Electrokinetic measurements of particles in lubricating oils: Implications for filtrations

Electrokinetic measurements of particles in lubricating oils: Implications for filtrations Tushar K. Misra* and Stanley M. Barnett Department of Chemical Engineering University of Rhode Island, Kingston,

and the FramlAllied RI, USA

Filtration

Research

Center,

Experimental evidence showing the importance ofelectrical dr~lthle-layerf~~rces injltration of lubricating oil is presented. These forces are shown to he present in a wide variety of lubricating oils and are seen to be a function of the particle andJiber size, the zeta potential of the particle andjber media, and their separation distance. The implications of these forces and their interaction with the London-van der Waals forces and the resrrltant interaction potential and its possible effects on the performance of oil jilters is discussed.

Keywords:filtration, liquid; double-layer forces; filters, fibrous; oil, lubricating; zeta potential Introduction The primary purpose of an oil filter is to protect the engine from abrasive wear. In this manner, peak performance is maintained and maintenance costs are kept low. From the standpoint of engine protection, the size and concentration of oil contaminants are as important as their chemical composition. Wear rates are related to the contaminant size distribution in the oil and degree of filtration. One of the primary differences between the filtration of gases and liquids is the viscosity of the flowing fluid. This causes the Stokes number for a given particle in a liquid to be about two orders of magnitude smaller than in the case of air, and the Peclet number to be about two orders of magnitude larger. The Stokes number signifies the importance of inertial impaction as a capture mechanism, which increases as the Stokes number increases. Similarly, a decreasing Peclet number signifies the increasing role of Brownian diffusion. Therefore, these mechanisms do not prove to be relatively less important as capture mechanisms for liquids, when compared with interception and other forces such as the electrical doublelayer forces and London-van der Waals forces. During filtration, inertia and diffusion are unimportant in the transport step, whereas the double-layer and Lon* Current address: Exxon Production 77077, USA Address reprint requests to Dr. Bamett cal Engineering and the Fram/Allied University of Rhode Island, Kingston, Received II February 1991; accepted

@ 1991 Butterworth-Heinemann

Research Co., Houston, TX at the Department of ChemiFiltration Research Center, RI 02881, USA. 21 May 1991

don-van der Waals forces are important in the attachment step. Double-layer repulsion or attraction is commonly considered in aqueous filtration. Among the limited investigations that have considered surface charge phenomena in nonaqueous systems, the work of van der Minne and HermanierJ shows that ions exist in nonpolar hydrocarbons, and suspended particles exhibit electrophoretic mobility. Particle charge can occur due to adsorption or surface orientation effects. Due to the low ionic strength the neutralizing electrical double layer can be two or three orders of magnitude thicker than corresponding aqueous situations. Hence, the electrical double-layer attractions in such systems can lead to strong attraction or repulsion between particles and fibers (collectors). Lubricating oil filtration to a nominal IO-pm particle size is no longer adequate as clearances between moving parts become smaller in high-pressure systems. A typical sample of unfiltered, used lube oil may contain carbon particles ranging in size from colloidal dimensions up to 2 pm together with particles of road grit that may be 120-150 pm across.3 Hence, the practice of formulating these oils is to hold contaminants in suspension, so that particles that could cause abrasive wear are removed by a filter, before the oil is fed to the bearings. This is accomplished by adding detergents to the oil during formulation. Typically, particles are charged through the use of ionic surfactants that also alter the system’s electrical properties. Detergency is a necessary property of lubricating oils for use in internal combustion engines. This provides the oil the caSeparations

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1991, vol. 1

267

Electrokinetic

measurements

in lubricating

oil particles:

pacity to carry away soot and other combustion chamber detritus, as well as their own decomposition products, in order that they will not accumulate and interfere with normal engine operation. In order to keep these suspensions stable in the nonpolar media (the lube oil base is generally paraffinic or napthenic), the repulsive double-layer forces and the generally attractive van der Waals forces set up an energy barrier that prevents the suspended particles from agglomerating. Few physical properties of lubricants have received as much concentrated attention as those relating to surface phenomena. There are many fields such as emulsification, foaming, and lubrication in which interfacial phenomena dominate. The large effect that a low concentration of certain substances exerts on the interfacial activity of a lubricating oil without concomitant change in its bulk properties is a very important criterion. The selective adsorption of certain surfuce active substances at the interface causes them to be active even in very low concentrations.4 In such cases, the detergents disassociate in the lube oil, affecting its ionic strength, and also adsorb at the surface of the particles. This imparts a certain charge, positive or negative, depending on the ion that is adsorbed at the surface of the particle. Generally, all the particles will acquire the same charge and will repel each other. Even in the presence of nonionic surfactants there is a repulsive force that keeps suspensions stable. This repulsion is known as “entropic repulsion” and is in reference to the loss of transformational freedom of the tails of adsorbed surfactant molecules when two adsorbed layers interpenetrate. This interpenetration leads to a loss of entropy and thus to repulsion. Now if such a system were made to flow through a fibrous filter bed, it is a moot point as to what the polarity of the charge will be that will develop on the surface of the fibers. This depends to a great extent on the concentration of the detergent in the lube oil. Green and Parfitt’ carried out zeta potential measurements in a system consisting of carbon particles sus-

T. K. Misra and S. M. Barnett

pended in p-xylene with Aerosol-OT (sodium di-2ethylhexyl sulphosuccinate) as the detergent. At a detergent concentration of 1 mol/m3, the zeta potential of a carbon particle was +48 mV. However, when the detergent concentration was increased to 50 mol/m3, the zeta potential value of the carbon particle changed to -90 mV. The Debye length (K) at these conditions was 18.9 x 108 m-r, and the diffuseness of the electric double layer was two to three orders of magnitude greater than the equivalent solute concentration in water. Hence, small changes in the chemical nature of the system alter the degree of ionization and consequently the nature of the double-layer force; in fact, this force can be either attractive or repulsive between a particle and a fiber. An attractive double-layer force would obviously help the process of particle capture and retention. However, when it is repulsive, it acts together with the London-van der Waals forces to set up an energy barrier that tends to inhibit particle capture (Figures 14). Only particles possessing sufficient radial kinetic energy, when approaching a fiber surface, will be able to overcome this energy barrier and become attached to the fiber. Energies in excess of this can cause bounce off and possibly allow the particle to escape. In order to determine the electrical doublelayer force, the zeta potential of the particle and the fiber material have to be determined by careful electrophoretic measurements. Zeta potential data have been related to filter performanceM and dispersion stability.9 Along with the double-layer force, there is an attractive component known as the London-van der Waals force. This force is generally a function of the material properties of the particle, fiber, suspending fluid, and the distance of separation between the particle and fiber. The interaction of the particle, fiber, and suspending fluid is lumped together in a constant known as the Hamaker constant, An*, where the subscripts I, 3, and 2 refer to the type of fiber, suspending fluid, and particle, respectively.1° Table f provides a

Notation A132

:

E

@ K

vr

268

Hamaker constant for the carbonSDB-lubricating oil system (ergs) particle radius (pm; x lo4 A) shortest distance between the surfaces of the carbon black and SDB narticle (urn; x lo4 A) dielectric’ constant of the test fluid, dimensionless interaction potential (kT units or J) Debye-Htickel reciprocal length parameter (m-r) surface potential (mV; mV X 10m3/ 300 esu for force calculations)

Separations

Technology,

1991, vol. 1

p R 5

test fluid viscosity (Pa.s, reported at 22°C) resistance (ohm) zeta potential (mV; or mV X 10d3/ 300 esu for force calculations)

Subscripts

refer to carbon and SDB particles, respectively London-van DL, LVW, R refer to double-layer, der Waals forces, and resultant potentials of interaction, respectively

PC7 Pf

Electrokinetic 3 wt%

Vulcan

measurements

XC-72

in lubricating

oil particles:

T. K. Misra and S. M. Barnett

5 wt%

in Oil

Aero~~per~e

15 In Oil

I8

1.500 o-o O-0

5w30 IOW30 low40 15w50HD30

A-A A-A o-o

I4 I2 A-A

IO

O-O

0

5w30 TOW30 lOW40 15w50HD30

6 4 2 0 -2 -4 -6 -8 -10 0

100

300

200

600

500

400

I

1 0

100

200

300

500

400

DISTANCE,6(ANGSTROMS)

