Journal of Magnetism and Magnetic Materials 323 (2011) 2701–2709

Contents lists available at ScienceDirect

Journal of Magnetism and Magnetic Materials

0304-88

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jmmm

Characterization of a ferrofluid-based thermomagnetic pump formicrofluidic applications

Souvik Pal a, Amitava Datta a, Swarnendu Sen b, Achintya Mukhopdhyay b, Kallol Bandopadhyay c,Ranjan Ganguly a,n

a Department of Power Engineering, Jadavpur University, Kolkata 700098, Indiab Department of Mechanical Engineering, Jadavpur University, Kolkata 700032, Indiac Machine Dynamics Division, BARC, Trombay, Mumbai 400085, India

a r t i c l e i n f o

Article history:

Received 11 February 2011

Received in revised form

3 June 2011Available online 16 June 2011

Keywords:

Micropump

Ferrofluid

Thermomagnetic convection

MEMS

Kelvin body force

53/$ - see front matter & 2011 Elsevier B.V. A

016/j.jmmm.2011.06.016

esponding author. Tel.: þ91 33 23355813; fa

ail address: ranjan@pe.jusl.ac.in (R. Ganguly).

a b s t r a c t

We experimentally characterize the performance of a miniature thermomagnetic pump, where suitably

imposed temperature and magnetic field gradients are used to drive ferrofluid in a 2 mm diameter glass

capillary tube, without application of any external pressure gradient. Such a pump can operate in a

hermetically sealed micro electromechanical system configuration without any moving part, and is

thus capable of handling microfluidic samples with little risk of contamination. In the experiment, the

ferrofluid in the capillary is exposed to a magnetic field using a solenoid; a small resistive heater

wrapped on the tube wall is used to create temperature gradient in such a way that the Kelvin body

force in the medium produces a net unbalanced axial component. This causes a thermomagnetic

pumping action, transporting the ferrofluid in the capillary tube from the colder end to the warmer end.

Performance of the thermomagnetic pump is investigated experimentally to characterize the pump

pressure head and discharge under different working conditions, namely, the magnetic field strength,

heating power, and ferrofluid properties. A comparison with two other field actuation pumps at

comparable length scales is also presented. The pump produces higher output at lower power supplies

and magnetic field compared to the other two pumps.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

The rapid expansion in the field of microfluidics has necessitatedthe study of micropumps capable of delivering extremely smallvolume of liquid intermittently or on a continuous basis in microelectromechanical (MEMS) devices. Such devices are expected toachieve better throughput at reduced error and cost. In theseportable biological and chemical analyses systems, one of the majorfunctional requirements is to control the flow of extremelysmall volume of fluid through micro and nano-channels. Due tothe miniaturization, micropumps find potential applications inBioMEMS devices, such as in MEMS-scale polymerase chain reaction(PCR) devices [1], micro total analysis systems (mTAS) [2], and otherbiological handling and analysis tools. Besides, such micropumpscan also offer a solution towards reliable passive cooling of state ofthe art microelectronic devices [3].

Micropumps offer probably the largest variety among all theMEMS components in terms of their operating principle, perfor-mance, size, and fabrication methods. An excellent review [4] onMEMS micropumps broadly classifies them into two categories, viz.

ll rights reserved.

x: þ91 33 23357254.

mechanical and non-mechanical. The first category involves movingparts for flow actuation while in the pumps belonging to the secondcategory, momentum is imparted to the fluid indirectly. Due to thepresence of the moving components, the mechanical pumps havelimitation in terms of miniaturization and hence offer steep fabrica-tion challenges in a MEMS network. This makes the principle of fieldactuation a better alternative for micropumps.

Traditional mechanical micropumps [5] rely on diaphragmdriven by piezoelectric [6,7], thermopneumatic [8], electrostatic[9], electrohydrodynamic [10], electromagnetic [11], and shapememory actuation [12]. Among these, the piezoelectric, thermo-pneumatic, and shape memory actuation can only producepulsatile flows. In some cases structural problems related tofatigue failure is not uncommon [13]. In contrast, field actuationprovides a continuous and variable flow. Electrostatic and elec-trohydrodynamic actuation have the disadvantage of very highvoltage (�1 kV) requirements, requiring very good insulation inmicrofluidic devices. Another limitation of electrohydrodynamicgas pump is the formation of ozone and other ions, which can leadto health hazards [14]. Thermomagnetic pumping poses neitherof the above mentioned shortcomings and thus provides a veryattractive option for flow control in microfluidic devices.

