Thin liquid film lubrication under external electrical ...Thin liquid film lubrication under...

Thin liquid film lubrication under external electrical fields: Roles of liquidintermolecular interactions

Guoxin Xie,a) Jianbin Luo,b) Shuhai Liu, Dan Guo, and Chenhui ZhangState Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China

(Received 10 January 2011; accepted 8 April 2011; published online 1 June 2011)

One of the important features of the nanoscale liquid film lubrication is the formation of ordered

layers at the solid/liquid interface. In this paper, the effect of the intermolecular interaction in

liquid lubricant films on the formation of ordered layers after applying external electric fields

(EEFs) has been investigated by measuring the central-film-thicknesses of liquids in concentrated

point contacts and then inferring the thin film rheology. It has been found that the film formation

properties of both pure liquid n-alkanes and liquid n-alcohols with relatively long chains have

weak responses to EEFs, while those of their mixed solutions could be enhanced more notably by

EEFs. In addition, the effect of the dispersive interactions between solvent molecules on the

formation of ordered layers in thin lubrication films under EEFs was also discussed. VC 2011American Institute of Physics. [doi:10.1063/1.3587477]

I. INTRODUCTION

Understanding the nanoscale properties of thin liquid

films is of great importance in a wide variety of applications

such as lubrication, spreading and adhesion, etc.1,2 With the

development of micro/nanomachining techniques, the tribo-

surfaces of ultraprecise instruments are often molecularly

smooth, and the gap between the surfaces is usually at the

nanometer scale. The lubricant film confined in such a nar-

row gap behaves differently from the expectations of classi-

cal lubrication theories.3,4 Different kinds of experimental

techniques have been applied to investigate the behaviors of

nanoscale liquid films in lubricated contacts.5–8 It has been

shown8–14 that the behaviors of the molecules in the confined

liquid film are dependent on the physicochemical properties

of the liquid, the solid surface properties, and the interactions

between the liquid molecules and the solid surfaces.

More recently, the effect of an external electrical field

(EEF) on the properties of thin liquid films has attracted

much attention.16–22 One of the important findings in these

studies was that the EEF could enhance the degree of molec-

ular ordering in polar liquid films or nonpolar solution films

with polar additives. As a result, similar to the action of an

EEF on liquid crystals, the electroviscosity effect of these

liquid films due to the formation of ordered layers or electri-

cal double layers near the surfaces of tribopairs could be

expected. However, the EEF effect on the liquid films with

longer alkyl chains was found to be less noticeable in our

previous study,20 suggesting that the interactions between

liquid molecules should be adequately considered for under-

standing the behaviors of confined liquid films. Therefore,

the investigations of how the interfacial ordered layers in liq-

uid films are affected by the interactions between liquid mol-

ecules under the influence of EEFs are very interesting.

However, it is rarely discussed. In the present study, the

relative optical interference intensity (ROII) technique was

employed to investigate the EEF effects on confined films of

simple n-alkanes, n-alcohols, and their mixtures by meas-

uring the central-film-thickness in the concentrated point

contact and then calculating the effective viscosity.

II. EXPERIMENT

The scheme of the experimental setup is shown in Fig.

1. A circular contact region is formed when a high precision

steel ball in a liquid cup is pressed against a glass disk coated

with a semireflective chromium (Cr) layer. The test liquid in

the oil cup is entrained into the contact region with the rota-

tion of the ball driven by the disk. Then, a liquid film is

established in the contact region. The state of the liquid film

in the contact region can be monitored with the interference

patterns obtained from a microscope and a charge coupled

device camera. The central-film-thickness, h, in the contact

region is obtained with the ROII technique. A detailed meas-

uring mechanism of the film thickness can be seen in Refs. 8

and 15. The roughness, Ra, values of the undeformed surfa-

ces of the steel ball and the Cr layer are about 3.7 nm and 1.0

nm, respectively. The Young’s moduli of the steel ball and

the glass disk are 205.8 and 72.0 GPa, respectively. The val-

ues of Poisson’s ratio of the steel ball and the glass disk are

0.30 and 0.17, respectively. The applied load was 28 N. The

corresponding contact radius was about 125.56 lm. The ball

was replaced and the disk was carefully cleaned after each

test. In order to expose the liquid film to an EEF, the nega-

tive pole of direct current power remains in contact with the

Cr layer, and the positive one is in contact with the steel ball.

