silicone oil dispersions

Journal of Colloid and Interface Science 392 (2013) 75–82

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Effect of surface properties on the electrorheological response of hematite/silicone oil dispersions Ozlem Erol a, María del Mar Ramos-Tejada b, Halil I. Unal a, Ángel V. Delgado c,⇑ a

Gazi University, Faculty of Science, Department of Chemistry, 06500 Ankara, Turkey University of Jaén, Department of Physics, 23071 Jaén, Spain c University of Granada, Faculty of Science, Department of Applied Physics, 18071 Granada, Spain b

a r t i c l e

i n f o

Article history: Available online 9 October 2012 Dedicated to Professor Egon Matijevic´ on the occasion of his 90th birthday. Keywords: Elastic modulus Electrorheological behavior Hematite Hydrophobization Oleic acid Viscous modulus Yield stress

a b s t r a c t In this work we present an investigation of the influence of particle surface characteristics on the electrorheological (ER) behavior of suspensions of either pure or modified hematite (a-Fe2O3) particles dispersed in silicone oil. The modification consisted of either dehydration or hydrophobization of the particles before preparing the suspensions. A comparison was performed between the electrorheological responses of suspensions with the same volume fraction of hematite particles having different surface properties. The effects of applied electric field strength on the viscosity, yield stress and dynamic moduli of these suspensions were examined. It was found that the usual positive ER response, that is, enhanced values of the yield stress and elastic modulus induced by the electric field were obtained for hematite and, to a lesser extent, for dried hematite suspensions. In contrast, a ‘‘negative ER effect’’, i.e., the reduction of yield stress and elastic modulus upon application of electric field was observed for hydrophobically modified (oleic acid coated) hematite. This means that the field produces destruction of structures rather than their build up, above a threshold electric field strength. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction Electrorheological (ER) fluids are defined as a class of smart materials characterized by special rheological properties, which undergo reversible and considerable changes upon application of an externally applied electric field (E). Generally, ER fluids are made of solid, polarizable particles or liquid crystals dispersed in an insulating solvent. Sometimes, they also contain small amounts of additives such as polar compounds or surfactants which can enhance the ER effect and/or the stability of the whole suspension [1]. In addition to their fundamental interest, ER materials have many potential applications, in such areas as vibration damping systems, semi-active actuators, and artificial muscles. But there are limitations for their wider applications, including: possible colloidal instability, environmental concerns, abrasion, corrosion, insufficient yield stress, etc. The ER performance of a material is closely related to the conductivity and polarizability of the materials involved, and it will hence be largely dependent of the physical properties of its components. In fact, many ER dispersions described in the literature show positive ER response when subjected to external electric fields of sufficient strength, i.e., the application of the

⇑ Corresponding author. Address: Department of Applied Physics, School of Sciences, Campus Fuentenueva, University of Granada, 18071 Granada, Spain. E-mail address: [email protected] (Á.V. Delgado). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.09.060

field provokes the appearance of plastic behavior characterized by a finite yield stress and a large increase in the effective viscosity. Several theories have been proposed to explain this phenomenon, but the most popular approach is the so-called polarization model [2–6]. This assumes that the ER effect results from the dielectric polarization of particles suspended in a non-conducting fluid, arising from the difference between the electric permittivities of the dispersed particles and the liquid medium. Field-induced polarization of the dispersed particles leads to dipolar interactions between them, which provoke their alignment into chains reinforcing the suspension, with subsequent viscoelastic behavior and field-dependent viscosity. The attractive force between the particles is proportional to the square of the product of the applied electric field E and the dielectric mismatch parameter be ¼ ðep  ef Þ=ðep þ 2ef Þ, where ep and ef are the real components of the relative electric permittivities of the particles and the host fluid, respectively. Because the model was found unsuitable for explaining all kinds of ER behaviors [7–10], some modifications were proposed, the most straightforward of which was the use of the complex, frequency-dependent permittivities (ep ; ef ) instead of their low-frequency, real values [11–15]. This permits to distinguish the mechanisms prevailing at low and high field frequencies. At high frequency ac field, the permittivity difference is the determinant quantity, and this is the core of the polarization model, whereas in the conduction model [16–18], it is the ratio of the conductivities

