Our Recent Publications (2005 - Present)



Polyimide Blend Alignment Layers for Control of Liquid Crystal Pretilt Angle through Baking
Hyo Kang, Jin Seol Park, Eun-Ho Sohn,  Daeseung Kang, Charles Rosenblatt, and Jong-Chan Lee
Polymer (submitted)
Polymer Blend Alignment Layer.pdf
Polyimide films were used for liquid crystal alignment layers to control LC pretilt
angles over the full range (8o-90o). The pretilt angles could be controlled using
polyimide films prepared from polyamic acid for vertical LC alignment and using
polyimide blend films prepared from two types of polyamic acids, one for vertical LC
alignment and the other for planar LC alignment, by changing the baking times ranging
from 40 to 180 min at 230 oC. The polyimide blend film could control the pretilt angle
better than the polyimide prepared from just one polymer component. The LC
alignment behavior was well correlated with the wettability of the polyimide films due
to the fragmentation of the long alkyl side group on the polyimide surfaces by the
baking process.
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Surface chirality-induced modulation of director twist in a nematic cell
Yoonseuk Choi, Timothy J. Atherton, and Charles Rosenblatt
Phys. Rev. Lett. (submitted)
Stripes-chiral-surface-7-with-figures.pdf
A substrate coated with a polyimide alignment layer is scribed bidirectionally with the stylus of an atomic force microscope to create an easy axis for liquid crystal orientation.  The resulting topography breaks 2D inversion symmetry and results in an amplitude modulation of an imposed twisted nematic state, which is observed optically as spatially periodic light and dark stripes. This behavior does not occur when the substrate is scribed unidirectionally.  A theory is presented that suggests how surface chirality, which implies an asymmetric anchoring potential, could give rise to such a modulation.
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Direct measurement of surface-induced orientational order parameter profile above the nematic -- isotropic phase transition temperature
Ji-Hoon Lee, Timothy J. Atherton, Valentin Barna, Antonio De Luca, Emanuela Bruno, Rolfe G. Petschek, and Charles Rosenblatt
Phys. Rev. Lett. 102 167801 (2009)
The spatial and temperature dependence of the surface-induced orientational order parameter S(z,T) was determined in the isotropic phase.
An optical fiber was immersed in a thin liquid crystal layer and the retardation was measured as a function of the fiber's height above the surface,
from which the model-independent S(z,T) was deduced with resolution of ~1-2 nm.  It was found that: i) S(z=0)<0.12 close
to the nematic transition temperature, ii) the suscpetibility is maen field like, and iii) S(z,T) deviates significantly from exponential spatial
decay.  The results are discussed in terms of a nonlocal potential.
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Phase separation induced polar electrooptical effect in a doped nematic liquid crystal
J.H. Lee, D. Kang, R.G. Petschek, and C. Rosenblatt
Appl. Phys. Express 1, 121801 (2008)

A cell prepared for vertical alignment was filled with a mixture of the liquid crystal octylcyano biphenyl, doped with a three-legged molecule, in the isotropic phase in the presence of an imposed temperature gradient perpendicular to the substrates.  After cooling the cell through the nematic phase and restoring the temperature uniformity, an electrooptic response was observed on applying an electric field across the cell in one direction but not in the other.  This unipolar effect was absent for cells prepared in the absence of a temperature gradient.  The results are discussed in terms of phase separation and preferential adsorption of the tripod molecule.

