Electric fields in the rheology of disperse systems. Yu. F. Deinega and G. V. Vinogradov. Institute of Colloid Chemistry, Ukrainian SSR Academy of Sciences, ...
Rheologica Acta
~eol Acta23:636-651(1984)
Electric fields in the rheology of disperse systems Yu. F. Deinega and G. V. Vinogradov Institute of Colloid Chemistry, Ukrainian SSR Academy of Sciences, Kiev and Institute of Petrochemical Synthesis, USSR Academy of Sciences, Moscow Abstract." In the present survey, the influence of electric fields on the structure and rheological properties of disperse systems as well as the effect of deformations on their electrical characteristics are discussed. The properties of these systems are considered in terms of the dielectric permittivity and electrification potential. The considerable thickness of the double electric layer around the disperse phase particles, which is characteristic of disperse systems with nonpolar hydrocarbon dispersion media, provides the possibility for strong electric fields to produce an electric nonuniformity on the surface of the disperse phase particles. The formation of hydrate layers on the particles creates the possibility of polarization of the disperse phase. In plastic disperse systems such as greases, a strong orientation effect is observed, which contributes to the creation of frozen flow patterns when the flow is suddenly stopped. The survey is concluded with a consideration of the process of formation of chain structures in the direction of the lines of force of the electric field whose orientation is normal to the direction of flow, which can lead to complete stoppage of the flow. Key words: Rheo-electric effect, electrorheological effect, electroviscous effect, dielectric power-loss factor, permittivity, double electric layer, electrocrystallization structures
1. Introduction Detailed investigations of the deformation and flow of disperse systems have in recent years attracted an ever increasing amount of attention. The importance of studying simultaneously the theological, optiGal, electrical and other properties is due to the fact that only such investigations can provide complete insight into the structure of disperse systems and the changes in the structure occurring as a result of deformations. Optical-polarization and rheological methods have proved to be effective for investigating the mechanism of deformation and revealing the relation between the mechanical properties and the structure of disperse systems. Rheo-optical investigations have led to the discovery of a new phenomenon, namely the formation of frozen oriented structures when the flow of greases is suddenly stopped [1]. In the investigation of the deformation of polymeric systems with the aid of the optical-polarization method the distribution of shear stresses during the flow of polymer out o f a flat capillary was established [2]. However, the opticalpolarization method is often inapplicable because of the low transparency of concentrated systems. 994
New and interesting possibilities for a detailed analysis of the structure of disperse systems and their changes in flow lie in a combined study of their electrical and rheological properties. Many specific features of the structure of disperse systems, which cannot be revealed by either of these methods used alone, become clear from the mutual influence of their electrical and rheological properties. Using electric fields one cannot only obtain information on the structure, but also radically alter the structure, i.e. influence the theological properties. In this connection, particular attention should be paid to disperse systems in hydrocarbon liquids, in which strong electric fields can be created due to the low conductance of the dispersion medium. According to the character of the effect of the mechanical and electrical forces on the system, one can distinguish between two main types of electrorheological investigations. 1. The study of the effect of mechanical deformations on the structure and electrical properties of diperse systems. Variation of electrical parameters such as conductance, dielectric permittivity and power loss factor as well as the production of an electric potential
Deinega and Vinogradov, Electric fields in the rheology of disperse systems under the influence of shear can be classified as rheoelectric effects. 2. The study of the effect of strong electric fields on the structure and rheological properties of disperse systems. Rapid reversible changes in the rheological properties under the action of an electric field are termed electrorheological effects [3]. The influence of electric charges on the disperse phase particles within disperse systems can lead to an increase in viscosity. This is known as an electroviscous effect. A survey of investigations of the electroviscous effect is given in [4].
2. Electric fields as an indicator of structural transformations in the deformation of disperse systems
Changes in the electrical and rheological properties occurring during deformations are both due to the same changes in the structure of the disperse medium. Therefore, a comparison of the electrical and theological characteristics during a deformation can provide im= portant information on the orientation, destruction and restoration of the structure.