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of the particles and host fluid, C = rp/rf that matters. Qualitative analyses [18,19] suggest that an aggregation of the particles to form chains between the two electrodes is possible only if U > 1. The attractive force responsible for particle chaining is then proportional En, were n ? 2 at low fields, and n ? 1 at high fields, the latter being a consequence of the field dependence of the liquid conductivity (rf = rf(E) [15–19]). Foulc et al. [20] propose an approximate mathematical expression for evaluating the force between the particles at high field, in the case of C  1:

" F ¼ 2pe0 ef r

2

(  1=2 )#2 10C 2E ln EEC p EC

ð1Þ

where r is the radius of the particle, e0 is the permittivity of vacuum, and the factor Ec is related to the following approximation of rf(E):

rf ðEÞ ¼ rf ðE ¼ 0Þ½1  A þ A expðE=EC Þ

ð2Þ

In this expression, the parameters A and EC are chosen in such a way that the Onsager’s law is best approximated, and its values are A = 0.1 and EC = 0.335 kV/mm for E in the 0.15–0.50 kV/mm [20]. Still more intriguing is the fact that a sort of negative ER effect: in such cases, the viscosity and yield stress are found to decrease with the increase in field strength. Thus, Trlica et al. [21] found such kind of behavior in suspensions of flat magnesium hydroxide particles, and ascribed it to the anisotropy in dielectric polarizability of the solids. Similar experiments were performed with aluminum hydroxide, with more rounded particles, and, because a standard positive ER effect was observed, it was suggested that the shape of particles plays a significant role on this effect. On the other hand, the effect of the chemical structure of the material on the ER response for urethane-based polymers having different terminal groups was considered by Uemura et al. [22], who observed either a positive or negative ER effect depending on the dielectric nature of the terminal groups. A model elaborated by Hao et al. [23] predicted a negative ER effect only if the temperature dependence of ep fulfills the condition dep/dT < 0. Boissy et al. [19] proposed instead that the electrophoretic phenomenon plays a key role in promoting the negative ER effect. These authors reported that when the conductivity of dispersed solid particles is smaller than that of the dispersant fluid, the particles migrate towards one electrode under the action of the electric field. Similar observations were published by Wu and Conrad [24] for teflon/silicone oil suspensions. Their theoretical analysis showed that in dc fields like those considered in the present investigation, the electrorheological response will be negative if rp < rf and ep < ef, or rp  rf and ep > ef. In a different study, Feng et al. [25] found that ZnO nanowires showed negative ER response and migrated towards both electrodes due to an electron transfer among them, despite the fact that rp > rf. Similar particle migration toward the both electrodes was observed for elongated goethite; and it was further reported that the addition of silica nanoparticles to the goethite suspension or the increase in the viscosity of the dispersant phase converted the negative ER effect to positive by hindering the particle charging and migration [26]. In addition to the electric properties of the particles and the medium, and the electric field strength, other quantities have been identified as factors establishing the ER response. Volume fraction, temperature, and water content of the particles or the fluid are among them. In this work, attention will be paid to the role of particle surface properties, which may influence not only the ER behavior, but also the dispersion stability. In fact, most ER fluids contain additives such as surfactants and promoters, aimed at improving the response of the suspensions to the applied field through control of the solid/liquid interfacial properties [27–30].

It can be expected that the wettability of the particles by the fluid can play an important role in the ER effect, considering that electric breakdown, stability and interfacial polarization are closely related to the wettability of particles [31]. Wang et al. [32] found that modification of TiO2 nanoparticles from hydrophilic to lypophilic led to enhancement of the ER effect. As another example, Shen et al [33] reported no measurable ER response in suspensions of barium titanyl oxalate coated with urea (BTRU) nanoparticles in hydrocarbon oil. Interestingly, by adding a small amount of oleic acid to the hydrocarbon oil, they achieved a high yield stress. They suggest that the surface tension between the particles and oil is greatly reduced due to the mediating effect of the oleic acid molecules, thus allowing the particles to disperse and to move close together upon the application of an electric field. According to these authors, the amount of added oleic acid is critical because the adsorbed oleic acid molecules should not prevent the close contact of the BTRU particles under the electric field. Fang et al. [34] investigated theoretically the effects of zero-field dispersity of colloidal particles and particle wettability on the ER effect, and found that well dispersed particles resulting from improved particle wettability experience much stronger attraction than unstable ones aggregated into clusters. In other works it is proposed that wetting characteristics themselves cannot be solely responsible for an improved ER effect. Wang et al. [35] point out the fundamental influence of the dielectric properties, including permittivity, conductivity and dielectric loss in obtaining high-performance ER materials. Shen et al. [36] proposed a different physical mechanism for ER fluids with nanoparticles coated with polar molecules. These fluids display very high yield stresses. When an electric field E is applied, the polarized particles in the fluid attract each other and align along the field direction to form chains. Sufficiently strong electric fields can effectively bring the particles together because the local field Eloc is capable of aligning the polar molecules in the interparticle gap. They suggest that the total attractive force between the polar molecule and the polarization charge on the particle, Fm–e, is, for a unit area of interface, given by:

