Our Recent Publications (2010 - Present)


Co-Revolving Topological Defects in a Nematic Liquid Crystal”
 A.L. Susser, S. Kralj, and C. Rosenblatt
Soft Matter, 17, 9616 (2021)

A patterned surface defect of strength m = +1 and its associated disclination lines can decompose into a
pair of surface defects and disclination lines of strength m = +1/2. For a negative dielectric anisotropy
liquid crystal subjected to an applied ac electric field E, these half-integer defects are observed to
wobble azimuthally for E > than some threshold field and, for sufficiently large fields, to co-revolve
antipodally around a central point approximately midway between the two defects. This behavior is
elucidated experimentally as a function of applied field strength E and frequency n, where the threshold
field for full co-revolution scales as nu^1/2. Concurrently, nematic electrohydrodynamic instabilities were
investigated. A complete field vs. frequency ‘‘phase diagram’’ compellingly suggests that the induced
fluctuations and eventual co-revolutions of the ordinarily static defects are coupled strongly to—and
driven by—the presence of the hydrodynamic instability. The observed behaviour suggests a Lehmann-like
mechanism that drives the co-revolution.

“Anchoring-mediated spontaneous topography in thin nematic liquid crystal films”
 A.J. Ferris, C. Rosenblatt, and T.J. Atherton,
Phys. Rev. Lett., 126, 057803 (2021)
A topography in a Newtonian fluid occurs if there is a disturbance near the surface. But what if there is
no such disturbance? We show by optical profilometry that a thin nematic film resting on a topological defect-
patterned substrate can exhibit a hill or divot at the opposing free (air) interface in the absence of a
topological disturbance at that interface. We propose a model that incorporates several material properties
and that predicts the major experimental features. This work demonstrates the importance of, in particular,
anisotropic surface interactions in the creation of a free-surface topography.

“Transition from Escaped to Decomposated Nematic Defects, and vice versa
A.L. Susser, S. Harkai, S. Kralj, and C. Rosenblatt
Soft Matter 16, 4814 (2020)

An escaped radial director profile in a nematic liquid crystal cell can be transformed into a pair of
strength m = +1/2 surface defects (and their associated disclination lines) at a threshold electric field.
Analogously, a half-integer defect pair can be transformed at a threshold electric field into a director
profile that escapes into the third dimension. These transitions were demonstrated experimentally and
numerically, and are discussed in terms of topologically discontinuous and continuous pathways that
connect the two states. Additionally, we note that the pair of disclination lines associated with the m =
+1/2 surface defects were observed to co-rotate around a common point for a sufficiently large electric
field at a sufficiently low frequency.


“Multiple twisted chiral nematic structures in cylindrical confinement”
M. Ambrozic, A. Gudimalla, C. Rosenblatt, and S. Kralj
Crystals 10, 576 (2020)

In this article, we theoretically and numerically study the chirality and saddle-splay elastic
constant (𝐾24)-enabled stability of multiple twist-like nematic liquid crystal (LC) structures in
cylindrical confinement. We focus on the so-called radially z-twisted (RZT) and radially twisted
(RT) configurations, which simultaneously exhibit twists in different spatial directions. We express
the free energies of the structures in terms of dimensionless wave vectors, which characterise the
structures and play the roles of order parameters. The impact of different confinement anchoring
conditions is explored. A simple Landau-type analysis provides an insight into how different model
parameters influence the stability of structures. We determine conditions for which the structures
are stable in chiral and also nonchiral LCs. In particular, we find that the RZT structure could exhibit
macroscopic chirality inversion upon varying the relevant parameters. This phenomenon could be
exploited for the measurement of 𝐾24.