2.1 Rheo-electric effects 2.1.1 Conductance For evaluating the formation of structures within systems consisting of a conducting disperse phase in a nonconducting disperse medium, for example suspensions of carbon black in hydrocarbons, measurements of conductance at rest and during the flow are employed [3, 5]. Destruction of carbon black structures under the influence of the flow and disturbance o f the bonding between the particles (conductors) cause a drop in the conductance. The higher the deformation rate is, the larger the drop in conductance. U p o n cessation of the flow the structure is restored and the conductance again increases. The appearance of conductance anisotropy in flow has been used to obtain information on the shape and size of particles. The method is based on the fact that the conductance during flow is not the same as that due to the orientation of the particles. When the solution stops flowing, its conductance changes because of relaxation processes. The theory of the method was considered by Schwarz [6]. Experimental verification was carried out by Heckman and Gots [7] in a specially designed Couette apparatus which allowed the conduction to be measured in three mutually perpendicular directions. An aqueous solution of sodium thymus nucleate with rod-like particles was one system and graphitic acid with disk-shaped particles was another. The results of the measurements were in good agreement with electron microscopy data. Investigations of aqueous solutions of sodium oleate showed that conduc-
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tance anisotropy first occurs above the temperature-concentration boundary, which corresponds to a transformation or aggregation of spherical micelles into anisodiametrical ones. Conductance anisotropy in flow has been studied in hydrocarbon suspensions of carbon black, as well as in aqueous solutions of polyphosphates and dispersions of gelkyds [8, 9]. Special attention has been paid to the behaviour of the conductance in the flow of hydrocarbon disperse systems, in which a double electric layer exists at the interface between the particles and the dispersion medium. The essence of the effect is that upon reaching a particular deformation rate the conductance or power loss factor increases at low frequencies. Figure 1 shows the results for the power loss factor (tan 6) for calcium cup grease at a frequency of 400 Hz. D e f o r m a tion of the system in a rotational plastoviscometercondenser [41] at relatively high deformation rates leads to a reduction in tan 6, which is associated with the destruction of the structural skeleton and a drop in the conductance. As the deformation rate increases, the variation in the power loss factor is reversed. The mechanism of conductance at high deformation rates is still not clearly understood. Bondi and Penther [10] suggested that the conductance increases as a result of the transfer of charges during collisions between the disperse phase particles. In [11] we put forward a suggestion that an increase in the dielectric losses is associated with the deformation and stripping of the ionic shells surrounding the particles and the ensuing increase in the concentration of free charges. Using these ideas one can account for the fact that the conductance starts to increase at lower deformation rates when the intensity of electrokinetic phenomena is stronger. One of the important advantages of the conductance method is the possibility of measuring the degree of structurization of the system without destroying it. A sudden stop in the flow of grease at low deformation rates causes an increase in tan 6, while such a stop at high deformation rates produces a decrease. The type of change in the power loss factor after cessation
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638 of the flow is determined by the rate of restoration of the structural skeleton and the ionic shells around the disperse phase particles, and the production of the anisotropic structure. The relative influence of these various processes depends on the deformation rate at which the flow is stopped. Thus, the conductance of plastic hydrocarbon systems is determined by the existence of the threedimensional structure and by the state of the interface, its electric charge and the degree of hydration. The mechanism responsible for the formation of charges on a hydrophobic surface in a hydrocarbon medium has not yet been adequately studied. By analogy with aqueous systems, dissociation of surface groups and adsorption of ionogenic surfactants have been postulated [15]. Another mechanism for the formation of surface charges is a proton exchange between the medium and the particles [12]. In hydrocarbon solutions of oil-soluble basic polar polymers, protons are captured by macromolecules. As a result, the surface of the particles becomes positively charged. In solutions of acidic polymers the latter donate protons so that the surface of the disperse phase acquires a negative charge. Hydrophilic particles adsorb water contained in minute amounts in the non-aqueous medium, and so can be coated with mono- or polymolecular layers of water [13]. Such layers have an enhanced conductance due to the increased concentration of ions (upon dissociation of the surface ionogenic groups). This is responsible for the surface conduction of the hydrocarbon disperse systems and their sensitivity to shear.
