Fabrication of Magnetically Rotating Active Micromixer and Evaluation of Mixing Performance

Abstract

The micro mixers are essential to mix the fluids in lab-on-a-chips which analyze the biological antigen-antibody reactions or chemical reactions. The fluids flowing in lab-on-a-chips have very low Reynolds number which is under 100. Fluids having a low Reynolds number are mainly depend on diffusion mechanism to mix when flowing in a micro channel. Therefore, the mixers using the diffusion mechanism need very long channel length and narrow channel width for efficient mixing. However, these kind of passive mixers using the diffusion mechanism have limitations in their sample size caused by the long channel length. Thus, researchers have studied to overcome the limitations of passive mixer to achieve good mixing performance in limited device size. One way to achieve this goal is generating chaotic advection in the microfluidic system. In this thesis, fabrication of magnetically rotating active micromixer was studied and the mixing performance was evaluated. The rotating motion is efficient to increase the contract area between the fluids and the rotor which is spun by external rotating magnetic field. The rotating magnetic field initiates the rotational movement of the rotor by generating magnetic torque. The rotor was fabricated by electroplating using a magnetic material which is Nickel-Cobalt alloy to enhance the magnetic torque. In addition, the micro fluidic channel was designed in simple Y-shape with two inlets and one outlet to evaluate the mixing performance. The different color dyed fluids are injected at each inlet and the fluids are mixed at the mixing chamber. The experimental setup except the micromixer consists of the three parts; syringe pumps, a high speed camera, and a rotating magnet connected with a DC motor. The motor generates the rotational field and the rotational speed can be controlled by changing RPM of the motor. The syringe pumps were used to inject and extract the fluids to the inlets and from the outlet, respectively. The mixing performance of the micromixer was evaluated by captured images using the high speed camera. The mixed fluids were analyzed to obtain the intensity of each pixel by converting the image into gray scale. The mixing performances are expressed by using standard deviation and normalized pixel intensity. The mixing performance was evaluated with various conditions. The flow rate was varied from 10 L/hr to 500 L/hr, and the used voltages of the motor were 6 V and 8 V. Reynolds numbers were calculated for each flow rate that are 0.064 and 0.0013 for 500 L/hr and 10 L/hr, respectively. If the standard deviation of color index with gray scale is zero, the mixing performance is defined to be 100%. The highest mixing performance was 90 percent with Reynolds number of 0.01. Different mixing performances along with the different rotor shapes were investigated with a constant voltage, 6V, into the motor. The mixing performances of the Z-shaped rotor and I-shaped were varied from 90 to 77 percent and 83 to 65 percent for 100 L/hr and 500 L/hr, respectively. Thus, the Z-shaped rotor showed better performance around 10% than the I-shaped rotor. However, when applying 8 V into the motor, the mixing performances were not much difference with the two different rotor shapes. For the Z-shaped rotor, the mixing performance was nearly saturated at 6 V so, when applying 8 V into motor, the mixing performance was barely changed. On the contrary, the I-shaped rotor was not reached at the saturated status at 6 V. Thus, the mixing performance of the I-shaped rotor was increased until 8 V and saturated at this condition. For this reason, when comparing the mixing performance of the Z-shaped and I-shaped rotors, the difference of each rotor was small when applying 8 V into the motor. ⓒ 2014 DGIST