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Full control of nematic pretilt angle using spatially homogeneous mixtures of two polyimide alignment materials
Ji-Hoon Lee, Daeseung Kang, Christopher M. Clarke, and Charles Rosenblatt
J. Appl. Phys. 105, 023508 (2009)
A mixture of two polyamic acids, one having an alkyl side chain and ordinarily
used for vertical liquid crystal alignment, and the other without a side chain and
ordinarily used for planar alignment, is deposited on two substrates and baked. It
is found that: i) the polymer alignment layer does not phase separate and ii) the
pretilt angle theta_0 of a cell filled with liquid crystal pentylcyanobiphenyl is a function
of the relative polymer concentration and can be controlled continuously over the
range 0◦ ≤ theta_0 <90◦. A model is proposed in which quartic terms in the surface
energy are responsible for the smooth variation of theta_0 with concentration.
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Polar horizontal vs. polar vertical reverse tilt domain walls:  Influence of pretilt angle below the nematic -- isotropic phase transition
Ji-Hoon Lee, Timothy J. Atherton, Daeseung Kang^{a}, Rolfe G. Petschek, and Charles Rosenblatt
Phys. Rev. E 78, 021708 (2008)
On cooling through the isotropic to nematic phase transition in a cell whose substrates induce a large pretilt angle θ₀ from the vertical direction but with no preferential azimuthal orientation, tilt domains appear. The boundary walls between reverse tilt domains are found to be bend/twist-like when θ₀(T=T_{NI}) is sufficiently large just below the isotropic -- nematic phase transition temperature T_{NI}, i.e., for a nearly planar orientation. Here the director becomes planar approximately midway through the wall, and we refer to this type of wall as "Polar Horizontal," which are topologically stable.  However, if θ₀(T=T_{NI}) is sufficiently small just below T_{NI}, i.e., closer to vertical orientation, a splay/twist-like domain wall obtains, where the director is vertically oriented approximately midway through the wall; we refer to this type of wall as "Polar Vertical," whose stability depends on the anchoring. On cooling through the nematic phase the pretilt angle θ₀ decreases, with the director aligning closer to the vertical orientation. Nevertheless, the structures of both types of domain walls remain unchanged on variation of θ₀ with temperature owing to topological constraints, and also are unchanged after the application and removal of a large electric field. We examine the structure of domain walls for the liquid crystal ZLI-4330 (Merck) as a function of pretilt angle θ₀(T=T_{NI}), and calculate a critical value θ₀^{c}(T=T_{NI}) of the pretilt angle just below T_{NI} for which the predominance of domain walls crosses over from Polar Horizontal to Polar Vertical.


Diverging Elasticity and Director Uniformation in a Nanopatterned Cell near the Nematic - Smectic-A Phase Transition
T.J. Atherton, R. Wang, and C. Rosenblatt
Phys. Rev. E 77, 061702 (2008)
The stylus of an atomic force microscope is used to scribe herringbone patterns of various wavelengths into a polyimide-coated substrate.  The patterns serve as a template for alignment of the liquid crystal octyloxycyanobiphenyl, and impose a bend distortion in the liquid crystal in the vicinity of the herringbone apices. The pretransitional behavior of the liquid crystal is observed by polarized microscopy as it is cooled through the nematic-smectic-A phase transition, facilitating direct visualization of the extrapolation length, which is related to the tradeoff between elastic and anchoring forces. Just above the phase transition temperature the expulsion of bend deformation is observed, and is shown to be in good quantitative agreement with continuum theory. Very close to the transition temperature a weak threshold behavior is observed, wherein the smectic-A phase forms a monodomain for short period herringbones, but breaks into multiple domains when the patterned period is large.

Nanoscale anisotropic patterning for alignment of a birefringent fluid and nanoimaging of its optical phase retardation
V. Barna, A. De Luca, and C. Rosenblatt
Nanotechnology 19, 325709 (2008)
Anisotropic nanopatterning, based on an atomic force microscopy scribing technique, of a polyimide film is used to generate an alignment layer whose topography depends on the writing direction. By means of a new nanotomographic approach, the optical retardation d of an alignable birefringent fluid that covers the scribed substrate is measured with unprecedented resolution of only a few tens of nanometers. Calculations for d  are in excellent agreement with both topographical and nanoimaging experimental results.
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Optical Nanotomography of liquid Crystals
Antonio De Luca, Valentin Barna, Timothy J. Atherton, Giovanni Carbone, Matthew E. Sousa, and Charles Rosenblatt
Nature Physics 4, 869 (2008)
The physical properties of anisotropic fluids can be made to vary on very short length scales of 100 nm or less by appropriate treatment of the confining substrate(s). This facilitates the use of ordered fluids, such as liquid crystals, in a variety of applications ranging from displays to switchable optical elements such as gratings and lenses.  Future advances will require a full understanding of the fluid’s structure at the nanoscale level.  But owing to diffraction limitations, three dimensional imaging of the liquid’s molecular orientation profile has been beyond the reach of extant optical techniques.  Here we demonstrate a powerful imaging approach based on the use of a tapered optical fiber with an aperture smaller than an optical wavelength.  We immerse the fiber’s tip inside a thin layer of an anisotropic fluid that sits atop a substrate and raster-scan the fiber at a series of heights above the surface.  From the collected images we are able to obtain a three dimensional visualization of the fluid’s structure with a resolvable volume two to three orders of magnitude smaller than attainable by extant methods.  As a demonstration we examine a nematic liquid crystal, whose molecular orientation is controlled by a nanoscopic pattern scribed into the underlying polymer-coated substrate.  For the first time we are able to observe directly the length scale of ~ 200 nm over which the molecular orientation relaxes due to the liquid crystal’s elastic forces.  This technique of acquisition and analysis of image slices may be applied to other soft systems, and offers the intriguing possibility of 3D nanoscale reconstruction for stacked lipid bilayers, lyotropic liquid crystals composed of macromolecules such as DNA or tobacco mosaic virus, and suspensions 



