Scientific and Technical Aerospace Reports Volume 39 April 6, 2001

cal transducers. Additionally, the ability to experimentally probe colloidal suspensions interacting through tunable anisotropic
potentials is of fundamental interest. In this talk we will describe our work toward a microscopic understanding of magnetorheological
behavior. Much of the rheological behavior arises from the cross-linked structure caused by defects and Landau-Peierls
thermal fluctuations of dipolar chains. We used the light scattering technique, diffusing-wave spectroscopy, to investigate the fluctuations
of dipolar chains. We have prepared monodisperse neutrally buoyant MR suspensions allowing us to probe the dynamics
of the dipolar chains using light scattering without complications due to gravitational forces and polydispersity. Optical gradient
force trapping techniques, or laser tweezers, have become increasingly important tools for studying the microscopic structure,
mechanics, and interactions in biological, colloidal, and macromolecular materials. We also present our study of the micromechanical
properties of dipolar chains and chain aggregates in a magnetorheological suspension. Using dual-trap optical tweezers,
we are able to directly measure the deformation of dipolar chains parallel and perpendicular to the applied magnetic field. We
observe the field-dependence of chain mechanical properties, such as tensile strain, chain reorganization, defect-annealing and
rupture. We discuss the role these forms of energy dissipation take in the yield stress and rheology of MR suspensions.
Author (revised)
Magnetorheological Fluids; Suspensions; Crosslinking; Colloids
20010024902 Colorado Univ., Dept. of Physics, Boulder, CO USA
Freely Suspended Smectic Filaments and the Structure of the B7 Phase of MHOBOW
Clark, N., Colorado Univ., USA; Link, D. R., Colorado Univ., USA; Maclennan, J. E., Colorado Univ., USA; Proceedings of the
Fifth Microgravity Fluid Physics and Transport Phenomena Conference; December 2000, pp. 337-339; In English; See also
20010024890; No Copyright; Abstract Only; Available from CASI only as part of the entire parent document
Our recent discovery of the spontaneous formation of chiral domains in fluid smectic phases of achiral bow-shaped molecules
opens up a wide variety of possibilities for new liquid crystal phases and phenomena. The basic, spontaneously chiral layer structure
of the highest temperature fluid smectic phases, the B2 and B7, are shown. One of the most intriguing aspects of this structure
is the plethora of possible phases coming from different stacking sequences of the polar ordering and tilt directions. The four possibilities
of next-nearest neighbor alternation are shown. In the original material studied, NOBOW, the ground states found are antiferroelectric,
either the racemic SmC(sub S)P(sub A) or the chiral SmC(sub A)P(sub A). We are currently studying MHOBOW,
synthesized by D. Walba which, by virtue of its methyl hexyloxy tail has a tendency to form anticlinic layer interfaces, in the hope
of finding a phase with a ferroelectric ground state, either SmC(sub A)P(sub S) or SmC(sub S)P(sub A), which can be obtained
in NOBOW only by applying a field. Preliminary observations of MHO-BOW have made its study, from the point of view of
understanding novel LC structures, extremely high priority. The following truly remarkable characteristics have been revealed:
(i) The smectic phase grows out of the isotropic in the form of helical ribbons. The resulting planar aligned textures of focal conics
with layers normal to glass plates exhibit bizarre modulations, including stripes and checker-boards. These have also been seen
in other materials suggesting that this is a new phase (tentatively called B7), which is a fluid smectic with some kind of in-layer
structure. (ii) It is virtually impossible to make freely suspended films of MHOBOW. Rather it makes the freely suspended filaments
which preliminary x-ray scattering experiments reveal to have the nested cylinder layer structure indicated; (iii) The powder
x-ray diffraction exhibits four resolution-limited smectic layering peaks, very close in layer spacing, which vary continuously with
T. This is further evidence for a more complex three dimensional structure than NOBOW, which has a typical single layering
reflection. (iv) The x-ray structure factor of the layering peak of the filaments is extraordinarily complex and rich. Varying in qL
(the scattering vector component along the filament axis) from a double slit-like pattern to modulated layer-like patterns, as qH
(the scattering vector component normal to the filament axis) is varied over the range where the four powder peaks are located.
These results suggest some kind of mosaic structure, perhaps with different layer spacings corresponding to the different stacking
sequences. Recent x-ray diffraction experiments show that the peaks are modulated in intensity upon translation along a filament,
in domains of several hundred microns dimension. These preliminary experiments suggest that the B7 is a fluid smectic with
extremely unusual and fascinating structures. of all of the many hundreds of fluid smectic materials we have attempted to study
in the freely suspended film geometry over the years, only a few have failed to form films, and none showed any great tendency
to form filaments, although this clearly should be a possible freely suspended smectic LC morphology. On several ocCASIons
in the past we have intentionally tried to make filaments from a variety of smectics without success. Thus the smectic filament
formation property makes the B7 phase unique. It seems quite likely that the stability of filaments is related to the in-plane structure.
The filaments exhibit other interesting structural and optical features. They are birefringent with a local optic axis which is
oblique and which can vary continuously along filament and which can be manipulated with an electric field applied normal to
the fiber, as if the field were causing a rotation of the optic axis about the fiber axis. Rapid displacement of the ends of the fiber
71