Thermomagnetic pumps use ferrofluids as working medium.In microfluidic architecture, the ferrofluid medium can be used

S. Pal et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 2701–27092702

either as the primary pumped fluid, or they can act as a liquidplunger in a micropump, pumping the secondary (working) fluidthrough the microchannels of the device. Ferrofluids are colloidalsuspensions of single domain magnetic nanoparticles [15]. Theparticles are typically of the order of 10 nm in diameter and are ina permanent state of magnetization, i.e., they behave as magneticdipoles even in the absence of an applied field. However, thefluid bulk does not exhibit magnetic behavior until an externalmagnetic field is imposed to align the thermally disorientedmagnetic moments of these particles. The particles are coated withadsorbed surfactant layers to prevent particle agglomeration due tointerparticle van der Waals force and dipole–dipole interactions.These surfactant layers maintain an adequate spacing between theparticles so that the attraction energy between adjacent particles issmaller than the disordering energy of their thermal motion, therebyproviding colloidal stability. Consequently, such a ferrofluid can beconsidered as a single homogeneous liquid. Flow fields establishedwith these fluids can be altered by applying external magnetic fields.The key advantages of using such a ferrofluid pump in microfluidicarchitecture are that (i) ferrofluids offer low-friction reciprocating orcontinuous motion under externally imposed magnetic field; (ii) dueto their fluid-like nature, ferrofluids offer ease of delivery to thepoint of requirement inside the microfluidic architecture, and hence,unlike a solid plunger, a ferrofluid does not require device-integra-tion at the microfabrication phase; (iii) there can be contact-lessactuation without any moving components; and (iv) a ferrofluidplug used as a plunger can also be considered as self sealing due tocapillary forces.

Temperature gradient in ferrofluid establishes a spatial gradi-ent in its magnetic susceptibility. Under an imposed magneticfield, this results in a net unbalanced component of magneticbody force on ferrofluid, which in turn leads to thermomagneticconvection. Thermomagnetic flow can be particularly useful tothe cases where the conventional pressure-driven and thermo-gravitational flows are inadequate, e.g., in microscale devices, orunder microgravity conditions. Numerical investigations of ther-momagnetic convection in forced flow [16] and square enclosure[17] configurations have shown that the thermomagnetic effectsare significant particularly at smaller length scales. This is alsosupported through scaling analyses [18]. In a suitably designedconfiguration, non-uniform magnetic and temperature fields canbe imposed such that the resulting thermomagnetic force pumpsthe magnetic fluid through a microchannel or capillary tube. Thequantity and direction of the bulk fluid flow in such a miniaturethermomagnetic pump can be altered on demand by appropri-ately controlling the magnetic field and the temperature fieldgradients.

Although there were previous attempts to design and charac-terize a ferrofluidic micropump, most of the methods aimed atusing ferrofluid plug as an actuator to pump a secondary fluid andare pulsatile in nature [19–22]. Mao and Koser [23] numericallyand experimentally analyzed an integrated high flow rate MEMSferrofluid pump. The operation of the pump requires spatiallytraveling time varying magnetic fields thereby requiring a complexsetup to achieve the same. Love et al. [24,25] described the conceptof thermomagnetic pumping in simple microfluidic geometry,though they did not fully characterize the thermomagnetic pumpperformance and its controllability so far as the individual influ-ences of the imposed magnetic field and the fluid heating areconcerned. Li et al. [26] used an ordered assembly of loop deviceconsisting of a permanent magnet, a heater, a heat sink, and atemperature-sensitive magnetic fluid to form a thermomagneticflow-assisted cooling device. The same group also proposed asimilar arrangement of an automatic energy transport device usingthermomagnetic convection, where they characterized the flowprofile within the device experimentally [27] and numerically [28]

and investigated the influence of parameters like magnetic field,heat supply and relative placement of the magnet and the heateron the flow velocity. A more recent design focused on a thermo-magnetic electronics cooling application [29] where the device wasfound to exhibit a self regulating feature, i.e., the flow velocity ofthe magnetic fluid increased with the increase of the heat load andvice versa. Each of these designs had a thermomagnetic pumpunit to drive the flow through the device, but the head-dischargecharacteristics of the pump units were not reported separately. Herewe have presented the operation of a miniature thermomagneticpump in terms of its head and discharge, which is traditionallytreated as the most useful representation of a pump performance fora generic microfluidic application. The pump performance is ana-lyzed experimentally for various operating conditions, e.g., the heatinput, magnetic field strength, and ferrofluid properties. The studyalso compares the performance of the pump with two ferrofluidshaving different magnetic properties. A comparison with other FHD(ferrohydrodynamic) and MHD (magnetohydrodynamic) pumps isalso provided at the end.