F m—e ¼

3/ 3/qm el2 E Nfm—e ¼ C 2 2pr pre0 ef d2

ð3Þ

in which

F m—e ¼

el 3

2pe0 ef dm—e

ð4Þ

where / is the volume fraction of the particles in suspension, N is the number of active polar molecules on each particle, qm is the area density of the polar molecules adsorbed on the particles, e is the fundamental charge, l and d are the dipole moment and the size of the polar molecule and C encloses information on the properties of the particles and the fluid as well as on the adsorption energy of the polar molecules. The authors conclude that the particle–particle force is much weaker than Fm–e, and therefore the ER effect comes mostly from the latter. In this work, the electrorheological behavior of hematite/silicone oil suspensions is investigated under dc electric fields. In order to evaluate the effects of surface properties on the ER response of suspensions of, they will be subjected to some treatments, including dehydration (aimed at studying the effect of adsorbed water) and hydrophobization (the target in this case is exploring if better wettability improves the response), before dispersing them in silicone oil. The effects of E on the yield stress and viscoelasticity of these suspensions will be investigated. Rheological data are completed by optical observations of the structures in the suspensions under the action of the field.

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2. Experimental

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the wetting liquid. Drops of water were deposited on compressed pellets of the hematite and coated hematite particles.

2.1. Materials The hematite, a-Fe2O3, particles used in this investigation (qp = 5.12 g cm3, average particle size 105 ± 25 nm [37]) were purchased from Sigma–Aldrich (USA). Oleic acid coating was the method employed for particle hydrophobization, and the materials used were oleic acid (purity 90%) and HNO3, both supplied by Sigma–Aldrich, and ultrapure water (Milli-Q Academic, Millipore, USA). The dispersing phase was always poly(dimethylsiloxane) (silicone oil, SO) manufactured by Sigma–Aldrich with nominal viscosity ranging between 10 and 100 mPa s. All chemicals were used as received.

2.2. Preparation of the ER fluids Hematite/silicone oil (H/SO) and dried hematite/silicone oil (DH/SO) suspensions with 10% volume fraction were prepared by slow addition of the powder to the oil under mechanical stirring (Vortex B1 plus, Boeco, Germany). Later, the suspensions were further homogenized in an ultrasonic bath (Selecta, Spain). The DH/SO suspensions were prepared as described above, but both hematite and silicone oil were previously maintained in an oven at 120 °C for 24 h. The silicone oil viscosity was 100 mPa s in all cases. The procedure followed for the preparation of oleic acid-coated hematite/silicone oil (CH/SO) suspensions consisted of the following steps: (i) 17 g of hematite was dispersed in 400 mL of deionized water under vigorous mechanical stirring for 10 min. (ii) 5 mL of oleic acid was added dropwise to the above solution at room temperature and the pH was adjusted to 6.5 using a solution of HNO3. The resulting solution was aged under vigorous mechanical stirring for at least 1 h at room temperature. At this pH value, the hematite surface is positively charged because its isoelectric point is at pHiep  8.5 [38], while the proportion of oleate ions is still high enough (the pKa of oleic acid, determined by titration, was reported to be between 8.0 and 8.5 by Cistola et al. [39]) to promote the adsorption of the oleate ions on the hematite particles. The coating produces hematite coagulation due to the hydrophobicity provided by the oleate layer (see the Supporting information: Fig. S1 illustrates the considerable change in the suspension appearance upon treatment). (iii) The precipitate was washed at least four times with acetone to remove water and non-adsorbed oleic acid. (iv) A specified amount of SO was added to the acetone-wet precipitate. The resulting suspension was taken to a vacuum chamber and heated to 35–50 °C under mechanical stirring for 3–4 h to evaporate acetone and homogenize the suspension and, finally, SO was added to the above suspension up to a final volume fraction of solids / = 10%. All hematite/SO suspensions were allowed to equilibrate for at least 24 h before experiments. They were homogenized again by stirring and ultrasonics immediately before measurements were performed.