"Electric field-driven reconfigurable multistable topological defect patterns"
S. Harkai, B.S. Murray, C. Rosenblatt, and S. Kralj
Phys. Rev. Research 2, 013176 (2020)

Topological defects appear in symmetry breaking phase transitions and are ubiquitous throughout Nature. As an ideal testbed for their study, defect configurations in nematic liquid crystals (NLCs) could be exploited in a rich variety of technological applications. Here we report on robust theoretical and experimental investigations in which an external electric field is used to switch between pre-determined stable chargeless disclination patterns in a nematic cell, where the cell is sufficiently thick that the disclinations start and terminate at the same surface. The different defect configurations are stabilised by a master substrate that enforces a lattice of surface defects exhibiting zero total topological charge value. Theoretically, we model disclination configurations using a Landau-de Gennes phenomenological model. Experimentally, we enable diverse defect patterns by implementing an in-house-developed Atomic Force Measurement scribing method, where NLC configurations are monitored via polarised optical microscopy. We show numerically and experimentally that an “alphabet” of up to 18 unique line defect configurations can be stabilised in a 4x4 lattice of alternating s=  surface defects, which can be “rewired” multistably using appropriate field manipulation. Our proof-of-concept mechanism may lead to a variety of applications, such as multistable optical displays and rewirable nanowires. Our studies also are of interest from a fundamental perspective. We demonstrate that a chargeless line could simultaneously exhibit defect-antidefect properties. Consequently, a pair of such antiparallel disclinations exhibits an attractive interaction. For a sufficiently closely-spaced pair of substrate-pinned defects, this interaction could trigger rewiring, or annihilation if defects are depinned.

"Electric field-induced crossover from 3D to 2D topological defects in a nematic liquid crystal: experimental verification"
A.J. Ferris, S. Afghah, R.L.B. Selinger, J.V. Selinger, and C. Rosenblatt
Soft Matter, 16, 642 (2020)
A substrate was patterned with two pairs of half-integer strength topological defects, (+1/2, +1/2) and
(+1/2, 1/2). In a sufficiently thick cell, a disclination line runs in an arch above the substrate connecting
the two half integer defects within each pair. The director around the disclination line for the like-sign
pair must rotate in 3D, whereas for the opposite-sign defect pair the director lies in the xy-plane parallel
to the substrate. For a negative dielectric anisotropy nematic, an electric field applied normal to the
substrate drives the director into the xy-plane, forcing the arch of the disclination line of the like-sign
pair to become extended along the z-axis. For sufficiently large field the arch splits, resulting in two
nearly parallel disclination lines traversing the cell from one substrate to the other. The opposite-sign
defect pair is largely unaffected by the electric field as the director already lies in the xy-plane.
Experimental results are presented, which are consistent with numerical simulations.

“Chiral Polymeric Nanocapsules and Their use for Conformational Deracemization of Liquid Crystal
A. Zoabi, M. Santiago, D. Gelman, C. Rosenblatt, D. Avnir, and R. Abu-Reziq

J. Phys. Chem. (Part C) 122. 17936 (2018)

We present the first preparation and properties of chiral nanocapsules. The chiral shell, a polyurea derivative, was obtained by interfacial emulsion
polymerization of L-lysine with polymethylene polyphenyl isocyanate. The chirality of these nanocapsules was manifested by its ability to induce conformational
deracemization of a liquid crystal. This induced chirality was measured using the “Raynes’ experiment”, in which the boundary conditions of cells impose a ±90° rotation
of the liquid crystal director from one surface to the other. Both left- and right-handed director twist domains appear on cooling from the isotropic to the nematic phase.
Owing to the weak induced chirality of the liquid crystal, one sense of director rotation is energetically more favorable and its domain size expands, resulting in curvature of the
domain walls. The curvature was measured as a function of capsule concentration and serves as a metric of the induction of chirality in the surrounding liquid crystal.

   “Geometry and external field positionally controlled nematic topological defects”
 P. Kurioz, M. Kralj, B.S. Murray, C. Rosenblatt, and S. Kralj
Beilstein J. Nanotech. 9, 109 (2018)

We study in a Landau–de Gennes approach impact of confinement topology, geometry and external fields on spatial positioning of nematic topological defects (TDs). In quasi two-dimensional systems we demonstrate that confinement enforced total topological charge m>>1 decays into elementary TDs bearing charge m=1/2. These assemble close to the bounding substrates to enable essentially bulk-like uniform nematic ordering in the central part of a system. This effect is reminiscent of the Faraday cavity phenomenon in electrostatics. We observe that in certain confinement geometries, varying order parameter correlation length size could trigger global rotation of an assembly of TDs. Finally, we show that an external electric field could be used to drag the boojum “finger tip” towards a confinement cell interior. Assemblies of TDs could be exploited as traps for appropriate nanoparticles, opening several opportunities for development of functional nanodevices.

