Our Recent Publications (2006 - Present)


“Nodal analysis of nonlinear behavior of the instability at a fluid interface”
M.-C. Renoult, C. Rosenblatt, and P. Carlès
Phys. Rev. Lett. 114, 114503 (2015)

The growth of nonlinearities in a 2D Rayleigh-Taylor (RT) instability for a single-mode sinusoidal
initial perturbation is studied in terms of amplitude and symmetry e ects. Because the interface de-
formation amplitude does not give access to the latter, we turn to the interface zero-crossings (nodes)
as a metric. A weakly nonlinear model is developed and compared to node position measurements
in RT magnetic levitation experiments. Our results show that the nodes metric is successful for
detecting the fi rst harmonic growth and exploring the transition to fully-developed nonlinearity.


Using Parabolic Flights to Examine Quantitatively the Stability of Liquid Bridges under Varying Total Body Force
G. DiLisi, R. Dempsey, R. Rarick, and C. Rosenblatt
Micrograv. Sci. and Tech. 27, 145 (2015)

Liquid bridges were flown aboard a Boeing 727-200 aircraft in a series of parabolic arcs that produced multiple periods of microgravity. During the microgravity portion of each arc, , the effective total body acceleration due to external forces, became negligibly small so that cylindrical liquid bridges could be suspended across two coaxial support posts. At the bottom of each arc,  slowly increased to a maximum of 1.84g, causing the liquid bridges to deform and in some cases collapse. Bridge-stability was examined for axial and lateral orientations with respect to  by measuring the slenderness ratio, as a function of Bond number, at which bridges collapsed. Results are compared with theory as well as with experimental results from a magnetic levitation technique and demonstrate that parabolic flights offer a viable alternative to existing methods for quantitative experiments on fluids in microgravity.


Liquid crystal quenched orientational disorder at an AFM-scribed alignment surface
J. S. Pendery, T. J. Atherton, M. Nobili, R. G. Petschek, E. Lacaze and C. Rosenblatt

Soft Matter, 11, 2220 (2015)

A polyimide substrate was scribed using the stylus of an atomic force microscope, then covered with a nematic liquid crystal.  The fiber from a near field scanning optical microscope was immersed into the liquid crystal and rastered approximately 80 nm above the surface, thereby obviating smearing effects that occur in thicker samples.  By appropriate averaging of multiple data sets, a histogram of the “frozen-in” director deviation Delta_fi from the average easy axis was obtained, having a full-width-half-maximum of ~ 0.02 rad.  Additionally, the positional autocorrelation function of Delta_fi was extracted, where the primary correlation length was found to be comparable to, but larger than, the liquid crystal’s extrapolation length.  A secondary characteristic length scales of a few mm was observed, and is thought to be an artifact due to material ejection during the scribing process.  Our results demonstrate the utility of nanoscale imaging of the interface behavior inside the liquid crystal.



Creating Arbitrary Arrays of 2D Topological Defects
B.S. Murray, R.A. Pelcovits, and C. Rosenblatt
Phys. Rev. E 90, 052501 (2014)
An atomic force microscope was used to scribe a polyimide-coated substrate with complex patterns that serve as an alignment template for a nematic liquid crystal.  By employing a sufficiently large density of scribe lines, two-dimensional topological defect arrays of arbitrary defect strength were patterned on the substrate.  When used as the master surface of a liquid crystal cell, in which the opposing slave surface is treated for planar degenerate alignment, the liquid crystal adopts the pattern’s alignment with a disclination line emanating at the defect core on one surface and terminating at the other surface.


Optical imaging of liquid crystals at the nanoscale
C. Rosenblatt
ChemPhysChem 15, 1261 (2014)
The instrumentation associated with Near Field Scanning Optical Microscopy (NSOM) can be exploited to provide three-dimensional structure and dynamic information about liquid crystals at scales not possible with diffraction-limited tools. This minireview will focus on our use of NSOM techniques to probe spatial variations of the nematic director and the nematic orientational order parameter on length scales as small as a few nanometers. 


Studies of Nanocomposites of Carbon Nanotubes and a Negative Dielectric Anisotropy Liquid Crystal
P. Kalakonda, G.S. Iannacchione, R. Basu, I.R. Nemitz, and C. Rosenblatt

J. Chem. Phys. 140, 104908 (2014)

The complex specific heat is presented over a wide temperature range for a negative dielectric
anisotropy alkoxyphenylbenzoate liquid crystal (9OO4) and carbon nanotube (CNT) composites as
a function of CNT concentration. The calorimetric scans were performed under near-equilibrium
conditions between 25 and 95 C, first cooling followed by heating for CNT weight percent ranging
from w = 0 to 0.2. All 9OO4/CNT mesophases have transition temperatures 1 K higher
and a crystallization temperature 4 K higher than that in the pure 9OO4. The crystal phase
super heats until a strongly first-order specific heat feature is observed, 0.5 K higher than in the
pure 9OO4. The transition enthalpy for the nanocomposite mesophases are 10% lower than that
observed in the bulk. The strongly first-order crystallization and melting transition enthalpies
are essentially constant over this range of w. Complementary electroclinic measurement on a
0.05 wt% sample, approaching the smectic-C phase from the smectic-A, indicate that the SmA-
SmC transition remains mean-field-like in the presence of the CNTs. Given the homogeneous and
random distribution of CNT in these nanocomposites, we interpret that these results as arising
from the LC-CNT surface interaction pinning orientational order uniformly along the CNT, without
pinning the position of the 9OO4 molecule, leading to a net ordering effect for all phases.

Nematic molecular core flexibility and chiral induction
Tzu-Chieh Lin, Ian R. Nemitz, Christopher J. McGrath, Christopher P. J. Schubert, Hiroshi Yokoyama, Robert P. Lemieux, and Charles Rosenblatt
Phys Rev. E 88, 042501 (2013)

Electroclinic measurements, in which an applied electric field E induces a rotation theta [~E] of the liquid crystal director about the electric field axis in a chiral environment, were performed on several configurationally achiral liquid crystals in the presence of an imposed helical director profile. This imposed twist establishes a chiral symmetry environment for the liquid crystal. It was observed that a conformationally racemic mesogen possessing a flexible phenyl benzoate core exhibits a measurable electroclinic response in the nematic phase. On the other hand, when the phenyl benzoate mesogen is mixed with a mesogen containing a rigid, conformationally achiral core (fluorenone), or with a racemic dopant with an axially chiral core that mimics a mesogen having rigid right- and left-handed conformations (2,2’-spirobiindan-1,1’-dione), the magnitudes of the electroclinic responses were found to decrease sharply, apparently going to zero when extrapolated to the pure 2,2’-spirobiindan-1,1’-dione or fluorenone limit. Note that neither of these additives possesses a nematic phase. The results suggest that the flexibility of the core and its ability to deracemize conformationally in order to compensate the elastic energy cost of the imposed twist is the primary mechanism behind the observed electroclinic response.

Gold nanoparticle self-assembly moderated by a cholesteric liquid crystal
Joel S. Pendery, Olivier Merchiers, Delphine Coursault, Johan Grand, Habib Ayeb,Romain Greget,g Bertrand Donnio, Jean-Louis Gallani, Charles Rosenblatt, Nordin Félidj, Yves Borensztein and Emmanuelle Lacaze
Soft Matter 9, 9366 (2013)

We show that the study of gold nanoparticle self-assemblies induced by a liquid crystal matrix reveals the
intimate distorted structure of the liquid crystal existing prior to nanoparticles' incorporation.We also show
how this intimate structure controls the spacing between nanoparticles in the self-assemblies. We have
created hybrid films of cholesteric liquid crystal (CLC) and gold nanoparticles, the CLC being deformed
by competing anchorings at its two interfaces. Whereas previous results have evidenced formation of
only slightly anisotropic clusters for large nanoparticles (diameter 20 nm), we now demonstrate for
smaller nanoparticles (diameter 4.2 nm) formation of long needles of lengths larger than 50
nanoparticles and widths smaller than 5 nanoparticles, on average oriented perpendicular to the
anchoring direction. The difference between the two kinds of nanoparticle aggregations is interpreted
by a modification of the balance between aggregation between nanoparticles and trapping by the
defects, favoured by the disorder induced by the alkylthiol molecules grafted around the nanoparticles.
This leads to a well-defined, anisotropic Localized Surface Plasmonic Resonance (LSPR) of the 4.2 nm
embedded nanoparticles. Interpretation of these optical properties using generalized Mie theory allows
for a comparison between CLC/gold nanoparticles and the same nanoparticles trapped within smectic
topological defects or deposited on the same substrate without a liquid crystal. A smaller spacing
between nanoparticles is demonstrated in the CLC system with an attraction between nanoparticles
induced by the CLC matrix, related to the additional disorder associated with the nanoparticles'
presence. The experimental observations allow us to estimate the disordered size of the liquid crystal
shell around the nanoparticles in the CLC to be of some nanometers. They also suggest that the CLC
distorted by competing anchorings is characterized by the presence of arrays of defects with topological
cores of width smaller than 5 nm that act as efficient anisotropic traps for the nanoparticles.


