Particle imaging techniques for volumetric three-component (3D3C) velocity measurements in microfluidics

Abstract

The reliable measurement of the 3D3C velocity field in microfluidic devices becomes more and more important for future optimization and developments for lab-on-a-chip applications or point-of-care medical diagnosis systems. In the past years, different particle-based imaging methods, such as confocal scanning microscopy, holography, stereoscopic and tomographic imaging or approaches based on defocused particle images or optical aberrations have been developed and applied successfully to measure velocity fields in microfluidic systems. The benefits and drawbacks of these methods will be discussed in detail as the proper understanding of the measurement principle is essential to select the most appropriate technique for a desired measurement application. Once an imaging method is chosen, the velocity can be estimated by correlation-based methods or tracking approaches. The advantages and disadvantages of both methods and the importance of image preprocessing will also be discussed in detail.

Particle focusing by AC electroosmosis with additional axial flow

“…The basic principle of astigmatism µ-PTV is that due to the anamorphic effect by inserting a cylindrical lens, particle images are deformed into ellipses. This ellipticity is directly related to the particle position normal to the focal plane (Liu et al 2014a;Cierpka et al 2010Cierpka et al , 2011Cierpka and Kähler 2012). A fluorescence microscope with a 20× Zeiss objective lens (numerical aperture of 0.4 and focal length of 7.9 mm) was used.…”

“…1 3 (Liu et al 2014a;Cierpka et al 2010Cierpka et al , 2011Cierpka and Kähler 2012). The vorticity is analyzed as function of the axial background velocity and the results are compared to a numerical model developed before (Liu et al 2014b).…”

Preface: Focus on imaging methods in granular physics

“…[25][26][27][28][29][30] If the volume fraction of the particles is sufficiently low, stereo-camera or volumetric methods can be used to capture three-dimensional (3D) dynamics. [31][32][33] If the particles are optically transparent, there exist two additional options. Where the interstitial liquid can be chosen freely, index matching provides a means to acquire 3D data.…”

“…Where it is not possible to identify individual particles, it is still possible to measure mean displacements (or velocity/deformation fields) using cross correlation techniques like particle image velocimetry (PIV). [31][32][33] These techniques work by subdividing individual frames into small interrogation areas. For these areas, the mean displacement is determined by cross correlation between subsequent frames.…”

Depth position detection for fast moving objects in sealed microchannel utilizing chromatic aberration

“…23 The reflection mirrors were embedded in a polydimethylsiloxane (PDMS) channel to determine the depth position of passing particle utilizing the captured top-view and side-view images by the embedded mirror. 24 This approach greatly eliminated the use of delicate optical components for depth detection.…”