DISTANCE,6(ANGSTROMS)

Figure 1 Resultant interaction potential vs. separation distance for 3 wt% Vulcan XC-72 in oil

Figure 4 Resultant interaction potential vs. separation distance for 5 wt% Aerosperse 15 in oil

set of Hamaker constants for a combination cle-fiber systems in lubricating oil. 5 wt%

Vulcan

XC-72

-i 5 ;

Materials A-A

1.000

10~40 16W50 HD30

A-A O-O

0.500

k P p

of parti-

Experimental procedures

In Oil

1.500 : . e”

600

0.000

0

-1.000

iL 0

I 100

200

300

400

600

500

DISTANCE.6(ANGSTROMS)

Figure 2 Resultant interaction potential vs. separation distance for 5 wt% Vulcan XC-72 in oil

3 wt%

Aerorperse

I5

in Oil

IO 0

The experiments were conducted using Huber Aerosperse 15 and Carbot Vulcan XC-72 carbon black. To determine the double-layer interactions between carbon black particles and polystyrene fibers, styrene divinylbenzene (SDB) latex particles of 11.9 pm diameter (Dow Chemical Company, Indianapolis, IN, USA) were chosen. This study was conducted to determine the double-layer and London-van der Waals interactions between carbon black particles and polystyrene (SDB is the fiber analogue) fibers. Generally, the streaming potential technique is used to measure the zeta potential of fibers in a fibrous bed, by flowing the test fluid through a combination of the filter bed sandwiched in between two porous electrodes while measuring the induced potential and the pressure drop across the bed. The porosity of these fibrous filter beds are generally very high (~97%) and hence, the streaming potential technique is questionable because of the low pressure drops across such beds. Therefore, in order to obtain the interaction potential between carbon particles and fibers, analogous materials such as SDB latex spheres were chosen to represent the fibers.

6 0-O

4

HD30

2

Table 1 tems

0

Values of the Hamaker constant, A13*, for different sys-

-2 -4

Fiber 1

Contaminant

2

Medium

3

Alx*,t

-6 -8 -10

0

100

200

300

400

500

600

SDB Glass SDB

Carbon black Carbon black ACFTD

Lube oil Lube oil Lube oil

0.15 4.2 0.01

DI~TANCE.~(ANQSTROMS)

Figure 3 Resultant interaction potential vs. separation distance for 3 wt% Aerosperse 15 in oil

* All Hamaker constants are given in J x lO+*O t Hamaker constants were calculated using a method outlined by Visser.lO

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1991, vol. 1

269

Electrokinetic Table 2

measurements

SAE SAE SAE SAE SAE

5W30 lube oil lOW30 lube oil lOW40 lube oil 15W50 lube oil HD30 lube oil

112 126 156 248 232

Contaminant Huber Aerosperse 15 Carbon black Cabot Vulcan XC-72 Carbon black Fiber material SDB latex spheres

Dielectric constant, E

Conductivityt

p* x IO-3 x 10m3 x 10M3 x 10e3 x 1O-3

1.37 1.37 1.39 1.40 1.37

x x x x x

Specific gravity

* t * 5

oil particles:

Properties of the test particles and fluids

Fluid and description 1. 2. 3. 4. 5.

in lubricating

IO-4 10-d IO-“ 10-a 1O-4

2.45 2.45 2.48 2.38 2.50

Average particle size*

1.8

0.32

1.8

0.03

1.085

11.9 (1.9)§

Viscosity is given in Pa.s. Conductivity is given in Rmlm-l. Particle size is the particle diameter given in pm. Standard deviation in the diameter of SDB latex spheres.

T. K. Misra and S. M. Barnett

recovered, and oven dried at 40°C for 2 days. These were added to the test fluids at concentrations of 8 wt%. The carbon black was added to the test fluids at concentrations of 3 and 5 wt%. The experiments were carried out at 22°C and the relative humidity of the laboratory was controlled at 35 t 5 %RH. The test oils and the carbon black were also stored in a vacuum desiccator. This was done to minimize any absorption of water vapor by the hygroscopic oils. It has been observed that trace quantities of water in hydrocarbon media have a marked effect on the surface charges owing to the tendency of water to adsorb at the interface, thus altering and sometimes even reversing the charge. Since weighing is employed as a means for electrophoretic mobility determination, the cell in which the test is carried out was first saturated with the pure test fluid for a week before a test on the fluid in question was conducted. The cell was stored in the pure test fluid when not in use. The conductivity of the test fluids were measured, the cell constant having been previously determined using the electrophoretic cell used in the experiments, with a O.lN KC1 solution of known conductivity. The cell constant was determined to be 99.5 m-l.

