Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session D37: Liquid Crystals and Complex Fluids |
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Sponsoring Units: DFD Chair: Philip Taylor, Case Western Reserve Univ Room: LACC 512 |
Monday, March 21, 2005 2:30PM - 2:42PM |
D37.00001: Simulation study of liquid crystal anchoring at a polymer surface Mehdi Hamaneh, Philip Taylor The process of rubbing a polymer substrate to induce planar anchoring has two effects. It aligns the polymer chains and also creates grooves in the surface. We have investigated which one of these effects is more important by conducting a series of simulations of molecules of 5CB in contact with a poly(vinyl alcohol) surface. The polymer surface was constructed from a set of parallel straight chains. It was then distorted to mimic the effect of grooves in a direction perpendicular to the chain direction, thus causing two opposing anchoring effects. It was found that the 5CB molecules ordered preferentially along the chain direction when the depth of the grooves was less than 20 percent of the distance between grooves. For grooves whose walls were more steeply pitched, the nematic ordering aligned with the grooves. [Preview Abstract] |
Monday, March 21, 2005 2:42PM - 2:54PM |
D37.00002: PVDF homopolymer and copolymer coated surfaces for liquid crystal alignment Hemang Shah, Adam Fontecchio We report on the liquid crystal (LC) surface alignment induced by polymers of vinylidene fluoride (PVDF). Ferroelectric polymers based on PVDF can align LCs by virtue of an inherent polarization. The polarization can be manipulated using different techniques to align the LCs in preferred directions for ON and OFF states. This is a promising feature for a potential alignment layer for LCs in electro-optic devices including displays. We have used PVDF homopolymer and copolymers of VDF with Trifluoroethylene (TrFE), tetrafluoroethylene (TeFE) and hexafluoropropylene for our study. The films are prepared using different solvents, and are subsequently spin-coated and baked. Depending on the concentrations of the solvent, films with different morphologies are obtained. In addition, PVDF films can be poled by application of an electric filed during the baking process. The alignment of the LCs is due to both the surface morphology, and dipole-induced dipole interactions at the surface. The alignment variations of LC based on the substrate will be presented. Also, the effects of rubbed polymer layers on LC alignment will be discussed. This research thus provides an insight into electro-optical devices including displays, memory elements and sensors based on LCs and ferroelectric polymers. [Preview Abstract] |
Monday, March 21, 2005 2:54PM - 3:06PM |
D37.00003: Dynamics of Evolution of Striped Structures in Liquid Crystals Deng-Ke Yang In liquid crystals, there are many types of striped structures, such as disclinations and domain walls as well as structural fingers. Depending on their free energy is positive or negative (compared with the background structure), they will shrink or grow with time. We can describe well their dynamic behavior by considering the balance between the line tension (free energy per unit length) of the striped structure and an effective viscosity force which is the product of the shrinking (or growing) speed and an effective translational viscosity coefficient. We calculate the translational viscosity coefficient in terms of the rotational viscosity coefficient and the area of the cross section of the stripe. This theory agrees well with experimental results. [Preview Abstract] |
Monday, March 21, 2005 3:06PM - 3:18PM |
D37.00004: Network and cellular pattern formation in a phase-separating lyotropic liquid crystal Yasutaka Iwashita, Hajime Tanaka Phase separation is the most fundamental physical phenomenon that produces various heterogeneous structures. Here we study a phase separation of lyotropic liquid crystal into ordered (lamellar) and disordered (sponge) phases. We found two types of novel pattern formation caused by the interplay between a phase separation and smectic ordering in addition to normal phase separation; (i) the formation of a transient network of lamellar phase which is similar to the network pattern observed in viscoelastic phase separation (VPS) and (ii) the formation of a metastable cellular structure of the lamellar phase. By analyzing their dynamics and internal structures, we conform that the viscoelastic contrast between the lamellar and the sponge phase, which is the origin of the VPS-like process, is due to the elastic nature of the smectic order. Furthermore, the cellular structure is turned out to be favored to attain macroscopic smectic order. We successfully control the morphology among droplet, network and foam by changing the heating rate. [Preview Abstract] |
