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one of the very early motivations for introducing confinement in experimental studies of liquids and glass formers was the general agreement that there might exist a typical lengthscale measuring the dynamical molecular cooperativity @xcite .
this quantity is suspected to increase with decreasing temperature , and to contribute to the observed anomalous properties of the structural relaxation such as super - arrhenian slowing - down in fragile compounds . introducing a maximum size to the system by means of a porous matrix
gives the opportunity to emphasize experimental evidence of those dynamical cooperative clusters .
up to now , most theoretical expectations and more or less direct experimental studies converge to state that the characteristic size of these cooperative rearranging regions ( crr ) does not exceed a few molecular sizes .
confinement effects have been mostly scrutinized in porous materials presenting cavities of nanometric dimension .
a large variety of materials has been used , often silicates within different geometries , either isotropic ( fractal aerogels , interconnected vycors , aerosils or gelsils ) or template - controlled ( mcm-41 , sba-15 ) .
several experimental difficulties are encountered in such studies due to the unavoidable powder averaging of the measured observables and to the huge surface to volume ratio that is reached .
indeed , the surface contribution to the free energy of the confined liquid can not be ignored anymore , and the awaited ultimate finite - size effects can be masked by dominant interfacial solid - liquid interactions @xcite .
experimental evidence of such surface contribution was reported on simple liquids like toluene @xcite , as a stretching of the dynamical self - correlation functions toward longer times , and a non - vanishing static contribution within experimental resolution .
these data were interpreted by a very large distribution of correlation times , starting from very slow reorientations imposed by a pinning at the pore surface , to bulk - like dynamics at the center .
such a broadening of molecular dynamics frequencies over several decades is supported by numerical simulations @xcite .
this onset of glass - like dynamics in simple liquids appears as an essential consequence of confinement , but is not the only one , for also the structure and the thermodynamical stable and metastable states are affected @xcite .
these interrelated effects remain partly misunderstood even for simple fluids .
a large effort towards a better understanding of more complex confined fluids is timely , for they are introduced in an increasing number of innovative applications of nanosciences ( lab - on - chip , nanofluidics ... ) .
liquid crystals ( lc ) are test - systems in order to investigate fluids with an increasing degree on complexity .
their structure is characterized by anisotropic order parameters giving rise to typical sequences of nematic or smectic phases .
however , their molecular nature allows one to confidently assign dynamical contributions in terms of rotations , translational diffusion or librations , accessible to a large number of techniques like dielectric spectroscopy @xcite , neutron quasielastic scattering or light scattering ( photon correlation spectroscopy , optical kerr effect @xcite ) .
both these structural and dynamical aspects have been widely studied on bulk samples , and make lc s model systems particularly suited to experimental investigations of confinement effects on complex fluids . like more simple liquids , the thermodynamics of confined lc s is also profoundly modified .
one of the most striking effect is the instability of the translational smectic ordering with respect to the quenched disorder field introduced by an irregular solid / lc interface @xcite .
this leads to a very important depression of the melting temperature of the confined crystalline phase , and the suppression of the nematic to smectic continuous transition , replaced by a frustrated short - range ( nanometric ) smectic ordering @xcite .
we report in this paper new experimental data depicting structural and dynamical features of a confined lc in a nanoporous material of very low dimensionality .
fully hydrogenated liquid crystal ( lc ) 4-octyl-4-cyanobiphenyl ( 8cb ) was purchased from sigma - aldrich and used without further purification .
columnar porous silicon ( psi ) matrices were obtained from a heavily doped ( 100 ) oriented silicon substrate by electrochemical etching @xcite .
a porosity of 60 % , made of a parallel arrangement of not - connected channels ( diameter @xmath0 300 ) is obtained , aligned in a layer of thickness @xmath0 30 @xmath1 m .
this very large aspect ratio induces a quasi one - dimensional character of the porous structure , preferentially aligned on the macroscopic lengthscale .
thermal oxidation of the structure can be further performed , leading to a transparent layer well suited to investigations by optical techniques .
