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Publication Title | Microfluid Nanofluid (2010) 8:709–726 DOI 10.1007/s10404-010-0588-1

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Microfluid Nanofluid (2010) 8:709–726 DOI 10.1007/s10404-010-0588-1

REVIEW

Advances and applications on microfluidic velocimetry techniques

Stuart J. Williams • Choongbae Park • Steven T. Wereley

Received: 26 June 2009 / Accepted: 19 February 2010 / Published online: 24 March 2010 Ó Springer-Verlag 2010

Abstract The development and analysis of the perfor- mance of microfluidic components for lab-on-a-chip devices are becoming increasingly important because microfluidic applications are continuing to expand in the fields of biol- ogy, nanotechnology, and manufacturing. Therefore, the characterization of fluid behavior at the scales of micro- and nanometer levels is essential. A variety of microfluidic velocimetry techniques like micron-resolution Particle Image Velocimetry (lPIV), particle-tracking velocimetry (PTV), and others have been developed to characterize such microfluidic systems with spatial resolutions on the order of micrometers or less. This article discusses the fundamentals of established velocimetry techniques as well as the tech- nical applications found in literature.

Keywords Micron-resolution particle image velocimetry (lPIV) Microfluidics Particle tracking velocimetry (PTV) Microchannels

1 Introduction

Microfluidic velocimetry techniques such as micron-reso- lution Particle Image Velocimetry (lPIV) and particle tracking velocimetry (PTV) measure fluid motion in a spatially resolved manner with length scales ranging from 10-4 to 10-7 m. Fluid motion is observed through

S. J. Williams

Department of Mechanical Engineering, University of Louisville, 200 Sackett Hall, Louisville, KY 40292, USA

C. Park S. T. Wereley (&)

Birck Nanotechnology Center, Mechanical Engineering, Purdue University, West Lafayette, IN 47907, USA e-mail: wereley@purdue.edu

examination of tracer particles within the fluid. These par- ticles are either artificially introduced into the fluid or are naturally occurring like red blood cells suspended within blood. Multiple experimental images are acquired and analyzed with particle tracking or spatial correlation methods to obtain the fluid’s velocity from the displacement of the particles. lPIV refers to a microscopic adaptation of established macroscopic Particle Image Velocimetry (PIV) techniques. lPIV is regarded as independent from macro- scopic PIV or PTV methods and has been widely accepted as a reliable microfluidic velocimetry technique.

Both microscale PIV and PTV techniques use tracer particles suspended in fluid, and a digital camera is used to acquire the location of suspended particles over time. PIV uses cross-correlation techniques to determine the particle displacement for an interrogation region within an image pair. High particle densities are usually a characteristic of PIV. PTV typically tracks single particles with nearest neighbor matching to determine particle displacement. In order to avoid particle misidentification, low particle seeding densities are common with PTV techniques. The appeal for high particle seeding density and avoidance of Brownian motion make PIV methods more desirable for nanoscale fluid mechanics.

Extensive reviews of PIV and PTV techniques and applications have been proposed by Adrian (1991, 1996, 2005), Raffel et al. (2007), Sinton (2004), and Lindken et al. (2009). Compared to macroscopic PIV, lPIV has considerably different optical and mechanical configura- tions. Fluorescent imaging is typically used to enhance the signal and overcome diffraction effects due to small par- ticle size (\1 lm). The resolution can be improved to less than one micrometer with the use of a microscope equipped with proper optics. Volume illumination is used in lPIV, compared to light sheet illumination common in

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