2.1.2 Dielectric permittivity The dependence of the dielectric permittivity on shear may be associated with the orientation of anisodiametric particles and the deformation and destruction of aggregates and "soft" particles. Parts [14] was one of the first to indicate a relation between the dielectric and rheological properties and the changes in the structure. The dielectric permittivity was observed to increase in the course of thixotropic restoration of the structure of an oil suspension of carbon black. The effect of dielectric permittivity variation in flows of non-aqueous suspensions and emulsions was investigated by Voet [16]. The dielectric properties of zinc, aluminium and titanium oxide dispersions in linseed and mineral oils, as well as pigments in low-viscosity organic liquids at rest and during shear flow were measured by means of a rotational coaxialcylindrical condenser-viscometer. A reduction in the dielectric permittivity and the appearance of Newtonian flow can be explained by the destruction of particle agglomerates. The dielectric behaviour of suspensions in shear was described by Bruggeman's equation e= el(1 + 3v), where e is the permittivity of the suspension, el the dielectric permittivity of the dispersion medium, and v the volume fraction. Deviations from the linear dependence during flow and at rest were taken into account by the form and agglomeration factors. A reduction in the dielectric permittivity of concentrated water-oil emulsions in a rotational condenser was noted by
Rheologica Acta, Vol. 23, No. 6 (1984) Hanai [17]. With an increase in the electric field frequency the rheo-electric effect diminished and at 1 MHz disappeared altogether. Contradictory results were obtained in the investigation of the dielectric properties of hydrocarbon dispersions of silicon dioxide. Klass and Martinek [18] found that the dielectric permittivity decreased with increasing shear rate and frequency of the electric field. Funt and Mason [21] suggested that this was due to the deformation of a double dielectric layer in the direction of flow. At low frequencies dielectric losses increase in shear due to the growth of conductance in the flow. In systems obtained by thickening mineral oils with silicon dioxide Bondi and Penther [10] obtained a strong dependence of the dielectric permittivity on the rate of shear. This was associated with the destruction of the aggregates of spherical aerosil particles. A considerable increase in the permittivity during deformation was observed for bentonite grease [19]. Stopping the flow resulted in a drop in e. The nature of this effect is not yet clear. In the flow of solutions of non-polar polymers in non-polar solvents the dielectric permittivity does not change. Rheoelectric effects have been found in shear deformation of benzene solutions of ethyl cellulose. However, whereas Junge [20] showed that the dielectric permittivity of concentrated (10-40%) solutions reduced in shear, Funt and Mason [21] found that e increased. After stopping the flow the dielectric permittivity was observed to be restored with time. As the temperature increased, the relaxation time for structural rearrangements in polymer solutions decreased. Deinega et al. [11] noted that the interaction between the hydrodynamic and electric orientation of the macromolecules in solution can affect the dielectric permittivity in different ways. D e p e n d i n g on the character of the structure of the macromolecules (the dipole moment can be directed either across or along the chain), the two types of orientation will either be additive or oppose one another. Consequently, dielectric permittivity in shear can either increase or decrease. Polar groups in ethyl cellulose molecules are thought to be directed perpendicular to the longer axis. Therefore, the orientation of the molecules in flow causes an increase in the dielectric permittivity. An increase in the dielectric permittivity of nitrocellulose n-butyl acetate solutions in shear was observed by Wendisch [22]. At higher temperatures the effect was smaller. Their data confirmed that nitro groups in the nitrocellulose molecule are arranged perpendicular to the axis of the molecule. The behaviour of polymeric solutions in combined shear and electric fields was considered theoretically using a model &rigid ellipsoids by Ikeda [23] and Saito and Kato [24]. To describe the influence of deformation and orientation on the dielectric polarization in the laminar flow of flexible macromolecular chains, Peterlin and Reinhold [25] used a necklace model. From their theory it follows that if the elementary dipole moment is directed along the chain, then the dielectric permittivity must be independent of the gradient of the flow. On the other hand, if the elementary dipole moment is perpendicular to the macromolecular chain, e must increase in proportion to the square of the flow gradient. Considerable changes in the dielectric properties of soaphydrocarbon systems under dynamic conditions were noted by Bondi and Penther [10]. The dielectric permittivity of systems
Deinega and Vinogradov, Electric fields in the rheology of disperse systems consisting of a mineral oil thickened with 6-10% technical lithium stearate decreases with an increase in the shear rate. This is thought to be related to the orientation of electrically anisotropic soap fibres. Upon removing the shear stress (stopping the rotor) the dielectric permittivity immediately regained its initial value. Detailed investigations of the dielectric and rheological properties of plastic soap greases of various chemical types showed considerable variations in the dielectric parameters in shear [26-31]. Figure 2 shows the dependence of the dielectric permittivity and shear stress sodium grease on the deformation rate. Curves 1 and 1' indicate a transition from lower to higher deformation rates, and curves 2 and 2' from higher to lower deformation rates (the dashed lines are the flow curves). The sensitivity of e to shear is determined by the orientation of the anisodiametric soap