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Stability of magnetorheological fluids in ionic liquids

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2011 Smart Mater. Struct. 20 045001

(http://iopscience.iop.org/0964-1726/20/4/045001)

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Smart Mater. Struct. 20 (2011) 045001 (10pp) doi:10.1088/0964-1726/20/4/045001

Stability of magnetorheological fluids inionic liquidsA Gomez-Ramırez, M T Lopez-Lopez, F Gonzalez-Caballero andJ D G Duran

Department of Applied Physics, Faculty of Sciences, University of Granada, AvenidaFuentenueva s/n, 18071 Granada, Spain

E-mail: jdgarcia@ugr.es

Received 2 December 2010, in final form 31 January 2011Published 8 March 2011Online at stacks.iop.org/SMS/20/045001

AbstractIn this work, magnetorheological (MR) fluids using ionic liquids (ILs) as carrier were prepared.With this aim, two different ILs, with low and high electrical conductivity, and two kinds of ironparticles, either with silica coating or without it, were used. The viscosity of the suspensionswas measured, and the results compared with the theoretical predictions of Batchelor’sequation. The deviations from Batchelor’s equation were used to make inferences on theaggregation state of the suspensions. Contact angle measurements on iron powder pellets werealso performed in order to analyze the wetting of the particles by the ILs employed as carriers.In addition, sedimentation and redispersion experiments were carried out and discussed in viewof the aggregation degrees of the suspensions inferred from the previously mentionedexperiments. From all these experiments, it could be concluded that the suspension consistingof silica coated iron particles dispersed in the low conductivity ionic liquid presents the beststability and redispersibility properties. Finally, microscopic observations of dilutedsuspensions, carried out upon magnetic field application, showed that the most regularfield-induced structure was also obtained for this suspension.

1. Introduction

Ionic liquids (ILs) are substances composed entirely of ions,generally non-centrosymmetric organic cations with complexanions, which are in the liquid state at room temperature [1, 2].Amongst the most relevant characteristics of ILs are theirnegligible vapor pressure, thermal stability, biodegradabilityand non-flammability [2–7]. These properties make ILssuitable for many technological applications (see [2, 3, 6–17]).Despite the broad range of applications developed in recentyears, a deeper knowledge of IL properties is necessary, sincetheir physicochemical characterization is still incomplete. Theinteraction of ILs with solutes and interfaces, and the study ofthe double electric layer formed in the solid–liquid interface,remain as crucial points [7]. Other points that require abetter understanding are the claimed greenness and non-toxiccharacter [6]. Furthermore, due to the extensive industrialpossibilities of ILs, their production processes need to becomecheaper, because the current price is too high for many oftheir applications. Many scientific efforts are now focused onspreading their potentialities to other scientific branches.

Recently, the use of ILs has been extended to the fieldof magnetorheological (MR) fluids [18]. MR fluids aresuspensions of magnetizable particles in a carrier liquid,characterized by the appearance of field-induced particlestructures upon magnetic field application [19–22]. Due to thepotential applications of MR fluids (electronic devices [23],biomedicine [24, 25], drug delivery systems [26], and shockabsorbers [27, 28]), there has been in recent decades a growinginterest in improving their properties, especially their stabilityagainst the aggregation and sedimentation of their constitutiveparticles [29–34]. Guerrero-Sanchez et al [18] were thefirst authors to propose the use of ILs as carriers in MRfluids. They reported an improvement in the stability ofMR fluids when ILs were employed as carrier liquids. It issuggested that the IL carrier plays the role of a stabilizingagent, since no other stabilizers (surfactants or polymers) wereadded [35, 36]. IL-based MR fluids could be used in systemswhere the temperature and pressure conditions are extreme (forexample shock absorbers in spacecraft). Furthermore, becauseof their non-corrosive nature, ILs can represent a promisingchoice as carriers in novel MR devices in which natural rubberor another elastomer is used as a major component of the

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Smart Mater. Struct. 20 (2011) 045001 A Gomez-Ramırez et al

device [37–39]. Apart from the work by Guerrero-Sanchezet al [18], only a few papers about ILs-based MR fluids havebeen published [40–42].