2.3. Contact angle measurements The advancing contact angle measurements were conducted at room temperature using a goniometer (DSA 100, Krüss, Germany) equipped with a microliter syringe. Ultrapure water was used as

2.4. Electrorheological measurements ER experiments were performed (at 25.0 ± 0.1 °C) in a Bohlin CS-10 rheometer (Malvern Instruments, Malvern, UK) using the ER cell designed by the manufacturer (a parallel-plate system, 40 mm in diameter with a gap of 1 mm between them). A copper wire (0.1 mm in diameter) was used to electrify the upper plate while the lower one was grounded. To avoid the passage of electric current through the rheometer, the upper plate of this ER cell was fixed by means of a plastic piece. The electric field (ranging between 0 and 3 kV/mm) was generated by a DC power supply (LD Didactic 521–535, Germany) connected to a Trek Model 606E6 High Voltage Amplifier (Trek, USA). All the samples were subjected to a 60 s pre-shear (shear rate 50 s1) and then allowed to equilibrate with the applied electric field (in absence of shear) for 60 s. Two kinds of experiments were then performed: (i): steady state viscous flow measurements consisting of shear stress ramps in which shear rate and viscosity data were collected while the electric field was kept at a constant strength for each test; (ii) in the oscillatory shear measurements, the complex modulus of the systems was measured as a function of the frequency (between 0.1 and 10 Hz) of the applied oscillating shear stress for different magnitudes of the electric field.

2.5. Dielectric measurements Because of the difficulty of directly measuring the dielectric properties of the particles, we used suspensions to carry out dielectric investigations. The permittivity of coated hematite dispersed in silicone oil (nominal viscosity 20 mPa s) suspension with / = 10% volume fraction was measured with an Agilent 4284A (USA) impedance meter. The measuring cell was the 16452A Liquid Test Fixture, also from Agilent. From the real and imaginary components of the impedance of the suspensions, their permittivity and conductivity were obtained, by previously calibrating the cell in air.

2.6. Microscopic observations For microscopic observations the samples (10% volume fraction) were placed them in a chamber obtained by closing one of the ends of a Philips PR 9510 (Eindhoven, The Netherlands) parallel-plate conductivity cell by gluing a glass slide. A scheme of the setup can be found in [26]. The distance between the platinized electrodes was 0.76 ± 0.02 cm, and the cell was filled up to 3 mm from the bottom (0.5 mL sample volume). The cell was placed on the stage of a Nikon SMZ800 (Amstelveen, The Netherlands) optical microscope (300 maximum magnification), and the image was captured with a PixeLink PL-A662 (Ottawa, Canada) CCD camera.

2.7. Zeta-potential measurements Zeta (f)-potentials of the colloidal dispersions were obtained from electrophoretic mobility data in a Malvern Nano-ZS zeta potential analyzer which works with the Laser Doppler Electrophoresis technique by using phase analysis light scattering. The self optimization routine (laser attenuation and data collection time) in the Zeta-Sizer software was used for all the measurements. The f-potentials of the materials colloidally dispersed in SO (g = 102 Pa s, Aldrich), having the concentration of 0.1 g/L, were measured using a dip-cell and calculated using the Hückel approximation.

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3. Results and discussion 3.1. Contact angle determination Water contact angle measurements were performed to check the changes in the wetting behavior of hematite after oleic acid treatment. Pictures of the water contact angle onto hematite (a), and coated hematite (b) surfaces are given in Fig. 1. The contact angle on hematite increased from 41 ± 4° to 96.3 ± 1.2°. The contact angle of water on pure hematite of both natural and synthetic origins has been reported to range from 15° to 46° [40–42]. The results provide evidence of the change of wettability: the introduction of the oleic acid molecules on the surface of hematite increased, as desired, the hydrophobicity of the iron oxide particles.