Three sets of liquid mixtures were used: (I) n-decanol

(Sinopharm Chemical Reagent, purity: >99%) in n-heptane

(Sinopharm Chemical Reagent, purity: >99%) mixtures with

solute concentrations of 5, 10, 25, 37.5, 50, and 75 vol. %;

(II) n-decanol in n-hexadecane (Haltermann, purity: >99%)

mixtures with solute concentrations of 10 and 50 vol. %;

(III) n-decanol in octamethylcyclotetrasiloxane (OMCTS)

a)Electronic mail: [email protected])Author to whom correspondence should be addressed. Electronic mail:

[email protected].

0021-8979/2011/109(11)/114302/6/$30.00 VC 2011 American Institute of Physics109, 114302-1

JOURNAL OF APPLIED PHYSICS 109, 114302 (2011)

Downloaded 05 Jul 2011 to 166.111.200.172. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

http://dx.doi.org/10.1063/1.3587477

http://dx.doi.org/10.1063/1.3587477

http://dx.doi.org/10.1063/1.3587477

(Fluka, purity: >99%) mixtures with solute concentrations

of 10 and 50 vol. %. The bulk viscosity, g0, was measured

using a rheometer (Anton Paar, MCR301). The boiling

points at atmospheric pressure of n-heptane, n-hexadecane,

and OMCTS are 98.4, 287, and 175.5 �C, respectively. The

relevant physical parameters of the test liquids are summar-

ized in Table I. All of the experiments were conducted at a

temperature of 25 6 1 �C.

III. RESULTS AND DISCUSSION

The central-film-thicknesses, h, of the n-decanol/n-heptane

mixtures against rolling speed at different external voltages are

shown in Fig. 2. According to the elastohydrodynamic (EHD)

lubrication theory, the film thickness, h, depends upon the

speed, u, and the lubricant bulk viscosity, g0, according to

Ref. 3

h ¼ a ug0ð Þb; (1)

where the constant, a, is related to the geometry and the elas-

tic modulus of the tribopair, the pressure-viscosity coeffi-

cient of the lubricant and load, etc. The index, b, normally

ranges from 0.6 to 0.75 for different liquids in the EHD

lubrication regime.9 The fluid washing effect is strong and

the fluid film acts predominantly in the lubricated contact in

this lubrication regime.8 It can be inferred from Eq. (1) that

the plot of log (h) versus log (u) should be linear. The theo-

retically fitted lines (dashed lines) of the film thicknesses at

higher speeds investigated under no EEF according to Eq.

(1) are plotted in Fig. 2. As shown, the straight lines in the

plots have different theoretical gradients due to the differen-

ces in the index, b, and the gradients for some liquids are

smaller than 0.6 within the investigated speed range. The

gradient smaller than the typical one in the EHD lubrication

regime may arise from the decrease in the fluid washing

effect and the increase in the effect of solid surface proper-

ties on the film formation properties. Furthermore, such an

analysis done by fitting the measured film thickness over a

power law of speed would bring some uncertainty to the

comparison between the film formation properties of differ-

ent liquids. However, it does not affect the estimation of the

changes in the film formation properties for a given liquid af-

ter exposure to EEFs.

The deviation of the measured film thicknesses in Fig.

2(a) from the theoretically fitted values of pure n-heptane is

unnoticeable, and the differences between the film thicknesses

at different voltages are very small. For n-decanol/n-heptane

mixtures of low solute concentrations (5, 10, 25, and 37.5 vol.

%), the relationships between log (h) and log (u) at low

speeds exhibit deviations from linearity, i.e., the gradients of

the plot of log (h) versus log (u) reduce [Figs. 2(b)–2(e)]. The

measured film thicknesses under no EEF are several nano-

meters larger than the theoretical ones. After exposure to the

EEFs, the films become thicker in varying degrees. Most dra-

matically, the film thicknesses increase by more than 10 nm

at 30 V for 25 and 37.5 vol. % n-decanol/n-heptane mixtures.