In the present work, the stability and redispersibility ofMR fluids prepared using different ILs as dispersing media areanalyzed and compared with these properties for conventional(oil-based) MR fluids. In order to make inferences on thepossible stabilization mechanisms, two different ILs and twokinds of iron microparticles (characterized by having or nothaving silica coating) were used. The use of silica coatedparticles was motivated because, as suggested by Guerreroet al [43], this change in the surface chemistry of theparticles can significantly improve the stability of the IL-based suspensions. Besides, two kinds of ionic liquids, whichmainly differ in their conductivity, were chosen in order toanalyze the possible effect of the carrier properties on thesuspension stability. Information about the wettability ofmetal oxide or silica surface by ILs is obtained from contactangle measurements. Furthermore, the aggregation state ofthe suspensions is inferred from different experiments thatinvolved measurements of viscosity, sedimentation velocity,and redispersion of the suspensions. Finally, microscopicobservations of these IL-based MR fluids, both in the absenceand in the presence of magnetic fields, are presented in orderto obtain a better picture about the aggregation state of thesuspensions.

2. Experimental methods

2.1. Materials

In this work, two commercial imidazolium-based ionic liquidswere used as carriers: 1-ethyl-3-methylimidazolium ethylsul-fate (IL1) and 1-ethyl-3-methylimidazolium diethylphosphate(IL2). Both ILs were supplied by Merck, with synthesis grade.The properties of this kind of liquids are determined by thenature of the constituting ions (cation and anion). As anexample, their water solubility depends on the length of thealkane chain [4], the cation structure has an important effect inthe melting point [2], and several physical properties (viscosity,density, conductivity) strongly depend on the volume of theconstitutive ions [5]. The conductivities of the ILs employedwere 3.95 and 0.66 mS cm−1 for IL1 and IL2, respectively.Table 1 shows other relevant properties. It is important to notethat the physical properties of ILs are also severely affectedby the presence of impurities (e.g. water and chloride ions)dissolved during their synthesis or handling [4]. At thispoint, it is interesting to note that, as mentioned in [40],the water dissolved in ILs can affect the stability of thesuspensions prepared with them as carriers. In consequence,the water content of the ILs employed in the preparationof the suspensions was estimated by drying the samples at60 ◦C under vacuum (manometric pressure −93 Pa) for 4 h.After this time, no significant changes in sample weights wereobserved (with a precision of 0.001 g). Besides, in order to testthe time required to absorb a significant water amount undernormal atmospheric conditions, the weight of the samples

Figure 1. Size distributions of iron particles: (a) Fe-CC and(b) Fe-HS. The solid lines represent Gaussian fits.

was measured before and after 48 h of exposure to air. Thequantities of water absorbed were 1.7% w/w and 2.5% w/wfor IL1 and IL2, respectively. In consequence, during theshort mixing time in contact with air required to prepare thesuspensions, it is expected that the ILs acquire negligible watercontent. Obviously, the suspensions prepared in this work weremaintained in a desiccator during the storage time.

Mineral oil (density and viscosity at 20 ◦C: 0.85 g cm−3

and 39.58 ± 0.16 mPa s, respectively) from Sigma-Aldrichwas also used as carrier. Mineral oil (MO) is a typicalcarrier employed in MR fluids and in this work it is used forcomparison with IL-based suspensions.

As solid phase two kinds of iron particles were used,characterized by either having or not having a silica coating.Figure 1 shows the size distributions of both kinds ofparticles, which were obtained from TEM (transmissionelectron microscopy) micrographs measuring the diameter of200 particles. The particles coated with silica (Fe-CC) have amean diameter of 1.4±0.6 μm, while the particles without anycoating (Fe-HS) have a mean diameter of 1.0 ± 0.7 μm.

Both iron samples (Fe-CC, Fe-HS) were supplied byBASF (Germany). Their densities (given by the manufacturer)were 7.5 and 4.1 g cm−3 for Fe-HS and Fe-CC samples,respectively. Figure 2 shows a picture, obtained by highresolution TEM (HREM), of the silica coated iron particles of

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Table 1. Physicochemical properties of the ILs.