Rayleigh-Taylor instability experiments with precise and arbitrary control of the initial interface shape
Z. Huang, A. De Luca, T.J. Atherton, M. Bird, C. Rosenblatt, and P. Carlès
Phys. Rev. Lett. 99 204502 (2007)
In a Rayleigh-Taylor instability a dense fluid sits metastably atop a less dense fluid, 
a configuration that can be stabilized using a magnetic field gradient when one fluid is
highly paramagnetic. On switching off the magnetic field, the instability occurs as
the dense fluid falls under gravity. By affixing appropriately shaped magnetically-permeable
wires to the outside of the cell, one may impose arbitrarily-chosen and well-controlled
initial perturbations on the interface. This technique is used to examine both the linear
and nonlinear growth regimes for which the perturbation amplitudes, growth rates,
and nonlinear growth coefficients are obtained.



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Naturally occurring reverse tilt domains in high-pretilt alignment nematic liquid crystal
Ruiting Wang, Timothy J. Atherton, Minhua Zhu, Rolfe G. Petschek, and Charles Rosenblatt
Phys. Rev. E  76, 021702 (2007)
A cell whose substrates were coated with the polyamic acid SE1211 (Nissan Chemical Industries)
and baked at high temperatures was filled with a nematic liquid crystal in the isotropic phase. 
On cooling into the nematic phase, naturally occuring and temporally and thermally robust reverse
 tilt domains separated by thin filament-like walls were observed.  The properties of the these structures are reported.
 
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The importance of fluctuations in fluid mixing
 K. Kadau, C. Rosenblatt, J.L. Barber, T.C. Germann, Z. Huang, P. Carlès, and B.J. Alder
Proc. Nat. Acad. Sci. 104, 7741 (2007)
A ubiquitous example of fluid mixing is the Rayleigh-Taylor instability, in which a
heavy fluid initially sits atop a light fluid in a gravitational field. The subsequent
development of the unstable interface between the two fluids is marked by several
stages. At first, each interface mode grows exponentially with time, before
transitioning to a nonlinear regime characterized by more complex hydrodynamic
mixing. Unfortunately, traditional continuum modeling of this process has
generally been in poor agreement with experiment. Here we indicate that the
natural, random fluctuations of the flow field present in any fluid, which are
neglected in continuum models, can lead to qualitatively and quantitatively better
agreement with experiment. We performed billion-particle atomistic simulations
and magnetic levitation experiments with unprecedented control of initial interface
conditions. A comparison between our simulations and experiments reveals good
agreement in terms of the growth rate of the mixing front, as well as the new
observation of droplet breakup at later times. These results improve our
understanding of many fluid processes, including interface phenomena that
occur, for example, in supernovae, the detachment of droplets from a faucet, and
ink jet printing. Such instabilities are also relevant to the possible energy source
of inertial confinement fusion, in which a millimeter-sized capsule is imploded to
initiate nuclear fusion reactions between deuterium and tritium. Our results
suggest that the applicability of continuum models would be greatly enhanced by
explicitly including the effects of random fluctuations.
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Continuous control of liquid crystal pretilt angle from homeotropic to planar
Karen Vaughn, Matthew Sousa, Daeseung Kang, and Charles Rosenblatt
Appl. Phys. Lett. 90, 194102 (2007)
A mixture of two polyamic acids, one having an alkyl side chain and ordinarily used for vertical liquid crystal alignment, and the other without a side chanin and ordinarily used for planar alignment, is deposited on two substrates and baked at high temperatures.  When the resulting cell is filled with the liquid crystal pentyl cyanobiphenyl, it is found that the pretilt angle θ₀ is a function of the baking temperature, and can be controlled continuously over the range 0°≤θ₀≲90°.
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Bend expulsion from the smectic-A phase:  Analogy to type-I superconductor
R. Wang, I.M. Syed, G. Carbone, R.G. Petschek, and C. Rosenblatt
Phys. Rev. Lett.  97, 167802 (2006)
Using an atomic force microscope to nanopattern a substrate for liquid crystal alignment, a bend distortion is imposed on a
liquid crystal.  In regions of large bend the smectic-A phase melts into the nematic phase, and the width of the melted region
is measured as a function of temperature.  The results are consistent with type-I superconducting (nematic -- smectic-A)
behavior, wherein a large magnetic field (bend or twist distortion) induces an order to disorder transition.
A model that accounts for non mean-field behavior is presented.
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Rubbing-induced anisotropy of long alkyl side chains at polyimide surfaces
H.D. Jayathilake, M.H. Zhu, C. Rosenblatt, A.N. Bordenyuk, C. Weeraman, and A.V. Benderskii
J. Chem. Phys. 125, 064706 (2006)
Molecular organization at polyimide surfaces used as alignment layers in liquid crystal displays was investigated using vibrational Sum Frequency Generation (SFG) Spectroscopy.  We focus on the orientation of the long alkyl side groups at the polymer surface using polarization-selected SFG spectra of the CH3- and CH2-stretch modes of the side chain.  Mechanical rubbing and baking, an accepted industrial procedure used to produce pretilt of the liquid crystal, was found to induce pronounced azimuthal anisotropy in the orientational distribution of the alkyl side chains.  Orientational analysis of the SFG vibrational spectra in terms of the azimuthal and tilt angles (in- and out-of-plane, respectively) of the alkyl side chains shows their preferential tilt along the rubbing direction, with the azimuthal distribution narrower for stronger rubbed polymer samples. 
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Electrically-switchable, polarization-independent diffraction grating based on negative dielectric anisotropy liquid crystal
Min Hua Zhu, Giovanni Carbone, Charles Rosenblatt
Appl. Phys. Lett. 88, 253502 (2006)
An atomic force microscope is used to scribe polymer-coated substrates to create an electrically-controlled
 polarization grating. The grating is nondiffracting in the voltage-off state and diffracting in the voltage-on state.
Based upon an optical phase difference of approximately π between adjacent pixels, the grating's efficiency
 is independent of optical polarization and can be prepared for diffraction in either one or two dimensions.
 
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Quasi-divergent nematic surface electroclinic coefficient
M.H. Zhu, G. Carbone, and C. Rosenblatt
Phys. Rev. E 73, 041701 (2006)
A polyimide coated substrate is treated so that vertical liquid crystal alignment (θ=0) obtains over the temperature range T_NA<T<T_a,
where T_NA is the nematic — smectic-A transition temperature.  When the cell is filled with a chiral liquid crystal, application of an
 in-plane electric field for T_NA<T<T_a induces a nonzero polar tilt θ ∝ E of the liquid crystal director at the surface,
where the tilted orientation propagates elastically into the bulk.  On heating toward T_a, this surface electroclinic response
becomes large, corresponding to the onset of a surface tilt transition from θ=0 to nonzero θ.
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Rayleigh-Taylor instability for immiscible fluids of arbitrary viscosities: Magnetic levitation investigation and theoretical model
P. Carlès, Z. Huang, G. Carbone, and C. Rosenblatt
Phys. Rev. Lett.  96, 104501 (2006)
A magnetic field gradient was used to draw down a low density but highly paramagnetic fluid below a more dense fluid in a Hele-Shaw cell. On
turning off the field a Rayleigh-Taylor instability was observed in situ, and the growth of the most unstable wavevector was measured as a
function of time.  A theory for the instability that permits different viscosities for two immiscible fluids was developed, and good agreement was
found with the experimental results.
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Smectic tilt susceptibility:  Anharmonic behavior in surface-induced smectic layers above the nematic -- smectic-A transition temperature
Z. Huang, G. Carbone, C. Xia, Ghanshyam P. Sinha, and C. Rosenblatt
Phys. Rev. E 72, 021708 (2005)
Freedericksz transition measurements were used to determine the quartic contribution to the
energy associated with molecular tilt relative to the layer normal in the surface-induced smectic
layers in the nematic phase above the smectic-A transition temperature T_NA.  Both the
quadratic and quartic coefficients are consistent with the scaling relationship (T-T_NA)^-3nu,
where nu is the correlation length exponent, and their ratio was approximately constant.
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Effect of flexible bimeosgenic dopant on the stability of the anticlinic liquid crystal phase
M.H. Zhu, R.G. Petschek, L. Komitov, N. Olsson, B. Helgee, J.M. Kim, and M.E. Neubert
Phys. Rev. E. 72, 021702 (2005)
Small quantities of the floppy bimesogen di(4PPB5)3Si were dissolved in an anticlinic liquid crystal
consisting of a mixture of left- and right-handed TFMHPOBC, with enantiomer excess X=0.2.
The bimesogen dopant was found to promote anticlinic order with an anticlinic interaction coefficient
per molecule u_{dopant} smaller than, but of the same order as, the rigid bent-core dopant P-7PIMB
 For both dopants u_{dopant} was found to be much larger than that due to a pair of TFMHPOBC
molecules in adjacent layers.  The results are examined in terms of both the flexibility of the group
linking the two legs of each dopant, as well as their chemical structure.
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Planar degenerate substrate for micro- and nanopatterned nematic liquid crystals
I.M. Syed, G. Carbone, C. Rosenblatt, and B. Wen
J. Appl. Phys. 98, 034303 (2005)
A micro- or nanopatterned planar aligned cell ordinarily requires a pair of mirror-image patterned
substrates that must be aligned in register to ≪1 μm. As an alternative we examine the director
orientation profile of a nematic liquid crystal in a cell composed of one substrate whose easy axis is patterned
