123 research outputs found
Domain Walls and Anchoring Transitions Mimicking Nematic Biaxiality in the Oxadiazole Bent-Core Liquid Crystal C7
We investigate the origin of secondary disclinations that were recently
described as a new evidence of a biaxial nematic phase in an oxadiazole
bent-core thermotropic liquid crystal C7. With an assortment of optical
techniques such as polarizing optical microscopy, LC PolScope, and fluorescence
confocal polarizing microscopy, we demonstrate that the secondary disclinations
represent non-singular domain walls formed in an uniaxial nematic during the
surface anchoring transition, in which surface orientation of the director
changes from tangential (parallel to the bounding plates) to tilted. Each
domain wall separates two regions with the director tilted in opposite
azimuthal directions. At the centre of the wall, the director remains parallel
to the bonding plates. The domain walls can be easily removed by applying a
modest electric field. The anchoring transition is explained by the balance of
(a) the intrinsic perpendicular surface anchoring produced by the polyimide
aligning layer and (b) tangential alignment caused by ionic impurities forming
electric double layers. The model is supported by the fact that the temperature
of the tangential-tilted anchoring transition decreases as the cell thickness
increases and as the concentration of ionic species (added salt) increases. We
also demonstrate that the surface alignment is strongly affected by thermal
degradation of the samples. The study shows that C7 exhibits only a uniaxial
nematic phase and demonstrate yet another mechanism (formation of secondary
disclinations) by which a uniaxial nematic can mimic a biaxial nematic
behaviour.Comment: 21 pages, 9 Figures, 1 Tabl
Filamentous phages as building blocks for reconfigurable and hierarchical self-assembly
Filamentous bacteriophages such as fd-like viruses are monodisperse rod-like
colloids that have well defined properties: diameter, length, rigidity, charge
and chirality. Engineering those viruses leads to a library of colloidal rods
which can be used as building blocks for reconfigurable and hierarchical
self-assembly. Their condensation in aqueous solution \th{with additive
polymers which act as depletants to induce} attraction between the rods leads
to a myriad of fluid-like micronic structures ranging from isotropic/nematic
droplets, colloid membranes, achiral membrane seeds, twisted ribbons,
-wall, pores, colloidal skyrmions, M\"obius anchors, scallop membranes to
membrane rafts. Those structures and the way they shape shift not only shed
light on the role of entropy, chiral frustration and topology in soft matter
but it also mimics many structures encountered in different fields of science.
On one hand, filamentous phages being an experimental realization of colloidal
hard rods, their condensation mediated by depletion interactions constitutes a
blueprint for self-assembly of rod-like particles and provides fundamental
foundation for bio- or material oriented applications. On the other hand, the
chiral properties of the viruses restrict the generalities of some results but
vastly broaden the self-assembly possibilities
Polarized light imaging of birefringence and diattenuation at highresolution and high sensitivity
Polarized light microscopy provides unique opportunities for analyzing the
molecular order in man-made and natural materials, including biological
structures inside living cells, tissues, and whole organisms. 20 years ago, the
LC-PolScope was introduced as a modern version of the traditional polarizing
microscope enhanced by liquid crystal devices for the control of polarization,
and by electronic imaging and digital image processing for fast and
comprehensive image acquisition and analysis. The LC- PolScope is commonly used
for birefringence imaging, analyzing the spatial and temporal variations of the
differential phase delay in ordered and transparent materials. Here we describe
an alternative use of the LC-PolScope for imaging the polarization dependent
transmittance of dichroic materials. We explain the minor changes needed to
convert the instrument between the two imaging modes, discuss the relationship
between the quantities measured with either instrument, and touch on the
physical connection between refractive index, birefringence, transmittance,
diattenuation, and dichroism.Comment: 21 pages, 5 figures, accepted for publication in Journal of Optic
Molecular engineering of chiral colloidal liquid crystals using DNA origami
Establishing precise control over the shape and the interactions of the
microscopic building blocks is essential for design of macroscopic soft
materials with novel structural, optical and mechanical properties. Here, we
demonstrate robust assembly of DNA origami filaments into cholesteric liquid
crystals, 1D supramolecular twisted ribbons and 2D colloidal membranes. The
exquisite control afforded by the DNA origami technology establishes a
quantitative relationship between the microscopic filament structure and the
macroscopic cholesteric pitch. Furthermore, it also enables robust assembly of
1D twisted ribbons, which behave as effective supramolecular polymers whose
structure and elastic properties can be precisely tuned by controlling the