Weightformula Cation Anion

Viscosity(mPa s)

Solubilityin water

Density (g cm−3)(from supplier)

Conductivity(mS cm−1)

IL1 C8H16N2O4S C6H11N+2 C2H5O4S− 107 ± 5 Yes 1.24 3.95 ± 0.20

IL2 C10H21N2O4P C6H11N+2 C4H10O4P− 317 ± 16 Yes 1.14 0.66 ± 0.03

Figure 2. HREM micrograph of the silica coated iron particles(Fe-CC).

sample Fe-CC. As observed, the thickness of the silica coatingis not completely uniform, but the whole particle surface iscovered by it.

First magnetization curves of both powders (Fe-CC andFe-HS) were obtained at 25 ◦C using a Squid Quantum DesignMPMS XL magnetometer (Quantum Design, USA). Figure 3shows the magnetization of the powders, M , as a function ofthe magnetic field strength, H . As observed, iron particlespresent a typical ferromagnetic behavior. By averaging theplateau at high H values, the saturation magnetization, MS ,of the particles was obtained, being 1587 ± 2 kA m−1 and870.9 ± 1.4 kA m−1 for Fe-HS and Fe-CC, respectively. Asexpected, the MS of Fe-CC powder is smaller than that of Fe-HS due to the silica coating of the former.

2.2. Preparation of the suspensions

Six different suspensions were prepared by combination ofboth kinds of iron sample (Fe-CC and Fe-HS) with the threecarriers selected. The solid volume fraction was 10% in allcases. The procedure to prepare the suspensions was alwaysthe same: (i) proper amounts of the powders were pouredinto the carrier, and the resulting mixture was homogenizedby hand and using a vortex mixer; (ii) then the suspensionswere immersed in an ultrasonic bath for 10 min; (iii) theywere shaken by hand for five extra minutes. Some suspensionswere homogenized more easily than others; for example,the suspension containing Fe-CC in IL2 was completelyhomogeneous after 2 min of mixing by hand.

As we mentioned previously, the suspensions must bemanipulated and kept with special care, since they tend toabsorb water from the atmosphere. Thus, to avoid waterabsorption, all the suspensions were kept in a desiccator.

2.3. Sedimentation experiments

In order to characterize the sedimentation behavior, thesuspensions were placed in test tubes (length 120 mm;

Figure 3. First magnetization curves of the Fe-HS and Fe-CCpowders.

diameter 10 mm). The sedimentation process was followed bymonitoring the evolution of the sediment–supernatant interfacegenerated as the particles settled.

2.4. Viscosity determination and redispersion experiments

The viscosities of the suspensions and liquid carriers weremeasured using a Bohlin CS10 controlled stress rheometer.The geometry employed was a vane in cup set, with a gap of1 mm. Measurements were performed at 25 ◦C.

Viscosity measurements were performed as follows.Firstly, a preshear of 200 s−1 was applied for 1 min, followedby a waiting time of another minute, to ensure the same initialconditions; afterward, a linear ramp of stresses was applied andthe instantaneous viscosity and shear rate were measured.

Redispersion experiments were performed with the samerheometer and geometry. The protocol consisted of monitoringthe time evolution of the shear rate under a constant shearstress applied to suspensions that had previously been at restfor different times. The measurements were carried out asfollows: (i) the suspension was placed in the measuring cell;(ii) a preshear of 200 s−1 was applied for 2 min; (ii) a waitingtime (T ) was elapsed; (iv) a constant stress was applied and,as the vane rotated, the time evolution of the shear rate wasmonitored, which gave information about the stiffness of thesediment; (v) at the end of each measurement, a preshearof 200 s−1 was applied for 3 min to ensure the completeredispersion of the sediment; (vi) the same procedure wasrepeated for different waiting times (T = 0, 15, 30, 60 min,and 24 h).

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2.5. Contact angle experiments

Contact angle experiments were carried out by setting dropsof ILs (both IL1 and IL2) on the iron surface (Fe-CC or Fe-HS). To obtain a compact and homogeneous surface, pelletswere formed by compressing Fe-HS and Fe-CC powders under104 kp cm−2 for 10 min. Pictures of the liquid drops werecaptured by means of a Nikon Pixelink LCD camera connectedto a Rame-Hart 100-07-00 goniometer. For each IL–ironsample, the contact angles were measured in a series of picturesand the mean value ± standard deviation calculated.

