MEMS-Nozzle

By feed of a fluid through a nozzle array, which is a plate with many tiny holes, so-called micro-plume injection into a micro channel can be achieved [51, 147]. Typically, the micro channel s floor is perforated in a section in this way and a closed-channel fraction follows for completion of mixing. Large specific interfaces can in principle be achieved depending on the nozzle diameter. This mixing concept benefits from conceptual simplicity and fits well to existing MEMS techniques. Furthermore, it consumes less footprint area and therefore does not create much dead space, which is one of the prime requirements during pTAS developments. 

The transfer of this principle also benefits from the characteristic conditions that count for microfluidic systems. By using MEMS technologies the geometry of the steam nozzles can be reduced drastically without losing relative accuracy. Thus, the overall dimensions of the pump and also the amount of steam that is necessary for operation is reduced. Another advantage of a micro-diffusion pump is that the capillary forces overbalance gravity forces, which are decisive in the macroworld. Hence it is possible to construct a pump that can be operated orientation-independent. 

The shell may be of metal (steel, alloy, or non-ferrous), plastic, wood or some combination which may require the addition of liners or inner layers of rubber, plastic or brick. The mechanical problems of attaching inner nozzles, supports and brick require considerable attention that is not an integral part of sizing the equipment. Figures 9-2A-C show a typical large steel brick-lined-mem-brane lined tower with corbeled brick support locations. In these towers, temperature and/or corrosive conditions usually dictate the internal lining, and the selection of the proper acid- (or alkali-) proof cements. 

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. 

Wang MR, Li ZX (2004) Numerical simulations on performance of MEMS-based nozzles at moderate or low temperatures. Microfluid Nanofluid l(l) 62-70... 

A photograph of the NASA/GSFC MEMS-based microthruster etched into a silicon wafer and placed on top of a US penny for purposes of scale. (Bottom) A scanning electron microscopy (SEM) image of the NASA/GSFC microthruster with the converging-diverging supersonic micro-nozzle outlined for clarity (See Hitt et al. [2] for details)... 

Supersonic Micro-nozzles, Table 1 Design parameters for the NASA/GSFC MEMS-based monopropellant supersonic micro-nozzle... 

Perhaps the most ubiquitous example of such systems is the air bag deployment mechanisms incorporated into most modern automobiles. These systems use microaccelerometers to tri er the air bag release at the appropriate time. Another common MEMS device is the printhead used in ink-jet printers, which includes the pump system and the nozzles. 

Baek et al. [5] have adopted two concepts to improve the nozzle density in the top-shooter design. First, the heater was placed on the sidewall of the ink inlet (see Fig. 2) and thus, the area taken by the heater could be minimized. Second, the manifold, the inlet, pressure chamber, and the nozzle were arranged in a line vertically with respect to the substrate. The nozzle density was improved in this new design although it was not easy to fabricate using MEMS technology. 

The thermal inkjet printhead is perhaps one of the most successful microelectromechanical systems (MEMS) products to date. The inkjet printhead uses microheaters (about 20 X 20 xm to 40 x 40 xm in size) under a train of electrical pulses of several microseconds (ps) to periodically generate microbubbles. The microbubble will expel a small droplet of ink through a nozzle at a high frequency to a specified position on a paper to cortpose the text and graphics. The quality of the printing depends on the thermal properties of the ink, the size of the droplets and... 

The experimental data agreed best with a model of an unsymmetrical conformation, Ci, of the Irve-mem-bered ring with the C(4) and S(3) atoms displaced from the AsS(l)C(5) plane in different directions by +0.455 and -0.454 A, respectively, but a C2 model could not be ruled out completely (see igures). The nozzle temperature was reported to be 453 °C, but it was probably 453 K. 

One of the most commercially successful MEMS devices has been the thermal inkjet. In this device a small resistor heats an ink to create a vapor bubble. As the vapor bubble expands it displaces a drop of ink out of a nozzle. We will consider the design of a thermal inkjet device in Chapter 6 on microfluidics, but here we consider the heater that generates the vapor bubble. The typical propellant in inkjet inks is water, which is superheated to approximately 330°C in a few microseconds. If we consider a polysilicon heater that is 25 pm on a side on top of oxide that is 2 pm thick, the thermal conductance of the oxide would be... 

Chen Y.C., B.Y. Shew, S.C. Tsaig and C.L. Kuo, 2003, A New Approach to Fabricate Nozzle Plates by Integration of OGA and MEDM Technology, Taiwan 7th Nano Mems Conf., pp253-256. 

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