Results The SDB particles were added to the test fluids and their zeta potentials measured. The implications of this choice and its effect on the results are presented in the discussion section. The latex spheres were supplied as 10 wt% stock suspensions. The lubricating oils (the test fluids) chosen for this test were commercially available lubricating oils (Mobil): (1) SAE 5W30, (2) SAE lOW30, (3) SAE lOW40, (4) SAE 15W50, and (5) SAE HD30. The viscosity of the test fluids was measured with a Brookfield LVT Viscometer (Brookfield Engineering Laboratories, Stoughton, MA, USA). Table 2 shows the required material properties of the test particles and the test fluids.

Zeta potential measurements

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Table 3 Zeta potential values of the contaminant logue in oil

1991, vol. 1

and fiber ana-

Zeta potential, 4”

methodology

The conductivity of the test fluids and the electrophoretie mobilities of the carbon black particles and the SDB spheres in lubricating oil were measured with a Zeta Potential Analyzer (Micromeritics Corporation, Norcross, GA, USA), which operates on the principle of the Hittorf method.” To recover the SDB latex spheres from the stock solutions, they were filtered using a 0.45~pm membrane. The recovered spheres were washed with a O.lN HCI solution several times, followed by several washes with a O.lN NaOH solution. The spheres were then washed continuously with distilled, deionized water that had been filtered through a 0.22~pm membrane for several minutes. This washing was continued, and the conductivity of the wash water was monitored until it was very low. This signified that any entrained acid or alkali had been washed away and the latex spheres were essentially clean. The sphere suspension was then centrifuged, 270

The zeta potentials of the test particles in the test fluids are shown in Table 3. These obtained values were averaged over a large number of observations (15 sets for each particle-fluid combination) to account for errors introduced by the Brownian motion of the particles. For sufficiently high particle concentration (> I%), the Hittorf method gives reproducible values for the electrophoretic mobility. The zeta potential values have

Contaminant Aerosperse

15

Vulcan XC-72

Fiber analogue SDB Latex particles

*All the numbers values.

Lube oil

3 wt%

5 vvt%

5w30 lOW30 1ow40 15w50 HD30 5w30 1ow30 1ow40 15w50 HD30

-16(4) -18(6) -23(9) -37(11) -34(7) -21(4) -18(5) -25(3) -33(5) -37(4)

-30(8) -25(8) -28(4) -42(7) -48(4) -28(2) - 24(3) -29(5) -36(8) -39(4)

-25(2) -36(5) -44(8) -54(6) -60(8)

5w30 1ow30 1ow40 15w50 HD30 in the parentheses

8 wt%

are standard

deviation

Electrokinetic

been reported

measurements

along with the standard

deviations

in lubricating

in

Tuble 3.

To determine the interaction potential of the carbon black particles and the analogous SDB particles, the method of Hogg et a1.i2 is used to determine the double-layer repulsive (or attractive) potential energy, between two spherical double layers is used. + SiPnfilarly, Hamaker13 derived the potential energy of attraction,
oil particles:

T. K. Misra and S. M. Barnett

lOW3O is essentially attractive, thereby presenting a very low energy barrier against particle capture and retention during the process of filtration. For the lOW40 oil, the interaction curve has a primary maximum of 0.495 kT for Vulcan XC-72 and 5.04 kT for Aerosperse 15. The interaction potential curve with the highest primary maximum of 15.17 kT is seen for the Aerosperse 15 in HD30 oil. It is seen that the interaction potentials are directly proportional to the particle radii, the zeta potentials, and inversely proportional to the interparticle distance.

