Micro piv

The transition from laminar to turbulent flow in micro-channels with diameters ranging from 50 to 247 pm was studied by Sharp and Adrian (2004). The transition to turbulent flow was studied for liquids of different polarities in glass micro-tubes having diameters between 50 and 247 pm. The onset of transition occurred at the Reynolds number of about 1,800-2,000, as indicated by greater-than-laminar pressure drop and micro-PIV measurements of mean velocity and rms velocity fluctuations at the centerline. 

PIV has become the most popular technique to measure velocity and turbulent properties (Figure 15.1). The movement of seed particles in a millimeter-thick laser sheet is measured by correlating two photos taken a few milliseconds apart. With two cameras, it is also possible to obtain a 3D vector of the velocity in that plane. The method gives, in general, very good resolution of the flow, but it requires optical access. Also, measurement close to walls can be problematic due to light reflections that disturb the measurements. One extension of PIV is the micro-PIV that uses fluorescent tracer particles, which allows all direct light, for example, reflections at the walls, to be filtered out [1]. 

Hagsater, S.M. et al. (2008) A compact viewing configuration for stereoscopic micro-PIV utilizing mm-sized mirrors. Exp. Fluids, 45 (6), 1015-1021. Bernard, P.S. and Wallace, J.M. (2002) Turbulent Flow Analysis, Measurement... 

Such a velocity distribution given by Eq. 31 was verified experimentally by Yan et al. [8] in which a method is proposed to simultaneously determine the zeta potentials of the channel surface and the tracer particles in aqueous solutions. This is achieved by carrying out microscale particle image velocimetry (micro-PIV) measurements of the electrokinetic velocity distributions of tracer particles in both open- and closed-end microchannels under the same water chemistry condition. 

This time scale, Ateff, usually is of the order of about 0(10-10) s depending on channel and reservoir sizes and is much larger than that of start-up EOF which is of the order of 0(Oh p/n) 0(10 ) s. The proposed model was verified experimentally using the micro-PIV technique [9]. 

Typically in fluid mechanics, a wealth of information about any given flow system can be fotmd from the velocity field, which is often visualized and quantified through the use of tracer particles. As mentioned above, micro-PIV and micro-PTV are both well-established tools for extracting quantitative information from microscale fluid systems [3], However, special care must be taken in applying these techniques to very nearwall flows with evanescent wave illumination. Both techniques require imaging the instantaneous positimis of tracer particles seeded in the flow at two different instances in time to infer fluid velocities. 

Kinoshita H, Kaneda S, Fujii T, Oshima M (2007) Three-dimensional measurement and visualization of internal flow of a moving droplet using confocal micro-PIV. Lab Chip 7(3) 338-346... 

In the main, flow visualization with TLCs offers some elementary advantages. It is a nonintmsive measurement technique, the TLCs are easy to handle, the measurement is nearly instantaneous, and the TLCs can also be used as tracers for 2D and 3D micro-PIV to determine the velocity profile. 

The micro-PIV measurement reported in this experiment was carried out with a 4x objective lens. With a CCD sensor size of 6.3 x 4.8 mm, the size of an image pixel is 2.475 pm and the size of the measured area is 1584 x 1188 pm. Fluorescent particles with a diameter of 3 pm were used to trace the flow. A microchannel with the cross section of 910 x 50 pm and the length of 5 mm was used. The liquids used in the experiment were the aqueous NaCl solution (concentration 10 " M) and aqueous glycerol (volume concentration 24 %). The integration area is 32 x 32 pixels. Previous studies showed that the entry length of liquid flow in microchannel was very short [11]. The measurement was taken at 1 mm downstream of the entrance thus stable velocity field was obtained. 

In the following, a method will be introduced for simultaneously determining the zeta potentials of both the microchannel surfaces and the tracer particles by using the microscale particle image velocimetry (micro-PIV) technique. 

Micro-PIV technique is used to measure the steady velocity of tracer particles in an electrolyte in both open- and closed-end microchannels. Under an applied DC electric field, the observed particle velocity, Wp, evaluated from the micro-PIV measurement is the superposition of the electrophoresis velocity of the charged particle, u. , and the electroosmotic velocity of the electrolyte,... 

As the microscope objective is focused on the midplane of the channel (i.e., 7=0) during the micro-PIV experiment, the dimensional electroosmotic velocity at the midplane is expressed as... 

Relationships Between the Micro-PIV-Measured Particle Velocity and the Zeta Potentials of the Channel Surface and the Particles in Open- and Closed-End Channels... 

As indicated by Eq. 4, the particle velocity measured from the micro-PIV technique is a combination of the electrophoresis velocity of the tracer particles which is related to the particle zeta potential, p, and the electroosmotic flow field which is associated with the zeta potential of the channel surface, If micro-PIV experiments are carried out in an electrolyte in open-end and closed-end microchannels, according to Eq. 4, we can write the expressions for the micro-PlV-measured velocity of the tracer particles in open-end and closed-end rectangular microchannels as below ... 

We define the particle mobility, Pp, measured by micro-PIV technique in an electric filed, E, as... 

In principle, Eqs. 25 and 26 show that if the distributions of the particle mobility measured by micro-PIV in the open-and closed-end channels are known, the zeta potentials of both the particles and the channel surface can be determined simultaneously. 

The micro-PIV setup consists of four main components an illumination system, an optical system, a coupled charge device (CCD) camera, and a control system. The control system consists of a peripheral component interface (PCI) card, and its corresponding software is implemented in a personal computer. The computer can control and synchronize all actimis related to illumination and image recording. The schematic of the setup is illustrated in Fig. 2. 

Fig. 2 Schematic of the micro-PIV setup. The PC controls and synchronizes the lasers for illumination, CCD camera for image recording, and high-voltage switch for turning on the high-voltage supply...

In micro-PIV measurement, the major uncertainty is due to the Brownian motion of tracer particles. In particular, the Brownian motion plays an important role when submicron tracer particles are used in PIV experiments with flow velocities of less than 1 mm/s. According to Einstein, the Brownian motion-induced random velocity can be estimated from... 

Micro-PIV-Based Diffusometry, Fig. 1 Comparison of (a) autocorrelation function, (b) cross-correlation function without any Brownian motion, and (c) cross-correlation function with Brownian motion... 

Micro-PIV-Based Diffusometry, Fig. 2 Broadening of the correlation function due to a velocity gradient... 

Finally, measurement and visualization methods are needed in order to analyze and utilize non-Newtonian microfluidic flows. Transparent materials such as glass and PDMS enable a host of optical techniques to be used. Many of these have been used to analyze non-Newtonian flows in microfluidic devices, most notably micro-PIV and related particle imaging techniques, and flow visualization using fluorescent dyes. Pressure taps have also been integrated to measure the non-Newtonian flow response simultaneous to flow visuaUzatimi [6, 9]. 

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