crystallites in the direction of flow (i.e. perpendicular to the electric field) and occurs in systems containing soaps of oxyacids, namely acids with a hydroxyl group in the hydrocarbon chain, and also hydrated soaps of mixtures of fatty acids. Polarization of such systems is associated with the existence within the soap crystallites of chains of hydroxyl groups connected by hydrogen bonds and proton transfer processes. When a grease flow is suddenly stopped, the dielectric permittivity values are fixed at the values observed in the flowing grease. Thus, dielectrical investigations confirm the ability of greases to "freeze" the structure of the flow when it is suddenly stopped. This is in agreement with results for the mechanical properties of greases that thermal motion has little influence on their structure. Weakening the structural skeleton with the aid of surfactants or diluting the system enhances the relax-
639
ation processes, which is reflected in a larger or smaller increase in e after stopping the flow. For systems whose dielectric permittivity is sensitive to shear, the dependence of e on frequency is characteristic. Figure 3 shows the frequency dependence of e and tan ~ for sodium grease with differently oriented frozen structures. With an increased orientation of the particles in the direction of flow (curve 2 was for a higher deformation rate) the frequency dependence of the dielectric permittivity is reduced. At higher frequencies the dielectric permittivity for differently oriented structures become approximately equal, i.e. the sensitivity to shear disappears. This means that a change in e due to the orientation of the particles is only possible at frequencies at which macrostructural polarization occurs. The sensitivity of the dielectric permittivity to shear and the possibility of fixing the structures in the flow open up the possibility of investigating orientation and relaxation processes at various stages of deformation after the cessation of the flow.
2.1.3 Electr!fi'cation potential The appearance of a potential difference (V) [10, 32] between the electrodes of a rotational plastoviscometercondenser can also be classified as a rheo-electric effect. As the deformation rate is increased in soap-hydrocarbon systems with a conductance not greater than 10 -1° Ohm -I cm -1 a reversal of the sign of the potential can be observed. Curve 1 in figure 4 was obtained with an increasing deformation rate and curve 2 with a decreasing deformation rate. Figure 4 shows that going from lower to higher deformation rates the positive
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Fig. 3. Frequency dependence of the dielectric permittivity and power loss factor for sodium grease (the same as in figure 2) with frozen oriented structure: curves 1 and 1' structure frozen at a deformation rate of 2.42 s-l; curves 2 and 2' structure frozen at a deformation rate of 312 s - 1
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Rheologica Acta, Vol. 23, No. 6 (1984)
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charge on the rotor reaches its maximum value and then falls to zero before acquiring a negative charge. Electrification of greases and reversal of the sign of the potential with an increasing deformation rate are associated with the double electric layer present at the interface and with a change in the structure of greases during deformation. The conductance of the system, which determines the rate at which the charge is removed, is important. The appearance of a positive charge on the rotor is determined by the existence of a near-wall layer on it enriched with the dispersion medium, which carries a positive charge in the case of sodium grease. As the deformation rate increases, two factors come into play. In the first place, the possibility of separation of charges in the double electric layer increases. Secondly, an increase in the deformation rate reduces the influence of the near-wall layer and, hence, of the dispersion medium on the charge on the rotor. The first factor tends to increase of the charge on the rotor. The relative role of these two factors varies in such a way that at some particular deformation rate a maximum positive charge is attained. At sufficiently high deformation rates the disperse phase particles contact the rotor surface very frequently, which explains the recharging of the rotor, since these particles are negatively charged. However, as stated above, at high deformation rates the conductance increases, which leads to a decrease in the electric potential. This accounts for the minimum in the potential versus deformation rate curve at high deformation rates. Going from high to low deformation rates an appreciable hysteresis in the electric potential is observed, which is the result of the destruction of the structure at high deformation rates. Homogenization is particularly pronounced when the potential is positive. In a grease that has undergone strong mechanical destruction the
near-wall layer becomes less isolated and the positive potential is sharply diminished. When the rotor is suddenly stopped, the positive potential falls either quickly or slowly to zero. However, at speeds of rotation at which the reversal in potential occurs, after stopping the negatively charged rotor, recharging can take place with a subsequent drop of the charge to zero [33]. This effect is evidently related to the change in the structure of the grease at the moment of rapid braking of the rotor, and in particular to the contraction of the structural skeleton under the influence of the inertia forces at the stator and the inflow of the dispersion medium to the rotor. When the action of inertia ceases, a resilient restoration of the structural skeleton occurs. It again comes into contact with the surface of the rotor and the isolated near-wall layer of the hydrocarbon medium disappears. The occurrence of the electrification potential in shear provides an opportunity for studying the structure of the boundary layer in the course of slippage along the wall.