In contrast, nearly linear relationships of log (h) versus log

(u) under no EEF can be observed for 50 and 75 vol. % n-dec-

anol/n-heptane mixtures within the investigated speed range

[Figs. 2(f) and 2(g)]. After exposure to the EEFs, the film

thicknesses increase by 2–3 nm, as a whole. For pure n-dec-

anol, the measured film thicknesses under no EEF are slightly

larger than the theoretical values at low speeds. However,

very small changes in the film thicknesses can be observed

under the influence of EEFs [Fig. 2(h)].

A further analysis of Eq. (1) indicates that if the pres-

sure-viscosity coefficient of the lubricant is assumed to be

invariant when the lubricant is confined in a narrow gap, the

deviation of the gradient of log (h) versus log (u) from the

typical value in the EHD lubrication regime at low speeds

can be ascribed to the change in the lubricant viscosity. This

is due to the fact that other operating conditions, e.g., load

and rolling speed, are unchanged. Thus, the effective viscos-

ity of the confined liquid film can be obtained by rearranging

Eq. (1) as,

geff �1

u

h

a

� �1=b

: (2)

In order to give a clearer demonstration of the EEF effect on

the characteristics of confined lubricant films, the effective

viscosity, geff, will be discussed in the following text. The

calculated effective viscosities of the n-decanol/n-heptane

TABLE I. Some relevant physical parameters of test liquids.

Bulk viscosity,

g0 (mPa � s)

Refractive

index

n-heptane 0.39 1.39

n-decanol 10.90 1.43

n-hexadecane 3.59 1.43

OMCTS 2.40 1.40

n-decanol/n-heptane (5 vol. %) 0.88 1.39



n-decanol/n-heptane (37.5 vol. %) 3.66 1.41



n-decanol/n-hexadecane (10 vol. %) 4.73 1.43

n-decanol/n-hexadecane (50 vol. %) 6.43 1.43

n-decanol/OMCTS (10 vol. %) 3.25 1.42

n-decanol/OMCTS (50 vol. %) 5.63 1.42

FIG. 1. The schematic diagram of the experimental setup.

114302-2 Xie et al. J. Appl. Phys. 109, 114302 (2011)


films by using Eq. (2) are shown in Fig. 3. Since the EEF

effect on the film thickness of pure n-heptane is not remark-

able and the data are also somehow scattered, the effective

viscosities of the n-heptane film were not calculated. As

shown in Fig. 3(a), the effective viscosity under no EEF

gradually increases with the decreasing film thickness for the

5 vol. % n-decanol/n-heptane film, and a double increase,

compared with the bulk value at the film thickness of 5 nm,

is present. Larger increases in the effective viscosity of the

5 vol. % n-decanol/n-heptane film can be observed after ex-

posure to EEFs. The effective viscosity at 30 V increases by

six times compared with the bulk value at the film thickness

of 8 nm. Similarly, the EEF effect on the effective viscosity

at the small film thickness is also very evident for the n-dec-

anol/n-heptane films with solute concentrations of 10, 25,

and 37.5 vol. % [Figs. 3(b)–3(d)]. Most dramatically, the

effective viscosity at 30 V increases by more than ten times

compared with the bulk value for the 25 vol. % n-decanol/

FIG. 2. (Color online) Central-film-

thicknesses vs rolling speed of the n-dec-

anol/n-heptane films with different sol-

ute concentrations: (a) 0, (b) 5, (c) 10,

(d) 25, (e) 37.5, (f) 50, (g) 75, and (h)

100 vol. % at various voltages. The theo-

retically fitted lines (dashed lines) of the

film thicknesses at 0 V at high speeds,

investigated according to Eq. (1), are

also shown.



n-heptane film. In the cases of the n-decanol/n-heptane films

with higher solute concentrations (50 and 75 vol. %) and the

pure n-decanol film, the EEF effects on the effective viscos-

ity are not as remarkable as those for films with low solute

concentrations, as shown in Figs. 3(e)–3(g).

