ORIGINAL PAPER

Effect of Alkyl Chain Length on the Orientational Behaviorof Liquid Crystals Nano-Film

Ming Gao1 • Liran Ma1 • Jianbin Luo1

Received: 8 November 2015 / Accepted: 19 February 2016 / Published online: 31 March 2016

� Springer Science+Business Media New York 2016

Abstract Different homologues of cyanobiphenyl liquid

crystals (CB LC) (nCB with n = 5, 6, 7) present an

interesting and novel material system. So far, their orien-

tational behavior at nanoscale has not been extensively

investigated yet. Here, we utilized a self-established in situ

ball-on-disk testing platform equipped with polarized

Raman spectroscopy, to study the ordering performance of

nCB LCs confined as a nano-thin lubricating film. The

results demonstrated that both external condition as shear

velocity and internal molecular alkyl length dramatically

affect the ordering process of nCB LC nano-lubricating

film, consequently the degree of anisotropy. A simple

model, along with detailed physical analysis, has been

proposed to explain the observed phenomena. Our findings

may provide new insights into controlling the alignment of

LCs during lubricating.

Keywords Nanotribology � Liquid crystal � Nano-lubricating film � Raman spectroscopy

1 Introduction

Microfluidics and rheology of complex fluids such as

liquid crystal (LC) play a fundamental role in many

fields, including microchips [1, 2], LC display (LCD)

technologies [3–5], and lubrication [6, 7]. During the past

decades, a great goal of many studies has been performed,

especially on the orientational quality of LC in a confined

microspace [8]. Due to its soft ordering and liquid nature,

LC also extends its application in tribological research area

[9]. Benefiting from their ability to form ordered molecular

films near substrate surfaces during the frictional process,

low friction coefficient could be obtained when LCs were

used as the lubricants [10–12]. Hence, LCs have become a

popular material for lubricants or lubricant additives, which

brings about the need to understand the effects of various

external and internal factors coupling on their ordering

under shearing.

As of now, a large amount of pioneering experiments

and theoretical investigations had been done to explore the

underlying tribological mechanism of LCs. The work done

by Gennes et al. showed that smectics LC confined

between surfaces will undergo surface-enhanced transla-

tional order, not just orientational, which likely has a sig-

nificant contribution to the lubrication effect [13]. Tian

et al discussed the ordering effect of 7CB confined in a

nanoscale carbon nanotube by its thermodynamic property,

revealing the relationship between the ordering effect and

its boundary lubrication property [14]. Matsumura et al.

[15] developed a small-size model of step bearing system

to the controllability and dynamic properties of 5CB liquid

crystal film. Kurihara et al. reported the effect of confine-

ment on the electric field-induced orientation of 6CB

between mica surfaces with the resonance shear measure-

ment (RSM) [16]. Recently, an in situ Raman spectroscopy

based on frictional testing platform has been built up by our

group, and related study has been done, focusing on the

molecular orientation of 5CB liquid crystal films confined

in contact region between a steel ball and silica glass disk.

This result opened up a new view on the measurement for

Electronic supplementary material The online version of thisarticle (doi:10.1007/s11249-016-0663-1) contains supplementarymaterial, which is available to authorized users.

& Liran Ma

maliran@tsinghua.edu.cn

1 State Key Laboratory of Tribology, Tsinghua University,

Beijing 100084, China

123

Tribol Lett (2016) 62:24

DOI 10.1007/s11249-016-0663-1

alignment anisotropy of molecule during friction process,

and it also revealed the dependence of friction coefficient

on molecular anisotropy [17].

However, few attention has been paid on the influence

of its own molecular structure on the orientation of LC

molecules in the confined flowing situation. In fact,

homologous LC always presents quite different characters

in melting points, mesophase morphology, transition tem-

perature from nematic to isotropic and optical properties

[18], which are strongly determined by the molecular

structure. Further, the flexible alkyl chain, which can adopt

many conformations, needs a special attention since it

enables influent the molecular entropy. Here, three nematic

LCs from the CB family (5CB, 6CB, and 7CB), which

differ in the length of the molecular CnH2n?1 side chain,

are investigated as the lubricants.

