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1. Deformation and orientational order of chiral membranes with free edges

   https://doi.org/10.1039/d1sm00629k  

   Ding, Lijie; Pelcovits, Robert A.; Powers, Thomas R. (July 2021, Soft Matter)

   null (Ed.)

   Motivated by experiments on colloidal membranes composed of chiral rod-like viruses, we use Monte Carlo methods to simulate these systems and determine the phase diagram for the liquid crystalline order of the rods and the membrane shape. We generalize the Lebwohl–Lasher model for a nematic with a chiral coupling to a curved surface with edge tension and a resistance to bending, and include an energy cost for tilting of the rods relative to the local membrane normal. The membrane is represented by a triangular mesh of hard beads joined by bonds, where each bead is decorated by a director. The beads can move, the bonds can reconnect and the directors can rotate at each Monte Carlo step. When the cost of tilt is small, the membrane tends to be flat, with the rods only twisting near the edge for low chiral coupling, and remaining parallel to the normal in the interior of the membrane. At high chiral coupling, the rods twist everywhere, forming a cholesteric state. When the cost of tilt is large, the emergence of the cholesteric state at high values of the chiral coupling is accompanied by the bending of the membrane into a saddle shape. Increasing the edge tension tends to flatten the membrane. These results illustrate the geometric frustration arising from the inability of a surface normal to have twist. 

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2. Chiral organosilica particles and their use as inducers of conformational deracemization of liquid crystal phases

   Cohen, Orit; Ferris, Andrew J.; Adkins, Raymond; Lemieux, Robert P.; Avnir, David; Gelan, Dmitri; Rosenblatt, Charles (April 2018, Chemical physics letters)

   Chiral organosilica particles of size ~200 nm were synthesized from an enantio-pure multi-armed chiral D-maltose organosilane precursor in the absence of co-condensation with an achiral monomer. Two distinct experiments were performed to demonstrate the particles’ ability to induce conformational deracemization of a host liquid crystal. The first involves an electric field-induced tilt of the liquid crystal director in the deracemized smectic-A phase. The other involves domain wall curvature separating left- and right-handed liquid crystal helical pitch domains imposed by the cells’ substrates. The results demonstrate unequivocally that enantio-pure organosilica nanoparticles can be synthesized and can induce chirality in a host. 

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3. Observation of a uniaxial ferroelectric smectic A phase

   https://doi.org/10.1073/pnas.2210062119  

   Chen, Xi; Martinez, Vikina; Nacke, Pierre; Korblova, Eva; Manabe, Atsutaka; Klasen-Memmer, Melanie; Freychet, Guillaume; Zhernenkov, Mikhail; Glaser, Matthew A.; Radzihovsky, Leo; et al (November 2022, Proceedings of the National Academy of Sciences)

   We report the observation of the smectic A F , a liquid crystal phase of the ferroelectric nematic realm. The smectic A F is a phase of small polar, rod-shaped molecules that form two-dimensional fluid layers spaced by approximately the mean molecular length. The phase is uniaxial, with the molecular director, the local average long-axis orientation, normal to the layer planes, and ferroelectric, with a spontaneous electric polarization parallel to the director. Polarization measurements indicate almost complete polar ordering of the ∼10 Debye longitudinal molecular dipoles, and hysteretic polarization reversal with a coercive field ∼2 × 10 5 V / m is observed. The SmA F phase appears upon cooling in two binary mixtures of partially fluorinated mesogens: 2N/DIO, exhibiting a nematic (N)–smectic Z A (SmZ A )–ferroelectric nematic (N F )–SmA F phase sequence, and 7N/DIO, exhibiting an N–SmZ A –SmA F phase sequence. The latter presents an opportunity to study a transition between two smectic phases having orthogonal systems of layers. 

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4. Surface anchoring as a control parameter for stabilizing torons, skyrmions, twisted walls, fingers and their hybrids in chiral nematics

   Tai, Jung-Shen; Smalyukh, Ivan I. (January 2020, Physical review)

   Chiral condensed matter systems, such as liquid crystals and magnets, exhibit a host of spatially localized topological structures that emerge from the medium’s tendency to twist and its competition with confinement and field coupling effects. We show that the strength of perpendicular surface boundary conditions can be used to control the structure and topology of solitonic and other localized field configurations. By combining numerical modeling and threedimensional imaging of the director field, we reveal structural stability diagrams and intertransformation of twisted walls and fingers, torons and skyrmions and their crystalline organizations upon changing boundary conditions. Our findings provide a recipe for controllably realizing skyrmions, torons and hybrid solitonic structures possessing features of both of them, which will aid in fundamental explorations and technological uses of such topological solitons. Moreover, with limited examples, we discuss how similar principles can be systematically used to tune stability of twisted walls versus cholesteric fingers and hopfions versus skyrmions, torons and twistions. 

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5. All twist and no bend makes raft edges splay: Spontaneous curvature of domain edges in colloidal membranes

   https://doi.org/10.1126/sciadv.aba2331  

   Miller, Joia M.; Hall, Doug; Robaszewski, Joanna; Sharma, Prerna; Hagan, Michael F.; Grason, Gregory M.; Dogic, Zvonimir (July 2020, Science Advances)

   Using a combination of theory and experiments we study the interface between two immiscible domains in a colloidal membrane composed of rigid rods of different lengths. Geometric considerations of rigid rod packing imply that a domain of sufficiently short rods in a background membrane of long rods is more susceptible to twist than the inverse structure, a long-rod domain in a short-rod membrane background. The tilt at the inter-domain edge forces splay, which in turn manifests as a spontaneous edge curvature whose energetics are controlled by the length asymmetry of constituent rods. A thermodynamic model of such tilt-curvature coupling at inter-domain edges explains a number of experimental observations, including a non-monotonic dependence of the edge twist on the domain radius, and annularly shaped domains of long rods. Our work shows how coupling between orientational and compositional degrees of freedom in two-dimensional fluids give rise to complex shapes and thermodynamics of domains, analogous to shape transitions in 3D fluid vesicles. 

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