Digital holographic microscopy applied to measurement of a flow in a T-shaped micromixer

Abstract

In this paper, we describe measurements of a three-dimensional (3D) flow in a T-shaped micromixer by means of digital holographic microscopy. Imaging tracer particles in a microscopic flow with conventional microscopy is accompanied by a small depth-of-field, which hinders true volumetric flow measurements. In holographic microscopy, the depth of the measurement domain does not have this limitation because any desired image plane can be reconstructed after recording. Our digital holographic microscope (DHM) consists of a conventional in-line recording system with an added magnifying optical element. The measured flow velocity and the calculated vorticity illustrate four streamwise vortices in the micromixer outflow channel. Because the investigated flow is stationary and strongly 3D, the DHM performance (i.e. accuracy and resolution) can be precisely investigated. The obtained Dynamic spatial range and Dynamic velocity range are larger than 20 and 30, respectively. High-speed multipleframe measurements illustrate the capability to simultaneously track about 80 particles in a volumetric measurement domain.

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

“…In a later work, Lee and Kim (2009b) successfully applied the system to measure the secondary flow in curved microchannels with a diameter up to 300 lm. Ooms et al (2009) applied holographic PTV with 2 lm particles to the flow in a T-shaped mixer. They made a thorough analysis of all the parameters for setting up a holographic microscopy experiment.…”

Micro-holographic PIV/PTV Technique

“…A comparable analysis performed in Ref. [5] reports the effect of virtual-image speckle noise on the signal-to-noise ratio ignoring the effect of the out-of-focus real particle images. The maximum number of particles that can be recorded on a digital sensor is…”

“…In practice, this limits the NA to a value of a few hundredths (if Dx ¼ 6.45 mm and l ¼ 0.6328 mm , NA ¼ 0.049). This leads to a large particle image depth of focus and consequently to a relatively large inaccuracy of measured longitudinal particle positions [5]. This problem could be overcome if cameras were available with pixel spacing of the order of the illumination wavelength (such cameras are not available and microscope objective offers an alternative solution).…”

“…The hologram is magnified by a microscope objective and imaged onto a CCD sensor. The intensity of hologram on focal plane of microscope objective, I h,focal (x, y), and on CCD sensor, I h,CCD (x, y), is related as [5] …”

A comparative analysis of the uncertainty of astigmatism-μPTV, stereo-μPIV, and μPIV

“…Thus, their applicability, especially for the tomographic methods, to micro-fluidic devices seems to be quite limited, and alternative imaging approaches are necessary. Recently, in-line holography (Lee and Kim 2009;Ooms et al 2009) was applied to 3D velocity field measurements in microscopic channels. However, the numerical reconstruction process is rather time consuming, and the optical setup has to be built with great care to have an acceptable accuracy of the out-of-plane velocity.…”