2. Materials and methods

2.1. The principle

The basic operating principle of a thermomagnetic pump relieson the fact that below the Curie temperature, the magneticsusceptibility of a ferrofluid decreases with increase in tempera-ture. In a spatially uniform magnetic field there is no gradient ofmagnetization in an isothermal ferrofluid. When a temperaturegradient is established, ferrofluid will be non-uniformly magne-tized due to the change in susceptibility. This will result innon-uniform Kelvin body force on the ferrofluid. With spatiallynon-uniform magnetic field, the temperature induced magnetiza-tion gradient will augment the force imbalance in the fluid.Therefore, in a non-uniformly heated space, colder ferrofluid willbe attracted towards high field gradient region with a larger forcethan the hotter ferrofluid. Thus, in a non-isothermal ferrofluidmedium, if the high field gradient is established near the hotferrofluid region, the warmer ferrofluid would be replaced by thecolder ferrofluid from the vicinity, eventually establishing a flow.The flow is continuous and perpetual as long as the magnetic fieldand temperature field gradients are maintained.

Flow and energy transport in the thermomagnetic pump isgoverned by the mass, momentum, and energy conservationequations, i.e.,

rV ¼ 0, ð1Þ

r @v

@tþVrV

� �¼�rpþmr2VþMrB, ð2Þ

and

rCp@T

@tþVrT

� �¼ kr2T : ð3Þ

Here, V represents the fluid velocity, p the pressure, T theabsolute temperature, while B denotes the magnetic field. Theferrofluid properties are denoted by k the thermal conductivity,Cp the specific heat, r the density, m the dynamic viscosity, andM the magnetization. The Kelvin body force per unit volume [13]appears as a volumetric source term last, and is represented bythe last term in Eq. (2). The magnetic equation of state follows therelation [14]

M¼ wmH, ð4Þ

where H¼ 1=m0 B=ð1þwmÞ. The total or integrated magneticsusceptibility wm of the ferrofluid is considered to be a function

S. Pal et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 2701–2709 2703

of temperature T. For small temperature range (about a referencetemperature Tn) the magnetic susceptibility may be assumed tovary in a Boussinesque fashion, i.e.,

wm ¼w0

f1þbpðT�TnÞg, ð5Þ

where bp ¼�½@ðw=w0Þ=@T� denotes the pyromagnetic coefficient ofthe ferrofluid.

2.2. The pump unit and its operation

Fig. 1a shows a schematic diagram of the thermomagneticpump. The pump unit comprises of a 2 mm inner diameter and11 cm long glass tube, filled with ferrofluid, a disk-shapedsolenoid (electromagnet) of length l¼20 mm mounted concentricto the tube, and a heater element of equal length at theimmediate downstream end of the electromagnet (Fig. 1b). Theheater section has nichrome wires wrapped around the tube andis covered by glass tape thermal insulation on the outside toprevent heat loss to the surroundings. A concentric mild steel coreis inserted in the space between the inner rim of the electro-magnet and the outer surface of the glass tube to intensify the

r

z

2 cm

z=-l

/2

z=+

l/2

z=3

l/2

Fig. 1. (a) Schematic of the thermomagnetic pump unit. (b) Close up of the heater.

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

z (m

)

Coil MS Core

Glass wall surfaces Inner Outer

Fig. 2. (a) Magnetic field contours and flux lines computed using COMSOL. (b) Measure

the tube.

local magnetic field and its gradients at the two ends (poles) ofthe electromagnet.

Fig. 2a shows the magnetic field produced by the electro-magnet (754 turns) for a current of 2.03 A as evaluated usingCOMSOL Multiphysics 3.3. Triangular mesh having 25,064 ele-ments and 12,375 nodes is used for the simulation. The grid wasclustered near the poles to capture the high field gradients inthese zones. The numerical results are verified (refer Fig. 2b) bymeasuring the magnetic field in the accessible part of the electro-magnet using a Hall probe Gaussmeter (DGM 900, Ferrites India),which show excellent agreement. It is evident from Fig. 2a thatthe field changes very rapidly near the two ends of the coil, thusproducing very large axial gradients of B or H. As the tubediameter is very small as compared to the length of the electro-magnet, the radial gradient of the field is negligibly small.

The origin of thermomagnetic force in the pump unit can beexplained from its configuration and the magnetic field distribu-tion. For illustration, 2 pairs of points, viz., A and B, and C andD – each pair being equidistant from their nearest poles – areconsidered on the coil axis (Fig. 1a). In isothermal condition, i.e.,when the heater is off, the field is setup in such a way that thetwo ferrofluid elements A and B (Fig. 1a) are equally attractedtowards the respective poles. Also fluid elements at C and D willfeel equally attracted towards their nearest poles but the magni-tude of attraction will be comparatively less due to the weakeningof field gradient towards the equatorial plane of the solenoid (seeFig. 2b). There will be no force at the middle (the equatorialplane), since the field gradient is zero there. Such symmetric forcedistribution does not induce any net flow of ferrofluid under thiscondition. As the fluid in the tube, from both inside and outsidethe coil zone, tries to rush towards the poles, there is a pressurerise in the fluid near the two poles (i.e., the ends of the solenoid).When the heater is switched on, the ferrofluid temperature willrise in the region to the right of the solenoid (Fig. 1a). Under thissituation, the attraction felt by a ferrofluid element at B becomesless than that felt by a fluid element at A (because the fluidmagnetization is less at B). Hence, a net resultant force sets uptowards the right, resulting in a flow in that direction. Consider-ing that the radial component of magnetic field and its gradientare negligible as compared to their axial components, the netaxial magnetic force on the ferrofluid inside the capillary tube can