“Observation of a streak texture in the hybrid-aligned Smectic-C phase”,
I.R. Nemitz, I. Gryn, N. Boudet, R.P. Lemieux, M. Goldmann, B. Zappone, R.G. Petschek, C. Rosenblatt, and E. Lacaze
Soft Matter  14, 460 (2018)

A novel structure was observed below the Smectic-A – Smectic-C phase transition in a very thin open cell having an air interface above and enforced planar anchoring at the substrate below. The structure appears as periodic dark and light streaks running perpendicular to the oily streaks, which are present in the Smectic-A phase [Soft Matter 12, 678 (2016)]. These new streaks form by extending from one oily streak to the next in discrete steps, eliminating optical evidence at visible wavelengths of the oily streaks. At lower temperatures the streaks can undulate and exhibit a sawtooth-like structure; such a structure is chiral in two dimensions. A possible scenario for the origin of these streaks is presented.

“Chiral organosilica particles and their use as inducers of conformational deracemization of liquid crystal phases”
 Orit Cohen, Andrew J. Ferris, Raymond Adkins, Robert P. Lemieux, David Avnir, Dmitry Gelman, and Charles Rosenblatt,
Chem. Phys. Lett. 696, 112 (2018)
Chiral organosilica particles of size ~ 200 nm were synthesized from an enantio-pure multi-armed chiral D-maltose organosilane precursor in the absence of co-condensation with an achiral monomer.  Subsequently, two separate experiments were used to demonstrate the particles’ ability to induce conformational deracemization of host liquid crystals.  One experiment exploits the reduced chiral symmetry of the liquid crystal’s smectic-A phase, where an applied electric field induces an optically measurable tilt of the liquid crystal molecules; this is the “electroclinic effect”.  The other experiment involves a measurement of the domain wall curvature separating left- and right-handed liquid crystal helical pitches imposed by the cells’ substrates; this is the “Raynes’ Geometry” experiment.  The results demonstrate unequivocally that enantio-pure organosilica nanoparticles can be synthesized and can induce chirality in a host liquid.

Decomposition vs. Escape of Topological Defects in a Nematic Liquid Crystal
BS Murray, S. Kralj, and C. Rosenblatt
Soft Matter 13, 8442 (2017)

Nematic cells patterned with square arrays of strength m = ±1 topological defects were examined as a function of cell thickness ( 3 < h < 7.5 m), temperature, and applied voltage.
 Thicker cells tend to exhibit an escape of the nematic director as a means of mitigating the elastic energy cost near the defect cores, whereas thinner cells tend to favor splitting
of the integer defects into pairs of half-integer strength defects.  On heating the cells into the isotropic phase and recooling into the nematic, the relaxation mechanism of some defects
 was found to reverse.  This is consistent with the system’s symmetry, which requires a first order transition between the two relaxation mechanisms.

Survival of smectic-A oily streaks into the nematic phase by UV irradiation of reactive mesogen dopant”,
I. Gharbi, A. Missaoui, D. Demaille, E. Lacaze, and C. Rosenblatt,

Crystals 7, 358 (2017)
Thin smectic liquid crystal films having competing boundary conditions (planar and homeotropic at opposing surfaces) form well-known striated structures known as “oily streaks”,  which are a series of hemicylindrical caps that run perpendicular to the easy axis of the planar substrate. The streaks vanish on heating into the nematic phase, where the film becomes uniform  and exhibits hybrid alignment. On adding sufficient reactive mesogen and polymerizing, the oily  streak texture is maintained on heating through the entire nematic phase until reaching the bulk isotropic phase, above which the texture vanishes. On cooling back into the nematic and  smectic-A phases, the oily streak structure cannot be retrieved, with the texture indicating a loss of 20 integrity of the skeleton. Both polarizing optical and atomic force microscopy data are presented.