It was previously proposed that the larger film thick-

nesses than the theoretically predicted ones under no EEF

are due to the formation of ordered layers in the liquid film

near solid surfaces.8 In the presence of an EEF, polar mole-

cules with permanent dipoles can be adsorbed and reoriented

to more easily increase the number of interfacial ordered

layers.15 However, the EEF effects on the adsorption and ori-

entation of the molecules in pure polar liquid films become

less pronounced when the alkyl chain is longer.20 Such phenom-

ena were proposed to be a result of the intermolecular interac-

tions in the liquid film, mainly including the dipole-dipole

FIG. 3. (Color online) Normalized

effective viscosity vs film thickness of

the n-decanol/n-heptane films with dif-

ferent solute concentrations: (a) 5, (b)

10, (c) 25, (d) 37.5, (e) 50, (f) 75, and

(g) 100 vol. % at various voltages. The

solid curve (0 V), long-dashed curve (10

V), short-dashed curve (20 V), and dot-

ted curve (30 V) were drawn by hand to

guide the eye.



interactions [the shadow circular regions of Fig. 4(a)] and

the dispersive interactions between alkyl chains [the regions

denoted as dashed circles of Fig. 4(a)]. Large intermolecular

interactions would result in high potential barriers for the

breaking of monomers and the following rotation of partially

liberated monomers. Hence, the EEF effect on the effective vis-

cosity of the pure n-decanol film is not very remarkable. For

the reasonably dilute n-heptane solutions in which n-decanol

is the solute (e.g., the solute concentrations are 5, 10, 25, and

37.5 vol. %), the intermolecular self-association can be mini-

mized in the following way: The dipole-dipole interactions,

or rather hydrogen bonds between alcohol molecules,

become weak in a dilute nonpolar n-alkane solution. Then,

the hydroxyl groups have a larger rotational freedom, and

the permanent dipoles can be reoriented more easily by the

electrical force. More ordered layers could be formed near

the solid surfaces due to the solid/liquid interfacial interac-

tions, e.g., the ion-dipole interaction. Therefore, a greater

effective viscosity and thus, a larger film thickness can be

expected, as schematically shown in Fig. 4(b).

Next, the mixtures of n-decanol in n-hexadecane, which

has a longer alkyl chain than n-heptane, will be discussed. The

dispersive interactions between alkyl chains in n-hexadecane

are stronger than those in n-heptane, and it can be intuitively

shown by the difference in the boiling points of the two

liquids. The n-hexadecane will have a lower solvation power

than n-heptane, and n-decanol will be less well solvated in

n-hexadecane.23 In other words, n-decanol molecules will be

more aggregated and curled up in n-hexadecane than in

n-heptane.24 Hence, the hydrogen bonds between n-decanol

molecules in n-hexadecane are stronger since the number of

hydroxyl groups per unit volume is constant at the same sol-

ute concentration. More energy is then needed to compensate

for the larger entropy loss associated with the alignment by

EEFs for n-decanol molecules in the n-hexadecane solution,

i.e., the n-decanol molecules display a lower capacity for

being reoriented by EEFs. Therefore, the n-decanol/n-hexa-

decane film should have a weaker response to the EEF than

the n-decanol/n-heptane film at the same solute concentration.

Such a speculation is in agreement with the experimental

results in Fig. 5, where the EEF effect on the effective viscos-

ity of the n-decanol/n-hexadecane films is less remarkable.

Finally, OMCTS, a model liquid with quasispherical

molecules, was used as the solvent. The relationships

between the effective viscosity and the film thickness of

OMCTS, and n-decanol/OMCTS mixtures with solute con-

centrations of 10 and 50 vol. % are shown in Fig. 6. The

effective viscosity of the pure OMCTS film under no EEF

becomes increasingly larger with the decreasing film thick-

ness when the film thickness is smaller than 12 nm. It is pri-

marily due to the formation of molecular layers in the

OMCTS film near the solid surfaces, and similar results were

also observed in previous studies9,10,25–27 However, no no-

ticeable change in the effective viscosity of the OMCTS film

can be observed after the film has been exposed to EEFs.