Monday, March 21, 2005 3:18PM - 3:30PM |
D37.00005: Routes to spatiotemporal chaos in the rheology of nematogenic fluids Moumita Das, Buddhapriya Chakrabarti, Sriram Ramaswamy, Chandan Dasgupta, Ajay Sood With a view to understanding the ``rheochaos'' observed in recent experiments in a variety of orientable fluids, we study numerically the equations of motion of the spatiotemporal evolution of the traceless symmetric order parameter of a sheared nematogenic fluid. In particular we establish, by decisive numerical tests, that the the irregular oscillatory behavior seen in a region of parameter space where the nematic is not stably flow-aligning is in fact spatiotemporal chaos. We outline the dynamical phase diagram of the model and study the route to the chaotic state. We find that spatiotemporal chaos in this system sets in via a regime of {\em spatiotemporal intermittency}, with a power-law distribution of the widths of laminar regions, consistent with the ideas of H. Chat\'{e} and P. Manneville, Phys. Rev. Lett. {\bf 58}, 112 (1987). Further, the evolution of the histogram of band sizes shows a growing length-scale as one moves from the chaotic towards the flow aligned phase. Finally we suggest possible experiments which can observe the intriguing behaviors discussed here. [Preview Abstract] |
Monday, March 21, 2005 3:30PM - 3:42PM |
D37.00006: Rupture of an Amphiphile layer on air-water interface Mahesh Bandi, John Cressman, Walter Goldburg An amphiphile layer is decorated with tracer particles (mean diameter 50$\mu$m, specific gravity 0.25) on the surface of a tank of water. A jet of water is forced up from the underlying bulk. The jet forces a hole in the amphiphile layer. Its shape forms the subject of this study. The interface between the particle covered and particle free regions is highly ramified presenting a jagged structure that fluctuates in space and time. An attempt is to interpret this observation as the fracture of a quasi two dimensional surface. [Preview Abstract] |
Monday, March 21, 2005 3:42PM - 3:54PM |
D37.00007: Nucleated growth patterns in binary phase-separating liquid gas systems Christopher Pooley, Anna Balazs, Julia Yeomans We construct a model to simulate systems that have gas in coexistence with a binary liquid. We present a statistical mechanical approach similar to that used to derive the Van der Waals equation of state. However, in this case, we introduce two species of particle with different interactions between similar and dissimilar types, and from this we derive the Landau free energy density. In equilibrium, the system minimizes this free energy, and this allows us to construct the phase diagram. Using a binary lattice Boltzmann algorithm, we numerically model this system. In particular, we focus on the discovery of intriguing tentacle structures observed in a small area within the nucleation region. We show how these structures grow in time, and how this evolution is changed in the inertial, viscous and diffusive limits. [Preview Abstract] |
Monday, March 21, 2005 3:54PM - 4:06PM |
D37.00008: The formation and growth of myelin figures. Ling-Nan Zou Myelin figures are $\mu$m-sized cylindrical structures which develop at the interface between water and the concentrated lamellar phase of certain surfactants, such as phosphatidylcholines (PC). Here, we describe an experiment to observe the onset and growth of single myelin figures originating from the water/surfactant interface at the contact line of a sessile drop. We find that {\it isolated} myelin figures grow in length linearly with time $L \propto t$, in contrast to the $L \propto \sqrt t$ growth previously observed in dense myelin bundles [1]. Using fluorescence microscopy to image material transport, we find that growth is primarily due to the addition of surfactant into the myelin figure at its point of attachment to the concentrated phase. A simple model, based on differential hydration of the surfactant, gives both the $L \propto t$ and $L \propto \sqrt t$ behaviors. As expected from this model, the application of external osmotic pressure strongly suppresses myelin figure growth.\\ \newline [1] M. Buchanan \textit{et al}. \textit{Langmuir}, \textbf{16}, 3718. (2000) [Preview Abstract] |
Monday, March 21, 2005 4:06PM - 4:18PM |
