Shear-induced undulation of smectic- $\mathsf{A}$ : Molecular dynamics simulations vs. analytical theory

Soddemann, Thomas; Auernhammer, Günter K.; Guo, Hongxia; Dünweg, Burkhard; Kremer, Kurt

doi:10.1140/epje/e2004-00045-0

Cited by 56 publications

(68 citation statements)

References 43 publications

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“…13 shows D velocity > D vorticity > D gradient for strain values lower than 3000 which suggests undulations with a wave vector along the vorticity axis, in agreement with theoretical predictions 89,90 and molecular dynamics simulations. 91 Altogether, the diffusion results are consistent with the following pathway of onion formation: starting from flat layers, first an undulation with the wave vector along the vorticity axis is introduced. Eventually, undulations with wave vectors along the velocity directions are added.…”

Section: Intermediate Structuresupporting

confidence: 77%

Planar lamellae and onions: a spatially resolved rheo–NMR approach to the shear-induced structural transformations in a surfactant model system

et al. 2011

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The shear-induced transformations between oriented planar lamellae and a state of closely packed multilamellar vesicles (MLVs) in a lyotropic nonionic surfactant model system were studied by the combination of nuclear magnetic resonance (NMR) spectroscopy and diffusometry with magnetic resonance imaging (MRI). 2H NMR imaging confirmed the discontinuous nature of the transition from onions to planar lamellae, revealing the spatial coexistence of both states within the gap of the cylindrical Couette geometry. On the other hand, NMR diffusion measurements in three principal directions and at various values of strain strongly suggest that a multi-lamellar cylindrical or undulated intermediate structure exists during the continuous and spatially homogeneous transition from planar lamellae to MLVs.

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Section: Intermediate Structuresupporting

confidence: 77%

Planar lamellae and onions: a spatially resolved rheo–NMR approach to the shear-induced structural transformations in a surfactant model system

et al. 2011

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“…[20]). The SmA phase becomes, then, nematic under shear, in contrast to what has been found in systems with homeotropic alignment or intrinsic layering [22]. Although the dynamic transition to the helicopter rotation seems to take place at a similar interaction strength as the appearance of I-SmA in equilibrium, we find no direct connection between the two features.…”

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confidence: 85%

Helicopter rotation and smectic-isotropic coexistence of strongly attractive rods

Ripoll

2011

Phys. Rev. E

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Hydrodynamic simulations of strongly attractive rodlike colloids are performed with and without shear flow. In the absence of flow, the isotropic-nematic coexistence becomes isotropic-smectic A, and the interfacial properties clearly vary with increasing attraction strength. In the presence of shear flow, a new collective rotation appears in which the director rotates in the vorticity-flow plane in a similar fashion to the movement of the rotor of a helicopter. DOI: 10.1103/PhysRevE.83.040701 PACS number(s): 64.70.M−, 82.70.Dd, 64.60.−i, 83.50.−v Liquid crystals in equilibrium, and in the presence of flow fields, show a very rich structural and dynamical behavior. Equilibrium phase diagrams display coexistence between isotropic (I), where rods are randomly oriented, nematic (N), where rods have a orientational preference, smectic (Sm), with both orientational and positional order, and crystalline phases, where the positional order is long ranged [1]. Attraction induced by depletion interactions has been considered in analytical calculations and experiments [2,3] of rods with large aspect ratios (rod length L over rod diameter σ ). The broadening of existing coexistence regions like I-N and N-Sm has been determined, together with the appearance of new ones as I-I, N-N, and eventually also I-Sm. Computer simulations are a powerful tool that can provide a detailed description of these systems. Numerous simulations with repulsive rods of small aspect ratios, typically from 2 to 6, have studied phases with positional order (see, e.g., Ref.[4]). Simulations of thermotropic systems of Gay-Berne colloidal rods [5] with increasing attraction have reported the appearance of different phases as a function of the potential asymmetry. Gay-Berne forces become unrealistically thin at the ends with increasing aspect ratio. I-Sm coexistence has been obtained with such rods by keeping the two phases at different temperatures [6]. In the presence of shear flow, the phase diagram is fundamentally altered [7]. Isotropic phases tend to align with flow, and oriented phases may undergo orchestrated motions that have been characterized about two decades ago [8]. Nematic phases have shown rotations in the flow-gradient plane (similar to the blades of a mill) or with an angle tilted in the vorticity direction (similar to the paddle in a kayak). These rotations are known as tumbling and kayaking [9]. Emerging questions are the effect that shear has when interfaces of varying softness are present or when applied to phases with positional order. Examples of attractive colloids where shear alignment is interesting for industrial applications are carbon nanotubes [10], wormlike micelles [11], or polymers.In this Rapid Communication, we investigate by means of computer simulations the effect of large attractions on the I-N coexistence of systems of rods with aspect ratio 20. At rest, increasing the attraction transforms the nematic phase into the smectic and linearly decreases the interface width. In the presence of shear flow, a qualitativ...

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“…Molecular dynamics simulations on lyotropic smectics place this in a regime of flow rates well above those we have applied in the present study 14,15 . The critical shear rate for thermotropic smectics that has been given in a theoretical approach 16 is slightly above ours.…”

Section: Discussionsupporting

confidence: 55%

Rheology of lamellar liquid crystals in two and three dimensions: a simulation study

et al. 2012

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We present large scale computer simulations of the nonlinear bulk rheology of lamellar phases (smectic liquid crystals) at moderate to large values of the shear rate (Peclet numbers 10 − 100), in both two and three dimensions. In two dimensions we find that modest shear rates align the system and stabilise an almost regular lamellar phase, but high shear rates induce the nucleation and proliferation of defects, which in steady state is balanced by the annihilation of defects of opposite sign. The shear rateγ c at onset of this second regime is controlled by thermodynamic and kinetic parameters; we offer a scaling analysis that relatesγ c to a critical "capillary number" involving those variables. Within the defect proliferation regime, the defects may be partially annealed by slowly decreasing the applied shear rate; this causes marked memory effects, and history-dependent rheology. Simulations in three dimensions show instead shear-induced ordering even at the highest shear rates studied here. This suggests thatγ c shifts markedly upward on increasing dimensionality. This may in part reflect the reduced constraints on defect motion, allowing them to find and annihilate each other more easily. Residual edge defects in the 3D aligned state mostly point along the flow velocity, an orientation impossible in two dimensions.

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Shear-induced undulation of smectic- $\mathsf{A}$ : Molecular dynamics simulations vs. analytical theory

Cited by 56 publications

References 43 publications

Planar lamellae and onions: a spatially resolved rheo–NMR approach to the shear-induced structural transformations in a surfactant model system

Planar lamellae and onions: a spatially resolved rheo–NMR approach to the shear-induced structural transformations in a surfactant model system

Helicopter rotation and smectic-isotropic coexistence of strongly attractive rods

Rheology of lamellar liquid crystals in two and three dimensions: a simulation study

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