Microfluid Nanofluid DOI 10.1007/s10404-011-0903-5 REVIEW PAPER Micro-magnetofluidics: interactions between magnetism and fluid flow on the microscale Nam-Trung Nguyen Received: 15 September 2011 / Accepted: 30 October 2011 Ó Springer-Verlag 2011 Abstract Micro-magnetofluidics refers to the science and Keywords Micro-magnetofluidics Á Magnetism Á technology that combines magnetism with microfluidics to Microfluidics Á Magnetic bead Á Ferro fluid gain new functionalities. Magnetism has been used for actuation, manipulation and detection in microfluidics. In turn, microfluidic phenomena can be used for making tun- 1 Introduction able magnetic devices. This paper presents a systematic review on the interactions between magnetism and fluid flow Magnetofluidics traditionally refers to a class of devices that on the microscale. The review rather focuses on physical and utilize a magnetic fluid for sensing and actuating functions. engineering aspects of micro-magnetofluidics, than on the These devices were used as sensor for applications such as biological applications which have been addressed in a hearing aid and accelerometer. However, the term of number of previous excellent reviews. The field of micro- ‘‘magnetofluidics’’ is used here for the broader research field magnetofluidics can be categorized according to the type of involving magnetism and fluid flows. Figure 1 shows the the working fluids and the associated microscale phenomena basic relationships between the four principal fields of of established research fields such as magnetohydrodynam- physics with the most applications: fluidics, electrics, optics, ics, ferrohydrodynamics, magnetorheology and magneto- and magnetism. The links between these fields cover most phoresis. Furthermore, similar to microfluidics the field can modern technologies, especially micro/nanotechnologies. also be categorized as continuous and digital micro-mag- However, efforts on the exploration of magnetofluidics in netofluidics. Starting with the analysis of possible magnetic microscale and its applications have been scattered. Many forces in microscale and the impact of miniaturization on possibly interesting phenomena have been neglected due to these forces, the paper revisits the use of magnetism for the lack of a systematic approach. Compared to an electric controlling fluidic functions such as pumping, mixing, field, a magnetic field has various advantages in microfluidic magnetowetting as well as magnetic manipulation of parti- applications. Magnetic manipulation can utilize external cles. Based on the observations made with the state of the art magnets that are not in direct contact with the fluid. Non- of the field micro-magnetofluidics, the paper presents some magnetic molecules and cells can be attached to magnetic perspectives on the possible future development of this field. beads, so that they can be sorted and detected by an external While the use of magnetism in microfluidics is relatively magnetic field. In contrast to electric concepts, magnetic established, possible new phenomena and applications can manipulation and detection are not affected by other be explored by utilizing flow of magnetic and electrically parameters such as surface charges, pH and ion concentra- conducting fluids. tion. In most cases, magnetic manipulation does not induce heating and does not require expensive external systems as compared to optical concepts. N.-T. Nguyen (&) A number of excellent reviews on applications of School of Mechanical and Aerospace Engineering, magnetism in microfluidics exist in the literature. However, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore the scopes of these reviews are limited to phenomena with e-mail: [email protected] existing biological applications. Gijs (2004) reviewed the 123 Microfluid Nanofluid aspects of micro-magnetofluidics, than on the biological Electrokinetics applications which have been addressed in the above- Fluidics Electrowetting mentioned excellent reviews. The field of micro-mag- s netofluidics can be categorized according to the type of the c i d i working fluids, and the microscale phenomena of estab- u Magnetofluidics l o t p lished research fields such as magnetohydrodynamics O Magnetism Electrics (MHD), ferrohydrodynamics (FHD), magnetorheology Electromagnetism (MR) and magnetophoresis (MP). Furthermore, the field Magneto optics can also be categorized as continuous and digital micro- magnetofluidics as in traditional microfluidics. Figure 2 shows an overview of micro-magnetofluidics and its sub- Optics Optoelectronics fields. In general, the field can be categorized according to Photonics the properties of the fluids or to the ways the fluids are Fig. 1 The basic domains of physics and their interfaces handled. The majority of magnetofluidic phenomena