In this study, we aim to reveal the effect of alkyl ‘‘tai-

lor’’ on the molecular ordering performance of nano-liquid

crystal film. A ball-on-disk testing platform equipped with

a Raman spectrometer was employed, with a series of

nematic LCs used as the lubricants. Once the lubricating

process began, a nanoscale LC lubricating film was formed

between the ball and the surface of silica glass plate.

Molecular orientation was mapped in the inlet region, and

its quantitative anisotropic degrees are calculated by the C–

C aromatic vibration mode at 1606 cm-1. Further analysis

was carried out to demonstrate the observed experimental

phenomena. Our findings may have implication in

improving LC ordering performance and related lubricat-

ing properties.

2 Experimental Section

2.1 Materials

Three kinds of nematic-type liquid crystals (5CB: p-n-

pentyl-p’-cyanobiphenyl; 6CB: p-n-hexyl-p’-cyanobiphe-

nyl; and 7CB: p-n-heptyl-p’-cyanobiphenyl, purchased

from Huarui Ltd., China) owning a different length of alkyl

chain length were used as the lubricant. For easy reference,

the lubricants and their properties are listed in Table 1. All

solutes had a purity of at least 98% and were used without

further purification. Each liquid crystal has a rod-like

shape, containing three parts in its molecule: the flexible

alkyl chain, the rigid cyclobenzene group, and the cyano

group, as shown in Table 1 [19].

2.2 In Situ Nanoscale LC Film Alignment

Measurement Apparatus

Figure 1a shows a schematic diagram of the apparatus used

to form the nanoscale LC film and measure its alignment

behavior. A ball-on-disk testing platform was set up to

create a rolling point contact frictional condition, in which

a steel cup mounted with a force lever here served as a

container for lubricant, while the ball was supported by a

three bearings and rotated by the rotating silica plate. In

addition, the state of motion pair was pure rolling. During

the frictional process, a nanoscale lubricated film was

formed between the loaded steel ball with a diameter of

12 mm and the flat surface of a smooth silica glass disk.

According to AFM measurements, the roughness of the

ball and disk was both approximately 5 nm. The surfaces

of the ball and the disk have not been anchored yet; the LC

molecules lie on their surfaces randomly. The alignment

behavior of CB family liquid crystals under shearing was

initially examined by using a Raman spectrometer and an

optical microscopy, a CCD detector, and a He–Ne laser

(k = 514 nm emission wavelength), through which Raman

scattering was performed in the backscattering geometry

(180� scattering angle). Once the rolling friction started,

lubricants were entrained into the contact area between the

ball and disk, forming a lubricant film. As displayed in the

microscopic image (Fig. 1b), a micrograph of the nano-

sized lubricating film could be observed once load had

been applied on the steel ball against silica glass plate,

along with the contact area region (marked by purple cir-

cle) in the center, the outlet region in the front (marked by

yellow dashed line), and the inlet region.

2.3 Procedure

In a typical experimental process, the disk and ball were

carefully cleaned with ethanol and deionized water. The

applied normal load was 2.94 N, leading to the diameter of

contact area to be 150 lm. Raman spectra were collected

in the inlet region, with a spectral resolution of 2 cm-1 for

values of the angle h (formed by the light polarization

direction and the rotating speed direction, see Fig. 1b),

which vary from 0� up to 360�, with steps of 15�. Themeasurement was taken in the inlet region range from 0 to

200 lm, and the influence of the surface curvature of ball

could be neglected here. The measured spots are also

marked in Fig. 1b by the blue dots, from (0, 100) to (0,

200). The integral time for each spectrum was 10 s, and the

laser power was 150 mW. The amplitude order of the

measured film thickness is 1–10 nm, while the measure-

ment depth of Raman spectroscopy is 2 lm, so the mea-

surement is across the whole film thickness. Before each

Raman exposure process, the state of the lubricant film was

observed by using the microscope and charge-coupled

device camera. The rotating linear velocity of the ball

increases from 5 to 250 mm/s. The C–C aromatic vibration

mode of CB family liquid crystal at 1606 cm-1 was used to

calculate the orientational anisotropy, S, and characterize

24 Page 2 of 7 Tribol Lett (2016) 62:24

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the orientation ordering of liquid crystal molecule via

calculating the polarization anisotropy.