2.6. Microscopic observations

For microscopic observations the MR samples were placedbetween two glass slides. These observations were carriedout in the absence and presence of an applied magneticfield, in order to know whether there were differences in theaggregation state and in the magnetic-induced particle chainingwhen different liquids (IL1, IL2, MO) were used as carriers.For this aim a Nikon SMZ800 microscope (Japan) with a 6×objective was employed. The magnetic field was applied eitherperpendicular or parallel to the microscope objective axis. Theperpendicular field was applied with a pair of Helmholtz coils,whereas the parallel one was applied using a coil. To carry outthis experiment, the suspensions were diluted up to 0.75 vol%,since suspensions containing 10 vol% were opaque and thusnot suitable for microscopic observations.

3. Results and discussion

3.1. Viscosities of the suspensions

As described in the previous section, the viscosities ofthe suspensions and carrier liquids were measured. Fromsuch experiments we can obtain the relative viscosity of asuspension, that is the suspension viscosity divided by thecarrier viscosity. Figure 4 shows the values of the relativeviscosity for the different suspensions, and the theoreticalprediction of Batchelor’s equation [44]:

ηr = η

ηs= 1 + 2.5φ + 6.2φ2. (1)

Here, φ is the volume of solid (iron powder), η theviscosity of the suspension, ηs the carrier viscosity and ηr

the relative viscosity. Note that Batchelor’s equation holdsgood for suspensions with a solid volume fraction φ �0.1. This equation takes into account the effect of two-bodyhydrodynamic interactions between rigid spheres, which areresponsible for the term φ2. Thus, when a good fit betweenthe experimental results and Batchelor’s prediction is obtained,it is expected that only the hydrodynamic interactions areimportant in the suspensions. On the contrary, when theexperimental curves are far from the theoretical prediction, itcan be deduced that there are other significant interactions. Itis not clear which the dominant interparticle interactions are inthe IL carriers. Altin et al [45] suggested that particles could bestabilized by electrostatic and steric repulsion. Nevertheless,Guerrero et al [18] consider that only the steric repulsion could

Figure 4. Relative viscosities of suspensions containing a solidvolume fraction φ = 10%. The solid lines correspond to Fe-CCsuspensions, and dashed lines to Fe-HS suspensions. The carriersused are indicated in the graph. The theoretical prediction ofequation (1) is included ( ).

be relevant for this aim, in agreement with Ueno et al [46]. Inthis last paper, it is suggested that the electrostatic repulsion isinefficient in ILs because of the high ionic atmosphere whichscreens the particle surface charge. On the other hand, Khareet al [47] proposed that the formation of surface coordinativecompounds between iron and IL anions could lead to thestabilization of the particles.

In a recent work [48], it was shown that in MR suspensionsa larger deviation with respect to Batchelor’s formula isassociated with a higher aggregation degree in the suspension.As clearly seen in figure 4, suspensions of either Fe-HS orFe-CC in mineral oil show the largest deviation with respectto Batchelor’s prediction. The lack of homogeneity and thepresence of other than two-body hydrodynamic interactionsin these suspensions, which results in a high aggregationdegree, could explain such behavior. When suspensions in MOwere prepared, it was really difficult to obtain homogeneousmixtures: a transparent supernatant was observed just a fewseconds after suspension preparation. When IL1 was used ascarrier, the relative viscosity of the suspension was closer tothe theoretical predictions. The differences observed betweenthe two kinds of iron powders are not significant in this case.Finally, suspensions in IL2 show the best results; for Fe-CC inIL2 the relative viscosity of the suspension fits almost perfectlyto Batchelor’s prediction. It is noticeable that the relativeviscosities for suspensions of Fe-CC are closer to Batchelor’sprediction than those obtained for suspensions of Fe-HS, forall the liquid carriers employed, pointing out that the silicalayer plays a significant role in obtaining less aggregatedsuspensions.

The differences found between suspensions in IL1 andIL2 are likely due to the different volumes of the anions ofthese liquids. In a recent work [5], it was reported that theconductivity of ILs diminishes as the volume of the anionincreases. In our work, the conductivity of IL1 is larger thanthat of IL2 and, consequently, the volume of the anion ofIL2 (diethylphosphate) is larger than that of IL1 (ethylsulfate).