0Bz (mT)

COMSOL simulationGaussmeter reading

Upper edge of solenoid

Lower edge of solenoid

20 40 60 80

d and simulated values of the z component magnetic field (Bz) along centerline of

FlexibleConnecting

Pipe

T2T1

T3

T2T3

T1 Data Acquisition System

MeasuringFlask

P

Capillary Tube (Pump Unit)

ElectromagnetCoil

Heating Coil

FerrofluidContainer

DC Power Supply Unit

T3T1T2

120

Spout

Connecting pipe

Fig. 3. (a) Experimental setup for measurement of flow, head and temperature in

an operating position. (b) Photograph of the thermomagnetic pump setup

displaying its different components.

S. Pal et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 2701–27092704

be expressed as

Fm ¼ Am0

Z 1�1

M@H

@z

� �� �dz� Am0

Z zF

�zF

ðw�z�wzÞ@ðH2Þ

@zdz

� ðA=m0Þ

Z zF

�zF

ðDwÞdðB2z Þ, ð6Þ

where A denotes the cross-section area of the capillary tube, andthe limits �zF and zF denote the axial distances beyond which themagnetic field drops down nearly to zero (see Fig. 2b). Theintegral in Eq. (6) depends on the B–z profile (Fig. 2b) as well asthe difference between the magnetic susceptibilities of the hotand cold ferrofluids at equidistant points on either sides of themid-point of the solenoid (treated as the origin here). The termDw depends on the local fluid temperature, magnetic field and thepyromagnetic coefficient bp of the ferrofluid. Under steady stateof operation, the flow rate Q through the thermomagnetic pumpwould be obtained through a balance between the drivingmagnetic force, and the combination of pressure force Dp devel-oped by the pump and the fluid friction force Fr offered by thepump, i.e.,

Fm ¼ ðDpÞAþFrðQ Þ: ð7Þ

Since the friction force is a function of the flow rate Q, accurateprediction of thermomagnetic pumping (e.g., flow rate at a givenpressure head) would warrant correct estimation of local ferro-fluid temperature at every axial location within the capillary tube.When such an estimation is not feasible (as in the present case),thermomagnetic pump characteristics can be obtained from thetraditional head-discharge curves.

2.3. The experimental setup and procedure

Fig. 3a shows a schematic of the experimental setup, whileFig. 3b shows an enlarged photograph of the thermomagneticpump. The heater and tube assembly and the solenoid are securedto an adjustable stand such that the tube and solenoid axes canbe inclined with horizontal at any desired angle (the angle can bemeasured by a protractor mounted on the base). Two separate DCsupplies having ranges 0–30 V, 0–5 A and 0–7 V, 0–0.95 A wereused to feed the solenoid and the heater, respectively.

First, the ferrofluid is filled in the pump unit from theferrofluid container through a 20 cm long and 5 mm innerdiameter flexible connecting pipe (see Fig. 3a). The containerheight is adjusted to fill up the pump unit for different levels ofthe inclination. Initially the ferrofluid meniscus in the pump hasthe same level as in the container as shown in Fig. 3a. The level inthe tube would rise due to thermomagnetic pumping whenthe solenoid and the heater are switched on. The difference inlevel is the head developed by the pump at a given inclination andmeasured from the graduations on the inclined tube as h¼ Lcosy,where L is the distance along tube traversed by the ferrofluid (seeFig. 3a). The capillary tube has a 1201 bend at the upper end (seeFig. 3b) that forms a ‘‘spout’’ so that the ferrofluid does not creepback along the outer wall of the pump and is easily dischargedinto the measuring cylinder. During operations under differentinclination angle y, the pump unit is rotated around its axis suchthat the outlet of the spout is at the same level of the bend. This isdone to prevent any siphonic action. The two inputs to the pumpare field current in the solenoid producing the field and heatercurrent providing the heat input through Joule heating. For a fixedheater and field current the heat input and the field are constant.For a given heater and field current the pump can lift ferrofluid upto a certain maximum level corresponding to zero discharge. Wecall this head as shut-off head of the pump for the correspondinginput settings.