The variations in the effective viscosity of the n-decanol/

OMCTS films with the film thickness become less noticeable

under no EEF, indicating that n-decanol probably destroys

the molecular ordering in the OMCTS film. However, the

EEF effect on the effective viscosities of the n-decanol/

OMCTS films is more remarkable than that of the OMCTS

film. For instance, the effective viscosity of the 50 vol. % n-

decanol/OMCTS film increases dramatically with the

decreasing film thickness at 30 V. Moreover, it can be found

that the response of the effective viscosities of the n-decanol/

OMCTS films to EEFs is more pronounced than that of the

n-decanol/n-hexadecane films but less than that of the n-decanol/

n-heptane film at the same solute concentration. Such a phenom-

enon is primarily due to the difference in the strengths of the

FIG. 4. (Color online) The schematic diagrams of (a) pure n-decanol and

(b) n-decanol/n-heptane films with low solute concentrations under EEFs.

FIG. 5. (Color online) Normalized effective viscosity vs film thickness of (a) n-hexadecane film, (b) 10 vol. % n-decanol/n-hexadecane film, and (c) 50 vol. %

n-decanol/n-hexadecane film at various voltages. The exponents, b, used in Eq. (2) were 0.60, 0.56, and 0.60 for these lubricant films, respectively. The solid

curve (0 V), long-dashed curve (10 V), short-dashed curve (20 V), and dotted curve (30 V) were drawn by hand to guide the eye.



dispersive interactions between solvent molecules. It can be

inferred from the boiling points that the dispersive interactions

between OMCTS molecules are smaller than those between n-

hexadecane molecules, while larger than those between n-

heptane molecules. As discussed above, the larger the dispersive

interactions between solvent molecules, the lower the solvation

power of the solvent and the stronger the hydrogen bonds

between the hydroxyl groups in the n-decanol/solvent films.

IV. CONCLUSIONS

In summary, the effect of intermolecular interactions on

the formation of ordered layers in nanoliquid lubrication

films under EEFs has been investigated through measuring

the film thickness under pure rolling conditions. Experimen-

tal results indicate that the film formation properties of a rea-

sonably dilute n-alkane solution in which the solute is a long

liquid alcohol can be enhanced more remarkably by EEFs

than those of the pure solvent and the pure solute due to the

reduction in the intermolecular self-association. After en-

hancing the dispersive interactions between solvent mole-

cules, the number of the interfacial ordered layers in the

nanoliquid film decreases and the promotion of the film for-

mation properties by EEFs would weaken. The present work

suggests that properly incorporating the intermolecular inter-

actions into the lubricant design is of practical significance

for desirable control over the lubrication properties of thin

liquid films by EEFs.

ACKNOWLEDGMENTS

The work is financially supported by the National Natu-

ral Science Foundation of China (Grant Nos. 51021064 and

50823003). The authors are also grateful to the NSK Ltd. for

providing high precision steel balls.

1D. T. Wasan and A. D. Nikolov, Nature (London) 423, 156 (2003).2M. Urbakh, J. Klafter, D. Gourdon, and J. Israelachvili, Nature (London)

430, 525 (2004).3B.J. Hamrock and D. Dowson, ASME J. Lubr. Technol. 99, 264 (1977).4F. P. Bowden and D. Tabor, The Friction and Lubrication of Solids(Oxford University Press, London, 1954).

5I. Krupka, M. Hartl, and M. Liska, Trans. ASME, J. Tribol. 127, 890 (2005).6F. Guo and P. L. Wong, Proc. Inst. Mech. Eng., Part J: J. Eng. Tribol. 216,

281 (2002).7H. A. Spikes and P. M. Cann, Proc. Inst. Mech. Eng., Part J: J. Eng. Tribol.

215, 261 (2001).8J. B. Luo, S.Z. Wen, and P. Huang, Wear 194, 107 (1996).9H. A. Spikes and M. Ratoi, Tribol. Interface Eng. Ser. 38, 359 (2000).