"Decomposition of strongly charged topological defects"
S. Kralj, B.S. Murray, and C. Rosenblatt
Phys. Rev. E 95, 042702 (2017)

We study decomposition of geometrically enforced nematic topological defects bearing relatively large
defect strengths m in effectively two-dimensional planar systems. Theoretically, defect cores are analyzed
within the mesoscopic Landau - De Gennes approach in terms of the tensor nematic order parameter. We
demonstrate a robust tendency of defect decomposition into elementary units where two qualitatively differ-
ent scenarios imposing total defect strengths to a nematic region are employed. Some theoretical predictions
are verified experimentally, where arrays of defects bearing charges m = ±1, and even m = ±2, are en-
forced within a plane-parallel nematic cell using an AFM scribing method.

"Influence of a dispersion of magnetic and non-magnetic nanoparticles on the magnetic Fredericksz transition of 5CB"

Ahmed Mouhli, Jerome Fresnais, Habib Ayeb, Ian R. Nemitz, Joel S. Pendery, Olivier Sandre, Tahar Othman, Charles Rosenblatt, Vincent Dupuis, and Emmanuelle Lacaze
Phys.Rev E, 96, 012706 (2017)

Long time ago, Brochard and de Gennes predicted the possibility of significantly decreasing the critical magnetic field of the
Fredericksz transition (the magnetic Fredericksz threshold) in a mixture of nematic liquid crystals and ferromagnetic particles,
the so-called ferronematics. This phenomenon has rarely been measured, usually due to soft homeotropic anchoring induced at
the nanoparticle surface. Here we present an optical study of the magnetic Fredericksz transition and a light scattering study
of the classical nematic liquid crystal, 5CB, doped with 6 nm diameter magnetic and non-magnetic nanoparticles. Surprisingly,
for both nanoparticles, we observe at room temperature a net decrease of the threshold field of the Fredericksz transition at low
nanoparticle concentrations, which appears associated with a coating of the nanoparticles by a brush of polydimethylsiloxane
copolymer chains inducing planar anchoring of the director on the nanoparticle surface. Moreover the magnetic Fredericksz
threshold exhibits non-monotonic behaviour as a function of the nanoparticle concentration for both types of nanoparticles, first
decreasing down to a value from 23% to 31% below that of pure 5CB, then increasing with a further increase of nanoparticle
concentration. This is interpreted as an aggregation starting at around 0.02 weight fraction that consumes more isolated nanoparticles
than those introduced when the concentration is increased above c = 0:05 weight fraction (volume fraction 3.5 x 10-2).
This shows the larger effect of isolated nanoparticles on the threshold with respect to aggregates. From dynamic light scattering
measurements we deduced that, if the decrease of the magnetic threshold when the nanoparticle concentration increases is
similar for both kinds of nanoparticles, the origin of this decrease is different for magnetic and non-magnetic nanoparticles. For
non-magnetic nanoparticles, the behavior may be associated with a decrease of the elastic constant due to weak planar anchoring.
For magnetic nanoparticles there are non-negligible local magnetic interactions between liquid crystal molecules and magnetic
nanoparticles, leading to an increase of the average order parameter. This magnetic interaction thus favors an easier liquid crystal
rotation in the presence of external magnetic field, able to reorient the magnetic nanoparticles along with the molecules.


“Interface Coupling and Growth Rate Measurements in Multi-layer Rayleigh-Taylor instabilities”
 R. Adkins, E.M. Shelton, M.-C. Renoult, P. Carlès, and C. Rosenblatt
(Phys. Rev. Fluids, 2, 062001 (R) (2017)

Magnetic levitation was used to measure the growth rates of a Rayleigh-Taylor instability in a three layer fluid system
 a first step in the elucidation of interface coupling in finite layer instabilities.  For a three layer system having two interfaces, the unstable
 mode growth rate was found to decrease as the height of the middle fluid layer was reduced and the wavelength of the disturbance
 was increased, indicating strong interface coupling.  The ratios of the three layer to the classic two layer growth rates are in
 excellent agreement with calculations based on linear stability theory.