2.2 Kinetics of shear stress and dielectric parameters: variation due to deformation of disperse systems The sensitivity of the permittivity and electrification potential to shear was used for studying relaxation processes for stresses smaller than the shear strength, irreversible changes of the structure when the ultimate strength is surpassed, and the mechanism responsible for the formation of the near-wall layer.
2.2.1 Permittivity For studying the influence of orientation effects on the processes of deformation, use was made of the ability of plastic disperse systems to fix the structure of the flow upon rapid cessation of the flow [1]. The results of a simultaneous investigation of the rheological and dielectric properties of the frozen structures of sodium grease in a rotational plastoviscometer-condenser are shown in figure 5. Shear stress r versus time t and permittivity e versus time curves in figure 5 a are for the structure pre-oriented at the shear rate used for measuring e and z (~=2.42s-I). The monotonically increasing curve for r (t) shows that the system displays a near-wall effect which masks the brittleness of its structural skeleton in the bulk. The permittivity does not change during deformation, which is an indication that the orientation effect is absent. The influence of the orientation of the disperse phase particles on the shear stress kinetics is shown in figure5b. Frozen oriented structures were obtained by rapidly stopping the rotor from a speed correspond-
Deinega and Vinogradov, Electric fields in the rheology of disperse systems
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Fig. 5. Shear stress (solid line) and dielectric permittivity (dashed line) kinetics for the same grease as in figure 2 at a constant shear rate of 2.42 s-l: a - initial structure produced by ~= 2.42 s- ~;b - oriented structure produced by ~= 20.2 s- 1 ing to ?>= 20.2s -1. After the orientation, the rotor was slowly rotated (~= 2.42s -I) in the same direction. In this case the gradual increase in the stress is followed by the appearance of a more or less pronounced maximum in the curve r (t). At the same time, before the ultimate strength is surpassed, disorientation of t h e disperse phase particles occurs, this being inferred from the change in permittivity. Pre-orientation in the direction opposite to the subsequent slow rotation enhances the influence of the orientation effect on the shape of the r (t) and e (t) curves. The changes in permittivity occurring before the ultimate strength is surpassed confirm the suggestion [34] that the specific features of the properties of grease-type plastic disperse systems are determined by the existence within them of both rather large particles, which are only weakly influenced by thermal motion, and small particles, which are subject to thermal motion. The changes in the structure occurring before the ultimate strength is reached are initially associated with the rupture of weak bonds in the structural skeleton, which leads to changes in the positions of the small-size particles. Surpassing the ultimate strength is accompanied by the destruction of large particles, which are only weakly affected by thermal motion. These particles are responsible for the weak tendency of greases to show stress relaxation, and they determine the rigidity of the structural skeleton and the anisotropy of the frozen structure of the flow. Small-size particles are responsible for the resilient after-effect, the relaxation of stresses and the thixotropic restoration when their values are high.
The appearance of a positive potential in hydrocarbon systems with a negatively charged disperse phase makes it possible to investigate the formation of the near-wall layer structure at low deformation rates [35]. Figure 6 shows the results of a simultaneous investigation of the shear stress and electric potential kinetics for differently oriented structures of sodium grease. The curves shown in figure 6 a are for weakly oriented structures produced with ~= 2.12s -1 and the curves in figure 6 b are for anisotropic structures produced at a high deformation rate with ~ = 312 s -1. The monotonic increase in the curves for ~ (t) and V(t) in figure 6a shows that this system displays a considerable near-wall effect. An enrichment of the nearwall layer with the dispersion medium is accompanied by an increase in the positive potential to the values corresponding to the established flow. When grease containing the frozen structure from a rapid flow are subjected to deformation, the character of the shear stress and electric potential dependence are changed markedly. In this case the process of gradual loading is accompanied by the appearance of a clearly pronounced maximum in the curve for V(t). At initial times small variations in the potential are observed on the potential versus rotor rotation time curve. It is probable that elastic strains are important under these deformation conditions. The development of flow in the near-wall layer is accompanied by a dramatic increase in the positive potential on the rotor. With stresses close to the ultimate strength, structural changes take place which are accompanied by a sharp
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Rheologica Acta, Vol. 23, No. 6 (1984)
drop and then an increase in the positive potential on the rotor. In the course of gradual loading under stresses approaching the ultimate strength, it has been shown by dielectric measurements that disorientation and reorientation of the particles takes place in the fuzzy near-wall region. Rotation of the disperse phase partides in the flow leads to an increase in the number of their collisions with the rotor. Since soap crystallites in sodium grease are negatively charged, an increase in their contact with the rotor lowers its positive potential. The second maximum on the V(t) curves lies to the right of the ultimate strength. This is evidently associated with the fact that the maximum of the positive potential is determined by both the state of the nearwall layer and the velocity gradients in it. After the system has passed through the ultimate strength, the relative influence of the near-wall effect diminishes, but the velocity gradient in the near-wall layer (determined using a soft dynamometer) increases due to the reverse movement of the instrument body. Therefore the positive potential can grow to a definite value. Subsequent lowering of the potential is associated with the reduction of the velocity gradient in the near-wall layer and its gradual destruction. The phenomena considered show that the electric properties of greases are particularly susceptible to changes in the structure of the near-wall layer.