3.2. Electrorheological response: effect of electric field strength Fig. 2 shows shear stress (s) vs. shear rate c_ data, for various electric field strengths (E) in the case of (a) untreated hematite suspension (H/SO), (b) dried hematite suspension (DH/SO) and (c) coated hematite suspension (CH/SO). H/SO and DH/SO suspensions show shear-thinning behavior, the viscosity decreasing rapidly with increasing shear rate. On the other hand, the effect of the applied field is more pronounced at low shear rates, when hydrodynamic interactions are weaker. Finally, at high-shear rates, curves corresponding to different field strengths tend to merge and reach the values attained in the absence of field, showing that the electric field ceases to have an effect on the rheogram. Furthermore, in Fig. 2a it is clearly seen that the values of shear stress of H/SO suspensions increase with increasing applied electric field strength, particularly at low and moderate shear rates. The same behavior is observed, to a lesser extent, for DH/SO, Fig. 2b. On the contrary, the CH/SO system behaves differently under increasing E (Fig. 2c): for these systems, the ER response only increases with the field when this grows up to E = 1 kV/mm, and then decreases with further growth of the electric field strength. It can be said that there is a critical threshold field strength above which the field-induced reinforcing of the suspension disappears. These facts are best appreciated if the yield stress is obtained for each system and field strength. In Fig. 3 we analyze the static yield stress of the suspensions, sy (computed as the shear stress corresponding to the plateau of the s  c_ plots in a log–log representation [43–45]). Fig. 3 shows the plot of sy as a function of E. Such dependence in the case of H/SO and DH/SO was evaluated by fitting the data to a power-law of the type sy = pEq (solid lines in Fig. 3). The best-fit parameters are collected in Table 1. An almost linear relationship can be observed for H/SO. On the other hand, DH/SO suspensions show an intermediate behavior between linear and quadratic. Also, the negative ER response of CH/SO system is clearly seen in Fig. 3.

3.3. Viscoelasticity

Fig. 1. Pictures of water drops deposited on the surface of compressed discs of (a) hematite, and (b) oleic acid coated hematite particles.

Small amplitude oscillatory shear experiments are most effective for investigating viscoelastic phenomena associated to the existence of fibrillar structures. This is so because the application of small strains allows one to probe the particle interactions while minimizing the influence of the external flow field [46]. The linear viscoelastic region was first checked via amplitude sweeps in oscillatory stress tests at a fixed frequency of 1 Hz as a function of E. The results obtained are plotted in Fig. 4. A critical shear stress amplitude (sc) can be identified beyond which the elastic modulus G0 ceases to be constant, and falls rapidly with amplitude. The effect of the field strength on the values of this critical stress is depicted in Fig. 5. Without electric field (Fig. 4), the dynamic moduli were constant only up to very low shear stress values. At

Fig. 2. Steady state rheograms of (a) H/SO, (b) DH/SO and (c) CH/SO suspensions at various electric field strengths. Volume fraction of solids: 10% in all cases.

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Fig. 3. Dynamic yield stress vs. applied electric field strength for all the suspensions. Lines correspond to least squares fittings of the data.

Table 1 Best fit parameters of the yield stress of hematite suspensions to the power-law sy = pEq (sy in Pa, E in kV/mm) for H/SO and DH/SO (Fig. 3). r2 is the determination coefficient. Suspension

p

q

r2

H/SO DH/SO

39 ± 2 12.0 ± 1.9

1.16 ± 0.06 1.4 ± 0.2

0.99193 0.95785

increasing electric field strengths, sc raises in the case of H/SO and DH/SO, unlike CH/SO, where the limiting shear stress value increases just up to E = 1 kV/mm, and then decreases beyond this field. The field dependence of the critical shear stress for H/SO and DH/SO was evaluated by fitting the data to a power-law of the type sc = mEn (solid lines in Fig. 5). The best-fit parameters are collected in Table 2. The fitted n values agree with those found in the yield stress fitting, Table 1. Fig. 6 shows the frequency dependence of storage (G0 ) and loss (G00 ) moduli for different electric field strengths, and each sample. The amplitude of the applied oscillatory stresses were chosen in order to ensure linearity in all cases of frequency sweep (f = 0.1– 10 Hz). From Fig. 6, it can be said that the results agree with those of steady state rheograms. Without applied E, it is observed that G0 values were slightly higher than G00 ones for all samples examined. This suggests a not fully elastic, solid-like structure in the samples. With applied E, G0 values increase and begin to dominate significantly over G00 values, except in the case of the CH/SO system at high field. These increments were significant for H/SO. On the other hand, for the CH/SO system it can be seen that the elastic modulus is lower with than without applied field, and, furthermore, no plateau was observed for E = 3 kV/mm. The viscous properties of this system at high E values dominate over the elastic ones at moderate frequencies.