In order to measure the pump discharge we establish a contin-uous flow through the pump. This is achieved by adjusting the initiallevel of ferrofluid by moving the ferrofluid container to a level suchthat the head developed by ferrofluid at that inclination and currentis within the shut-off head of that configuration. This ensures a finitenon-zero discharge of the ferrofluid from its open end. The dischargeis measured by collecting the ferrofluid in a measuring cylinder overa finite time.

The head is varied by controlling both the inclination and theinitial ferrofluid level while the head-discharge characteristics ofthe pump are drawn at different input conditions viz. field andheater currents. Three K type thermocouples are placed tomeasure the ferrofluid temperature at three different axial posi-tions marked by T1, T2 and T3 in Fig. 3. Real time temperaturesare recorded at a sampling rate of 2 Hz from these thermocouplesusing a 16 channel, 22 bit data acquisition system (Agilent34970A).

2.4. The ferrofluids

Experiments are conducted with two different types of ferro-fluids, EFH3 (Ferrotec, USA) and a thermally sensitive ferrofluid(designated here as TSF, supplied by Bhavnagar University, India).These two ferrofluids have different magnetic and thermophysicalproperties that are listed in Table 1. The susceptibility of theferrofluids was measured by modified Gouy method, which involvesdirect measurement of the Kelvin body force on a given volume ofspherical ferrofluid element in a known magnetic field gradient [30].

S. Pal et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 2701–2709 2705

The experimentally obtained M–H curves are shown in Fig. 4 for thetwo ferrofluids. The accuracy of the results was established bytaking consecutive runs and calculating the uncertainty factor of theavailable data. The instrument was able to provide reliable andrepeatable measurements of the magnetic body force with anabsolute uncertainty within 6.1%. It can be observed from Fig. 4that EFH3 has superior saturation magnetization over TSF. It alsoemerges from Fig. 4 that the maximum ratio of magnetization ofEFH3 and TSF is �10.5 corresponding to H�4775 A/m and beyondH�16000 A/m the ratio is almost constant at a value of 6.5. It is also

Table 1Ferrofluid properties (from suppliers’ data).

Property Sample A (EFH3) Sample B (TSF)

Saturation

magnetization (G)

400 150

Density (g/ml) 1.28 1.03

Viscosity 6 cp at 27 1C N.A.

Flash point 92 1C N.A.

Curie point N.A. 156 1C

Pyromagnetic

coefficient

N.A. 0.0073/K (within a

temperature

range 23–100 1C)

0

10000

20000

30000

40000

50000

60000

0Magnetic field strength (H) (A/m)

Mag

netiz

atio

n (M

) (A

/m)

TSFEFH3

20000 40000 60000 80000 100000

Fig. 4. Experimentally (modified Gouy method) obtained M–H curves for EFH3

and TSF.

23283338434853586368

0Time (s)

Tem

pera

ture

(°C

)

T3T2T1

EFH3, Head=0.35 cm, I_field=2.03A, V_heater=5 V

50 100 15

Fig. 5. Experimentally obtained curves for (a) temp

evident that even with an applied field of 80,000 A/m (E100 mT)EFH3 is not saturated.

3. Results and discussions

3.1. Thermomagnetic response of the pump unit

In order to characterize the behavior of the thermomagneticpump unit, transient response of the ferrofluid temperatures anddischarge are recorded after the heater and coil are simulta-neously switched on. Since the ferrofluid receives heat from boththe solenoid coil (due to Joule heating) and the heater, itstemperature rises with time. A typical temperature vs. timeplot for thermocouples T1, T2 and T3 for the base case, i.e.,head¼0.35 cm, field current¼2.03 A, heater input 5 V, 0.7 A(3.5 W) with EFH3 is shown in Fig. 5a. The rate of temperaturerise at the downstream of the heater section (T3) is the maximum,while the temperature T1 at the upstream of the electromagnetrises the minimum. This clearly shows evidence of advectivetransport in the direction shown in Fig. 1a.

Fig. 5b shows the ferrofluid volume collected from the outlet ofthe thermomagnetic pump as a function of time. The slope of thiscurve signifies the discharge rate of the pump for the given powerinputs and head. The volume collected varies linearly with time,corresponding to a steady discharge of the pump. This does notcomply with the trend in temperature change in Fig. 5a where thetemperature difference DT¼(T2�T1) is found to increase with time.As the discharge rate is observed through a direct measurement, wesuspect that the thermocouple measurements did not accuratelyportray the fluid temperature. Since the thermocouple wires pene-trate the glass surface, there might be conduction along thethermocouple wires. This may have resulted in the increase in DT

with time. However, the maximum operating temperature of theferrofluid should be limited by the maximum allowable operatingtemperature to ensure the prevention of ferrofluid vapor formationsince that has associated health hazards.

