Microfluidic Flow Visualization

Device characterization Flow visualization Inertial microfluidics Microparticle focusing Microparticle separatitm... 

To be able to design devices based on microfluidics and nanofluidics, it is crucial to quantitatively visualize the flow of fluids in the microfluidic and nanofluidic channels. There have been many flow visualization methods being developed for macroscale fluid flow (e.g., hot-wire anemometry), but most of them are not suitable for micro- and nanoscale measurements because they are too intrusive for micro- and nanoscale fluid flows [3]. Fluorescence measurements are very suitable for quantitatively visualizing flow in micro- and nanoscales, because it is nonintrusive and it allows for measurements with a high spatial resolution. 

Sinton D (2004) Microscale flow visualization. Microfluid Nanofluid 1(1) 2—21... 

Fluorescence measurements are very important in studies on microfluidics and nanofluidics, with main applications on flow visualization and single-molecule detection. To achieve measurements with higher spatial resolution, which becomes more significant with the rapid development of nanofiuidics, research efforts should be focused on developing more advanced fluorescence microscopy setups. The particularly useful setups will be the ones that can break the classic optical diffraction limit. 

Flow visualization is a branch of fluid mechanics that provides visual perception of the dynamic behavior of fluids flows. The fundamental principle of any flow visualization technique lies in the detection of fluid transport by altering the fluid properties while leaving the fluid motion unaltered. Microscale flow visualization focuses on imaging microfluidic flows, with the most common techniques broadly classified into particle-based and scalar-based methods. 

Microscale flow visualization has played a key role in the development of the microfluidics and LOC fields. It is central to fundamental understanding of microflows, developing novel microfluidic processes, investigating nonideal and nonlinear behaviors, and providing data for numerical simulations. Many microfluidic applications such as mixing, pumping, and filtering require flow visualization to characterize device efficiency. To date, most microscale flow... 

As mentioned earlier, most microscale flow visualization techniques have focused on determination of flow velocity in liquids (or velocimetry). Fluorescence-based techniques, e.g., LIF and PF, have been extensively used for this application. However, in this section, we will look at another microfluidic application for which the development of microscale flow visualization techniques have been critical-micromixing. 

The field of microscale flow visualization has been receiving wide attention in the recent years due to developments in microfluidics and LOCs. Most of the developed techniques are capable of high spatial and velocity resolution, with p-PIV and fluorescence imaging being the most widely used. 

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]. 

Velocimetry is the measurement of fluid velocity. In the context of microfluidics and nanofluidics, velocimetry involves the determination of the velocity field in small-scale internal flows. Most commonly, velocimetry involves optical tracking of a fluid marker. In such cases, the terms flow visualization and velocimetry are used interchangeably. A variety of velocimetry methods have been developed for small-scale flows. Visualization-based methods can be divided into particle-based techniques such as microparticle image velocimetry and scalar-based techniques such as molecular tagging. Nonvisualization-based velocimetry methods have also been developed such as electrochemical velocimetry, where fluid velocity is determined via generation of a redox species. 

In the context of microfluidics and nanofluidics, visualization implies the determination of fluid velocity in small-scale internal flows. Visualization requires an optically detectable fluid marker that does not alter the local fluid velocity of interest. A variety of visualization methods have been developed for small-scale flows, many of which were derived from methods originally developed for macroscale flows. Molecular tagging describes one class of methods that involve molecules being rendered optically differentiable from the bulk fluid to serve as optically detectable fluid markers. The molecular dimensions of these fluid markers make them well suited to small-scale flows. Molecular tagging is commonly achieved by select exposure to light, although the specific photochemical mechanisms vary over the four techniques described here. 

Several variations of microflow visualization have been developed for microfluidic applications such as particle-based flow velocimetry and scalar-based flow velocimetry [5]. In terms of the zeta potential measurement, these visualization techniques such as micro-PlV are used to measure the velocity profile and flow rate under electroosmotic flow. Once the velocity is known, the zeta potential can be calculated from Eq. 6. The main advantage of using a flow visualization technique is that the electroosmotic velocity can be measured directly and in real time. In general, the small amount of particles or dye used has a negligible effect on the electroosmotic flow being measured. The oifly significant disadvantage of this technique is that the extent and cost of the hardware may be prohibitive. 

Transport phenomena on the microscale have gained particular importance due to an increasing demand for more efficient and sustainable processes. Especially the bridge between nano- and micro technologies requires a deep understanding of multiscale coherences. Advances in microfluidic and nanofluidic technologies have been paralleled by advances in methods for direct optical measurement of transport phenomena on these scales. A variety of methods for microscale flow visualization have appeared and evolved since the late 1990s. These methods and their applications to date are reviewed here in detail, and in the context of the fundamental phenomena that they exploit and the fundamental phenomena that they are applied to measure. 

This technique for calculating the out-of-plane component was recently applied to microfluidic flows [4, 6]. With the knowledge of the third velocity component iv it is possible to visualize the 3D structure in the entrance region of a T-shaped micromixer, as shown in Figure 4.6. The out-of-plane component w is unequal to zero. 

D. Sinton, Microscale flow visualization. Microfluids Nanojluids, 2004, 3, 2-21. 

To be able to design devices based on microfluidics and nanofluidics, it is crucial to quantitatively visualize the flow of fluids in the microfluidic and nanofluidic channels. There have been many flow visualization methods being... 

Plug Flow Reactor. Since plug flow can be visualized as a flow of small batch reactors passing in succession through the vessel, macro- and microfluids act... 

Fig. 16 Microfluidic genetic analysis (MGA) system, (a) Dyes are placed in the channels for visualization Scale bar. 10 mm). Domains for DNA extraction yellow), PCR amplification red), injection green), and separation blue) are connected through a network of channels and vias. SPE reservoirs are labeled for sample inlet ST), sidearm ( 4), and extraction of waste (EW). Injection reservoirs are labeled for the PCR reservoir PR), marker reservoir (MR), and sample waste (5W). Electrophoresis reservoirs are labeled for the buffer reservoir (BR) and buffer waste (BW). Additional domains patterned onto the device included the temperature reference TR) chamber and fluorescence alignment (FA) channel. The flow control region is outlined by a dashed box. Device dimensions are 30.0 x 63.5 mm with a total solution volume < 10 pL Scale bar. 10 mm), (b) Flow control region. Valves are shown as open rectangles. VI separates the SPE and PCR domains. V2 and V5 are inlet valves for the pumping injection, V3 is the diaphragm valve, and V4 is an outlet valve, (c) Device loaded into the manifold, (d) Intersection between SI and SA inlet channels, with the EW channel tapering to increase flow resistance Scale bar. 1 mm).

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