3.4. Microscopic observations Rheological data were completed by microscopic observations of the structures presented by the dispersions with the presence of the dc electric field (Fig. S2). Although such kind of observations are usually carried out after diluting the suspensions, in this study we rather intended to perform them in the original concentrated suspensions, having volume fraction of 10%. Separation of the particles was not observed for H/SO and DH/SO suspension systems (Fig. S2a and b), but in Fig. S2c particle migration toward the electrodes is

Fig. 4. Plots of the elastic modulus as a function of the stress amplitude in oscillatory tests, for (a) H/SO, (b) DH/SO, and (c) CH/SO suspensions at various electric field strengths. Volume fraction of solids: 10% in all cases. Frequency: 1 Hz.

clearly seen for CH/SO system and this observation is supported by the rheological measurements. A comparison of the time evolution of the particle structures in H/SO and CH/SO was possible by loading half of the cell with each of the samples and applying the field to the cell electrodes. Fig. 7 clearly shows the differences between H/SO (top) and CH/SO (bottom) systems: while the former remains dark, indicating that no noticeable migration of the particles to the electrodes occurs, the latter clearly undergoes phase separation and particle accumulation in the vicinity of the electrode area.

3.5. Dielectric properties For a proper description of ER effects, in the case of dc applied electric field, we need to know the conductivity of the particles, rp, and the permittivity and the conductivity of the dispersion medium. Our unknown is rp of coated hematite, since for the

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Fig. 5. Change of critical shear stress amplitude (sc) with E for all samples, / = 10%. Lines correspond to least squares fittings of the data.

Table 2 Best fit parameters of the critical stress of hematite suspensions to the power-law sc = mEn, (sc in Pa, E in kV/mm) for H/SO and DH/SO (Fig. 5). r2 is the determination coefficient. Suspension

m

n

r2

H/SO DH/SO

62 ± 8 10.7 ± 1.2

0.97 ± 0.15 1.42 ± 0.12

0.94465 0.98178

silicone oil it has been reported that ef = 2.6 and rf = 1014 S/m [47], and for bare hematite the literature data are ep = 12 [48], and rp = 2.6 107 S/m [49]. As already mentioned, in order to gain information about rp for CH particles, we measured the complex dielectric constant of coated hematite/silicone oil suspensions in the presence of ac fields of different frequencies, ranging from 103 to 105 Hz, as shown in Fig. 8. Note the constancy of the real part of the permittivity, indicating that no interfacial relaxation occurs in the frequency range investigated. From this, we can conclude that the observed frequency dependence of e0 must be associated to the conductivity of the suspension, which, considering that rp/rf  1, will be roughly given by rp/. Hence:

e00ðf Þ ¼

rp / 2pf e0

ð5Þ

The least-squares fitting of the data in Fig. 8 yielded the result

rp = (1.7 ± 0.5) 109 S/m, and the average real part of the dielectric constant can be estimated as ep = 11.4 ± 0.6, assuming a simple mixture formula e0 = ep/ + ef(1  /). Note that even for coated hematite the condition C = rp/rp  1 is still fulfilled, but the value of the conductivity ratio is greatly reduced: this will reduce the tendency of the particles to chain. As a consequence, a weaker ER effect can be predicted. We will come to this point in Section 4. 3.6. Zeta-potential measurements Electrophoretic mobility and zeta (f)-potentials of the hematite and coated hematite particles dispersed in SO (g = 102 Pa s, Aldrich) have been measured and the results are shown in Table 3. We can see that CH particles have higher charge than H particles. 4. Discussion The rheological properties of the suspensions investigated in the absence of the field depend on the suspension volume fraction and on the degree of particle aggregation as withany colloidal suspension.

Fig. 6. Storage modulus (G0 ), full symbols, and loss modulus (G00 ), open symbols, vs. frequency, at different field strengths, for (a) H/SO, (b) DH/SO and (c) CH/SO.