Experiments were repeated at different operating conditions anderror analyses were performed on the discharge data after excludingthe extraneous results. The average uncertainty is evaluated as

uðxÞ ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1

nðn�1Þ

Xn

i ¼ 1

ðxi�xÞ2" #

,

vuut ð7Þ

where xi and x denote a sample data and the mean data, respectively,and n the population size. Based on 10 readings corresponding to aparticular parametric condition, the uncertainty of the reporteddischarge data remained between 73.4% over the mean values.

0

0.2

0.4

0.6

0.8

1

1.2

0Time (s)

Vol

ume

colle

cted

(ml)

020 40 60 80

erature vs. time (b) volume collected vs. time.

S. Pal et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 2701–27092706

3.2. Head-discharge characteristic

For determining the head-discharge curves, the pump head isset prior to the experiment by setting the inclination of the tube.The discharge is measured for a given ferrofluid at specifiedvalues of the field current and heater current. The head-dischargecharacteristics (scatter plots, their fits and corresponding errorbars denoting the uncertainty of reading) of the pump with theEFH3 ferrofluid at a heater input of 5 V, 0.7 A (3.5 W) for twodifferent field currents 2.03 and 1.70 A are shown in Fig. 6a. Thehead-discharge curves show a strong drooping nature at lowhead, but the slopes reduce in magnitude at higher heads. Thisindicates that the thermomagnetic pump is essentially a low-head device. It is also apparent from Fig. 6a that as the fieldcurrent reduces, the curve shifts marginally towards the leftresulting in a reduced head at the same discharge (i.e., lesspumping effect). Fig. 6a also shows the variation of head withrespect to discharge for EFH3 at field current 2.03 A and a heaterinput of 6.7 V, 0.92 A (6.16 W) i.e., at a higher heat input but samefield compared to the other two head-discharge characteristics ofFig. 6a. It is evident that the effect of increased heat input ondischarge is more significant in the low discharge region.

Fig. 6b shows a comparison of head-discharge characteristics ofEFH3 and TSF at same field and heat input. The trend of the twocurves shown in the Fig. 6b are quite similar although with EFH3, thepump shows superior performance over TSF as it produces higherhead for the same discharge with the same input. For a fixed head ofthe pump EFH3 produces on an average 2.5 times more dischargethan TSF. This could be attributed to the higher magnetization of

0

0.005

0.01

0.015

0.02

0.025

0.03

0Head (cm)

Dis

char

ge (m

l/s)

EFH3,I_field 2.03A,V_heater 5VEFH3,I_field 1.70 A,V_heater 5VEFH3,I_field 2.03 A,V_heater 6.7V

0.5 1 1.5 2

Fig. 6. (a) Head-discharge characteristics of EFH3 ferrofluid for different input settings. (

setting.

0.0020.0040.0060.008

0.010.0120.0140.0160.018

0.02

0.5Field Current (A)

Dis

char

ge (m

l/s)

head=0.35 cmhead=0.65 cm

1 1.5 2 2.5

Fig. 7. (a) Discharge versus field current plot at a fixed heat input 3.5 W and different

2.03 A and different fixed heads for EFH3.

EFH3 under the same magnetic field strength. In spite of thedifference in magnitudes, the basic nature of the head-dischargecurves is quite similar for the two ferrofluid samples, which closelyfollows a rectangular hyperbolic curve. The power output from pumpis expressed as P¼gQh, where g denotes the specific weight (¼rg) ofthe fluid, Q is the discharge of the pump and h is the head developed.With a rectangular hyperbolic nature of the h–Q curve, the hydro-dynamic power output of the pump remains constant (since theproduct of h and Q is constant). As the input into the system alsoremains constant at all h and Q, Fig. 6 indicates that the thermo-magnetic pump efficiency curves would be nearly flat in nature.

3.3. Flow control by heat input and field current regulation

The flow through the thermomagnetic pump can be controlledby varying the heat input and the field current. Fig. 7a shows thevariation of flow rate as a function of the field current at twodifferent fixed heads (viz., 0.35 and 0.65 cm) and a fixed heat inputof 3.5 W for EFH3. Ideally, as the field current increases, the Kelvinbody force and hence the discharge will increase due to the increasein ferrofluid magnetization until it saturates. However, the presentexperiment operates at a field (maximum B¼61.3 mT for I¼2.03 A)much below the saturation regime for EFH3, and hence thedischarge increases almost linearly with field current (Fig. 7a). It isalso evident from Fig. 7a that the flow control by field currentregulation becomes less effective as the system head increases. Theless steep curve corresponding to the higher head testifies this fact.In Fig. 7a, as head increases from 0.35 to 0.65 cm, the slope of thelinear fit of the plot reduces from 0.006 to 0.004 ml/s/A, i.e., by

0

0.005

0.01

0.015

0.02

0.025

0Head (cm)

Dis

char

ge (m

l/s)

EFH3,I_field 2.03 A,V_heater 5VTSF,I_field 2.03 A,V_heater 5V

0.5 1 1.5 2

b) Comparison of head-discharge characteristics of EFH3 and TSF under same input

0

0.005

0.01

0.015

0.02

0Heat Input (W)

Dis

char

ge (m

l/s)

head=0.65 cmhead=0.347 cm

2 4 6

fixed heads for EFH3. (b) Discharge versus heat input plot at a fixed field current

0

0.5

1

1.5

2

2.5

3

1Field current (A)

Shu

t off

head

(cm

)

EFH3 at V_heater=7 V

1.5 2 2.5

Fig. 9. Shut-off head variation with field current for EFH3 (heater power¼6.16 W).

