More Like this

1. Effects of spatially-varying substrate anchoring on instabilities and dewetting of thin nematic liquid crystal films

   https://doi.org/10.1039/d0sm01416h  

   Lam, Michael-Angelo Y.-H.; Kondic, Lou; Cummings, Linda J. (November 2020, Soft Matter)

   null (Ed.)

   Partially wetting nematic liquid crystal (NLC) films on substrates are unstable to dewetting-type instabilities due to destabilizing solid/NLC interaction forces. These instabilities are modified by the nematic nature of the films, which influences the effective solid/NLC interaction. In this work, we focus on the influence of imposed substrate anchoring on the instability development. The analysis is carried out within a long-wave formulation based on the Leslie–Ericksen description of NLC films. Linear stability analysis of the resulting equations shows that some features of the instability, such as emerging wavelengths, may not be influenced by the imposed substrate anchoring. Going further into the nonlinear regime, considered via large-scale GPU-based simulations, shows however that nonlinear effects may play an important role, in particular in the case of strong substrate anchoring anisotropy. Our simulations show that instability of the film develops in two stages: the first stage involves formation of ridges that are perpendicular to the local anchoring direction; and the second involves breakup of these ridges and formation of drops, whose final distribution is influenced by the anisotropy imposed by the substrate. Finally, we show that imposing more complex substrate anisotropy patterns allows us to reach basic understanding of the influence of substrate-imposed defects in director orientation on the instability evolution. 

   more » « less
2. Electric field driven reconfigurable multistable topological defect patterns

   https://doi.org/https://doi.org/10.1103/PhysRevResearch.2.013176  

   Harkai, S.; Murray, B.S.; Rosenblatt, C.; Kralj, S. (January 2020, Physical review research)

   Topological defects appear in symmetry breaking phase transitions and are ubiquitous throughout Nature. As an ideal testbed for their study, defect configurations in nematic liquid crystals (NLCs) could be exploited in a rich variety of technological applications. Here we report on robust theoretical and experimental investigations in which an external electric field is used to switch between predetermined stable chargeless disclination patterns in a nematic cell, where the cell is sufficiently thick that the disclinations start and terminate at the same surface. The different defect configurations are stabilized by a master substrate that enforces a lattice of surface defects exhibiting zero total topological charge value. Theoretically, we model disclination configurations using a Landau-de Gennes phenomenological model. Experimentally, we enable diverse defect patterns by implementing an in-house-developed atomic force measurement scribing method, where NLC configurations are monitored via polarized optical microscopy. We show numerically and experimentally that an “alphabet” of up to 18 unique line defect configurations can be stabilized in a 4 × 4 lattice of alternating s = ±1 surface defects, which can be “rewired” multistably using appropriate field manipulation. Our proof-of-concept mechanism may lead to a variety of applications, such as multistable optical displays and rewirable nanowires. Our studies also are of interest from a fundamental perspective. We demonstrate that a chargeless line could simultaneously exhibit defect-antidefect properties. Consequently, a pair of such antiparallel disclinations exhibits an attractive interaction. For a sufficiently closely spaced pair of substrate-pinned defects, this interaction could trigger rewiring, or annihilation if defects are depinned. 

   more » « less
3. Confinement and magnetic-field effect on chiral ferroelectric nematic liquid crystals in Grandjean-Cano wedge cells

   https://doi.org/10.1103/PhysRevE.109.054702  

   Thapa, Kamal; Iadlovska, Olena S; Basnet, Bijaya; Wang, Hao; Paul, Ayusha; Gleeson, James T; Lavrentovich, Oleg D (May 2024, Physical Review E)

   We explore the structure and magnetic field response of edge dislocations in Grandjean-Cano wedge cells filled with chiral mixtures of the ferroelectric nematic mesogen DIO. Upon cooling, the ordering changes from paraelectric in the cholesteric phase 〖 N〗^* to antiferroelectric in the smectic SmZ_A^* and to ferroelectric in the cholesteric N_F^*. Dislocations of the Burgers vector b equal the helicoidal pitch P are stable in all three phases, while dislocations with b=P/2 exist only in the 〖 N〗^* and SmZ_A^*. The b=P/2 dislocations split into pairs of τ^(-1/2) λ^(+1/2) disclinations, while the thick dislocations b=P are pairs of nonsingular λ^(-1/2) λ^(+1/2) disclinations. The polar order makes the τ^(-1/2) disclinations unstable in the N_F^* phase, as they should be connected to singular walls in the polarization field. We propose a model of transformation of the composite τ^(-1/2) line-wall defect into a nonsingular λ^(-1/2) disclination, which is paired up with a λ^(+1/2) line to form a b=P dislocation. The SmZ_A^* behavior in the in-plane magnetic field is different from that of the N_F^* and N^*: the dislocations show no zigzag instability, and the pitch remains unchanged in the magnetic fields up to 1 T. The behavior is associated with the finite compressibility of smectic layers. 

