Author: Rabia Rana Atlan, University of Ankara, Department of Biomedical Engineering
Macro/Micro-channel applications have been researched for many years by their advantages in improving mass and heat transfer. After the development of microsystems which was predicated on macro systems, the flow and heat transfer techniques in channels are getting more and more understood. Nowadays, microchannels are widely used in the automotive industry, the cooling of microprocessors and especially in biomedical systems. The use of mini and micro terms for channels has made the classification of channels necessary according to their diameters.
The first classification was made in 2000 thereby, the hydraulic diameter channel from 1 to 100 μm was named as microchannel . Today, it was accepted that microchannels have characteristic dimensions with a hydraulic diameter below 1 mm in microtechnology.
There are several materials to be fabricated such as molding, micromachining, etc. Regardless of fabrication techniques, microchannels offer some opportunities which are benefited from their high surface-to-volume ratio. For instance, MEMS (Micro-Electro-Mechanical Systems) devices are used for biological and chemical analysis in microscale. High-precise match with the scale of biological structure and also the possibility for placing several functions in a chemical process of MEMS are considered as essential advantages of microscale chips . Within this scope, this review focuses on flow characteristics in microchannels.
In fluid mechanics, some flows are quite turbulent, while others are smooth and regular. The regular movement of flow which is specified by smooth flow lines is called laminar flow. The motion of high viscous fluids at low speeds is generally laminar. On the other hand, a high degree of irregular fluid motion at high speeds is defined as turbulent. Since the transition from laminar flow the turbulent flow does not occur suddenly, between these flow regimes there is another type of fluid characteristic which is called transient.
Reynolds Number & Laminar Flow Patterning in Microchannels
Using laminar flow pattern is a novel method in microchannels that allows two or more fluids (e.g. samples, analytes, biological fluids, solutions, etc.) to move side-by-side in a channel without interference [3, 4]. This terminology is applied in microfluidic systems as a result of their microscale dimensions and correspondingly low Reynolds number flow.
Osborne Reynolds, the British scientist, found that all flow regimes depend on surface roughness, geometries, the flow velocity, and the type of material in addition to other factors.
After detailed experiments, he defined a non-dimensional number which is called Reynolds number as the ratio of inertial forces to viscous forces in a significant flow configuration :
ρ is the density of the fluid
u is the velocity of the fluid
L is the typical length scale
μ is the dynamic viscosity of the fluid
ν is the kinematic viscosity of the fluid
Examples in Microfluidics Utilizing Laminar Flow
Micro-fuel cells are being considered as an alternative power source at microscale range. The two flows (which are oxidant and fuel streams) are distinguished in a single microchannel.Laminar flow allows a controllable diffusion region for oxidant and fuel streams in micro-fuel cells . Microfluidics is also used for the diagnostic device in chemical processes e.g. measurement of diffusion coefficients. In these devices (T-cells), two streams are introduced from the input. Then, the flows are run by laminar flow, it means that transportation only occurs via diffusion. This feature helps to find diffusion coefficients of materials .
Laminar flow is mostly utilized in lab-on-a-chip devices. It exploits of easily generated laminar flows to move fluids and to deliver them for patterning. Patterning ability of growth medium is the unique feature which provides to modeling delicate structures of mammalian cells .Conclusion
To sum up, several investigations have been undertaken in the current literature to better understand fluid flow in microfluidics and how to be utilized. Furthermore, these researches expanded with macro and microscale studies. As opposed to macro systems, microscale ones provide much more fit to the purpose of fluidic systems such as fuel cells, lab-on-a-chip devices, etc.
The flow regimes are specified by Reynolds number which depends on density, velocity, and viscosity of the fluid. Turbulent flow is a type of fluid regimes in which the fluid forms irregular fluctuations. In microfluidic systems, the dominant fluid regime is laminar flow which moves in smooth layers or path. It just offers diffusion transportation between two fluids, otherwise given two different streams cannot be mixed through the microchannels. These features will continue to open new research fields in science.
 Mehendale S., et al., (2000), doi: 10.1115/1.3097347
 Sharp, K.V., et al., (2019), ISBN: 9780849321061
 Takayama, S., et al.,(1999), 10.1073/pnas.96.10.5545.
 Jiang, X., et al., (2014), ISBN: 9780123983701
 Launder, B.E., (2011), ISBN: 978-0-521-19868-4
 Tanveer, M. and K.Kim, (2018), doi: 10.3390/mi9100479
 Kamholz, A.E. and P.Yager, (2001), doi: 10.1016/S0006-3495(01)76003-1