geometry of the elemental building blocks. Our results demonstrate the
potential synergy between DNA origami technology and colloidal science, in
which the former allows for rapid and robust synthesis of complex particles,
and the latter can be used to assemble such particles into bulk materials
Polarized light microscopy in reproductive and developmental biology
Author Posting. © The Author(s), 2013. This is the author's version of the work. It is posted here by permission of John Wiley & Sons for personal use, not for redistribution. The definitive version was published in Molecular Reproduction and Development (2013), doi:10.1002/mrd.22221.The polarized light microscope reveals orientational order in native molecular structures inside
living cells, tissues, and whole organisms. Therefore, it is a powerful tool to monitor and analyze
the early developmental stages of organisms that lend themselves to microscopic observations. In
this article we briefly discuss the components specific to a traditional polarizing microscope and
some historically important observations on chromosome packing in sperm head, first zygote
division of the sea urchin, and differentiation initiated by the first uneven cell division in the
sand dollar. We then introduce the LC-PolScope and describe its use for measuring birefringence
and polarized fluorescence in living cells and tissues. Applications range from the enucleation of
mouse oocytes to analyzing the polarized fluorescence of the water strider acrosome. We end by
reporting first results on the birefringence of the developing chick brain, which we analyzed
between developmental stages of days 12 through 20.This work was supported by funds from the National Institute of General Medical Sciences
(grant 1R01GM100160-01A1 awarded to TT) and the National Institute of Biomedical Imaging
and Bioengineering (grant EB002045 awarded to RO)
Polarization microscopy with the LC-PolScope
Author Posting. © The Author(s), 2003. This is the author's version of the work. It is posted here by permission of Cold Spring Harbor Laboratory Press for personal use, not for redistribution. The definitive version was published in Live Cell Imaging : A Laboratory Manual, edited by R. D. Goldman and D. L. Spector, :205-237. Cold Spring Harbor Laboratory Press, 2005. ISBN: 9780879696825.In the current chapter we describe the use of a new type of polarized light microscope which
we started to develop at the Marine Biological Laboratory about ten years ago. The new “PolScope” is based on the traditional polarized light microscope and enhances it with the use of liquid-
crystal devices and special image processing algorithms. The LC-PolScope measures polarization
optical parameters in many specimen points simultaneously, in fast time intervals, and at the highest resolution of the light microscope. It rapidly generates a birefringence map whose
pixel brightness is directly proportional to the local optical anisotropy, unaffected by the specimen
orientation in the plane of view, as well as a map depicting the slow axis orientation of the
birefringent regions. The basic LC-PolScope technology can be adapted to most research grade
microscopes and is available commercially from Cambridge Research and Instrumentation (CRI,
http://www.cri-inc.com) in Woburn, Massachusetts, under the trade name LC-PolScope.Financial support from the National Institute of General
Medical Sciences and from the National Institute of Biomedical Imaging and Bioengineering
through grants GM49210 and EB002045, respectively
Kinetochore-driven outgrowth of microtubules is a central contributor to kinetochore fiber maturation in crane-fly spermatocytes
© The Author(s), 2014. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Molecular Biology of the Cell 25 (2014): 1437-1445, doi:10.1091/mbc.E14-01-0008.We use liquid crystal polarized light imaging to record the life histories of single kinetochore (K-) fibers in living crane-fly spermatocytes, from their origins as nascent K-fibers in early prometaphase to their fully matured form at metaphase, just before anaphase onset. Increased image brightness due to increased retardance reveals where microtubules are added during K-fiber formation. Analysis of experimentally generated bipolar spindles with only one centrosome, as well as of regular, bicentrosomal spindles, reveals that microtubule addition occurs at the kinetochore-proximal ends of K-fibers, and added polymer expands poleward, giving rise to the robust K-fibers of metaphase cells. These results are not compatible with a model for K-fiber formation in which microtubules are added to nascent fibers solely by repetitive “search and capture” of centrosomal microtubule plus ends. Our interpretation is that capture of centrosomal microtubules—when deployed—is limited to early stages in establishment of nascent K-fibers, which then mature through kinetochore-driven outgrowth. When kinetochore capture of centrosomal microtubules is not used, the polar ends of K-fibers grow outward from their kinetochores and usually converge to make a centrosome-free pole.This work was supported by Grant EB002045 from the National Institute of Biomedical Imaging and Bioengineering awarded to R.O
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