on micrometer length scales λ and a second substrate treated for planar degenerate alignment using
polymethyl methacrylate. For cell thickness <λ, the experimentally measured director profile in the
bulk corresponds approximately to that imposed at the micropatterned substrate.
The results are compared with theoretical calculations.  This method provides an excellent
alternative to the use of a pair of substrates aligned in close register.
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Polar anchoring strength of a tilted nematic:  Confirmation of the dual easy axis model
Giovanni Carbone and C. Rosenblatt
Phys. Rev. Lett. 94, 057802 (2005)
The polar anchoring strength coefficient W was measured as a function of polar pretilt angle
theta_0 for the liquid crystal 5CB.  It was found that W is proportional to (theta_0)^2 over the
range 0<theta_0<35 degrees.  The results provide a confirmation of the dual easy axis model,
wherein the liquid crystal director adopts an equilibrium orientation theta_0 at the substrate
that is determined by competition between a pair of preferred orienatation directions.
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Possible structures for the lameller-isotropic (Lam-I) and lameller-nematic Lam-N)  phases
N.M. Patel, I.M. Syed, C. Rosenblatt, M. Prehm, C. Tschiersek
Liquid Crystals 32, 55 (2005)
The three refractive indices of a liquid crystal that exhibits lamellar
analogs of the three dimensional isotropic, nematic, and smectic-A phases
are reported as a function of temperature for the Lam-I and Lam-N
phases.  Coupled with x-ray results, the data suggest a number of striking
behaviors: the orientational director n of the mesogenic moiety
is nearly three dimensionally disordered in the Lam-I phase; 
the two dimensional order associated with n in the Lam-N phase is
weak; and conformational changes in the sidechain result in an increase in
the refractive index perpendicular to the lamellae on decreasing temperature in
the Lam-N phase.
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International Collaborations in Liquid Crystal Research:  Personal Experiences
Charles Rosenblatt
Facets 4, No. 2, 14 (2005)
Collaborations.pdf
Liquid crystal research falls under the broad category of “soft materials,” and is conducted
at universities, government laboratories, and in industry around the
world.  The field has been particularly conducive to collaborative interactions,
with many national and regional liquid crystal societies operating under the broad umbrella of the International
Liquid Crystal Society.  In this article I shall describe a very small but representative slice of current liquid
crystal research — that performed by my own research group — and show how it
has been impacted by collaborations with colleagues around the world.
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Large polar pretilt for the liquid crystal homologous series alkycyanobiphenyl
Zhibin Huang and C. Rosenblatt
Appl. Phys. Lett. 86, 011908 (2005)
Sufficiently strong rubbing of the polyimide alignment layer SE-1211 (Nissan Chemical Industries, Ltd.)
 results in a large pretilt of the liquid crystal director from the homeotropic orientation. 
The threshold rubbing strength required to induce nonzero pretilt is found to be a monotonic
 function of the number of methylene units in the homologous liquid crystal series alkylcyanobiphenyl.
The results are discussed in terms of the dual easy axis model for alignment.
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For a complete list of publicationsandpatents (1977 - Present), click here




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