8cb was confined into psi by capillary wetting from the liquid phase under vapour pressure in a vaccuum chamber and at a temperature of 60c , well above the n - i transition temperature .
the excess of lc was then removed by squeezing the wafers between whatman filtration papers .
the complete filling of the pores was characterized by confocal microraman spectroscopy @xcite .
neutron diffraction experiments were performed on the g6.1 double - axis diffractometer ( cold neutron guide ) at the laboratoire lon brillouin ( llb , cnrs / cea , saclay ) , with an incident wavelength of 4.71 .
quasielastic incoherent neutron scattering experiments were carried out on the time - of - flight ( tof ) spectrometer g6.2 at the llb , with a resolution of 107 @xmath1ev fwhm . the covered momentum transfer ( q ) range was about 0.4 to 1.9 @xmath2 .
the temperature was controlled to better than 0.1 k over a range from 100 k to 340 k using a cryoloop .
numerical deconvolution of tof spectra were carried out following standard procedures using the `` qensh '' software provided by the llb .
for both diffraction and tof experiments , eight filled porous silicon wafers were stacked parallel to each other in a cylindrical aluminium cell , representing a total amount of confined lc of about 20 mg .
ellipsometry experiments were carried out with a horiba jobin - yvon uvisel spectrometer , in the visible range ( @xmath3 from 410 to 830 nm ) .
the measured ellipsometric angles @xmath4 and @xmath5 are related respectively to the amplitude ratio and to the phase difference between the complex reflexion coefficients ( @xmath6 and @xmath7 ) of the sample ( resp . parallel or perpendicular to the incident plane ) . from these two angles , new quantities are defined according to :    @xmath8    these functions @xmath9 and @xmath10 were fitted versus the wavelength using the horiba `` deltapsi2 '' software provided with the apparatus .
the sample was modelled as a porous silica layer top of a silicon substrate .
the dielectric dispersions of silicon and silica were described using standard library data .
the dielectric dispersion of the organic liquid crystal was modeled by a uniaxial anisotropic single oscillator .
the porosity of the layer was described using a classical anisotropic bruggeman model @xcite mixing silica and void dispersions for the empty matrix , or silica and 8cb dispersions for the filled matrix .
brillouin scattering was performed with a triple pass tandem of fabry - perot ( sandercock model ) with a kr@xmath11 ion laser ( coherent ) at a wavelength of 647 nm .
laser power was kept at a few tens of mw in order to avoid a too big heating due to absorption of the bulk silicon supporting the film . at too high counting rates a shutter obturates the photomultiplier inducing discontinuities in the spectra .
typical spectra were accumulated during several hours .
spectra were fitted by an apparatus function convoluted to damped oscillator profiles ( brillouin lines ) and a debye relaxor .
all results presented hereafter were obtained close to room temperature , in the smectic phase of 8cb ( bulk or confined ) @xcite . in this temperature range ,
the liquid crystal 8cb confined in psi experiences smectic ordering that remains short - range ( sro , @xmath12 ) due to quenshed disorder effects imposed by solid - liquid interfacial interactions @xcite .
these effects destroy the bulk second order nematic to smectic phase transition . as a consequence ,
the bragg peak associated to this smectic sro and appearing at @xmath13 ( at a working wavelength of 4.7 ) is significantly broader than the experimental resolution .
figure [ ellipso ] displays ellipsometric spectra measured on empty porous silica and porous silica filled with 8cb .
the interference fringes are due to multiple reflections at interfaces of the porous layer .
it is therefore possible to deduce an estimation of the layer thickness , and typical values between 2.5 and 5 @xmath1 m ( depending on the sample ) were found , in good agreement with independent sem results .
a simple comparison of both curves shows strong differences in the amplitude modulation of those fringes , as a consequence of the dispersion of the optical index of the confined liquid crystalline phase .
this observation can be used as a direct characterization of a proper filling of the porous layer with 8cb .
a semi - quantitative analysis was performed using the model described in section [ expellipso ] .
best fits agree with an optical uniaxial birefringence of the filled porous layer of about @xmath14 , with an optical axis aligned with the direction of the nanopores .
the important differences in the ellipsometric data allow to separate unambiguously this additional birefrengence due to the liquid crystal from the natural optical anisotropy of the empty porous layer .
these two contributions can be further more distinguished through the temperature dependence of the liquid crystal contribution .
from the orientation of this measured optical anisotropy , it can be concluded that the orientational order parameter of the confined 8cb strongly couples to the one - dimensional character of the porous matrix .