Nematic Twist Cell: Strong Chirality Induced at the Surfaces
T-C Lin, I.R. Nemitz, J.S. Pendery, C.P.J. Schubert, R.P. Lemieux, and C. Rosenblatt
Appl. Phys. Lett. 102, 134101 (2013)
A nematic twist cell, with easy axes forming an angle θ0 = 20° and thickness d varying
continuously across the cell, was filled with a mixture containing a configurationally achiral
liquid crystal and a chiral dopant. A linear electrooptic effect, which requires a chiral
environment, was observed on application of an ac electric field. This “electroclinic effect”
varied monotonically with d , changing sign at 0 d = d where the chiral dopant exactly
compensated the imposed pitch. The results indicate that a significant chiral electrooptic effect
always exists near the surfaces of a nematic twist cell containing molecules that can be
conformationally deracemized. Additionally, this approach can be used to measure the helical
twisting power (HTP) of a chiral dopant in a liquid crystal.

2D Rayleigh-Taylor Instability:  Interfacial arc-length vs. deformation amplitude
M.-C. Renoult, P. Carles, S. Ferjani, and C. Rosenblatt
Eur. Phys. Lett. 101, 54001 (2013)Fluid interface instabilities are usually studied through the time evolution of the
amplitude of deformation of the interface. While this approach is convenient, it often fails to
fully describe the evolution of a deforming interface, especially when the interface cannot be
represented as a single-valued function of a space coordinate. Here, we present new experimental
data on Rayleigh-Taylor 2D instability for immiscible fluids, obtained through the use of magnetic
levitation. We observe that new information can be retrieved by using an alternate metric to the
amplitude, viz., the total arc-length of the interface (in 2D), or equivalently its total surface area
(in 3D). In particular, we identify a master curve for the evolution of the arc-length over time,
following three different regimes and on which all our data points fall. We conjecture that the
exploration of such alternate metrics will yield equally promising results on a broad range of
interface instabilities.


Probing the Pore Structure of a Chiral Periodic Mesoporous Organosilica Using Liquid CrystalsVallamkondu Jayalakshmi, Thomas Wood, Rajratan Basu,  Jenny Du, Thomas Blackburn, Charles Rosenblatt, Cathleen M. Crudden and Robert P. Lemieux

J. Mater. Chem. 22, 15255 (2012)

Periodic mesoporous organosilicas (PMO) are prepared by the surfactant-templated condensation of bridged organosilsesquioxane monomers.  By controlling the nature of the organic segment, the type of surfactant and the condensation conditions, one can control the physical and chemical properties of the resulting PMO and produce highly ordered porous structures with a periodicity on the nanometer scale. The development of chiral PMO materials has been of significant interest given their potential in heterogeneous asymmetric catalysis, chiral chromatography and non-linear optics.  Characterization of the chirality of pore structures in these materials thus far has been achieved by indirect methods including polarimetry and solid-state circular dichroism.  We report herein a general and convenient approach to probe directly the pore structure of chiral PMO materials based on their interactions with inexpensive liquid crystalline solvents, which result in the induction of measurable chiral properties in the nematic (N) and smectic A (SmA) phases of the liquid crystals.  The templated co-condensation of a biphenylene organosilsesquioxane monomer and a chiral binaphthyl organosilsesquioxane monomer produced a new chiral PMO material that was investigated as dopant in two different liquid crystal hosts.  Measurements of induced circular dichroism and helical pitch in the nematic phase of the cyanobiphenyl liquid crystal 5CB, and the measurement of an induced electroclinic effect in the SmA phase of the phenyl benzoate liquid crystal 9OO4 were carried out.  The induced chiral properties measured in these experiments are consistent with chirality transfer taking place inside the pores, and suggest that the inner structure of the pores in the PMO material is indeed chiral.