%L =

’ ’- exp(-KS)) +hl(l In(1 eXp(-K8)

exp(-I&))

1

Discussion (1)

and

a LVW =

-

Awprrpf

6(r,, + rpfF

Following the convention, the attractive Londonvan der Waals force is assigned a negative value. The resultant potential energy of interaction is given by @R

= @'Dr. + @LVW

and this resultant potential can be entirely attractive or both attractive and repulsive, depending on the magnitudes of the surface potential, Hamaker constant, and double-layer thickness. In Equation 1, the zeta potentials are used in the place of surface potentials, & and {f (instead of qor and zI’,,f). According to Usmi and Yamasaki,i4 the results of their coalescence experiments with mercury droplets could be quantitatively interpreted in terms of the DLVO theory using the potential of the outer Helmholtz plane !P\I~,,which is almost identical with q0 (Yr,,. or q,,). That is, the assumption of *‘Of(or vO,) = tf (or &) is reasonable. This is valid for a nonaqueous system with a low dielectric constant and where the surface potential decays very slowly with distance. It is also assumed that the variation of the ion concentration in the system has little effect on K. Since the lubricating oil is a complex system, consisting of detergents and many additives such as oxidation and corrosion inhibitors, rust preventives, pour point depressants, viscosity index improvers, antifoamers, etc., it is very difficult to obtain the value of the Debye-Htickel parameter by calculating the ion concentration of the detergent additive. Hence, the Walden’s law approximation6 is used to calculate a value for K based on the test fluid’s conductivity. A value of K = 5.9 X lo7 m-r was obtained with the use of this approximation. Figures Z-4 show the resultant interaction potential of the carbon black particles and SDB spheres at different concentrations in various lubricating oils. The interaction potential is expressed in units of kT (4.072 x 10m2rJ at 22°C) and is plotted as a function of the shortest distance, 6, between the surfaces of the carbon black and SDB particles. It is seen in all four figures that the resultant potential for the 5W30 and the

Usually, it is consideredi that an energy barrier of about 15 kT is necessary for long-term stability, i.e., to prevent the particles from agglomerating. The highest primary maximum for the system is visualized in which the carbon particles are being filtered by flowing through a bed of polystyrene fibers; in order for particle capture to occur, the filtration mechanisms such as inertia, interception, and the surface forces such as double-layer forces and London-van der Waals forces act in concert. If a particle flowing toward a fiber has a radial kinetic energy that is greater than 15.17 kT, it will overcome the repulsive energy barrier and impact with the fiber. For the 5W30, lOW30, and lOW40 oils, the maximum repulsive potential is quite low. It is then quite possible that a particle with a low radial kinetic energy (25.04 kT) would be sufficient to overcome the energy barrier and lead to possible collision and capture. Seryassol and DavisI have shown that several collision scenarios are made possible by the presence of interparticle forces. First, if a repulsive potential barrier is present and the particle inertia is relatively weak, then the particle is unable to pass over the potential barrier as it approaches the fiber; they remain separated in the secondary minimum. Then, the particle inertia is sufficient for it to pass over the potential barrier but does not have inertia to deform during the collision with the fiber, or to rebound out of the primary minimum. Finally, as the inertia is increased further, the particles have enough inertia to deform and then rebound over the potential barrier into the secondary minimum. The temperature and flow velocities within a lube oil filter have been observed to be quite high, with flow regimes approaching turbulence. It is, therefore, quite probable that contaminant particles can aquire such kinetic energies to ensure attachment. An assumption was made that polystyrene fibers (SDB analogue) would develop similar zeta potentials in the oil, a fact that is seen in the work of Jaisinghani and Verdegan’ where the streaming potential experiments yielded zeta potentials of the same magnitude. The formulas provided by Rajagopalan and Tienr7 for the interaction of a spherical double layer (corresponding to the contaminant particle) and a cylindrical double layer (corresponding to the fiber) were used to calculate the interaction potential. The values obtained varied only within * 10% of those shown in Figures lSeparations

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Electrokinetic

measurements

in lubricating

oil particles:

4. This result then supports the employment of analogous spherical particles in place of the fibers for the zeta potential determination. The process of oil filtration is multipass in nature, i.e., the same lubricating oil is repeatedly circulated through the filter to remove the contaminants. It is safe to speculate that the data seen in Figures 1-4 are applicable only during the initial stages of filtration where the fiber surface is relatively clean. However, with time there is a gradual buildup of particles on the fiber. If the particles and the fiber have the same sign of the zeta potential, the particle accumulation would lead to a rise in the zeta potential of the fibers. This could lead to larger resultant interaction potential and could possibly be high enough to prevent the particles from approaching the fibers. This possibly may be high enough to prevent the particles from approaching the fibers. This would then translate to a drop in filter performance. Such a phenomenon was observed by Jaisinghani and Verdegan.7 One method of counteracting this problem would be to surface treat the fibers so that the sign of the zeta potential developed on the fiber would be opposite to that of the contaminant particles. This would lead to borh the double-layer and London-van der Waals forces being attractive.

