Reconfigurable self-assembly through chiral control of interfacial tension

Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature 481 (2012): 348–351, doi:10.1038/nature10769.From determining optical properties of simple molecular crystals to establishing preferred handedness in highly complex vertebrates, molecular chirality profoundly influences the structural, mechanical, and optical properties of both synthetic and biological matter at macroscopic lengthscales1,2. In soft materials such as amphiphilic lipids and liquid crystals, the competition between local chiral interactions and global constraints imposed by the geometry of the self-assembled structures leads to frustration and the assembly of unique materials3-6. An example of particular interest is smectic liquid crystals, where the 2D layered geometry cannot support twist, expelling chirality to the edges in a manner analogous to the expulsion of a magnetic field from superconductors7-10. Here, we demonstrate a previously unexplored consequence of this geometric frustration which leads to a new design principle for the assembly of chiral molecules. Using a model system of colloidal membranes11, we show that molecular chirality can control the interfacial tension, an important property of multi-component mixtures. This finding suggests an analogy between chiral twist which is expelled to the edge of 2D membranes, and amphiphilic surfactants which are expelled to oil-water interfaces12. Similar to surfactants, chiral control of interfacial tension drives the assembly of myriad polymorphic assemblages such as twisted ribbons with linear and circular topologies, starfish membranes, and double and triple helices. Tuning molecular chirality in situ enables dynamical control of line tension that powers polymorphic transitions between various chiral structures. These findings outline a general strategy for the assembly of reconfigurable chiral materials which can easily be moved, stretched, attached to one another, and transformed between multiple conformational states, thus enabling precise assembly and nano-sculpting of highly dynamical and designable materials with complex topologies.This work was supported by the National Science Foundation (NSF-MRSEC-0820492, NSF-DMR-0955776, NSF-MRI 0923057) and Petroleum Research Fund (ACS-PRF 50558-DNI7).2012-07-0

Rheo-PIV of a shear-banding wormlike micellar solution under large amplitude oscillatory shear

We explore the behavior of a wormlike micellar solution under both steady and large amplitude oscillatory shear (LAOS) in a cone–plate geometry through simultaneous bulk rheometry and localized velocimetric measurements. First, particle image velocimetry is used to show that the shear-banded profiles observed in steady shear are in qualitative agreement with previous results for flow in the cone–plate geometry. Then under LAOS, we observe the onset of shear-banded flow in the fluid as it is progressively deformed into the non-linear regime—this onset closely coincides with the appearance of higher harmonics in the periodic stress signal measured by the rheometer. These harmonics are quantified using the higher-order elastic and viscous Chebyshev coefficients e [subscript n] and v [subscript n] , which are shown to grow as the banding behavior becomes more pronounced. The high resolution of the velocimetric imaging system enables spatiotemporal variations in the structure of the banded flow to be observed in great detail. Specifically, we observe that at large strain amplitudes (γ [subscript 0] ≥ 1), the fluid exhibits a three-banded velocity profile with a high shear rate band located in-between two lower shear rate bands adjacent to each wall. This band persists over the full cycle of the oscillation, resulting in no phase lag being observed between the appearance of the band and the driving strain amplitude. In addition to the kinematic measurements of shear banding, the methods used to prevent wall slip and edge irregularities are discussed in detail, and these methods are shown to have a measurable effect on the stability boundaries of the shear-banded flow.Spain. Ministerio de Educación y Ciencia (MEC) (Project FIS2010-21924-C02-02

Recent experimental probes of shear banding

Recent experimental techniques used to investigate shear banding are reviewed. After recalling the rheological signature of shear-banded flows, we summarize the various tools for measuring locally the microstructure and the velocity field under shear. Local velocity measurements using dynamic light scattering and ultrasound are emphasized. A few results are extracted from current works to illustrate open questions and directions for future research.Comment: Review paper, 23 pages, 11 figures, 204 reference

Gender differences in the use of cardiovascular interventions in HIV-positive persons; the D:A:D Study

Peer reviewe

A quest for shear banding in ideal and non ideal colloidal rods

© 2018 IOP Publishing Ltd. We assess the possibility of shear banding of semidilute rod-like colloidal suspensions under steady shear flow very close to the isotropic-nematic spinodal, using a combination of rheology, small angle neutron scattering, and laser Doppler velocimetry. Model systems are employed which allow for a length and stiffness variation of the particles. The rheological signature reveals that these systems are strongly shear thinning at moderate shear rates. It is shown that the longest and most flexible rods undergo the strongest shear thinning and have the greatest potential to form shear bands. Although we find a small but significant gradient of the orientational order parameter throughout the gap of the shear cell, no shear banding transition is tractable in the region of intermediate shear rates. At very low shear rates, gradient banding and wall slip occur simultaneously, but the shear bands are not stable over time.status: publishe

Sedimentation of large particles in a suspension of colloidal rods

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Capillary nematisation of colloidal rods in confinement

We confine a colloidal liquid crystal between parallel plates separated down to several times the rod length. By connecting the system to a reservoir we effectively create a grand canonical system, in which the liquid crystal displays an isotropic phase in the reservoir, but upon strong confinement becomes nematic between the parallel plates. This capillary nematisation transition can be followed down to the single particle level by means of laser scanning confocal microscopy. We compare the experimental findings to density functional theories (DFTs), within the Zwanzig model as well as a more advanced DFT, in which the effect of rod flexibility is taken into account

Capillary nematisation of colloidal rods in confinement

© 2018, © 2018 Informa UK Limited, trading as Taylor & Francis Group. We confine a colloidal liquid crystal between parallel plates separated down to several times the rod length. By connecting the system to a reservoir we effectively create a grand canonical system, in which the liquid crystal displays an isotropic phase in the reservoir, but upon strong confinement becomes nematic between the parallel plates. This capillary nematisation transition can be followed down to the single particle level by means of laser scanning confocal microscopy. We compare the experimental findings to density functional theories (DFTs), within the Zwanzig model as well as a more advanced DFT, in which the effect of rod flexibility is taken into account.status: publishe

Capillary nematisation of colloidal rods in confinement

We confine a colloidal liquid crystal between parallel plates separated down to several times the rod length. By connecting the system to a reservoir we effectively create a grand canonical system, in which the liquid crystal displays an isotropic phase in the reservoir, but upon strong confinement becomes nematic between the parallel plates. This capillary nematisation transition can be followed down to the single particle level by means of laser scanning confocal microscopy. We compare the experimental findings to density functional theories (DFTs), within the Zwanzig model as well as a more advanced DFT, in which the effect of rod flexibility is taken into account

Direct visualization of flow-induced conformational transitions of single actin filaments in entangled solutions

While semi-flexible polymers and fibres are an important class of material due to their rich mechanical properties, it remains unclear how these properties relate to the microscopic conformation of the polymers. Actin filaments constitute an ideal model polymer system due to their micron-sized length and relatively high stiffness that allow imaging at the single filament level. Here we study the effect of entanglements on the conformational dynamics of actin filaments in shear flow. We directly measure the full three-dimensional conformation of single actin filaments, using confocal microscopy in combination with a counter-rotating cone-plate shear cell. We show that initially entangled filaments form disentangled orientationally ordered hairpins, confined in the flow-vorticity plane. In addition, shear flow causes stretching and shear alignment of the hairpin tails, while the filament length distribution remains unchanged. These observations explain the strain-softening and shear-thinning behaviour of entangled F-actin solutions, which aids the understanding of the flow behaviour of complex fluids containing semi-flexible polymers