D37.00009: Why Myelins Form: Shape Instability of Flat Lamellae Due to Interlayer Repulsion Jung-Ren Huang, Thomas Witten We show theoretically that a multilamellar tube (myelin) may be more stable than a flat lamellar stack as a result of interlayer repulsion. This is a direct consequence of the geometrical confinement: Given the same amount of lipid and water, the tube morphology allows larger spacing and hence lower interlayer repulsion than a flat lamella. For example, a confined 200-layer lamellar DMPC stack is unstable and favors the formation of myelin tubes when the interlayer repulsion arising from the confinement is greater than 0.3atm. Our finding may provide an explanation for the myelin formation often seen during the dissolution of dry surfactants. [Preview Abstract] |
Monday, March 21, 2005 4:18PM - 4:30PM |
D37.00010: Viscoelasticity of Networks of Wormlike Micelles with Encapsulated Single Wall Carbon Nanotubes M.F. Islam, L.A. Hough, A.M. Alsayed, A.G. Yodh We investigate the viscoelastic properties of networks of wormlike micelles formed from the surfactant cetyltrimethylammonium tosilate (CTAT) with encapsulated single wall carbon nanotubes (SWNTs). In these experiments we hold the concentration of CTAT fixed at 4 wt{\%}, and then vary the concentration of SWNTs from 0.1 wt{\%} to 0.4 wt{\%}. The rheological measurements of these networks exhibit several striking features: 1) We observe a 10 fold increase of the elastic plateau modulus with the addition of 0.4 wt{\%} of SWNTs. 2) The low shear viscosity increases abruptly ($\sim $10 fold) when the number density of bare SWNTs exceeds the number density of CTAT crosslinks. 3) The addition of SWNTs increases the extensional properties of the CTAT wormlike micelles, allowing fibers to be drawn from the CTAT-SWNT suspension that are stable for several minutes. We consider these observations in the context of flexible and semiflexible/rigid rod polymers. This work has been partially supported by the NSF through MRSEC Grants DMR 00-79909 and DMR-0203378, and by NASA Grant NAG8-2172. [Preview Abstract] |
Monday, March 21, 2005 4:30PM - 4:42PM |
D37.00011: Formation of localized shear induced states in wormlike micelle solutions using magnetic nanowire probes Nathan Cappallo, Clayton Lapointe, Robert Leheny, Daniel Reich Under certain conditions, surfactant molecules in aqueous suspension can form long cylindrically shaped micelles that entangle, leading to complex non-linear rheological behavior. We report the formation of spatially localized shear induced states in the wormlike micelle system, cetylpyridinium chloride/sodium salicylate (CPCl/NaSal), through the rapid rotation of colloidal wires. Ferromagnetic Ni nanowires with radius 150 nm and lengths between 5 and 50 microns suspended in the micellar fluid are subjected to magnetic torques via rotating external magnetic fields. For low rotation frequencies the nanowires unwind after removal of the field, as expected for a viscoelastic fluid. After removal of higher frequency fields, however, the wires tip rapidly out of the plane of rotation due to a torque generated by the fluid. We interpret this phenomenon in terms of the nucleation of a region of nematic order in the micelles. The temperature and frequency dependence of this behavior, as well as the use of the nanowires to monitor the transient anisotropic elastic properties of the shear-induced state, will be discussed. [Preview Abstract] |
Monday, March 21, 2005 4:42PM - 4:54PM |
D37.00012: Structure and Dynamics of Microemulsions/Micelles in the Presence of a Monolayer Interface in the Ternary Amphilphilic Systems: A Computer Simulation Study Hongxia Guo, Monica Olvera de la Cruz Ternary amphiphilic systems such as amphiphilic block copolymers in selective homopolymer blends and surfactants in oil and water solutions are of great importance in many physical and biological systems. The amphiphilic molecules segregate into the interface of the corresponding immiscible (or incompatible) majority components. The amphiphilic molecules may also solubilize the homopolymers or water and/or oil into a microemulsion/micelle phase. We analyzed microemulsification of block copolymers at an interface by a computer model consisting of A homopolymer, B homopolymer and A-C copolymer in which the C segments have strong favorable interaction with the B homopolymer. By tuning the interaction between B and C components and adjusting the A-C concentration above the critical micelle concentration, we find segregation of A-C copolymers into the interface between A and B homopolymers, the stable polymer microemulsion consist of the B hompolymer in the core and A-C copolymer in the corona suspended in the A homopolymer matrix. The phase behavior and structure is examined as a function of concentrations and intermolecular interactions of the components. [Preview Abstract] |