For each nematic crystal, the temperature was controlled

in the range of nematic state. For thermotropic liquid

crystals, their order parameter would be affected by the

temperature, but we focused on the anisotropy of the

lubricant films confined in/near contact area, where their

thickness is around 10*90 nm, depending on the shearing

velocity. Under this situation, the shearing force and the

anchoring energy of the surface are the main force driving

the alignment of liquid crystal molecules rather than the

temperature. In addition, during the measurement, the

thickness of lubricant film was mainly determined by the

viscosity of lubricants when the external conditions, such

as load and shearing velocity, were same. Thus, it is

necessary to keep the viscosity of liquid crystal be a steady

value. As reported in the previous studies, the viscosity of

nCB was hardly variable when the T–TCN\ 6 �C [20–22].

As a result, we choose the temperature which is just 5�Cabove the TCN for each liquid crystal as the experimental

temperature, that was 30 �C for 5CB, 25 �C for 6CB, and

35 �C for 7CB.

3 Results and Discussion

3.1 Effect of Shear Speed

The most important quality of liquid crystal molecules is

the orientational performance. Subjected to the shearing

Fig. 1 a Schematic diagram of

the experimental apparatus.

b Micrograph of lubricating

contact. h is the angle between

the direction of laser

polarization and rotating speed.

Raman scattering signal was

collected in the inlet area, from

(0, 100) to (0, 200), marked by

the blue dot

Table 1 Chemical and physical properties of CB family nematic crystals

LC TCN (�C) TNI (�C) Molar mass (g mol-1) Molecular length (A) 3D Structure

5CB 24 35.3 249 16.3

6CB 14.3 30.1 263 17.6

7CB 30 42.8 277 18.7

TCN Tcrystalline–nematic, TNI Tnematic–isotropic

Tribol Lett (2016) 62:24 Page 3 of 7 24

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force, nCB molecules aligned themselves in the inlet

region [23], where the lubricating film thickness (e.g.,

h & 400 nm) is sufficiently large with respect to the sur-

face roughness. Here, we recorded the intensities of the C–

C vibration (at 1606 cm-1) under various shear velocity

during the lubricating process, and then, we drew out the

relative intensity via a radar chart for each measured point

and calculated the related anisotropy. To guarantee the

accuracy, the test rig was operated for 10 min at each linear

speed till a steady state before recording data.

As depicted in Fig. 2 (A-1) to (A-3), (B-1) to (B-3), and

(C-1) to (C-3), the relative Raman intensity of the C–C

vibration measured at point (0, 100) under specific velocity

was marked in the formation of radar chart. Maximum

Raman intensity is obtained when the laser polarization is

along the direction of velocity (h = 0�); minimum is

observed when the laser polarization is perpendicular to

velocity (h = 90�). The geometrical shape of the curve

clearly represented the ordering degree of nCB molecules.

As the anisotropy increased with growing speed, the radar

curve gradually became a rod-like ellipse, which suggested

that the confined nCB molecules in inlet region oriented

along the direction of rotation.

The orientation anisotropy S of nCB LC under different

rotating speeds is also displayed in Fig. 2. Figure 2a–c

shows the degree of anisotropy for 5CB, 6CB, and 7CB at

different positions in inlet region under a series of rotating

speed (v = 5, 10, 25, 50, 100, 180 mm/s). Figure 2a indi-

cates that under same shear speed, the anisotropy of 5CB at

different positions in the inlet region is generally stable.

When the applied speed grew gradually, the anisotropy S

increased substantially. As shown in Figs. 2b and 3c,

similar phenomena and laws were observed for 6CB and

7CB. For all kinds of nCB, the anisotropy S increased with

linear speed and then reached to a stable value. The distinct

difference between 5CB, 6CB and 7CB was that under

considerably low linear velocity of 5 and 10 mm/s, no

obvious ordered alignment was observed for 7CB, and

orientation occurred till linear velocity increased up to

25 mm/s.

