Iskanje po katalogu digitalne knjižnice

Opis: 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 �=±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.
Ključne besede: line defects, topological defects, nematic liquid crystals, electric field, atomic force microscopy, numerical techniques, polarized optical microscopy
Objavljeno v DKUM: 18.11.2024; Ogledov: 0; Prenosov: 4
Celotno besedilo (3,79 MB)
Gradivo ima več datotek! Več...

Opis: We study numerically the reconfiguration process of colliding |m|=1/2 strength disclinations in an achiral nematic liquid crystal (NLC). A Landau–de Gennes approach in terms of tensor nematic-order parameters is used. Initially, different pairs {m1,m2} of parallel wedge disclination lines connecting opposite substrates confining the NLC in a plane-parallel cell of a thickness h are imposed: {1/2,1/2}, {−1/2,−1/2} and {−1/2,1/2}. The collisions are imposed by the relative rotation of the azimuthal angle θ of the substrates that strongly pin the defect end points. Pairs {1/2,1/2} and {−1/2,−1/2} “rewire” at the critical angle θ(1)c=3π/4 in all cases studied. On the other hand, two qualitatively different scenarios are observed for {−1/2,1/2}. In the thinner film regime ℎ<ℎc, the disclinations rewire at θ(2)c=5π/4. The rewiring process is mediated by an additional chargeless loop nucleated in the middle of the cell. In the regime ℎ>ℎc, the colliding disclinations at θ(2)c reconfigure into boojum-like twist disclinations.
Ključne besede: liquid crystals, topological defects, disclinations, reconfiguration
Objavljeno v DKUM: 28.03.2024; Ogledov: 177; Prenosov: 25
Celotno besedilo (7,16 MB)
Gradivo ima več datotek! Več...

Opis: V diplomskem seminarju obravnavamo difuzijsko omejeno agregacijo. Predstavimo matematični model, s katerim generiramo agregate, ki jih karakteriziramo s parametri, kot so število delcev, temperatura, velikost delcev, razdalja med robom in agregatom ter jakost zunanjega polja. Prikažemo vpliv različnih parametrov na kvalitativne lastnosti agregatov s poudarkom na fraktalni strukturi.
Ključne besede: difuzijsko omejena agregacija, model brez mreže, Brownovo gibanje, fraktal, fraktalna dimenzija
Objavljeno v DKUM: 22.10.2014; Ogledov: 3998; Prenosov: 237
Celotno besedilo (1,85 MB)