Monday, March 21, 2005 4:54PM - 5:06PM |
D37.00013: Dynamics of self-assembling rigid rods Johan Dubbeldam, Paul Van der Schoot The current understanding of the dynamics of living polymers is based on the pioneering work of Cates and co-workers, who proposed that the scission and recombination kinetics of the polymeric chains can be described by a single time scale $\tau _{break}$, equal to the lifetime of a chain of mean length. For scission-recombination processes that are not exceedingly fast, one theoretically finds that stress relaxation takes place at times $\tau $ = ($\tau _{rep} \quad \tau _{break})^{1/2}$, at least in semi-dilute solution where the regime where reptation is the dominating mode of stress relief, characterized by the relaxation time $\tau _{rep}$. Comparison with experiments on giant surfactant micelles shows that this picture is qualitatively correct on sufficiently long time scales, but that for shorter times deviations do emerge. In order to study the breakdown of the dynamical mean-field approximation, we present a simplified, so-called end-evaporation model for the association kinetics of rod-like equilibrium polymers that takes into account the diffusive motion of the assemblies, and that allows for the re-absorption of monomers that have split off from the assemblies. The model can be solved exactly in the limit of small temperature or T jumps. We find that diffusion modifies the relaxation to the new equilibrium after a T jump significantly. We derive expressions for the time-dependent average chain length and for the size-dependent chain concentrations, and compare these with results of a numerical study. [Preview Abstract] |
Monday, March 21, 2005 5:06PM - 5:18PM |
D37.00014: The Interactions Between Two Rotating Dipoles Inside Electrorheological Fluids Y. C. Lan, R. Tao Interactions between rotating dipoles are important in study of electrorheological (ER) fluids since the induced electric dipoles in ER fluids may rotate under the shearing flow. There have been some theoretical predictions that regardless of the hydrodynamic complexion, the rotation, even very slow rotation, will significantly reduce the dipolar interaction and make the interaction vanishing as the rotation speed increases. The actual ER experiments are very difficult to interpret or compared with these theoretical predictions since the rotation definitely induces a hydrodynamic force, which is usually significant. To clarify the issue, we conducted an experiment with two electric-field induced dipoles in dry argon gas. One of the dipoles rotated in the direction perpendicular to the electric field and the other was connected to a microbalance to measure the dipolar interactiion. The following three different situations were investigated: (a) both dipoles were metal spheres; (b) one dipole was a metal sphere and the other was a dielectric sphere; (c) both dipoles were dielectric spheres. The experiment finds that the attractive force between the two dipoles indeed decreases with the rotation. However, in case with two metal spheres, the dipolar force tends to a non-zero constant as the rotation speed increases. In the metal-dielectric case and dielectric- dielectric case, the dipolar force monotonically decreases with the increasing rotation speed. [Preview Abstract] |
Monday, March 21, 2005 5:18PM - 5:30PM |
D37.00015: Structure of Concentrated Suspensions of Square Crosses Carlos Hernandez, Thomas Mason We make dispersions of micron-sized regular square crosses by patterning a thin polymer film on a flat substrate using photolithography and lifting off the crosses into solution. Micron-sized dispersions of monodisperse crosses can be used to model the phase behavior of molecular liquid crystals, since thermal energy is still important, yet optical microscopy can be used to examine positional and orientational structure at the particle scale. When the volume fraction of the crosses is raised, they interact through excluded volume, and there is a potential for jamming into disordered configurations or creating phases that have varying degrees of translational and orientational order. We measure the structure of concentrated suspensions of square crosses confined to a monolayer (i.e. two dimensions), and we also report the structure of bulk suspensions as a function of volume fraction. [Preview Abstract] |
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