S. Pal et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 2701–2709 2707

�33%. It is important to note here that the flow control throughmagnetic field current regulation has the inherent disadvantage ofJoule heating of the solenoid, resulting in weakening of the tem-perature gradient across the coil. As a result, at higher field currents,the increase in discharge is not proportionally high, though thedeviation from linearity in Fig. 7a is small.

Fig. 7b shows the variation of discharge with respect to heatinput from the heater at two different fixed heads of 0.35 and0.65 cm and a fixed field current 2.03 A with EFH3. The variation ofdischarge with respect to heat input, as shown in Fig. 7b, is non-linear. Although the pump discharge increases with the heaterpower, the slope of the discharge vs. heater power curve decreasesat larger heater power. This can be attributed to the increased meanferrofluid temperature at large heater power, which decreases theaverage ferrofluid susceptibility, thereby weakening the thermo-magnetic effect. Like the previous case (Fig. 7a), the discharge vs.heater power curves becomes less steep at higher heads (Fig. 7b).For example, at a heat input of 2.24 W, the slope of the curve inFig. 7b reduces from 0.00318 to 0.00272 ml/s/W, i.e., by 14.4% as thehead is increased from 0.35 to 0.65 cm. For a higher heat input, e.g.,at 4.98 W, the reduction in slope of the curve for the same change inhead is 11% (from 0.00139 ml/s/W for a head of 0.35 cm to0.00124 ml/s/W at 0.65 cm head). This suggests that the sensitivityof flow control by heat input regulation (represented by slope of thedischarge vs. heat input curve) is less influenced by the change inhead in the higher heat input region.

3.4. Comparison of flow control of EFH3 and TSF

The relative influence of heater power and the field currents onthe TSF and EFH3 are compared in Fig. 8. For a fixed heat input of3.5 W and head of 0.35 cm, the discharge with different fieldcurrents for EFH3 is on an average 2.9 times higher than thatobtained with TSF (Fig. 8a). In Fig. 8b, for the same head and a fixedfield current of 2.03 A, discharge under different heater powerinputs with EFH3 is found to be 2.5 times than that with TSF. Italso ensues from Fig. 8a and b that the flow control with both fieldcurrent and heat input is better achieved in EFH3. However, TSF canbe a suitable choice where a relatively constant discharge has to bemaintained against changing input circumstances. Also, TSF exhibitsa better linearity in the relationship of discharge with field currentor heat input compared to the discharge variation for EFH3.

3.5. Shut-off head variation

A separate study for variation of shut-off head (i.e., the head atzero discharge) for different magnetic field at fixed heater current isalso performed. The shut-off head of the thermomagnetic pump alsochanges as the input changes, which is described in Fig. 9. The

00.0020.0040.0060.008

0.010.0120.0140.0160.018

0.02

0.5Field current (A)

Dis

char

ge (m

l/s)

TSF at head=0.35 cmEFH3 at head=0.35 cm

1 1.5 2 2.5

Fig. 8. (a) Discharge versus field current plot for EFH3 and TSF at a fixed heat input 3.5 W

important consideration in shut-off head measurement is thetemperature. During the experiment for measurement of shut-offhead, there is no net flow of ferrofluid in the pump unit. Thus, theferrofluid is constantly heated by the heater, leading to a steady risein temperature. For the present experiment the allowable maximumtemperature was set at 100 1C. Shut-off head increases with fieldcurrent almost linearly in case of EFH3. The highest pressure thatthis pump develops with a field current of 2.33 A and for a heaterinput of 6.16 W (7 V, 0.88 A) is 2.75 cm, which corresponds to345 Pa. Fig. 6(a) indicates that the pump was operated at amaximum discharge of 0.02 ml/s at a heater power of 6.16 W anda field current of 2.03 A. The shut-off head for the same heaterpower and field current is 2.12 cm (Fig. 9). Assuming a fullydeveloped flow through the capillary tube and the connecting pipeand ferrofluid viscosity Z¼1.012 Pa s and density r¼1278 kg/m3,the pressure drop is estimated as hf ¼ 128ZLQ=prgd4 ¼ 5:36 mm,which is approximately 27% of the shut-off head under the sameheater load and field current. With TSF the maximum shut-off headcorresponding to the same field current of 2.33 A and heater input of6.7 V, 0.92 A (6.16 W) was found to be 1.6 cm.

