Microfluidics is the science of handling small amounts of liquids, inside micrometer scale channels. On a microfluidic chips, fluids are moved, mixed, or otherwise processed. Such applications use on chip passive or active flow control techniques, such as capillary forces, micro pumps or micro valves.
But a large number of applications relies on external actuation devices, which includes flow control devices to inject and move the fluid, and valves to switch and route the fluids.
Moreover, some measurements methods are often necessary to control flow parameters (i.e. pressure and flow rate). Selecting the best flow control method for your application is very important, since the performance of a microfluidic device depends highly on the flow control system.
FLOW CHARACTERISTICS AT MICROMETER SCALE
Flow behavior strongly differs at microscale from its macroscale behavior as some phenomena, such as gravity, become negligible, while other, such as capillarity, become preponderant.
Some of these new properties are very unintuitive. One of the most interesting is characterized by the Reynolds number, which compares the effect of momentum of a fluid to the effect of viscosity. In a microfluidic device, this number is very low. As a consequence, the flow become laminar, which means that side-by-side flows are not going to mix. They will exchange molecules only by diffusion. This property is often used in microfluidic device, to generate concentration gradients for example.
The drawback is that mixing can be quite difficult. Specific designs are used when mixing of several fluids is needed.
FLOW CONTROL SYSTEMS
Flow control is one of the key parameters for microfluidic experiments. Several techniques allow to put fluids inside the chip in motion.
The simplest one is to use hydrostatic pressure. It has the advantage of suppressing external actuation devices. However, the flow stability for long term experiments is a strong limitation, since the change of level in the inlet and outlet reservoirs cause a change of flow rate. Moreover, this technique is very sensitive to air bubbles.
Then volumetric pumps such as syringe pumps or peristaltic pumps can be used. This technique consists in applying a mechanical movement in order to modify the volume and create a flow rate. The major drawback of this application is the instability in the flow rate created by the moving mechanical parts of the pump.
Pressure control is also widely used for microfluidic flow actuation. The fluid movement is created by a pressure difference, as defined by the following law: Flow rate = Pressure / Resistance, where the resistance depends on the fluidic path geometry and fluid viscosity.
Other methods, such as electro-osmotic flow or integrated micro pumps are also used for flow actuation in microfluidic devices.
FLOW PARAMETERS MEASUREMENT
Sensors are often used to precisely know flow parameters, such as pressure of flow rate, in a specific point of the fluidic path.
Flow rate can be measurement with several different techniques: calorimetric, mechanical, acoustic, electro magnetic or optical measurement. To know more about these techniques, please check our dedicated review.
Pressure can also be measured inside the fluidic path, since pressure drops often occur inside microfluid setup. A variety of technique to measure pressure inside the microfluidic chip has been reported, such as membrane displacement measurement, or air bubbles volume measurement or optical interface tracking. External inline pressure sensors are also available.
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FLOW SWITCHING & ROUTING
For microfluidic applications which involve injecting different liquids, a system for flow switching and routing is necessary. This system can be directly integrated into the chip or be external to the chip.
The most famous on chip valves are Quake valves. They involvs a bilayer PDMS microfluidic chip. Liquid flows inside the bottom layer while the upper layer integrates an air network. When activated, this last can selectively compress and clog channels of the fluidic layer, which enables fluid motion’s control. Other types of on chip valves are described in our dedicated short review.
External valves are also oftenly used, and allow to reduce chips complexity. Among these valves, rotative, 2/2 and 3/2 valves are used. One important requirement in microfluidics is to use low internal volume valves, which are more compatible with microfluidic applications.
You will find hereafter a short list of microfluidic publications about fluid handling. If you wish to add a specific publication to this list, please contact us!
- Lee, C. Y., Chang, C. L., Wang, Y. N., & Fu, L. M. (2011). Microfluidic mixing: a review. International journal of molecular sciences, 12(5), 3263-3287.
- Stone, H. A., Stroock, A. D., & Ajdari, A. (2004). Engineering flows in small devices: microfluidics toward a lab-on-a-chip. Annu. Rev. Fluid Mech., 36, 381-411.
- Pittman, J. L., Henry, C. S., & Gilman, S. D. (2003). Experimental studies of electroosmotic flow dynamics in microfabricated devices during current monitoring experiments. Analytical chemistry, 75(3), 361-370.
- Lien, V., & Vollmer, F. (2007). Microfluidic flow rate detection based on integrated optical fiber cantilever. Lab on a Chip, 7(10), 1352-1356.
- Rasmussen, A., Mavriplis, C., Zaghloul, M. E., Mikulchenko, O., & Mayaram, K. (2001). Simulation and optimization of a microfluidic flow sensor. Sensors and Actuators A: Physical, 88(2), 121-132.
- Unger, M. A., Chou, H. P., Thorsen, T., Scherer, A., & Quake, S. R. (2000). Monolithic microfabricated valves and pumps by multilayer soft lithography.Science, 288(5463), 113-116.