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Table 2. Contact angles for ILs on Fe-CC and Fe-HS pellets.

Liquid Fe-CC Fe-HS

IL1 (high conductivity) 40◦ ± 4◦ 34◦ ± 6◦IL2 (low conductivity) 23.8◦ ± 1.7◦ 33◦ ± 5◦

Thus, as suggested by Khare et al [47], it is expected thatthe anions of IL2 are able to form a thicker steric barrier,via formation of surface complexes, which would justify thefinding that suspensions in IL2 are more stable.

On the other hand, contact angle measurements can help todiscriminate the wettability of the solid surface by ILs. Table 2shows the contact angles formed by the IL droplets on Fe-CCand Fe-HS pellets.

The lowest contact angle was obtained for droplets of IL2on Fe-CC, which means that the best wettability is reached forFe-CC particles dispersed in IL2. Consequently, in this case,the dispersion of particles should be easier. This result agreeswith the fact that the dispersion of Fe-CC in IL2 was relativelyeasy, as observed during the preparation of the suspensions.On the contrary, in the other cases (IL1-Fe-CC, IL1-Fe-HS,and IL2-Fe-HS) the wettability of the solid particles is worsethan for Fe-CC in IL2.

3.2. Sedimentation results

The sedimentation of a single particle inside a fluid dependson the gravitational and viscous forces. Taking into accountStokes’ law, the terminal velocity of a particle in a fluid can bewritten as follows:

vl = 2g(ρp − ρf)R2

9η(2)

where vl is the terminal velocity, g is the gravitationalacceleration, R is the particle radius, η is the carrier fluidviscosity and ρp and ρf are the density of particle and carrier,respectively.

From equation (2), it is clear that as the carrier viscosityincreases the particles will settle at a lower rate. In this work,different carrier liquids and powders were used. Consequently,in order to scale the sedimentation results, a normalized timet∗ was defined as follows:

t∗ = t (ρp − ρf)R2g

ηh0. (3)

In expression (3) t is referred to the time elapsed in theexperiment, h0 is the initial height of the suspension in the testtubes (in all cases 10.6 ± 0.1 cm). Using this normalized timethe sedimentation behavior of different particles in differentliquids can be safely compared. Note that t∗ takes into accountthe density and size of the particles and also the viscosity ofthe carrier.

Figure 5 shows the evolution of the normalized height(h/h0, h being the height of the sediment–supernatantinterface) as a function of the normalized time t∗. As canbe seen, h/h0 decreases with time towards a final plateau,which corresponds to the stationary state. The slopes of these

Figure 5. Normalized sediment height as a function of thenormalized time (see equation (3)) for suspensions containing Fe-CC(a) and Fe-HS (b). Carrier: , IL2; ��, IL1; ×, MO.

curves depend on the aggregation state of the suspension andon the possible friction between particles and/or aggregateswith the walls of the tube. In a previous work [29], it wasfound, from similar experiments, that the settling rate wasconsiderably reduced when the particles in the suspensionswere aggregated. This was associated to the fact that thepresence of large aggregates that spanned the test tube couldreduce the settling rate. Furthermore, the final stationary heightof the sediment–supernatant interface can also be used formaking inferences on the aggregation state of the suspension:the packing of aggregates is worse than that of individualparticles, thus increasing the final height of the sediment.

In figures 5(a) and (b), it can be observed that thetendencies are similar when Fe-CC or Fe-HS is used. Adecreasing curve is obtained, which corresponds to the settlingof the particles; the final plateau corresponds to the stationarystate. The curves obtained for IL1 and IL2 are approximatelysuperimposed, both for Fe-CC (figure 5(a)) and Fe-HS(figure 5(b)), while the value of h/h0 obtained for mineral oildecreases at a lower rate. In addition, the height of the sedimentis higher in MO than in ILs, which means that the sediment isless compact. According to the previous discussion, this couldbe associated to a higher degree of aggregation in MO-basedsuspensions.

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Figure 6. Evolution of the normalized sediment height with thenormalized time for the indicated suspensions.

