Sensors and Actuators A 153 (2009) 267–273 Contents lists available at ScienceDirect Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna Rapid mixing between ferro-nanofluid and water in a semi-active Y-type micromixer Tsung-Han Tsai, Dar-Sun Liou, Long-Sheng Kuo, Ping-Hei Chen ∗ Department of Mechanical Engineering, National Taiwan University, Taipei 10617, Taiwan article info Article history: Received 25 September 2007 Received in revised form 20 April 2009 Accepted 6 May 2009 Available online 15 May 2009 Keywords: Ferrofluid Nanofluid Mixing Micromixer abstract This study investigates the mixing phenomena between ferro-nanofluid (a water solution with suspended Fe 3 O 4 nanoparticles) and water in a Y-type semi-active micromixer. A permanent magnet with a magnetic strength of 2200G was installed directly under the mixer at a location behind the junction of the Y-type micromixer to improve the mixing between ferro-nanofluid and water. The permanent magnet is 9.6 mm long, 9.6 mm wide and 1.6 mm thick. Equal volumetric flow rates for both ferro-nanofluid and water were tested in two different channel widths, 300 m and 500 m. Three different volumetric flow rates of 3 L/min, 6 L/min, and 10 L/min were analyzed to determine the effect of flow rate on the mixing of fluids in the micromixer. Since Fe 3 O 4 nanoparticles in the ferro-nanofluid are strongly affected by a relatively uniform magnetic field from the permanent magnet, measured results show that a rapid mixing between ferro-nanofluid and water can be achieved immediately downstream of the permanent magnet for all volumetric flow rates and all channel widths tested in this study. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Microfluidic devices show great potential in applications involv- ing micro total analytic systems (-TAS). A -TAS requires microfluidic devices to enable several essential processes: sample pumping, sample mixing, sample reaction, and sample separation. Higher mixing efficiency between different samples undoubtedly reduces -TAS operation time and then increases the performance of target molecule detection in the -TAS. Numerous studies have previously discussed improving mixing efficiency in micromixers [1–9]. These micromixers can be classified into two categories: active and passive mixers. An active mixer uses an external force, typically an electric power input, to stir the samples in the microchannel. Passive mixers, on the other hand, have either small protrusions in the microchannel or rough surfaces on the microchannel wall to enhance mixing without requiring any exter- nal energy input. Most active mixers perform better than passive mixers [6,7]. However, active mixers usually result in higher power consump- tion and fabrication costs. Additionally, the Joule heating generated in active mixers may damage biological samples during the mixing process. An important task in this field is to reduce both power con- ∗ Corresponding author at: Department of Mechanical Engineering, National Tai- wan University, No. 1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan. Tel.: +886 2 23670781; fax: +886 2 23670781. E-mail address: phchen@ntu.edu.tw (P.-H. Chen). sumption and fabrication costs while improving mixing efficiency in an active mixer. Few prior studies have presented data on the mixing between a ferro-nanofluid (fluid with suspended magnetic particles) with other sample solutions. Nevertheless, some papers have shown that manipulating the conjugate of magnetic particles and biomolecules (or a function group) in a microfluidic device can easily amplify the electrical signal of concentration of target biomolecules by utilizing properties of magnetic particles [10,11]. This approach becomes more and more popular in the costly and time-consuming Polymerase Chain Reaction (PCR) process. However, this approach requires a mixing process to obtain the conjugate of magnetic par- ticles and biomolecules. The conventional method of conjugating the biomolecules and magnetic particles, which is dependent on the diffusion, needs a lot of time. Since magnetic particles in the sample can be affected by an external magnetic field from a per- manent magnet, a high mixing efficiency can be achieved between ferro-nanofluid and another sample solution in a micromixer if the permanent magnet is properly installed at the right location outside the micromixer. With the presence of ferro-nanofluid, the whole biomolecules detecting process can be integrated into a microflu- idic chip, which realizes the lab on a chip. The advantages for using a permanent magnet in the micromixer are that it avoids consuming external power and generating heat in the sample, and achieves a high mixing efficiency between the ferro-nanofluid and the other solution in the micromixer. Since no external electrical power is supplied to the micromixer, such an approach which uses a permanent magnet to improve mixing 0924-4247/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2009.05.004