3.2 Effect of Alkyl Chain

To further confirm the effect of the alkyl chain length on

orientation behavior of nCB molecules, pairs of experi-

ments have been repeated for each nCB liquid crystal.

According to the results mentioned above, under the same

shear speed, the anisotropy for each nCB liquid crystal at

different positions of the inlet region was almost invariable.

Hence, we took the average value of the anisotropy S of all

Fig. 2 a Degree of anisotropy for 5CB at different positions in inlet

region under a series of rotating speed, (A-1) to (A-3) shows the

relative value of Raman intensity at different h recorder at position (0,

100) under rotating velocity of 5, 25, and 180 mm/s. Similar meaning

of b and c for 6CB and 7CB, respectively

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measured positions in the inlet region for each pair of

experiment carried under a specific velocity.

Figure 3 illustrates the relationship between the aniso-

tropy of nCB LC and rotating speed. Firstly, it presented

the positive effect of linear velocity on the ordering of all

the tested nCB nanoscale lubricating film. Considering the

growth rate of anisotropy, we found that slope of curve

decreased with the rotating speed, the anisotropy S of 5CB,

6CB, and 7CB would finally reach a constant value of

approximately 0.61, 0.56, and 0.54. What’s more, it can be

noticed that the anisotropy decreased with the increasing C

numbers in the flexible alkyl chain part. This phenomenon

was more evident in the speed region from 5 to 25 mm/s.

As described above, the anisotropy of 7CB was near 0 at 5

and 10 mm/s, indicating no ordering was occurred. Based

on the observations, it could be speculated that both

shearing velocity and the alkyl chain length play an

important role in determining the orientational property of

nCB LC molecules confined in a nanoscale space.

3.3 Model and Physical Analysis

We have developed a model to explain the observed polar

orientation of nCB molecules in the nanosized space. As

illustrated in Fig. 3a, when the shear force increased with

increasing shear speed, LC molecules were entrained into

the confined space between two surfaces. We firstly cal-

culated the thickness of nCB liquid crystal film, and then,

we can get the shearing stress in the inlet region. According

to Hamrock–Dowson formula [24], the center-film thick-

ness is:

h ¼ 2:69a0:53ðg0mÞ0:67ðE0 Þ�0:073

R0:467W�0:067ð1� 0:61e�0:73kÞ ð1Þ

where h is the center-film thickness, a is the pressure–

viscosity coefficient, g0 is the absolute viscosity at atmo-

spheric pressure and constant temperature, m is the speed,

E0 is the material modulus of elasticity, R is the radius of

the ball, W is the load, and k is the ellipticity parameter.

For our system, E’ = 115.36 Gpa, R = 12 mm,

W = 2.94 N, hence.

h � 0:0352a0:53ðg0mÞ0:67 ð2Þ

From Eq. (2), it could be inferred that when the film

thickness increases with the growing velocity, more liquid

crystal molecules would be confined into the nanogap

between the ball and plate. In this situation, more propor-

tion of molecules would be affected by shearing stress

other than the anchoring energy of the solid surface,

leading the anisotropy S of the whole liquid crystal film

increases. In addition, as marked by the green arrows in

Fig. 4a, the lubricating film was formed near the inlet

region, roughly half of it comes from the surface of plate

and other half comes from the surface of ball. Before the

formation of the lubricating film, the shearing force has

been already applied. So here the shear rate r could be

calculated as Eq. (3):

r ¼ m0:5h

¼ 56:8v0:33a�0:53g�0:670 ð3Þ

As the shearing speed was in the range of 5–180 mm/s,

the corresponding shear rate is in the range of 1.29 9 106–

4.2 9 106 S-1.Then, the shear stress s in the nCB liquid

crystal film was computed as Eq. (4):

s � g0r � 56:8ðg0vÞ0:33a�0:53 ð4Þ

As reported in previous measurements [25], pressure–

viscosity coefficient a and absolute viscosity g0 of 5CB,

6CB, and 7CB were nearly same due to their similar

molecular structure; the value of a was about 10 Gpa-1;

and the value of g0 was around 30 mPas. From Eq. (4), it

could be deduced that the shear stress s increased along

Fig. 3 Anisotropy parameter S dependence on rotating speed in the

CB family

Fig. 4 a Alignment of nCB LC molecules in nano-lubrication film.