Considering that silicone oil is hydrophobic, whereas untreated hematite is hydrophilic, bare particles will tend to aggregate [50] when dispersed in SO. On the other hand, modified hematite becomes more hydrophobic after treatment processes, so a decrease in aggregation tendency of the coated particles in silicone oil can be expected. This difference could modify the rheological response in absence of electric field. However, we have not found significant differences between the steady state rheograms of the three suspensions (Fig. 2), unlike the oscillatory measurements (Fig. S3, Supplementary information). In these it is found that CH/SO suspensions show higher G0 than DH/SO and H/SO, this indicating that there is a stronger structure in the CH/SOcase. This could be due to a largerconcentration of flocsin suspensions of untreated particles, whereas the hydrophobic treatment hinders the aggregation of the coated particles. On the contrary, no significant differences can be found between bare and dried particles concerning their zero-field rheological behavior.

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Fig. 7. Comparison between microscopic observations of H/SO (top) and CH/SO systems (bottom) of 10% volume fraction for E = 1.8 kV/mm at different indicated times.

Fig. 8. Real (e0 ), and imaginary (e00 ) components of the relative permittivity of CH/ SO suspensions of 10% volume fraction, as a function of frequency.

Table 3 Electrophoretic mobility and zeta (f)-potential, calculated using the Hückel approximation, of hematite and oleic acid coated hematite in SO with nominal viscosity 0.01 Pa s . Sample

Electrophoretic mobility (108 m2 s1 V1)

Zeta-potential (mV)

Hematite Coated hematite

(8 ± 4) 103 (18 ± 5) 103

50 ± 20 120 ± 30

Concerning the effect of the applied field, Figs. 2–6 show that H/ SO suspensions undergo a positive ER effect. A linear relationship has been found between both the yield stress (Table 1) and the critical shear stress amplitude (Table 2) with the field strength E. Two different interactions can contribute to this behavior: the force (per unit interfacial area) Fm–e between the adsorbed polar molecules of water (in the case of H/SO systems) or oleic acid (CH/SO) and the polarization charge on hematite particles, and the force between neighbor particles through their induced charges, Fp–p. In order to evaluate the relative significance of these two interactions, an estimation of their strength has been done according to Eqs. (1), (3), considering that the volume fraction is / = 10%; lwater = 6.18

1030 C m; loleic acid = 8.35 1030 C m [51]; N = 1, E = 1 kV/mm. In addition, we have considered dm–e equal to the polar molecule size: dwater = 2.75 Å; doleic acid = 1.97 nm. For the calculations involved in Eq. (1), the rp value obtained from dielectric spectra of the coated suspensions (Fig. 8) has been used. The results are shown in Table 4. It is clear that both sources of interaction undergo a considerable strength reduction when the particles are coated, that is, this kind of particles will show a lesser tendency to form aggregates under the action of the field, this contributing to a weaker ER effect. The other pre-treatment described, dehydration, also produces a decrease in the ER response of hematite. This is a well known behavior [52–56]. A possible contribution to this behavior can come from the increase in particle polarizability when containing low water concentrations: if water, having a relative permittivity of 80, is adsorbed on the particle surfaces, the resulting particle will have a large dipole coefficient and consequently a large ER response. Moreover, the presence of water increases the conductivity of hematite particles, and so C is larger. According to Eq. (1), the interparticle force will be stronger and so will the ER effect. Another argument can also be given: the absence of water implies the lack of Fm–e so a lower ER response can be expected for DH/ SO, in agreement with experimental findings. Another interesting difference between the electrorheological behaviors of H/SO and DH/SO suspensions is that the latter show a nonlinear (in fact, intermediate between linear and quadratic) relationship between both the yield stress (Table 1) and the critical shear stress amplitude (Table 2) and the field strength E. The lower C ratio associated to de-hydrated samples seems to reduce the conductive component of the ER response, and gives more weight to the polarization mechanism, typically quadratic. It is clear that adsorbed water plays a central role in the linear response of the systems. With regard to the oleic acid effect onto the rheological response of the hematite suspensions, we note that, although oleic acid has been used with other systems (nanoparticles of barium titanyl oxalate coated with urea [33]) for inducing a giant ER effect, this is not found in the case of hematite. This could be explained by keeping in mind that our hematite particle size is one order of magnitude larger than the barium titanyl oxalate particles used by Shen et al. [33], and that Fm–e is proportional to the inverse of the particle radius. Using Eq. (3) we have estimated that the force is some 1000 times larger than it is in our systems (Table 4). Note that, in our case, such interaction is two orders of magnitude lower than the estimated force between CH particles, and this means that Fm–e has not a relevant role on the ER response. CH/SO suspensions showed a slight ER positive behavior at low applied electric fields and a negative ER effect at higher ones. This could be due to the existence of two simultaneous and competitive responses of the system upon the application of the field: the above mentioned forces between particles, and the charge generation on the particles, and their subsequent electrophoretic migration towards the electrodes. Different sources for the generation of surface charge on the particles in non-aqueous media can be mentioned [57–59], including presence of traces of water, dissociation of impurities in the solvent, proton dissociation on the particles themselves, ion generation in the fluid by the Debye–Falkenhagen effect. According to Felici [60], a very important factor has to be added,