3. An electric field as a tool for varying the structural and rheological properties of disperse systems The application of strong electric fields to disperse systems, in particular hydrocarbon systems, with a view
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to changing their structures and properties has recently received much attention. This is partly due to the discovery of a number of new phenomena in hydrocarbon systems and the interest in their practical application.
3.1 Structure of hydrocarbon disperse systems in e[ectricfie[ds The specific features of the behaviour of hydrocarbon disperse systems in electric fields are determined by the possibility of creating strong surface layers and influencing their structures. In particular, the thickness of the double electric layer in hydrocarbon media is two or three orders of magnitude larger than in aqueous media, which is responsible for their high deformability. As a rule hydrate layers are formed on the surface of hydrophilic phases in hydrocarbon media and these play an important role in the formation of charges and polarization in disperse systems. Furthermore, the hydrocarbon medium creates favourable conditions for the occurrence of electrical non-uniformities on the surface. The possibility of continuously adjusting the disperse phase charge from positive to negative values is extremely important for creating a generalized picture of the behaviour of hydrocarbon systems in electric fields. On the basis of procedures elaborated for adjusting the charge with the aid of surfactants [36, 37] and investigation of the influence of charge on the structure of hydrocarbon disperse systems within a wide range of concentrations and intensities of the electric field, it is possible to present the following diagrammatic electrokinetic picture of their behaviour [38] (figure 7). Here the upper row relates to dilute systems and the lower row to concentrated plastic disperse systems.
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Fig. 7. Changes in the structure of a disperse system (the same grease as in figure 2) in an electric field and the dependence on the electric charge and disperse phase concentration, a - d dilute systems, e - h concentrated systems: a - anaphoresis; b - double electrophoresis; c-interelectrode oscillation; d - cataphoresis; e - electrosyneresis at the anode; f - interelectrode extension; g - interelectrode contraction; h - electrosyneresis at the cathode
Deinega and Vinogradov, Electric fields in the rheology of disperse systems If the surface of the disperse phase carries a charge (in this case negative), then upon application of an electric field in dilute systems anaphoresis is observed, i.e. particles precipitate at the anode (figure 7a). In concentrated systems syneresis is observed, namely attraction of the structural skeleton to one of the electrodes and isolation of the dispersion medium at the other (figure 7e). Lowering the charge to the isoelectric point brings the system to the state shown in figure7b. The uncharged disperse phase, probably because of the field non-uniformity caused by the space charge, precipitates on both electrodes, that is double electrophoresis occurs. In concentrated systems an extension of the structural skeleton and an increase in the concentration of the disperse phase in the nearelectrode region can be observed (figure 7 f). When the intensity of the electric field reaches a particular value, interelectrode oscillation of the disperse phase particles occurs in the isoelectric state (figure 7 c). This phenomenon is associated with charging and recharging of the particles at the electrodes. Recharging in the electric field can occur due to various mechanisms: firstly, contact charging of metallic partides; secondly, recharging of hydrated particles as a result of electrochemical reactions of the decomposition of water on the electrodes; thirdly, recharging without contact of the particles with the electrode. The mechanism of non-contact recharging of particles is considered in [39]. Under these conditions in concentrated systems interelectrode contraction of the structural skeleton occurs (figure 7g), which was first observed by us. When a positive charge is induced in dilute systems, cataphoresis is observed (figure 7 d), while in concentrated systems electrosyneresis at the cathode occurs (figure 7h). Obviously, there exist a large number of states intermediate to those indicated in figure 7. In addition to electrokinetic phenomena, an important factor in the behaviour of disperse systems in electric fields is dielectric polarization, that is the formation of induced dipole moments of the disperse phase particles. The polarization of hydrocarbon disperse systems can lead to orientation and structure
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formation (figure 8 a), spontaneous rotation of disperse phase particles (figure 8b), which is based on the induction under definite conditions of a dipole moment with an orientation opposite to that of the field, and dielectrophoresis (figure 8 c), that is motion of uncharged polarized particles in the nonuniform electric field. Variations in the structure of disperse systems under the action of an electric field substantially influence their theological properties. Depending on the character of the structural variations, electric fields can enhance or lower the effective viscosity of disperse systems.