From the results in figure 5, it could be concluded thatthere are no appreciable differences in the settling rates ofsuspensions of the same kind of powder in the different ILs.Probably, this method, based on monitoring the sediment–supernatant interface, is not sensitive enough to obtain preciseinformation on the different states of aggregation that takeplace in the different ILs. If we look back to figure 4and table 2, approximately the same results were obtainedfor Fe-HS dispersed in IL2 and IL1, which agrees withthese sedimentation experiments. On the contrary, for Fe-CCsuspensions, significant differences in the relative viscosity andcontact angle were found when IL1 or IL2 was used as carrier.

Figure 6 shows a comparison for both kinds of powders inIL2. It is clear that there is an appreciable difference whenFe-CC is used instead of Fe-HS. When Fe-CC is used thesedimentation rate is higher, which means a lower state ofaggregation in the suspension. Besides, the final sedimentheight is lower with this silica coated iron powder. Therefore,the silica coating of Fe-CC particles should play an importantrole in the stability of suspensions. Similar results were foundin the case of IL1 (not shown here for brevity). In order tocomplete this discussion, the curve corresponding to Fe-CC inMO is also included in figure 6 for comparison. It is interestingto note that this curve is close to that corresponding to Fe-HSin IL2.

Finally, we will briefly turn to the sediment height inthe stationary state. For the case of Fe-CC in IL1 or IL2 itcan be affirmed that well compacted sediments were formed:the final height was 1.5 cm, which, compared with the initialheight (10.6 cm), gives a sediment volume fraction close to10% (the actual particle volume fraction). On the contrary,for suspensions of Fe-HS the final sediment height in ILs wasaround 3 cm, which means a more aggregated state.

3.3. Redispersion results

In section 2 the protocol followed to analyze the redispersibil-ity of suspensions was described. It is important to note thatno normalization time will be used in this section, since thestress applied in each case was that required for reaching a

Figure 7. Normalized shear rate (shear rate divided by the plateaushear rate value at zero waiting time) as a function of the measuredshearing time for suspensions of Fe-HS (full symbols) and Fe-CC(open symbols) in MO, and for the indicated waiting times.

shear rate of approximately 100 s−1 at zero waiting time. Thus,the effects of viscosity, particle size and density were alreadytaken into account by this stress. Such stresses were obtainedfrom the viscosity curves shown in figure 4. Since the shearrate reached for the different waiting times was not exactly100 s−1, results will be normalized by the plateau shear ratevalue at zero waiting time.

Figure 7 shows the results obtained for suspensions ofFe-HS and Fe-CC in mineral oil. As observed, all thecurves present a similar trend, characterized by an initialsharp increase, where most of the aggregates are likely brokenand redispersed, followed by a plateau, which represents theequilibrium between destruction and reconstruction of suchaggregates. As the waiting time increases, the normalizedshear rate in the plateau becomes smaller in value, which couldbe associated to the formation of sediments in the bottom of thecup. After an hour of waiting time, the normalized shear ratereached is less that a half of the initial value. This means thatthe rotation of the vane is considerably hindered by the settledaggregates.

Let us first center our attention on the comparison betweenthe suspensions of both kinds of iron powders dispersed in MO.

(i) For waiting times of 1 h or shorter, the suspension ofFe-HS reaches higher normalized shear rates than thosereached for the Fe-CC suspensions. To explain theseresults we must recall that the suspensions are not inthe stationary state: 1 h of waiting time in figure 7corresponds to normalized times in figure 5 of t∗ =0.014 and 0.012 for suspensions containing Fe-CC andFe-HS, respectively. For these t∗ values, it is clear thatthe suspensions have not reached the stationary state (seefigure 5), although there are more particles settled in theFe-CC suspension than in the Fe-HS one (h/h0 = 0.6 forFe-CC; h/h0 = 0.9 for Fe-HS). Thus, taking into accountthat both suspensions in MO present a high aggregationdegree, it could be concluded that in the case of Fe-CCthe vane rotation is hindered by the presence of boththe sediment and the irreversible aggregates remaining

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Figure 8. Normalized shear rate as a function of the measuredshearing time for suspensions of Fe-HS (full symbols) and Fe-CC(open symbols) in IL2, for the indicated waiting times. The curvecorresponding to the suspension containing Fe-CC in IL1 at 24 hwaiting time is included for comparison.

in suspension. On the other hand, in the case of theFe-HS sample, the shearing must be mainly hindered bythe aggregated particles present in suspension, since theredoes not exist a significant phase separation.