   more » « less
4. Shear-induced polydomain structures of nematic lyotropic chromonic liquid crystal disodium cromoglycate

   https://doi.org/10.1039/D0SM01259A  

   Baza, Hend; Turiv, Taras; Li, Bing-Xiang; Li, Ruipeng; Yavitt, Benjamin M.; Fukuto, Masafumi; Lavrentovich, Oleg D. (January 2020, Soft Matter)

   Lyotropic chromonic liquid crystals (LCLCs) represent aqueous dispersions of organic disk-like molecules that form cylindrical aggregates. Despite the growing interest in these materials, their flow behavior is poorly understood. Here, we explore the effect of shear on dynamic structures of the nematic LCLC, formed by 14 wt% water dispersion of disodium cromoglycate (DSCG). We employ in situ polarizing optical microscopy (POM) and small-angle and wide-angle X-ray scattering (SAXS/WAXS) to obtain independent and complementary information on the director structures over a wide range of shear rates. The DSCG nematic shows a shear-thinning behavior with two shear-thinning regions (Region I at  < 1 s −1 and Region III at  > 10 s −1 ) separated by a pseudo-Newtonian Region II (1 s −1 <  < 10 s −1 ). The material is of a tumbling type. In Region I,  < 1 s −1 , the director realigns along the vorticity axis. An increase of  above 1 s −1 triggers nucleation of disclination loops. The disclinations introduce patches of the director that deviates from the vorticity direction and form a polydomain texture. Extension of the domains along the flow and along the vorticity direction decreases with the increase of the shear rate to 10 s −1 . Above 10 s −1 , the domains begin to elongate along the flow. At  > 100 s −1 , the texture evolves into periodic stripes in which the director is predominantly along the flow with left and right tilts. The period of stripes decreases with an increase of  . The shear-induced transformations are explained by the balance of the elastic and viscous energies. In particular, nucleation of disclinations is associated with an increase of the elastic energy at the walls separating nonsingular domains with different director tilts. The uncovered shear-induced structural effects would be of importance in the further development of LCLC applications. 

   more » « less
5. Imaging the Water Snowline around Protostars with Water and HCO ⁺ Isotopologues

   https://doi.org/10.3847/1538-4357/ac3080  

   van ’t Hoff, Merel L.; Harsono, Daniel; van Gelder, Martijn L.; Hsieh, Tien-Hao; Tobin, John J.; Jensen, Sigurd S.; Hirano, Naomi; Jørgensen, Jes K.; Bergin, Edwin A.; van Dishoeck, Ewine F. (January 2022, The Astrophysical Journal)

   Abstract The water snowline location in protostellar envelopes provides crucial information about the thermal structure and the mass accretion process as it can inform about the occurrence of recent (≲1000 yr) accretion bursts. In addition, the ability to image water emission makes these sources excellent laboratories to test indirect snowline tracers such as H 13 CO + . We study the water snowline in five protostellar envelopes in Perseus using a suite of molecular-line observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA) at ∼0.″2−0.″7 (60–210 au) resolution. B1-c provides a textbook example of compact H 2 18 O (3 1,3 −2 2,0 ) and HDO (3 1,2 −2 2,1 ) emission surrounded by a ring of H 13 CO + ( J = 2−1) and HC 18 O + ( J = 3−2). Compact HDO surrounded by H 13 CO + is also detected toward B1-bS. The optically thick main isotopologue HCO + is not suited to trace the snowline, and HC 18 O + is a better tracer than H 13 CO + due to a lower contribution from the outer envelope. However, because a detailed analysis is needed to derive a snowline location from H 13 CO + or HC 18 O + emission, their true value as a snowline tracer will lie in the application in sources where water cannot be readily detected. For protostellar envelopes, the most straightforward way to locate the water snowline is through observations of H 2 18 O or HDO. Including all subarcsecond-resolution water observations from the literature, we derive an average burst interval of ∼10,000 yr, but high-resolution water observations of a larger number of protostars are required to better constrain the burst frequency. 

   more » « less