Asymmetric Flow Patterns - Sara Bruni
Background
In general, mixing in a fluid is a process in which one immiscible liquid is incorporated into another immiscible liquid. In microfluidics, the goal is to achieve a thorough and rapid mixing within devices. These mixing schemes can be described as active or passive. Active mixing is characterized by an external energy source to achieve mixing, while passive mixing occurs in the absence of additional energy. Passive mixing uses different microchannel configurations to restructure the flow and induce mixing. Asymmetric flow patterns include different methods of active and passive mixing used to speed up the mixing process at the microscale. In order for mixing to occur, flow patterns must be asymmetric. This section will focus on the different methods of active mixing.
Mixing on the Microfluidic Scale
Microfluidic systems and devices are becoming more common in a variety of applications and are revolutionizing the biomedical, and drug development industries. In standard macroscale systems, mixing is achieved through turbulent flow. However, due to the scale of these microfluidic systems, turbulence is not attainable as a result of the low Reynolds numbers. Reynolds number is the relationship between inertial forces and and viscous forces within a system.
Reynolds number is found using the following equation:
- [math]\displaystyle{ \mathrm{Re} = \frac{\rho v L}{\mu} }[/math]
where,
- [math]\displaystyle{ \rho }[/math] is the density of the fluid
- v is the velocity of the fluid
- L is the characteristic length of the fluid
- [math]\displaystyle{ \mu }[/math] is the dynamic viscosity of the fluid
Laminar Flow
Laminar flow is characterized by a Reynolds number less than 2,000. In this flow pattern, parallel lines flow linearly and there is an absence of mixing. In a microfluidic system, the characteristic length dominates the equation and leads to a low Reynolds number. Flow within microfluidic systems is laminar, and since there is no turbulence in these systems, mixing relies solely on diffusion which is a very slow and inefficient process. The goal of these microfluidic mixing patterns is to promote mixing through different means, like introducing additional forms of energy through active mixing like magnetic stirring or acoustic micro-mixing.
Methods of Active Mixing
Magnetic Stirring
Magnetic stirring is an active mixing method which utilizes a magnetic stir bar and a rotating magnet to conduct mixing. The stir bar is placed into the liquid and its motion is driven by another rotating magnetic in the device beneath the vessel containing the liquid. Magnetic mixers are often seen in laboratories, and in microfluidics, they are on a smaller scale. This method of mixing achieves rapid mixing without additional wires intruding the system. However, problems arise in regards to the stir bar length, as mixing does not extend very far past the end of the stir bar. Designing a stir bar of optimal length or using multiple stir bars can help to solve this problem.
Acoustic Micro-mixing
Acoustic micro-mixing (AMM) is a method of active mixing which uses sound waves to promote mixing and increase the rate of diffusive mixing. Bubble induced acoustic micro-mixing uses an air bubble in the liquid. This bubble is then exposed to vibration through sound waves, which generates frictional forces around the bubble and produces strong liquid circulation flow.
Bubble induced acoustic micro-mixing can significantly reduce mixing times from 6 hours to 1.75 minutes. This application is also useful as it uses a single apparatus and is easy to implement, and it has a relatively low cost. However, it is also difficult to integrate into microfluidic systems and it is limited based on the size of the mixing chamber.
Pneumatic valves and mixers
Different pneumatic valve and mixer systems are another method of active mixing through liquid manipulation on the microfluidic scale. These valves apply pressure to the membrane which actuates liquid flow and are a common type of microvalve controlled by gas pressure. They can be grouped and synchronized to work as pumps or mixers and achieve different forms of liquid flow. This flow method is widely used in microfluidics since it is easy to integrate into different systems. However, these microvalves are very sensitive to the specific device configuration and pressure values, making their application limited in certain fields. They are widely used in drug discovery and cell analyses.
Applications
Achieving mixing within a microfluidic system is essential for a variety of applications, like increasing the rate of reaction. When compared to passive mixing methods, active mixing offers better control over the rate of mixing and the level of mixing. The plethora of active mixers available also provides a suitable option for many different systems.
References
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