@xmath2 ) by 8cb confined into porous silicon . the inset sketches the experimental setup .
the sample macroscopic orientation is defined by the angle @xmath15 between the silicon wafer plane and the incident beam . ]
figure [ mosa ] shows the intensity of the bragg peak associated to the smectic sro measured at constant @xmath16 @xmath2 , while rotating the sample plane by an angle @xmath15 with respect to the neutron incident beam .
the best fit using an arbitrary lorentzian form indicates that the maximum of the bragg reflexion is peaked at @xmath17 , which corresponds to the sample orientation @xmath18 where the @xmath19 vector is parallel to the nanochannels of the porous silicon layer .
the linewidth is about 17hwhm . within this rather small mosaicity ,
the director of the smectic layers is obviously well aligned with the silicon channels axis .
both ellipsometry and neutron diffraction results show that 8cb confined in porous silicon displays in the meantime orientational and translational order parameters that are macroscopically one - dimensionally oriented through a strong coupling with the peculiar low dimensionality of the matrix .
accordingly , the molecular dynamics of the confined phase might be also affected by this anisotropic ordering . whereas translational diffusion should in some extent depend on the local properties of the smectic sro domains
, the rotational molecular relaxations should be more sensitive to orientational parameters . in this section ,
we report brillouin and quasielastic neutron scattering results , that complementary probe the molecular relaxations in the picosecond time range .
figure [ brillouin ] shows the comparison of brillouin spectra obtained on porous silica filled with 8cb or on the separated constituents .
the black line is the best fit according to the procedure detailed in section [ expbri ] .
the brillouin doublets appearing on the spectra have different origins : the one at the lowest frequency is a longitudinal mode propagating tangentially to the surface , whereas at the highest frequency appears a longitudinal mode propagating mainly along the pores perpendicular to the surface .
the response of the bulk liquid crystal consists in a single quasielastic lorentzian line of about 5 ghz hwhm ( i.e. @xmath20 ps ) .
the symmetry of the experimental setup suggests to assign this line to uniaxial rotation of the molecules around their long axis .
the response of an empty porous silica matrix reveals strong brillouin modes , sitting on a very broad quasielastic contribution ( more than 35 ghz ) .
comparatively , the brillouin spectrum of a filled porous layer can be described by a weighted linear combination of these individual responses , assuming a broadening of the acoustic modes .
this damping is due to the coupling of the solid matrix to the viscosity of the liquid crystal ( poroelasticity effect )
. a quasielastic contribution of about the same width as for the bulk 8cb can be seen on the data , suggesting that the uniaxial molecular rotations are only weakly affected by the confinement .
quasielastic neutron scattering experiments were performed , in order to probe additional relaxations .
figure [ mibemol ] shows the tof spectra measured on bulk 8cb and 8cb / psi .
spectra were fitted using one lorentzian plus one elastic ( resolution limited ) components . for bulk 8cb
, the quasielastic contribution has a linewidth of about 0.25 mev ( @xmath21 ps ) .
its occurence at large values of momentum transfer together with the presence of an elastic contribution would suggest that it also reflects localized reorientations ( rotations , end - chain relaxations ... ) .
@xmath2 , on ( a ) bulk 8cb , and ( b ) 8cb confined in porous silicon . ]    for 8cb confined in porous silicon , figure [ mibemol](b ) reports a significant increase of the elastic contribution , together with a decrease of the quasielastic linewidth down to 0.16 mev .
this moderate slowing down of the fast reorientational motions agrees with the brillouin results , and confirms that rotational molecular dynamics seem to be only weakly disturbed by the anisotropic confinement in porous silicon . on the other hand
, the increase of the elastic contribution suggests that slow relaxations ( slower than the tof energy resolution ) or long range translational diffusion might be much more affected .
this is expected from a heterogeneous propagation of a surface induced slowing - down through the inner pore volume .
we report in this note new experimental results on a smectic liquid crystal confined into a low dimensional , macroscopically oriented porous silicon layer , which has been introduced recently @xcite .
the combination of a large panel of complementary techniques allows one to probe in parallel the structural and dynamical behavior of the confined molecules .
it is shown that both orientational and translational order parameters of the confined liquid crystal couple to the anisotropy of the silicon matrix , and give rise to unusual structural properties reminiscent for some aspects of electric or magnetic field effects on bulk liquid crystals .
the picosecond molecular dynamics of this oriented short - range ordered confined smectic phase were probed by light and neutron scattering techniques , suggesting that mainly large amplitude motions like long axis tumbling or long range translational diffusion , should be affected by anisotropic confinement effects .
the authors are thankful to j .-
m . zanotti and b. frick , for expert advice on neutron scattering experiments and fruitful discussion , and also m. stchakovsky for precious help on spectroscopic ellipsometry measurements .
structural order parameters of a smectic liquid crystal confined into the columnar form of porous silicon are studied using neutron scattering and optical spectroscopic techniques . 
 it is shown that both the translational and orientational anisotropic properties of the confined phase strongly couple to the one - dimensional character of the porous silicon matrix . 
 the influence of this confinement induced anisotropic local structure on the molecular reorientations occuring in the picosecond timescale is discussed .