Chiral induction in thioester and oxoester liquid crystals by dispersed carbon nanotubes
Rajratan Basu, Charles Rosenblatt, and Robert P. Lemieux
Liq. Cryst. 39, 199 (2012)

Multi-walled carbon nanotubes were dispersed at low concentrations into various achiral liquid crystals having
either a thioester or oxoester linkage group in the core. The presence of the carbon nanotubes resulted in chiral
signatures being observed in the liquid crystals, including an electroclinic effect (a rotation of the liquid crystal
director perpendicular to, and linear in, an applied electric field) in both the nematic and smectic A phases, and
a macroscopic helical twist of the liquid crystal director in the nematic phase. For both experiments the chiral
signatures for the thioester liquid crystals were found to be an order ofmagnitude larger than those of the oxoesters.
We speculate that the much larger strength of the thioester’s chiral properties is a result of stronger non-covalent
interactions between the liquid crystal molecule and carbon nanotube.


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Surface induced weak orientational order and role of isotropic-nematic interface fluctuations in the appearance of an induced nematic film
Elena S. Pikina and Charles Rosenblatt
Eur. Phys. J. E 35, 87 (2012)
Recently the nontrivial spatial and temperature dependence of the surface-induced weak planar orientational order parameter S(z,T) was determined just above isotropic-nematic (IN) phase transition point [Ji-H. Lee et al, Phys. Rev. Lett 102, 167801 (2009)]. In this paper we present a theoretical explanation of the observed behaviour. We obtain expressions for the short-range and long-range contributions to the interface potential of the induced nematic film and specify the repulsive character of interaction between the soft IN-interface and the external bounding substrate. It is shown that the small value of the IN-interfacial tension results in the renormalization of the repulsive interaction potential due to the thermal fluctuations of the soft IN-interface. This leads to an increase of the equilibrium thickness of the induced nematic film and appearance of a step-like orientational order parameter profile. We find that only renormalized short-range and thermal pseudo-Casimir interactions are essential for the appearance of the induced nematic film, which provide the observed thickness h∼30 nm of this film. The long-range van der Waals interaction is shown to be negligibly small and the dominant role is played by the renormolized short-range repulsion. Fitting of the experimental order parameter profiles [Ji-H. Lee et al, (2009)] with the expressions based on these interactions makes it possible to determine the material parameters of the system, including the amplitudes of the surface interaction, the IN-interfacial tension and the nematic correlation radius. The agreement between theory and experiment confirms the importance of the interface fluctuation renormalization of the interface potentials for the soft interfaces.



Surface topography and rotational symmetry-breaking

R. Basu, I.R. Nemitz, Q. Song, R.P. Lemieux, and C. Rosenblatt
Physical Review E 86, 011711 (2012)
A nematic twist cell, in which the two polymer-coated substrates are rubbed and then rotated by an angle theta_0, results in a strong chiral environment at the surfaces.  In the presence of an applied electric field, a combination of the chiral environment and the rub-induced breaking of the Cinifnity rotation axis at the surfaces results in a rotation of the molecular director in the substrate plane, viz., a surface electroclinic effect.  Using this twist cell geometry, we separate out and quantify the strength of the rub-induced two-fold rotational symmetry from that of the chiral symmetry. Our primary result is that the strength of the mechanically-induced C2 rotational symmetry, which is proportional to the electroclinic response, scales linearly with the rub-induced topographical rms roughness and increases with increasing rubbing strength of the polymer.  Our results also suggest that the azimuthal anchoring strength coefficient is relatively insensitive to the strength of the rubbing.


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Macroscopic torsional strain and induced molecular conformational chirality
R. Basu, J. Pendery, R.G. Petschek, R.P. Lemieux, and C. Rosenblatt
Phys. Rev. Lett. 107, 237804 (2011)
A macroscopic helical twist is imposed on an achiral nematic liquid crystal by controlling the azimuthal alignment directions at the two substrates.  On application of an electric field the director rotates in the substrate plane.  This electroclinic effect, which requires the presence of chirality, is localized at the two substrates and increases with increasing imposed twist distortion.  We present a simple model involving a tradeoff among bulk elastic energy, surface anchoring energy, and deracemization entropy that suggests the large equilibrium director rotation at the surfaces induces a deracemization of chiral conformations in the molecules, quantitatively consistent with experiment.