The performance of oil filters is strongly affected by the double-layer interaction between contaminant (carbon black) and the filter media (SDB particles). This interaction shows a primary maximum, which is a repulsive energy barrier, that keeps the particle from contacting the fiber. It is seen to be dependent on the particle size, material properties of the particle, fiber, and suspending fluid, detergent concentration, and corresponding zeta potential. Besides these interactions, the filter performance is also a function of the porosity, flow velocity, temperature, etc. However, it is clear that through their concomitant actions, electrical double-layer and Londonvan der Waals forces tend to reduce removal. They effectively reduce the size of the contaminant particles by preventing agglomeration. These phenomena have yet to be fully understood and formulated completely on a theoretical basis.

Separations

Acknowledgments The authors are grateful to Mr. P. Aboytes and Mr. J. W. Balentine of J. M. Huber Corporation and to Mr. J. R. Hagerstrom of Cabot Corporation, respectively, for the generous, free samples of carbon black and also to Mr. Barry Verdigan of Nelson Industries for his valuable suggestions and helpful discussions.

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Technology,

1991, vol. 1

van der Minne,

J.L.

and Hermanie,

P.H.J.

J.

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SC;. 1952, 7, 612-615

2. 3. 4. 5. 6.

7.

van der Minne, J.L. and Hermanie, P.H.J. J. Colloid Sci. 1953, 8, 38-52 Braithwaite, E.R. ed. Lubricution und Luhricunts. New York: Elsevier, 1967 Bondi, A. Physicul Chemistry of Luhricuting Oils. New York: Rheinhold, 1951 Green, J.H. and Parfitt, G.D. Purticulate Science und Technology. vol. 5. 1987, p. 289 Chowdiah, P., Wasan, D.T. and Gidaspow, D. Electrokinetic phenomena in the filtration of colloidal particles suspended in nonaqueous media. AIChE J. 1981, 27,975-984 Jaisinghani, R.A. and Verdegan, B.M. Electrokinetics in hydraulic oil filtration-The role of anti-static additive. Proc. World Filtrution

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Onorato, F.J. and Tien, C. The effect of surface interactions on particle deposition. Chem. Eng. Commun. 1980, 7, 363376

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Conclusions

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Kitahara, A. Zeta potential in nonaqueous media and its effect on dispersion stability. Progress in Orgunk Coutings 1973/1974, 2, 81-98 Visser, J. On Hamakerconstants:

A comparison between Hamaker constants and Lifshitz-van der Waals constants. Adu. Colloid Interfuce Sci. 1972, 3, 331-363 Long, R.P. and Ross, S. An improved mass-transport cell for measuring electrophoretic mobilities. J. Co/bid Sri. 1965, 20, 438-447

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Hogg, R., Healy, T. W., and Furstenau, D.W. Mutual coagulations of colloidal dispersions. Truns. Furuduy Sot. 1966,62,

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Hamaker, H.C. The London-van der Waals attraction between spherical particles. Physiccr 1937, 4, 1058 Usmi, S. and Yamasaki, T. J. Co/bid Interfuce Sci. 1%9,29,

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Verwey, E.J.W. and Overbeek, J.T.G. Theory of Stuhility of Colloids. Amsterdam: Elsevier. 1948, p. I71 Serayssol, J.M. and Davis, R.H. The influence of surface interactions on the elastohydrodynamic collision of two spheres. Physicochem. Hydrodyn. 1987, 9, 41-52 Rajagopalan, R. and Tien. C. The theory of deep bed filtration. In Wakeman, R.J., ed. Progress in Filtrufion und Sepurution. New York: Elsevier. 1979, p. 196

Lyophobic

16.

17.