176 N.M. Kimura et al. / Physics Letters A 370 (2007) 173–176
Table 1
Values of the characteristic thermal times, thermal diffusivities and ratio of the thermal diffusivities in the discotic nematic sample at T = 25
◦
C
Phase t
co
(ms)
t
co⊥
(ms)
D
(10
−8
) m
2
/s
D
⊥
(10
−8
) m
2
/s
D
/D
⊥
(measured)
D
/D
⊥
(estimated)
N
D
2.62 ± 0.05 2.43 ± 0.05 4.40 ± 0.08 4.74 ± 0.08 0.93 0.88
parameters, into account, we obtain the parallel (D
) and per-
pendicular (D
⊥
) thermal diffusivities defined, respectively, in a
direction parallel or perpendicular to the director of the nematic
sample, and the ratio D
/D
⊥
. These important parameters are
given in
Table 1.
As can be seen in Table 1, the ratio D
/D
⊥
is smaller than
1 in this discotic nematic phase. To our knowledge, there are
no independent measurements of this ratio with this discotic
nematic phase in the literature. However, some time ago the
thermal diffusivity in a LLC was studied in a calamitic nematic
phase
[14] and it was proposed, as a first approach, that this
phenomenon could be understood through a relation given by
(2)
D
D
⊥
=
L
L
⊥
(1 + 2S) + (2 − 2S)
L
L
⊥
(1 − S) + (2 + S)
.
The parameters L
and L
⊥
defined in Eq.
(2) are, respec-
tively, the molecular dimensions parallel and perpendicular to
the director direction and S is the scalar order parameter. An
amazing aspect of this expression is that it predicts that if a
calamitic nematic phase is replaced by a discotic one, the ra-
tio D
/D
⊥
would change from a number greater than 1 to
a number smaller than 1. This simple theoretical prediction
is consistent with the experimental results determined in this
work. Taking the ratio between the parameters L
/L
⊥
∼
=
0.76
into account, obtained in this N
D
phase via experiment of X-ray
diffraction
[19], and using S
∼
=
0.5 [20] we obtain, from Eq. (2),
D
/D
⊥
∼ 0.88. This theoretical prediction, despite some limi-
tations, is consistent with our experimental data as indicated in
Table 1. It is important to mention that the symmetry of the mi-
celles and their average distances were not taken into account
in the theoretical approach of Eq. (2). Likewise, the fact that
the micelles do not have a rigid structure and change the mi-
cellar shape configuration under temperature and concentration
conditions of amphiphilic molecules present in the lyotropic
mixture must be also further considered. One possible way to
overcome these difficulties could be the use of the Hess ap-
proach [21,22], which was originally conceived to study the
viscosity of the nematic liquid crystals. This approach, which
considers that the geometry generated by the interacting poten-
tial is the essence of the anisotropic behavior found on the liq-
uid crystals phenomenology, has successfully described many
theological problems and does not suffer from the limitations
presented above.
To sum up, we have carried out a thermal diffusivity study
in discotic nematic phase of deuterated lyotropic mixtures
(KL/DeOH/D
2
O). To the best of our knowledge, this exper-
iment presents a first investigation of this parameter, partic-
ulary in the discotic nematic phase. The experimental ratio
D
/D
⊥
< 1 obtained for N
D
phase is discussed in terms of a
simple theoretical approach consistent with the nature of shape
anisotropy of micellar aggregates characteristic of the lyotropic
discotic nematic phase. The repeatability of our data, under the
described experimental conditions, was checked and verified by
this result. In this way, we have also determined this ratio for a
calamitic nematic phase and the obtained result is the same as
the one existing in the literature
[14].
Acknowledgements
We are thankful to the Brazilian Agencies CAPES, CNPq
and Fundação Araucária (PR) for the financial support of this
work.
References
[1] F. Simoni, Nonlinear Optical Properties of Liquid Crystals and Polymer
Dispersed Liquid Crystals, World Scientific, Singapore, 1997.
[2] I.C. Khoo, S.T. Wu, Optics and Nonlinear Optics of Liquid Crystals,
World Scientific, Singapore, 1993.
[3] M. Sheik-Bahae, A.A. Said, T.H. Wei, D.J. Hagan, E.W. Van Stryland,
IEEE J. Quantum Electron. 26 (1990) 760.
[4] N.M. Kimura, P.A. Santoro, P.R.G. Fernandes, R.C. Viscovini, S.L.
Gómez, A.J. Palangana, Phys. Rev. E 74 (2006) 062701.
[5] Y. Hendrikx, J. Charvolin, M. Rawiso, M.C. Holmes, J. Phys. Chem. 87
(1983) 3991.
[6] L.J. Yu, A. Saupe, Phys. Rev. Lett. 45 (1980) 1000.
[7] P.A. Santoro, J.R.D. Pereira, A.J. Palangana, Phys. Rev. E 65 (2002)
057602.
[8] N.M. Kimura, P.A. Santoro, P.R.G. Fernandes, A.J. Palangana, Liq.
Cryst. 31 (2004) 347.
[9] P.A. Santoro, A.R. Sampaio, H.L.F. da Luz, A.J. Palangana, Phys. Lett.
A 353 (2006) 512.
[10] W. Urbach, H. Hervet, F. Rondelez, J. Chem. Phys. 78 (1983) 5113.
[11] M. Marinelli, F. Mercuri, U. Zammit, F. Scudieri, Phys. Rev. E 58 (1998)
5860.
[12] G. Vertogen, W.H. de Jeu, Thermotropic Liquid Crystals, Verlag, Berlin,
1988.
[13] F.L.S. Cuppo, A.M. Figueiredo Neto, S.L. Gómez, P. Palffy-Muhoray, J.
Opt. Soc. Am. B 19 (2002) 1342.
[14] A.C. Bento, A.J. Palangana, L.R. Evangelista, M.L. Baesso, J.R.D.
Pereira, E.C. da Silva, A.M. Mansanares, Appl. Phys. Lett. 68 (1996)
3371.
[15] C.A. Carter, J.M. Harris, Appl. Opt. 23 (1974) 225.
[16] S.L. Gómez, A.M. Figueiredo Neto, Phys. Rev. E 62 (2000) 675.
[17] A.R. Sampaio, N.M. Kimura, R.C. Viscovini, P.R.G. Fernandes, A.J.
Palangana, Mol. Cryst. Liq. Cryst. 422 (2004) 57.
[18] A.M. Figueiredo Neto, Y. Galerne, A.M. Levelut, L. Liébert, J. Phys.
(Paris) Lett. 46 (1985) L-499.
[19] Y. Galerne, A.M. Figueiredo Neto, L. Liébert, Phys. Rev. A 31 (1985)
4047.
[20] Y. Galerne, J.P. Marcerou, Phys. Rev. Lett. 51 (1983) 2109.
[21] D. Baalss, S. Hess, Phys. Rev. Lett. 57 (1986) 86.
[22] H. Ehrentraut, S. Hess, Phys. Rev. E 51 (1995) 2203.