3.2 Near-wall effect in electric fields The theological behaviour of disperse systems in electric fields is determined by a combination of electrokinetic and polarization phenomena. The influence of an electric field on the rheological properties of disperse systems with an electrokinetic transfer of phases and polarization interaction between particles is shown diagrammatically in figure 9. The curve 0AB describes the kinetics of the shear stress development when the deformation rate is constant and the electric field absent. After the ultimate strength is surpassed at point A, the shear stress drops until steady conditions are reached. If a reasonably sized jump in potential exists in the system at the interface, then after application of a field at point B the shear stress drops sharply - an effect of slippage along the wall (curve BD). After the field is cancelled, the shear stress grows, the ultimate strength is surpassed, and the shear stresses fall to the values corresponding to steady flow (not shown in figure 9). A different situation occurs with polarizable systems, in which electrokinetic phenomena are suppressed. In this case the application of an electric field causes a sharp increase in the shear stress. This is an electrotheological effect, represented in figure 9 by curve BE. After the ultimate strength is surpassed, the flow becomes steady. The shear stress in this case considerably exceeds the initial value.
Fig. 8. Changes in the structure of disperse systems in electric fields related to dielectric polarization: a - orientation and structure formation; b - spontaneous rotation; c - dielectrophoresis
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Rheologica Acta, Vol. 23, No. 6 (1984) changes its radial position slowly, in the course of one experiment the ultimate strength can be surpassed several times. As the layer under consideration gradually approaches the body, its spatial distinctiveness diminishes: it becomes fuzzy. Therefore, in long-term experiments on the continuous deformation of grease the sawtooth shear stress versus time dependence gradually becomes smoothed out. Thus, any separation of the phases in an electric field appreciably diminish the resistance of the system to deformation due to the appearance of near-wall effects.
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Fig. 9. Influence of an electric field on the rheological properties of plastic disperse systems. A-ultimate strength; B point of application of electric potential; BC - intermittent processes of slippage and seizure inside the system; B D near-wall effect; BE - electrorheological effect
If a system with a negatively charged disperse phase is held for a short period of time in a high-voltage field, a layer slightly enriched with the dispersion medium will be formed at the negatively charged rotor of a plastoviscometer-condenser. Upon recharging of the rotor and deformation of the grease continuous fluctuations of the shear stress may be observed (curve BC in figure 9) [40]. During prolonged deformation the amplitude of the shear stress fluctuations on such a sawtooth curve gradually diminishes. The alternate appearance of maxima and minima in the shear stress during continuous deformation of the system after the change of the sign of the charge on the rotor from negative to positive can be explained in the following manner. After a certain increase in the concentration of the dispersion medium at the surface of the negatively charged rotor it slowly moves towards the body under the influence of the recharging of the rotor. In the zones within the grease in which the dispersion medium concentration is increased, the structural skeleton of the dispersion system weakens, and the ultimate strength is easily surpassed in these regions. However, the dispersion medium continues moving, which results in the lowering of its concentration in the zone in which the structural skeleton is destroyed. This hinders the process of destruction and increases the resistance of the system to deformation. The shear stress then keeps on growing until the accumulation of the dispersion medium in some new zone becomes such that the ultimate strength is surpassed. Consequently, since the grease layer with an increased concentration of the dispersion medium
3.3 Electrocrystallization structures The electrorheological effect is based on the formation of electrocoagulation structures. However, with the aid of an electric field it is possible to exert a considerable influence on the development of condensation-crystallization structures. The idea of the action of electric fields on these structures is based on the fact that during the formation of the disperse phase during cooling of a solution or melt a number of systems go through the liquid-crystalline states. Liquid crystals are characterized by a high sensitivity to electric and magnetic fields, in particular, these fields exert an orientation effect. The action of an electric field at this stage of formation of the system substantially changes the structure and leads to the appearance of anisotropy in the mechanical and optical properties of the