Table 4 Estimation of the interaction force Fm–e (per unit interface area) between the adsorbed polar molecules and the polarization charge of the particles, and the force between neighbor particles through their induced charges, Fp–p at E = 1 kV/mm for H and CH particles in SO. Suspension

Fm–e (Pa)

Fp–p (Pa) (E = 1 kV/mm)

H/SO CH/SO

1430 5.3

1410 770

82

O. Erol et al. / Journal of Colloid and Interface Science 392 (2013) 75–82

namely, injection of charge carriers by the electrodes: this universal phenomenon is associated to the transfer of one or more electrons from or to the molecules in the solvent. As a consequence of this electrochemical process, such ionized (either positive or negative) molecules will adsorb onto the particles and provide them with a non-zero zeta potential and electrophoretic mobility. In reality, such phenomena are far from being understood in full detail [61]. To understand the differences on the ER response of H/SO and CH/ SO suspension it is furthermore necessary to keep in mind that the estimated Fm–e between hematite and water molecules is three orders of magnitude higher than Fm–e between hematite and oleic acid molecules. Moreover, Fp–p is higher in the case of H/SO suspensions than in the case of CH/SO. So the forces on H/SO suspensions are large enough to hide the particle charging effect and create a structure between the hematite particles, producing a positive ER. But in the case of CH/SO, the particle charging effect is predominant at high applied electric fields, and the structure is broken because the particles migrate towards the electrodes as shown in Figs. 6 and 7. The differences found between DH/SO and CH/SO suspensions are more difficult to explain. In both cases the forces Fp–p are similar, and Fm–e must be zero (DH/SO) or negligible (CH/SO); in addition, their behaviors at low field are also similar. We may wonder why CH/SO displays negative ER effects at high fields, unlike DH/ SO. If uncontrolled water adsorption on DH particles can be discarded, the reason must be in the easier charging of CH particles at high fields. Note that this possibility is in agreement with the higher zeta potentials of CH as compared to H particles. To this we may add the general mechanism of oleic acid coating hindering the formation of chains by steric reasons and by the better wettability (and lesser tendency to aggregation) of CH particles in SO. 5. Conclusions

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The electrorheology of suspensions of hematite, dried hematite and modified hematite (/ = 10%) were determined under steady state (viscometry) and oscillatory conditions. The treatment process affected the ER response of hematite. Heat treatment (DH) and modification with oleic acid (CH) caused a decrease in the ER response of hematite. A proper amount of physically adsorbed water was found to be necessary for an appreciable ER effect. On the other hand, CH/ SO system showed lower rheological properties than observed in the absence of electric field after a critical electric field is reached thus showing a ‘‘negative ER effect’’. Reasons for this are discussed in terms of particle–particle interactions either direct or mediated by (induced particle charge)-(charge in the fluid) interactions.

[35]

Acknowledgments

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One of us, A.V.D., enjoyed the great opportunity of working as a research fellow under the direction of professor Matijevic´ at Clarkson University. The freezing weather in Potsdam was more than compensated by Egon and Bozˇica’s warm welcome, and by the energy transmitted by him in the lab, all the time. His teaching and example were determinant for my research career. Larga vida y gracias, profesor. The authors thank the European Science Foundation through COST Action D43, and the Turkish Scientific and Technological Research Council, for the scholarship provided to O. Erol (BIDEB). Financial support by Junta de Andalucía, Spain (Project PE2008FQM3993) and Spanish Ministry of Science and Innovation (FIS2010-19493) is gratefully acknowledged. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2012.09.060.

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