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Spatially controllable surface chirality at the nanoscale
J. Pendery, S. Ferjani, C. Rosenblatt, and R.G. Petschek
Europhys. Lett. 96, 26001 (2011)
Abstract – We demonstrate a mechanical approach for manipulating surface chirality at nanoscopic length scales. We use an atomic force microscope to scribe a step pattern, which is chiral in two dimensions, into a polymer-coated substrate, and control chiral strength by varying the steps’ length-to-width ratio R.  We determine the chiral strength by coating the surface with a liquid crystal and measuring its rotation on applying an electric field.  The chiral strength vs. R is nonmonotonic:  zero for R=1, then reaching a maximum, and tending to zero as R goes to infinity.  Our results demonstrate that chiral handedness and strength can be precisely controlled mechanically on nanoscopic length scales

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Chiral induction in thioester and oxoester liquid crystals by dispersed carbon nanotubes
Rajratan Basu, Charles Rosenblatt, and Robert P. Lemieux
Liq. Cryst. 39, 199 (2012)
Multi-walled carbon nanotubes were dispersed at low concentrations into various achiral liquid crystals having either a thioester or oxoester linkage group in the core.  The presence of the carbon nanotubes resulted in several chiral signatures being observed in the liquid crystals, including an electroclinic effect (a tilt of the liquid crystal director perpendicular to, and linear in, an applied electric field) in both the nematic and smectic-A phases, and a macroscopic helical twist of the liquid crystal director in the nematic phase.  For both experiments the chiral signatures for the thioesters were found to be larger by an order of magnitude than for the oxoesters.  We speculate that the relatively large strength of the thioester’s chiral signatures is a result of a stronger non-covalent bonding between the liquid crystal molecule and carbon nanotube

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Nematic electroclinic effect in a carbon nanotube-doped achiral liquid crystal
Rajratan Basu, Rolfe G. Petschek, and Charles Rosenblatt
Phys Rev. E 83, 041707 (2011)
A small quantity of carbon nanotubes dispersed in an achiral liquid crystal matrix transmits
chirality a short distance into the LC, and the LC+CNT mixture is found to exhibit a bulklike
electroclinic effect in the nematic phase. The magnitude of the effect increases rapidly on
cooling, showing significant pretransitional behavior on approaching the nematic – smectic-A
transition temperature (TNA) from above. The variation of the electroclinic coefficient is
negligible over the frequency range 100 Hz – 100 kHz in the in the nematic phase well above
TNA and in the smectic-A phase, whereas the electroclinic coefficient falls off significantly
with increasing frequency just above TNA.
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Deforming Static Fluid Interfaces with Magnetic Fields : Application to the Rayleigh-Taylor Instability
Marie-Charlotte Renoult,  Rolfe G. Petschek, Charles Rosenblatt, Pierre Carles
Experiments in Fluids
51, 1073 (2011)
Shaping arbitrary fluid interfaces opens interesting perspectives for fluid-based processes and experiments.
We demonstrate an experimental method to create non-planar static interfaces of almost arbitrary shape
between two fluids, one of which is made highly magnetically permeable by the addition of a magnetic compound.
By relying on spatially modulated magnetic fields, a nonhomogeneous
magnetic force is added to Earth’s gravitational force, and a non-planar static interface can be
stabilized. Precision experimental measurements are possible because we have developed a general method that allows
us to predict numerically the shape of the interface, thereby facilitating the optimal experimental design before actually
implementing it. As a first example, we apply this method to the Rayleigh–Taylor instability between two immiscible
fluids. The results we obtain demonstrate the feasibility of the experimental method and the accuracy of the numerical
predictions.
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Carbon nanotube-induced macroscopic helical twist in an achiral nematic liquid crystal
Rajratan Basu, Chia-Ling Chen , and Charles Rosenblatt
J. Appl. Phys. 109, 083518 (2011)
An achiral nematic liquid crystal was doped with a small quantity of carbon nanotubes, and the mixture was found to induce an average mechanical twist over macroscopic dimensions. The nanotube-induced chiral pitch length P was determined as a function of average nanotube concentration by measuring the radii of curvature of reverse twist disclination lines in 90° twisted nematic cells. The results reveal information about the nanotubes’ spatial distribution inside the cells. A concentration for the onset of significant aggregation of the nanotubes can be quantified from the apparent saturation of P-1 at higher concentrations. The macroscopic helical twisting power of the nanotubes has been estimated from the results. .