“Chiral oily streaks in a smectic-A liquid crystal”
 I.R. Nemitz, A.J. Ferris, E. Lacaze, and C. Rosenblatt
Soft Matter 12, 6662 (2016)

The liquid crystal octylcyanobiphenyl (8CB) was doped with the chiral agent CB15 and spin-coated onto a substrate treated for planar alignment of the director, resulting in a film of thickness several hundred nm in the smectic-A phase.  In both doped and undoped samples, the competing boundary conditions — planar alignment at the substrate and vertical alignment at the free surface — cause the liquid crystal to break into a series of flattened hemicylinders to satisfy the boundary conditions.  When viewed under an optical microscope with crossed polarizers, this structure results in a series of dark and light stripes (“oily streaks”) of period ~ 1 mm.  In the absence of chiral dopant the stripes run perpendicular to the substrate’s easy axis.  However, when doped with chiral CB15 at concentrations up to c = 4 wt-%, the stripe orientation rotates by a temperature-dependent angle j  with respect to the c = 0 stripe orientation, where j increases monotonically with c.  j is largest just below the nematic – smectic-A transition temperature TNA and decreases with decreasing temperature.  As the temperature is lowered, j relaxes to a steady-state orientation close to zero within 1 o C of TNA.   We suggest that the rotation phenomenon is a manifestation of the surface electroclinic effect:  The rotation is due to the weak smectic order parameter and resulting large director tilt susceptibility with respect to the smectic layer normal near TNA, in conjunction with an effective surface electric field due to polar interactions between the liquid crystal and substrate.

“Electroclinic effect in a chiral paranematic liquid crystal layer above the bulk nematic to isotropic transition temperature”
 I.R. Nemitz, E. Lacaze, and C. Rosenblatt,
Phys. Rev. E 93, 022701 (2016)

Electroclinic measurements are reported for two chiral liquid crystals above their bulk chiral
isotropic – nematic phase transition temperatures. It is found that an applied electric field E induces
a rotation θ [∝ Ε] of the director in the very thin paranematic layers that are induced by the cell’s
two planar-aligning substrates. The magnitude of the electroclinic coefficient dθ/dE close to the
transition temperature is comparable to that of a bulk chiral nematic, as well as to that of a
parasmectic region above a bulk isotropic to chiral smectic-A phase. However, dθ/dE in the
paranematic layer varies much more slowly with temperature than in the parasmectic phase, and its
relaxation time is slower by more than four orders of magnitude than that of the bulk chiral nematic
electroclinic effect.

"Chiral Periodic Mesoporous Organosilica in a Smectic-A Liquid Crystal:  Source of the Electrooptic Response"
 I.R. Nemitz, K. McEleney, C.N. Crudden, R.P. Lemieux, R.G. Petschek, and C. Rosenblatt,

Liq. Cryst. 43, 497 (2016)

Chiral periodic mesoporous organosilica (PMO) materials have been shown to deracemize a configurationally achiral, but conformationally racemic liquid crystal in which the PMO is embedded.  In particular, application of an electric field E in the liquid crystal’s smectic-A phase results in a rotation of the liquid crystal director by an angle proportional to E, which is detected optically — this is the so-called “electroclinic” effect.  Here we present results from electroclinic measurements as a function of frequency and temperature, which allow us to distinguish the component of optical signal that arises from liquid crystal chirality induced within the PMO’s chiral pores from that induced just outside the silica colloids.  Our central result is that the overwhelming source of our electrooptic signal emanates from outside the PMO, and that the contribution from the liquid crystal embedded in the chiral pores is much smaller and below the noise level.

“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.


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.


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.


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


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

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.
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
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.
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.
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.
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.

For a complete list of publicationsandpatents (1977 - Present), click here

Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation