piezoelectric Actuation in Multiphase Microfluidics, Fig. 7 (a) A microscope image of a lateral cavity acoustic pump with the main channel filled with DI water and polystyrene beads, (b) The pumping pressure generated by a 1-, 5-, and 10-cavity pair device with a maximum input voltage of 25 Vpp. The pressure output is calculated using the measured average flow velocity and the calculated hydrodynamic resistance of the microfluidic system. 

Brevig T, Kruhne U, Kahn RA, Ahl T, Beyer M, Pedersen LH (2003) Hydrodynamic guiding for addressing subsets of immobilized cells and molecules in microfluidic systems. BMC Biotechnol 3(10)... 

Disadvantages of conventional microchips include resistance to hydrodynamic flow (backpressure), adsorption of some biomolecules on walls, and limited robustness (e.g., clogging narrow channels). Digital microfluidic systems overcome the problems with mixing reagents, which are normally associated with conventional microfluidic devices that use laminar hydrodynamic flow. Such microscale platforms are normally based on the electrowetting-on-dielectric (EWOD) principle [66]. In these digital microchips. 

Physics mathematics engineering chemistry suspension mechanics hydrodynamics computational fluid dynamics microfluidic systems coating flows multiphase flows viscous flows. 

In centrifugal microfluidic systems, the Navier-Stokes equation is most conveniently expressed within the reference frame where the substrate rotating at a frequency = 27ri> is at rest (Fig. 1). Due to the non-inertial nature of this frame of reference, the centrifugal force density the Euler force density and the Coriolis force density/c additionally appear in the hydrodynamic equation... 

Scaling laws are simple. However, the conclusion based on scaling is not always directly applicable to microsystem. Let us consider an example of mixing in mierosystems. The question is whether mixing time can be reduced in microfluidic system by agitating the fluids. This can be answered using the mixing timescale by convection and diffusion mechanisms. We can write the hydrodynamic transport time as... 

FIGURE 3.33 Geometry and hydrodynamic characteristics of the microfluidic capillary system (CS). (a) top view of a CS. (b) The flow of liquid (arrows) is superimposed on the cross section (not to scale) of the CS. CRV capillary retention valve CP, capillary pump [459]. Reprinted with permission from the American Chemical Society. 

Microfluidics is the manipulation of fluids in channels, with at least two dimensions at the micrometer or submicrometer scale. This is a core technology in a number of miniaturized systems developed for chemical, biological, and medical applications. Both gases and liquids are used in micro-/nanofluidic applications, ° and generally, low-Reynolds-number hydrodynamics is relevant to bioMEMS applications. Typical Reynolds numbers for biofluids flowing in microchannels with linear velocity up to 10 cm/s are less than Therefore, viscous forces dominate the response and the flow remains laminar. 

In laminar flow, the viscous force of the fluid dominates over the inertial force so that in microfluidics, hydrodynamic focusing techniques can be used to increase the sensitivity of the system. Rodriguez-Trujillo et al. [61, 62] used a sheath flow with a lower conductivity than the central sample flow to concentrate the electric field lines into the impedance sensing region. A 3D hydrodynamic focusing scheme was proposed by Scott et al. 

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