b Model of single nCB LC molecule

Tribol Lett (2016) 62:24 Page 5 of 7 24

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with the shear speed, while under the same shear speed, the

shear stress s of 5CB, 6CB, and 7CB could be regarded as

equal.

Then, we considered the moment of inertia I of a single

liquid crystal. Here, we assumed it to be a rigid rod (its

length and width were marked as l and w), as depicted in

Fig. 4b. The calculation of I was shown in Eq. (5):

I ¼ ml2

3ð5Þ

According to the physical parameters of nCB LC dis-

played in Table 1, the moment of inertia I progressively

increased when the alkyl chain augmented. Based on the

above calculation, under the same shear speed, the applied

shear stress s was nearly identical for each nCB LC, while

moment of inertia I grew up with the increased alkyl

‘‘tailor.’’ Consequently, for a single molecule, less angular

rotation would occur from 5CB to 7CB. In addition, the

dispersion force between neighboring molecules would

increase as alkyl chain length got longer; meanwhile,

entanglement would occur frequently between molecules,

forming an aggregation occasionally, which stand in the

way of molecular orientational ordering. As a result, longer

nCB LC needs higher ‘‘power’’ to be arranged in order.

4 Conclusion

In summary, the alignment performance of nano-nCB

family liquid crystal film was investigated on a self-fabri-

cated ball-on-disk tribological platform via in situ polar-

ized Raman spectroscopy. A series of velocity were applied

to form the nano-lubrication film and align the LCs

molecules. Our results presented that the degree of aniso-

tropy for the same nCB liquid crystal grew in a logarithmic

form along with the rotating speed, eventually approached

to a stable maximum value. A significant difference on the

ordering behavior was also observed among these studied

liquid crystals, we attributed it to the length of alkyl chain.

We found that longer the alkyl chain owned by the liquid

crystal molecule, lower the anisotropy would be induced

under same shear speed. A proposed physical analysis

demonstrated that owning a higher molecular weight and

dispersion force, longer liquid crystal molecules needs

greater shearing force to be aligned orderly; meanwhile,

entanglement between molecules would occur more fre-

quently. Our results provided important supplementary for

the study of nanoconfined LC liquid orientational behavior

and understanding the effect of molecular structure on the

alignment performance.

Acknowledgments The work was financially supported by the

National Natural Science Foundation of China (51305225), the

National Key Basic Research Program of China (2013CB934200),

Research Fund of the Tsinghua University (20131089320).

References

1. Hassanzadeh, A., Lindquist, R.G.: Liquid crystal sensor micro-

chip. IEEE Sens. J. 12(5), 1536 (2012)

2. Hird, M.: Fluorinated liquid crystals–properties and applications.

Chem. Soc. Rev. 36(12), 2070 (2007)

3. Sengupta, A., Tkalec, U., Ravnik, M., Yeomans, J.M., Bahr, C.,

Herminghaus, S.: Liquid crystal microfluidics for tunable flow

shaping. Phys. Rev. Lett. 110(4), 048303 (2013)

4. Kim, J.H., Yoneya, M., Yokoyama, H.: Tristable nematic liquid-

crystal device using micropatterned surface alignment. Nature

420(6912), 159 (2012)

5. H. Kikuchi, Y. Haseha, S. Yamamoto, T. Iwata and H. Higuchi,

Sid Symposium Digest of Technical Papers 15(2), 578 (2009)

6. Luo, J.B., Shen, M.W., Wen, S.Z.: Tribological properties of

nanoliquid film under an external electric field. J. Appl. Phys.

96(11), 6733 (2004)

7. Mori, S., Iwata, H.: Relationship between tribological perfor-

mance of liquid crystals and their molecular structure. Tribol. Int.