(ii) The most important comments come from the resultsobtained for the suspensions already in the stationary state.Note that 24 h of waiting time corresponds to a normalizedtime (figure 5) of approximately t∗ = 0.26 (Fe-HS) andt∗ = 0.28 (Fe-CC). For these normalized times most ofthe particles are already in the sediment, and, thus, thehardness of the sediment will be evaluated. As can be seenin figure 7, the suspension of Fe-CC particles developsa higher normalized shear rate (softer sediment) than thesuspension of Fe-HS particles (harder sediment).

Summarizing, it could be concluded that even in the MO-based suspensions the silica coating hinders to some extent theirreversible aggregation of the particles, easing the formationof soft sediments.

Let us now analyze the redispersibility for the suspensionsof both iron samples in IL2 (figure 8). At first glance, it couldbe thought that there is no important difference between thetwo kinds of iron powders, for the waiting times studied. Thisagrees well with the results of the sedimentation measurementsat short times: for times shorter than 1 h, which correspondsto normalized times of t∗ = 0.0016 (Fe-HS) and t∗ =0.0013 (Fe-CC), there were no important differences in thesedimentation rate (see figure 5).

However, although for a waiting time of 24 h (t∗ =0.025 31 for Fe-CC, and t∗ = 0.0294 for Fe-HS in figure 5)the curves of figure 8 are practically superimposed, in this casethere were appreciable differences in the sedimentation state:while the suspension of Fe-CC was close to the stationarystate (h/h0 = 0.25, figure 5), the suspension of Fe-HS wasfar from it (h/h0 = 0.64). Thus, since the rotation of thevane is likely more hindered by the presence of the sedimentthan by the presence of irreversible aggregates in suspension,the apparent controversy is just explained because the Fe-CC

suspension is in a more advanced settling state than the Fe-HSone. For that reason, for equal shearing times, the vane rotationmust be relatively more hindered in the suspensions of Fe-CCas observed in figure 8.

As a final point, in figure 8 the curve for Fe-CC in IL1, at24 h waiting time, is included. In this case, the normalizedshear rate reached is much lower than those reached forsuspensions in IL2. From this, and similar results, not shownhere, the redispersibility of the suspensions in IL1 presents anintermediate behavior between that observed for suspensionsin IL2 and MO.

3.4. Microscopic observations

In order to analyze the initial degrees of aggregation of thesuspensions, and the differences in the magnetic field-inducedstructures, microscopic observations were carried out. Figure 9shows pictures obtained in the absence of magnetic field. Themain features in these pictures can be summarized as follows.(i) The suspensions in IL2 are the most homogeneous forboth iron powders: no aggregation is observed in pictures 9(a)and (d). (ii) The suspensions in MO (9(c) and (f)) present ahigh aggregation degree. This is expected: when suspensionsin IL2 were prepared the homogeneity was reached very easilywhereas it was difficult to homogenize the suspensions inMO. And (iii) for IL1 it could be observed that the state ofaggregation is intermediate between those observed in IL2 andMO, in agreement with results of the previous sections.

In figure 10, the structures formed under a magneticfield of 13.2 mT, applied perpendicularly to the microscopeobjective axis, are shown. When pictures with differentcarriers and the same particles are compared, it can beobserved that the most homogeneous chain structures (uponmagnetic field application) were formed for the case of IL2.For suspensions in MO, a non-homogeneous pattern wasobtained, characterized by the presence of large aggregatesforming entangled chains. The suspensions in IL1 present anintermediate behavior between these.

In order to gain more information about the field-inducedstructures and to try to discriminate between the chainingpatterns in Fe-CC and Fe-HS, observations with a magneticfield of 88 mT applied parallel to the microscope objectiveaxis were taken. In this case, the particle volume fractionwas φ = 0.02 vol%. Figure 11 shows the results obtainedfor suspensions of Fe-CC and Fe-HS in IL2 and MO, whichconfirm the previous statement: there are thicker aggregates inMO suspensions, which hinders the formation of regular field-induced structures (figure 11(b)). Furthermore, the differencesbetween Fe-CC and Fe-HS are easier to see in pictures takenwith the vertical magnetic field, since when using a horizontalmagnetic field different particle chains were on differentfocal planes, and the pictures are less clear (see figure 10).Comparing figure 11(a) for both kinds of iron, it can beobserved that for Fe-CC the chain distribution appears moreregular than in the case of Fe-HS. This fact suggests a loweraggregation degree in the suspension of Fe-CC in IL2 carrier,which is consistent with the results obtained in the previousexperiments.