3.6. Comparison of thermomagnetic pump performance with

other pumps

The present device is compared for performance with two othermicrofluidic pump configurations that use field actuation as shownin Table 2. The Configuration 1 [31] uses magnetohydrodynamic

0

0.005

0.01

0.015

0.02

0.025

0Heat input (W)

Dis

char

ge (m

l/s)

TSF at head=0.35 cmEFH3 at head=0.35 cm

2 4 6

. (b) Discharge versus heat input plot EFH3 and TSF at a fixed field current 2.03 A.

Table 2Comparison of thermomagnetic pump with other two different field actuation pumps at comparable length scale.

Config. 1 [29] Config. 2 [19] Config. 3 (present work)

Type Field actuation (MHD) Field actuation (FHD) Thermomagnetic convection

Operating principle Conducting fluid driven by Lorentz force

in the direction perpendicular to both

magnetic and electric fields

Pumping a secondary liquid

(e.g., water) by driving a ferrofluid plug

using a rotating permanent magnet

Ferrofluid driven by a net unbalanced

Kelvin body force as the ferrofluid is

exposed to non-uniform magnetic and

temperature fields

Dimension of pumpingchamber

Microchannel having rectangular

cross-section (1 mm�0.4 mm)

Microchannel having rectangular

cross-section (2 mm�0.25 mm)

Glass tube having inner diameter 2 mm

Power input 10–60 V DC 6.1 V, 1.9 mA DC for the motor

(corresponding to highest speed), 0.8 V,

500 mA for electromagnet coil.

2 V, 0.28 A DC to 7 V, 0.98 A DC for

heater 3.4 V, 0.63 A DC to 12.6 V, 2.3 A

DC for electromagnetic coil

Maximum B (T) 0.44 0.35 0.07

Maximum Q (ml/min) 63 45.8 1200

Maximum h 18 mm of sea water (177 Pa) 135 mm of water (1.3 kPa) 27.5 mm of EFH3 (345 Pa)

Limitations Deterioration of working fluid as a result

of bubble formation

Gradual weakening of ferrofluid plug

due to surface wetting

(i) Long term operation limited by the

maximum allowable temperature

(ii) Unwanted Joule heating from the

solenoid

S. Pal et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 2701–27092708

actuation of sea water in a microchannel while the Configuration 2[19] uses ferrohydrodynamic force to manipulate a ferrofluid plug ina microchannel to pump a secondary fluid. The Configuration 3 ofTable 2 represents the present thermomagnetic pump. It is evidentfrom Table 2 that thermomagnetic pump offers much higherdischarge compared to the other two types. The head developedby thermomagnetic pump is better than the MHD pump. Also,compared to the other two categories, our thermomagnetic pumphas the least magnetic field requirement. Besides, the thermomag-netic pump offers high regulation of flow control. The average rateof change of flow rate for field current regulation is 0.36 ml/min/mAand for heat input regulation 0.12 ml/min/mW with EFH3. Onechallenge in the thermomagnetic pumping is the maximum operat-ing temperature constraint. The other major difficulty is theunwanted Joule heating in the electromagnet, which can be resolvedusing a permanent magnet. However such a configuration wouldlose the flow control by field current regulation feature.

4. Conclusions

A table-top version of thermomagnetic pump is fabricated and itsperformance is analyzed experimentally using two different types offerrofluids. Thermomagnetic pumps are essentially low-head highdischarge devices. The study shows that the pump is capable ofproducing pressure head up to 2.75 cm of ferrofluid column (withEFH3) i.e., almost equal to 345 Pa corresponding to zero dischargecondition and maximum discharge of up to 0.02 ml/s (with EFH3)i.e., 1200 ml/min. The thermomagnetic pump design exhibits anearly hyperbolic head-discharge curve within the parametric rangeinvestigated, implying nearly constant thermomagnetic pump effi-ciency. An increase in heat input or the field strength of the solenoidimproves the performance of the pump. However, the extent ofperformance change due to each of the two inputs depends on thenature of the ferrofluid. For all the cases, the EFH3 was found toshow better performance than the TSF. The pump offers precise flowcontrol but high flow rate compared to similar pumps in itscategory. The results provide the basis for design of an on-demandthermomagnetic pumping system, which will be capable of beingintegrated into a microfluidic network.

Acknowledgment

The authors acknowledge the support of BRNS, Department ofAtomic Energy, for funding the work through the BRNS Research

Grant no. (2008/36/07-BRNS). Contribution of Mr. Srijit Chakrabortyin characterization of the ferrofluid samples is gratefullyacknowledged.

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