Three-dimensional active defect loops

We describe the flows and morphological dynamics of topological defect lines and loops in three-dimensional active nematics and show, using theory and numerical modeling, that they are governed by the local profile of the orientational order surrounding the defects. Analyzing a continuous span of defect loop profiles, ranging from radial and tangential twist to wedge ± 1 / 2 profiles, we show that the distinct geometries can drive material flow perpendicular or along the local defect loop segment, whose variation around a closed loop can lead to net loop motion, elongation, or compression of shape, or buckling of the loops. We demonstrate a correlation between local curvature and the local orientational profile of the defect loop, indicating dynamic coupling between geometry and topology. To address the general formation of defect loops in three dimensions, we show their creation via bend instability from different initial elastic distortions

Microfluidic control over topological states in channel-confined nematic flows

Compared to isotropic liquids, orientational order of nematic liquid crystals makes their rheological properties more involved, and thus requires fine control of the flow parameters to govern the orientational patterns. In microfluidic channels with perpendicular surface alignment, nematics discontinuously transition from perpendicular structure at low flow rates to flow-aligned structure at high flow rates. Here we show how precise tuning of the driving pressure can be used to stabilize and manipulate a previously unresearched topologically protected chiral intermediate state which arises before the homeotropic to flow-aligned transition. We characterize the mechanisms underlying the transition and construct a phenomenological model to describe the critical behaviour and the phase diagram of the observed chiral flow state, and evaluate the effect of a forced symmetry breaking by introduction of a chiral dopant. Finally, we induce transitions on demand through channel geometry, application of laser tweezers, and careful control of the flow rate

Topology of three-dimensional active nematic turbulence confined to droplets

Active nematics contain topological defects which under sufficient activity move, create and annihilate in a chaotic quasi-steady state, called active turbulence. However, understanding active defects under confinement is an open challenge, especially in three-dimensions. Here, we demonstrate the topology of three-dimensional active nematic turbulence under the spherical confinement, using numerical modelling. In such spherical droplets, we show the three-dimensional structure of the topological defects, which due to closed confinement emerge in the form of closed loops or surface-to-surface spanning line segments. In the turbulent regime, the defects are shown to be strongly spatially and time varying, with ongoing transformations between positive winding, negative winding and twisted profiles, and with defect loops of zero and non-zero topological charge. The timeline of the active turbulence is characterised by four types of bulk topology-linked events --- breakup, annihilation, coalescence and cross-over of the defects --- which we discuss could be used for the analysis of the active turbulence in different three-dimensional geometries. The turbulent regime is separated by a first order structural transition from a low activity regime of a steady-state vortex structure and an offset single point defect. We also demonstrate coupling of surface and bulk topological defect dynamics by changing from strong perpendicular to inplane surface alignment. More generally, this work is aimed to provide insight into three-dimensional active turbulence, distinctly from the perspective of the topology of the emergent three-dimensional topological defects.Comment: 7 figure

Broken particle-hole symmetry at atomically flat a-axis YBa2Cu3O7-d interfaces

We have studied quasiparticle tunneling into atomically flat a-axis films of YBa2Cu3O7-d and DyBa2Cu3O7-d through epitaxial CaTiO3 barriers. The junction heterostructures were grown by oxide molecular beam epitaxy and were carefully optimized using in-situ monitoring techniques, resulting in unprecedented crystalline perfection of the superconductor/insulator interface. Below Tc, the tunneling conductance shows the evolution of a large unexpected asymmetrical feature near zero bias. This is evidence that superconducting YBCO crystals, atomically truncated along the lobe direction with a titanate layer, have intrinsically broken particle-hole symmetry over macroscopically large areas.Comment: 15 pages, 4 figures; v2 includes minor changes in concluding paragraph to match PRL versio

Global optimization of freeform support structures

Postojeći su arhitektonski projekti usmjereni konstrukcijama slobodnih i neodređenih oblika. Veza između onoga što se želi postići projektom i izrade predstavlja izazov kod izvedbe potporne konstrukcije koja je geometrijski održiva, a trebala bi zadovoljiti određene estetske, izvedbene, toplinske zahtjeve i one koji se odnose na čvrstoću. Kako bi se osigurao kontakt rubova dodirnih izolacijskih panela cijelom njihovom debljinom, rubovi moraju biti odrezani pod različitim kutovima što dovodi do razlika u visini vrhova kuta te razlika u položaju unutarnjih metalnih ploča izolacijskih panela. Osnovni cilj predstavljenog istraživanja je razvoj postupka optimizacije pomoću kojega će se postići minimalna razlika u visini spoja kod svih spojišta, uzimajući u obzir sve površine slobodnog oblika pojedinog arhitektonskog projekta. Da bi se nadoknadile zaostale razlike u visini spojeva predlaže se korištenje držača razmaka različitih debljina. Mreže uglavnom četverokutne strukture konusnih svojstava zahtijevaju optimizaciju visine vrhova kutova kako bi se kod svih greda postigle približno minimalne razlike u visini spojeva. U članku se razmatra globalna minimizacija razlika u visini spojeva na uzorku arhitektonskog projekta slobodnog oblika, s mrežom uglavnom četverokutne strukture s konusnim svojstvima. Usporedba razlika u visini spojeva prije i poslije optimizacije pokazuje znatno poboljšanje.The trends in current architectural design are leading towards structures with free and irregular forms. The connection between the design intent and the fabrication presents a challenge when creating a support structure that is geometrically viable and should possess certain aesthetics, fabricational, thermal and strength requirements. To ensure the contact of edges of neighborhood insulation panels along their thickness, their edges must be cut under different angles which cause the differences in vertex heights and further the differences of the positions of the inner metal sheets of the insulation panels. The main goal of the presented research is the development of the optimization procedure by which the minimal joint height differences will be achieved in all the joints, taking into account all free form surfaces of the individual architectural design. To compensate for the residual joint height differences the usage of spacers of different thicknesses is proposed. Quad-dominant meshes with conical properties require optimization of the vertex heights to align all beams at approximately minimal joint height differences. The paper considers global minimization of joint height differences for a sample of free form architectural design, meshed with quad-dominant mesh with conical properties. The comparison of the joint height differences before and after the optimization shows the substantial improvement

Sculpting stable structures in pure liquids

Pure liquids in thermodynamic equilibrium are structurally homogeneous. In liquid crystals, flow and light pulses are used to create reconfigurable domains with polar order. Moreover, through careful engineering of concerted microfluidic flows and localized optothermal fields, it is possible to achieve complete control over the nucleation, growth, and shape of such domains. Experiments, theory, and simulations indicate that the resulting structures can be stabilized indefinitely, provided the liquids are maintained in a controlled nonequilibrium state. The resulting sculpted liquids could find applications in microfluidic devices for selective encapsulation of solutes and particles into optically active compartments that interact with external stimuli

Spontaneous magnetization and Hall effect in superconductors with broken time-reversal symmetry

Broken time reversal symmetry (BTRS) in d wave superconductors is studied and is shown to yield current carrying surface states. The corresponding spontaneous magnetization is temperature independent near the critical temperature Tc for weak BTRS, in accord with recent data. For strong BTRS and thin films we expect a temperature dependent spontaneous magnetization with a paramagnetic anomaly near Tc. The Hall conductance is found to vanish at zero wavevector q and finite frequency w, however at finite q,w it has an unusual structure.Comment: 7 pages, 1 eps figure, Europhysics Letters (in press