are based on electrically conducting fluids and magnetic fluids. The mutual interaction of magnetic field and the flow of use of magnetic beads for analytical applications. Later, Gijs electrically conducting fluids is covered by MHD, which is et al. (2010) followed up with a more comprehensive and well established and studied in the past hundred years updated review on applications of magnetic particles for (Davidson 2011). The fluids need to be electrically con- biological analysis and catalysis. Pamme (2006) extended ducting and non-magnetic, thus are limited to liquid metals, the scope in her review and covered a wider range of appli- plasmas and strong electrolytes. The traditional applica- cations of magnetism in microfluidics: pumping, mixing, tions of MHD are geophysics, astrophysics, plasmaphysic manipulation of partiles and magnetic detection. Pamme’s and metallurgy. review was the first attempt to look at the field from a broader Magnetic fluid consists of a carrier fluid and a suspen- perspective and to consider both ways of the interactions sion of magnetic particles. Depending on the size of the between magnetism and microfluidics. Since Pamme’s magnetic particles,the magnetic fluid behaves differently review was only based on reported works, many potentially leading to three main areas of FHD, MR and MP. If the interesting phenomena were neglected. Weston et al. (2010) magnetic particles are smaller than about 10 nm, the discussed the use of different types of magnetic forces for thermal energy dominates over the magnetic energy fluid motion. Weston’s review was based on the discussion of induced by an external magnetic field. Thus, the particles possible magnetic forces followed by their applications. can disperse well in the carrier fluid. The whole fluid Fisher and Ghosh reviewed the use of magnetism for pro- behaves as a paramagnetic liquid and is called ferrofluid. If pulsion of swimming particles (Fischer 2011). This minire- the magnetic particles is large enough, ranging from 10 nm view offers a new perspective on ‘‘smart’’magnetic particles. to 10 lm, they interact and react to the external magnetic Magnetic micro- and nano-structures with unique shapes field changing the viscosity of the fluid. The fluid is then other than the conventional sphere can be controlled with an called magnetorheological fluid. For magnetic particles on external magnetic field. Weddemann et al. (2010) reviewed the order of several microns or larger, the magnetic parti- the implementation of magnetic components in total analysis cles need to be considered individually as discrete entities, systems for biomedical applications. The scope of this leading to the field of magnetophoresis. review was limited to the detection and manipulation of According to the properties of the fluid flow, the research magnetic beads. Similarly, the review of Suwa and Watarai field can be categorized as continuous-flow and digital only focuses on the manipulation and detection of micro- MMF. In continuous-flow MMF, fluids are supplied or particles (Suwa 2011). Ganguly and Puri reviewed micro- manipulated in a continuous manner, where the fluids exist in fluidic transport of ferrofluid and magnetic particles in a single phase as in the case of MHD pumps and MHD mixers MEMS, Bio-MEMS devices (Ganguly 2010). Friedman and or in multiple phases such as emulsion. In digital MMF, Yellen discussed the physical fundamentals of magnetic fluids are manipulated as individual droplets or marbles, separation, manipulation and assembly using relatively which are droplets with a protective coating of hydrophobic simple but useful scaling analysis of the magnetic force and particles. Magnetic particles inside a droplet allow its con- its counter parts (Friedman 2005). trol and manipulation using a magnetic field. In this paper, micro-magnetofluidics (MMF) is under- Following fundamentals of magnetic forces in micro- stood as the science and technology that combines mag- scale, dimensionless numbers and their scaling laws are netism with microfluidics to gain new functionalities. The first discussed. Important phenomena are subsequently present review rather focuses on physical and engineering discussed according to the type of the fluid. 123 Microfluid Nanofluid - droplet-based - MHD micropumps - instabilities - ferrofluid droplets - MHD micromixers - MR fluid plug - magneto wetting - charged droplets Single-phase Multi-phase - droplets with magnetic beads - magnetic marbles Continous-flow micro magnetofluidics Digital micro magnetofluidics Micro magnetofluidics Electrically conducting fluids Magnetic fluids Magnetohydrodynamics Small magnetic particles Medium magnetic particles Large magnetic particles (MHD) d<10 nm 10 nm<d<1 m d>1 m DC-MHD AC-MHD RedOx-MHD Ferrohydrodynamics Magnetorheology