10M. Hartl, I. Krupka, and M. Liska, Sci. China Ser. A-Math., Phys.,

Astron. 44, 412 (2001). http://www.scichina.com:8081/sciAe/EN/abstract/

abstract376450.shtml.11S. H. Liu, L. R. Ma, C. H. Zhang, and X.C. Lu, Appl. Phys. Lett. 91,

253110 (2007).12S. H. Liu, G. X. Xie, D. Guo, and Y. H. Liu, J. Appl. Phys. 107, 104323 (2010).13Y. Z. Hu and S. Granick, Tribol. Lett. 5, 81 (1998).14A. Martini and A. Vadakkepatt, Tribol. Lett. 38, 33 (2010).15J. B. Luo, M. W. Shen, and S. Z. Wen, J. Appl. Phys. 96, 6733 (2004).16R. Verma, A. Sharma, K. Kargupta, and J. Bhaumik, Langmuir 21, 3710 (2005).17D. Bratko, C. D. Daub, K. Leung, and A. Luzar, J. Am. Chem. Soc. 129,

2504 (2007).18H. B. Zeng, Y. Tian, T. H. Anderson, M. Tirrell, and J. N. Israelachvili,

Langmuir 24, 1173 (2008).19D. Bratko, C. D. Daub, and A. Luzar, Phys. Chem. Chem. Phys.10, 6807

(2008).20G. X. Xie, J. B. Luo, D. Guo, and S. H. Liu, Appl. Phys. Lett. 96, 043112

(2010); G. X. Xie, J. B. Luo, S. H. Liu, D. Guo, and C. H. Zhang, Soft

Matter 7,4453 (2011).21I. Bou-Malham and L. Bureau, Soft Matter 6, 4062 (2010).22S. Srivastava, P. D. S. Reddy, C. Wang, D. Bandyopadhyay, and A.

Sharma, J. Chem. Phys. 132, 174703(2010).23H. J. Butt and M. Kappl, Surface and Interfacial Forces (Wiley-VCH,

Weinheim, 2010).24M. G. Martin, N. D. Zhuravlev, B. Chen, P. W. Carr, and J. I. Siepmann,

J. Phys. Chem. B 103, 2977 (1999).25J. Klein and E. Kumacheva, Science 269, 816 (1995).26Y. Zhu and S. Granick, Langmuir 19, 8148 (2003).27L. Bureau, Phys. Rev. Lett. 104, 218302 (2010).

FIG. 6. (Color online) Normalized effective viscosity vs film thickness of (a) pure OMCTS film, (b) 10 vol. % n-decanol/OMCTS film, and (c) 50 vol. % n-

decanol/OMCTS film at various voltages. The exponents, b, used in Eq. (2) were 0.67, 0.75, and 0.67 for these lubricant films, respectively. The solid curve (0

V), long-dashed curve (10 V), short-dashed curve (20 V), and dotted curve (30 V) were drawn by hand to guide the eye.



http://dx.doi.org/10.1038/nature01591

http://dx.doi.org/10.1038/nature02750

http://dx.doi.org/10.1115/1.3453074

http://dx.doi.org/10.1115/1.2032991

http://dx.doi.org/10.1243/135065002760364822

http://dx.doi.org/10.1243/1350650011543529

http://dx.doi.org/10.1016/0043-1648(95)06799-X

http://www.scichina.com:8081/sciAe/EN/abstract/abstract376450.shtml




http://dx.doi.org/10.1063/1.2827176

http://dx.doi.org/10.1063/1.3429074

http://dx.doi.org/10.1023/A:1019121103687

http://dx.doi.org/10.1007/s11249-009-9568-6

http://dx.doi.org/10.1063/1.1806259

http://dx.doi.org/10.1021/la0472100

http://dx.doi.org/10.1021/ja0659370

http://dx.doi.org/10.1021/la7017242

http://dx.doi.org/10.1039/b809072f

http://dx.doi.org/10.1063/1.3292213

http://dx.doi.org/10.1039/C0SM00971G

http://dx.doi.org/10.1039/C0SM00971G

http://dx.doi.org/10.1039/c0sm00377h

http://dx.doi.org/10.1063/1.3400653

http://dx.doi.org/10.1021/jp984583z

http://dx.doi.org/10.1126/science.269.5225.816

http://dx.doi.org/10.1021/la035155+

http://dx.doi.org/10.1103/PhysRevLett.104.218302

Thin liquid film lubrication under external electrical ...Thin liquid film lubrication under...

Documents

Transcript of Thin liquid film lubrication under external electrical ...Thin liquid film lubrication under...