Mechanically generated surface chirality:  Control of chiral strength
Sameh Ferjani, Joel Pendery, and Charles Rosenblatt
Appl. Phys. Lett. 97, 121905 (2010)
A substrate coated with an achiral polyimide alignment layer was scribed with the stylus of an atomic force microscope having a line-to-line force profile F_{A}F_{B}F_{C}F_{A}F_{B}F_{C}...  The strength of the resulting chiral surface was examined using the nematic liquid crystal electroclinic effect induced by the surface. The magnitude of the electroclinic effect was found to increase with increasing scribing force, which suggests a method for controlling the chiral strength.  Additionally, the electroclinic magnitude divided by the rms surface roughness was approximately constant with scribing force, suggesting that the azimuthal anchoring strength coefficient is nearly independent of the scribing force.
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Carbon nanotube-induced chirality in an achiral liquid crystal
Rajratan Basu, Krysta A. Boccuzzi, Sameh Ferjani, and Charles Rosenblatt
Appl. Phys. Lett. 97, 121908 (2010)
A small quantity of carbon nanotubes was dispersed in an achiral liquid crystal, and the mixture was found to exhibit a weak degree of chirality. The induced chirality in the LC was probed by means of the electroclinic effect in the liquid crystal's Smectic-A phase, which showed significant pretransitional behavior on approaching the Smectic-A -- Smectic-C transition temperature from above. The results suggest that there is a net chirality associated with the carbon nanotubes, which is transmitted into the liquid crystal.
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Direct visualization and measurement of the extrapolation length on cooling toward the nematic — smectic-A phase transition temperature
Yoonseuk Choi and Charles Rosenblatt
Phys. Rev. E  81, 051708 (2010)
A herringbone "easy axis" pattern is scribed into a polyimide alignment layer for liquid crystal orientation using the stylus of an atomic force microscope.  Owing to the liquid crystal's bend elasticity K₃₃, the nematic director is unable to follow the sharp turn in the scribed easy axis, but instead relaxes over an extrapolation length L=K₃₃/W₂^{ϕ}, where W₂^{ϕ} is the quadratic azimuthal anchoring strength coefficient.  By immersing a tapered optical fiber into the liquid crystal, illuminating the fiber with polarized light, and scanning the fiber close to the substrate, a visualization and direct measurement of L are obtained on approaching the nematic -- smectic-A phase transition temperature T_{NA} from above.  L is found to exhibit a sharp pretransitional increase near T_{NA}, consistent with a diverging bend elastic constant.
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Mechanically generated surface chirality at the nanoscale
Sameh Ferjani, Yoonseuk Choi, Joel Pendery, Rolfe G. Petschek, and Charles Rosenblatt
Phys. Rev. Lett. 104, 257801 (2010)
A substrate coated with an achiral polyimide alignment layer was scribed bidirectionally with the stylus of an atomic force microscope to create an easy axis for liquid crystal orientation.  The resulting non-centrosymmetric topography resulted in a chiral surface that manifests itself at the molecular level. To show this unambiguously, a planar-aligned negative dielectric aniostropy achiral nematic liquid crystal was placed in contact with the surface and subjected to an electric field E.  The nematic director was found to undergo an azimuthal rotation approximately linear in E. This so-called "surface electroclinic effect" is a signature of 2D chirality and was not observed when the polyimide was treated for a centrosymmetric topography, and therefore was nonchiral.
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Polyimide Blend Alignment Layers for Control of Liquid Crystal Pretilt Angle through Baking
Hyo Kang, Jin Seol Park, Eun-Ho Sohn, Daeseung Kang, Charles Rosenblat, and Jong-Chan Lee
Polymer 50, 5220 (2009)
Polyimide films were used for liquid crystal (LC) alignment layers to control LC pretilt angles over the full range (8°-90°). 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 °C. 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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Patterning-induced surface chirality and modulation of director twist in a nematic cell
Yoonseuk Choi, Timothy J. Atherton, Sameh Ferjani, Rolfe G. Petschek, and Charles Rosenblatt
Phys. Rev. E 80, 060701 (2009)
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 non-centrosymmetric topography breaks 2D inversion symmetry and results in a spatial amplitude modulation of an imposed twisted nematic state.  This is observed optically as spatially periodic light and dark stripes. When the alignment layer is scribed unidirectionally the centrosymmetric topography maintains inversion symmetry, and no stripes are observed.  The appearance of the twist modulation is consistent with a chiral term in the free energy.

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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 retardatio
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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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