system. Soap-hydrocarbon systems are unique regarding the diversity of lyotropic liquid-crystal phases. To obtain a crystallization structure, a melt of a soap in a hydrocarbon was poured into a plastoviscometer-condenser preheated to 200 ° and then cooled for two hours. For investigating the influence of an electric field on the formation of crystallization structure, the melt was cooled in an electric field having an intensity of 20 to 40 kV/cm [69]. The shear stress variation kinetics with a constant deformation rate for a system obtained by thickening mineral oil with 15% sodium oxystearate is shown in figure 10. Curve 1 relates to the grease produced in an electric field, curve 2 is for the grease produced in the absence of an electric field. The figure shows that the application of an electric field during the formation of the grease structure leads to an increase of both the ultimate strength and the shear stress corresponding to viscous flow. This effect is related to the formation of oriented anisotropic structures in the electric field; this was confirmed by the results of optical polarization and dielectric investigations. Electrocrystallization structures are characterized by the appearance of a
Deinega and Vinogradov, Electric fields in the rheology of disperse systems [Pal 250t
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Fig. 10. Shear stress variation kinetics at constant deformation rate (~=2.12s -1) for a grease based on sodium oxystearate: 1 - grease produced in an electric field; 2 - grease produced in the absence of an electric field birefringence effect and variation of the grease texture. The dielectric permittivity of soap-hydrocarbon structures oriented in the direction of a field is higher than those obtained in the absence of an electric field. The orientation of soap crystallites perpendicular to the direction of shear leads to the growth of the strength and viscous properties of the system. However, this influence of the action of an electric field on the theological properties of greases is also preserved after homogenization of the system, though to a smaller extent. This is an indication that the electric field influences not only the orientation but also the structure of soap crystallities and their interaction. According to X-ray analysis, the degree of crystallinity of soaps is increased by the action of an electric field. The orientation of liquid crystals, particularly polymeric liquid crystals, in electric and magnetic fields is very important for the production of high-strength oriented materials [70].
645
ous factors that influence the effect, in particular the intensity and type of electric field, the rate of deformation, the temperature, composition and concentration of the disperse phase and various additives; secondly, to elucidate the mechanism of viscosity growth in an electric field and thus suggest the construction of new electrorheological systems; thirdly, to determine the fields of practical application for the ERE [52]. The influence of some factors on the ERE is illustrated for the case of suspensions of polyvinyl alcohol and starch in a hydrocarbon medium [48 - 51]. Figure 11 shows the dependence of the ERE on the intensity of an electric field (E) for various concentrations of the disperse phase. Up to a certain threshold intensity of the electric field, the ERE is only weak. However, above this intensity the shear stress grows sharply with approximately a parabolic dependence on E and then a tendency to saturation. As can be seen from the figure, the ERE value increases as the content of the disperse phase grows. The higher the disperse phase concentration is, the more pronounced the dependence of the ERE on E. Figure 12 shows the dependence of the ERE on the shear rate. The effect diminishes with increasing deformation rate, and practically disappears for ~ above 1000s -1. The ERE is strongest at low deformation rates. The temperature dependence of the ERE is complicated (figure 13). At first increasing the temperature increases the shear stress but then it drops. It is noteworthy that the maximum not only increases for stronger electric fields, but its position also moves towards lower temperatures. One of the main problems of the electrorheology of disperse systems is the elucidation of the nature of the
3.4 Electrorheological effect Special attention has been paid to the action of electric fields on the structure of disperse systems with a view to increasing their viscosity. Winslow [42] was one of the first to observe a considerable increase of the shear stress in hydrocarbon dispersions of silica gel and semiconductors under the action of an electric field. Later the electrorheological effect was investigated by Klass and Martinek [43, 44]. Important investigations of the structure formation kinetics in combined electric and shear fields in connection with the study of the nature of the electrorheological effect were carried out by Shulman and Matsepuro [3, 45- 47]. Investigations of the electrorheological effect (ERE) have a number of aims: first, to study the role of vari-
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