29(1), 35 (1986)

8. Araki, T., Buscaglia, M., Bellini, T., Tanaka, H.: Memory and

topological frustration in nematic liquid crystals confined in

porous materials. Nat. Mater. 10(4), 303 (2011)

9. Borzsonyi, T., Szabo, B., Toros, G., Wegner, S., Torok, J.,

Somfai, E., Bien, T., Stannarius, R.: Orientational order and

alignment of elongated particles induced by shear. Phys. Rev.

Lett. 108(22), 228 (2012)

10. Amann, T., Kailer, A.: Ultralow friction of mesogenic fluid

mixtures in tribological reciprocating systems. Tribol Lett 37(2),343 (2010)

11. Nakano, K.: Scaling law on molecular orientation and effective

viscosity of liquid-crystalline boundary films. Tribol Lett 14(1),17 (2003)

12. Jimenez, A.E., Bermudez, M.D., Carrion, F.J., Martınez-Nicolas,

G., Amann, T.: Room temperature ionic liquids as lubricant addi-

tives in steel–aluminium contacts: influence of sliding velocity,

normal load and temperature. Wear 261(3–4), 347 (2006)13. De Gennes, P.G.: Interactions between solid surfaces in a pres-

mectic fluid. Langmuir 6(9), 1448 (1990)

14. Liu, X., Zhang, X., Tian, Y., Meng, Y.: Ordering of the 7CB

liquid crystal induced by nanoscale confinement and boundary

lubrication. Liq Cryst 39(11), 1305 (2012)

15. Matsumura, Y., Shiraishi, T., Morishita, S.: Stiffness and damp-

ing of liquid crystal lubricating film under electric field. Tribol.

Int. 54, 32 (2012)

16. Nakano, S., Mizukami, M., Kurihara, K.: Effect of confinement

on electric field induced orientation of a nematic liquid crystal.

Soft Matter 10(13), 2110 (2014)

17. Zhang, S., Liu, Y., Luo, J.: In situ observation of the molecular

ordering in the lubricating point contact area. J. Appl. Phys.

116(1), 014302 (2014)

18. Holbrey, J.D., Seddon, K.R.: The phase behaviour of 1-alkyl-3-

methylimidazolium tetrafluoroborates; ionic liquids and ionic

liquid crystals. J. Chem. Soc., Dalton Trans. 13, 2133 (1999)

19. Chmielewski, A.G., Lepakiewicz, E.: Rheological properties of

some biphenyl liquid crystals. Rheol. Acta 23(2), 207 (1984)

20. Jad _zyn, J., Czechowski, G., Bauman, D.: Static and dynamic

dielectric polarization and viscosity of n-hexylcyanobiphenyl in

the isotropic and nematic phases. Zeitschrift fur Naturforschung

A 55(9–10), 810 (2000)

24 Page 6 of 7 Tribol Lett (2016) 62:24

123

21. Ananthaiah, J., Sahoo, R., Rasna, M.V., Dhara, S.: Rheology of

nematic liquid crystals with highly polar molecules. Phys. Rev. E

89(2), 022510 (2014)

22. Oweimreen, G.A.: The effect of quasispherical solutes on the

smectic-A-nematic and nematic-isotropic phase equilibria in p-n-

Alkyl-p’-Cyanobiphenyl liquid crystals. J. Phys. Chem. B

105(35), 8410 (2001)

23. Luo, J., Wen, S., Huang, P.: Thin film lubrication. Part I. Study on

the transition between EHL and thin film lubrication using a rela-

tive optical interference intensity technique. Wear 194, 107 (1996)

24. Luo, J., Huang, P., Wen, S.: Characteristics of liquid lubricant

films at the nano-scale. J. Tribol. 121(4), 872 (1999)

25. Patricio, P., Leal, C.R., Pinto, L.F.V.: Electro-rheology study of a

series of liquid crystal cyanobiphenyls: experimental and theo-

retical treatment. Liq. Cryst. 39(1), 25 (2012)

Tribol Lett (2016) 62:24 Page 7 of 7 24

123