JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 14, NO. 5, OCTOBER 2005 987 Size Scale Effects on Cavitating Flows Through Microorifices Entrenched in Rectangular Microchannels Chandan Mishra and Yoav Peles Abstract—Cavitating flow of deionized water through various microorifices and microchannels has been investigated. Multifar- ious cavitating flow patterns, including incipient, choking and su- percavitation have been detected. Effects of microorifice and mi- crochannel size on cavitation have been discussed and results indi- cate the existence of strong size scale effects. Incipient and choking cavitation numbers are observed to increase with the area ratio between the microorifice and the microchannel while the orifice discharge coefficient plummets once cavitation activity erupts. Ad- ditionally, for a fixed microchannel width, the incipient and the choking cavitation numbers rise with the ratio between the hy- draulic diameters of the microorifice and the microchannel. In ad- dition, velocity and pressure effects on cavitation have been in- vestigated for several microorifices and the observed trends have been compared with established macroscale results. Furthermore, the flow patterns encountered at choking and supercavitation are significantly influenced by the microorifice and microchannel size. Flow rate choking occurs irrespective of the inlet pressures and is a direct consequence of cavitation inside the microorifice. The pre- dicted choked cavitation number is always higher than the exper- imental data. This discrepancy is suspected to be the result of ex- ceedingly small residence time for nuclei growth and the ability of the liquid to withstand low pressures at such scales. Flow and cav- itation hysteresis is observed but its effects are more pronounced for the smallest microorifice. [1404] Index Terms—Cavitation, choking, microorifice, microchannel, microfluidics, size scale effects, supercavitation. I. INTRODUCTION I N the last two decades, microelectromechanical systems (MEMS) research has received tremendous attention and registered prodigious growth. Rapid advancement in mi- crofabrication technologies coupled with the drive toward miniaturization of existing systems has resulted in the devel- opment of innovative MEMS and microfluidic systems for use in novel applications spread across diverse technological disciplines. A plethora of innovative microfluidic devices and systems have emerged namely microrockets [1], microcoolers [2], microrefrigerators [3], micromixers [4], drug delivery Manuscript received August 18, 2004; revised January 11, 2005. This work was partially supported by the National Science Foundation (NSF) (Program Officer: Dr. Triantafillos J. Mountziaris) under Contract 0520604. The micro- fabrication was performed at the Cornell NanoScale Facility (a member of the National Nanotechnology Infrastructure Network) which is supported by the National Science Foundation by Grant ECS-0335765, its users, Cornell Univer- sity, and industrial affiliates. Subject Editor S. M. Spearing. The authors are with the Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180 USA (e-mail: pelesy@rpi.edu). Digital Object Identifier 10.1109/JMEMS.2005.851800 systems [5], micropower systems including launch vehicles and high density power sources, electronic chip cooling systems, chemical microreactors [6], DNA synthesis and bio-MEMS systems [7]–[9]. The importance and utility of microfluidic devices continues to rise, and has led to the development of both passive microfluidic devices and power-MEMS [10] machinery like micro turbopumps etc. Hydrodynamic cavitation has been a cause for concern in the design of conventional scale hydraulic machinery [11]–[14]. The deleterious effects of cavitation on conventional fluid machinery are well documented and have been aggressively re- searched in the last century. Cavitation in hydraulic machinery can limit performance, lower efficiency, modify the hydro- dynamics of the flow, introduce severe structural vibration, generate acoustic noise, choke flow, and cause catastrophic damage [12]–[14]. Research on cavitation has contributed im- mensely toward improving the design of macroscale hydraulic machinery. All liquid handling devices are vulnerable to cavi- tation once apposite hydrodynamic conditions are encountered [12], [15]. Therefore, microfluidic systems like their large-scale counterparts are susceptible to the pernicious effects of cavi- tation. Moreover, cavitation can occur in all liquids including biological fluids. Cavitation has been documented in the in vitro testing of blood-handling devices [16]. Also, Lee et al. [17] ob- served cavitation related damage/ fracture in a millimeter-sized mechanical valve inside an artificial heart. Additionally, MEMS devices like Lab-on-chip systems, which handle biofluids, can also be influenced by cavitation since they contain microvalves [18] and silicon micropumps [19]. Vilkner et al. [20] provide a meticulous review of all Lab-on-chip systems and reports the development of mechanical (moving parts) and nonmechanical (passive) micropumps for use in micro-TAS systems. There is definitely a possibility of cavitation and bubble formation in micropumps and microvalves if apt hydrodynamic conditions develop. Woias [21] indicates that reciprocating micropumps are susceptible to problems with self-priming and bubble tol- erance. Furthermore, evidence of cavitation has been reported in MEMS turbopumps developed specifically for microrocket engines [22]. Clearly, cavitation effects cannot be ignored in the design of MEMS/ microfluidic systems. The pragmatic realization of these neoteric microfluidic systems depends on the establishment of a design framework acknowledging the pernicious effects of hydrodynamic cavitation at the microscale. Cavitation inside microfluidic systems has not received serious consideration previously. The presence of cavitation in flow through a microorifice has been recently identified by 1057-7157/$20.00 © 2005 IEEE