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Figure 9. Microscopic pictures in the absence of applied magnetic field for suspensions of Fe-HS ((a)–(c)) and Fe-CC ((d)–(f)), in IL2 ((a),(d)), IL1 ((b), (e)) and MO ((c), (f)). The particle concentration was approximately φ = 0.75 vol%.

Figure 10. Microscopic pictures upon magnetic field application (13.2 mT, perpendicular to the microscope axis) for suspensions of Fe-CCand Fe-HS in IL2 (a), IL1 (b), and MO (c). The particle concentration was approximately φ = 0.75 vol%.

4. Conclusions

In this paper we have reported an experimental study on thestability and redispersibility of IL-based iron suspensions. Wehave used two kinds of iron particles (either with silica coatingor without it) as the solid phase and two different ILs as carriers

(with low and high electrical conductivity). The behavior ofthese suspensions has been compared with that of conventional(oil-based) MR fluids.

From the whole series of experiments performed, it canbe concluded that the stability against aggregation and theredispersibility of concentrated iron suspensions are improved

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Smart Mater. Struct. 20 (2011) 045001 A Gomez-Ramırez et al

Figure 11. Microscopic pictures upon vertical magnetic field application for suspensions of Fe-CC and Fe-HS in IL2 (a) and MO (b). Theparticle volume fraction φ = 0.02 vol%.

for both kinds of powders when ILs are used as carriers. It isnoticeable that, when a low conductivity ionic liquid (IL2) isemployed as carrier, the suspensions show a better wettability,stability and redispersibility. Furthermore, the suspensionsprepared with silica coated iron particles dispersed in IL2 showthe best colloidal stability.

In order to explain the best stability observed in thislast suspension (silica coated particles/IL2), a number ofparameters must be considered, such as viscosity, meltingpoint, dielectric constant, wettability, conductivity, andcoordination ability of the IL-anion. In this work, thequantities employed in the representation of rheological andsedimentation data have been normalized to remove the effectof the density and viscosity of the particles and IL carriers.Thus, the differences observed in the stability of the differentparticle/IL combinations studied are restricted to three factors:the conductivity of the ILs (which is related with the anion

volume), the surface complexation, and the wettability ofthe solid surface by the ILs. Finally, we can conclude thatthe best stability observed in the Fe-CC–IL2 suspension isvery likely due to the combination of the better wettabilityachieved in such a particle/IL combination, and the formationof surface chemical compounds with the anion present in theIL2 carrier. In turn, the fact that the lower conductivity of IL2is related to a larger volume of the anion points towards stericrepulsion as the main stabilization mechanism. However, thebinding of the anions to the particle surface should not be areason for proposing an electrostatic stabilization in IL-basedsuspensions, because of the charge screening resulting from thehigh ionic atmosphere.

Summarizing, it seems apparent that low conductivityionic liquids (used as carriers) are capable of imparting ahigh colloidal stability to concentrated iron suspensions incomparison with oil carriers: that is, these highly polar liquids

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Smart Mater. Struct. 20 (2011) 045001 A Gomez-Ramırez et al

work as suitable stabilizing agents to avoid the aggregationamong metallic particles (dispersed in them), simultaneouslyeasing their redispersion. This fact reinforces the promisingfuture for ionic liquids in the formulation of environmentallyfriendly lubricants.

Acknowledgments

Financial support by Ministerio de Ciencia e Innovacion(Spain) under project no. FIS2009-07321 and by Junta deAndalucia (Spain) under project nos P08-FQM-3993 and P09-FQM-4787 is gratefully acknowledged. AG-R and MTL-L acknowledge financial support by Secretarıa de Estado deUniversidades e Investigacion (MEC, Spain) through its FPUprogram and by Universidad de Granada (Spain), respectively.The authors thank Dr C Guerrero-Sanchez for supplying theILs and for his valuable suggestions.

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