Uros Tkalec

Uros Tkalec
University of Ljubljana · Institute of Biophysics

PhD in Physics

About

32
Publications
7,506
Reads
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2,230
Citations
Introduction
Uroš Tkalec is currently working at three research institutions in Slovenia: Faculty of Medicine, University of Ljubljana, Faculty of Natural Sciences and Mathematics, University of Maribor, and Jožef Stefan Institute. He is specialized in experimental soft matter, ranging from nematic colloids to liquid crystal microfluidics, and laser tweezers manipulation.
Additional affiliations
January 2015 - present
Jožef Stefan Institute
Position
  • Research Associate
February 2012 - present
University of Maribor
Position
  • Professor (Assistant)
Education
November 2005 - June 2010
Jožef Stefan International Postgraduate School
Field of study
  • Physics, Soft Matter, Nanotechnology

Publications

Publications (32)
Article
Full-text available
Topological defects in the orientational order that appear in thin slabs of a nematic liquid crystal, as seen in the standard schlieren texture, behave as a random quasi–two-dimensional system with strong optical birefringence. We present an approach to creating and controlling the defects using air pillars, trapped by micropatterned holes in the s...
Article
Full-text available
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,...
Article
Full-text available
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...
Article
Full-text available
Achiral building blocks forming achiral structures is a common occurrence in nature, while chirality emerging spontaneously from an achiral system is usually associated with important scientific phenomena. We report on the spontaneous chiral symmetry-breaking phenomena upon the topographic confinement of achiral lyotropic chromonic liquid crystals...
Article
Full-text available
The ability to control both the mobility and chemical compositions of microliter-scale aqueous droplets is an essential prerequisite for next-generation open surface microfluidics. Independently manipulating the chemical compositions of aqueous droplets without altering their mobility, however, remains challenging. In this work, we address this cha...
Preprint
Full-text available
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 opto-thermal fields, it is possible to achieve complete control over the nucleation, growth,...
Preprint
Full-text available
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...
Article
Full-text available
Nematic braids are reconfigurable knots and links formed by the disclination loops that entangle colloidal particles dispersed in a nematic liquid crystal. We focus on entangled nematic disclinations in thin twisted nematic layers stabilized by 2D arrays of colloidal particles that can be controlled with laser tweezers. We take the experimentally a...
Article
Full-text available
We present a brief overview of recent development in the field of nematic colloids with an emphasis on the topology of colloidal structures and recently discovered topologically nontrivial defect configurations. Nematic colloids are complex soft-matter systems, in which the topology of defects, induced by colloidal inclusions, can be controlled and...
Article
Full-text available
We explore the flow of a nematic liquid crystal in microfluidic channels with a rectangular cross section through experiments and numerical modeling. The flow profile and the liquid crystal orientational profile show three distinct regimes of weak, medium, and strong flow as the driving pressure is varied. These are identified by comparing polarizi...
Conference Paper
Nematic braids formed by disclinations entangling colloidal particles in chiral and achiral nematic liquid crystals are geometrically stabilized and restricted by topology. We report how self-linking number enables a classification of entangled defect lines [1, 2] and how a simple rewiring scheme for the orthogonal crossing of two half integer disc...
Article
Full-text available
Tying knots and linking microscopic loops of polymers, macromolecules, or defect lines in complex materials is a challenging task for material scientists. We demonstrate the knotting of microscopic topological defect lines in chiral nematic liquid-crystal colloids into knots and links of arbitrary complexity by using laser tweezers as a micromanipu...
Article
Full-text available
We study the flow of a nematic liquid crystal through microchannels possessing degenerate planar anchoring conditions on the channel walls. Depending on the channel dimensions and the flow rate, the formation of different textures and topological defect structures is observed and studied using polarizing optical microscopy and fluorescence confocal...
Article
Full-text available
We have studied theoretically and experimentally the effects of various types of nanoparticles (NPs) on the temperature stability range [Formula: see text] T (BP) of liquid-crystalline (LC) blue phases. Using a mesoscopic Landau-de Gennes type approach we obtain that the defect core replacement (DCR) mechanism yields in the diluted regime [Formula:...
Article
Full-text available
The phase transition behaviour of the chiral liquid crystal CE6 doped with spherical surface-functionalised CdSe nanoparticles has been examined by means of high-resolution adiabatic scanning calorimetry and polarising microscopy. The addition of nanoparticles results in an essentially stabilised blue phase III. The phase diagram is displayed upon...
Article
Full-text available
Using an atomic force microscopy, we have measured the separation dependence of the force between an atomically flat mica sheet and a micrometer-sized glass sphere immersed in the nematic liquid crystal. As the mica surface induces a strong parallel alignment and the treated glass sphere induces a strong perpendicular alignment on the liquid crysta...
Article
Full-text available
We show that chiral ordering of the underlying complex fluid strongly influences defect formation and colloidal interactions. Nonsingular defect loops with a topological charge -2 are observed, with a cross section identical to hyperbolic vortices in magnetic systems. These loops are binding spontaneously formed pairs of colloidal particles and dim...
Article
Full-text available
The interactions between different types of colloidal particles are measured and analyzed. We use these interactions to build different self-assembled microstructures, such as dimers, chains, wires, crystals and superstructures. In the experiments we have used different size, different symmetry of colloids (elastic dipoles and quadrupoles) and diff...
Article
Full-text available
We present an experimental analysis of the topological properties of a nematic liquid crystal in the vicinity of small cylindrical objects (micro-rods), dispersed in the nematic liquid crystal 5CB (4-n-pentyl-4-cyanobiphenyl). Depending on the type of liquid crystal anchoring on the surfaces of micro-rods, we have observed two types of the symmetry...
Article
Full-text available
We describe and analyze experiments, where optical manipulation of small colloidal particles in the nematic liquid crystal (NLC) was used to create artificial colloidal structures, such as 1D chains and 2D colloidal crystals, and superstructures of different types of colloids. In all cases, the colloidal particles are strongly bound to each other,...
Article
Full-text available
We show that colloidal superstructures could be assembled in mixtures of large and small colloidal particles dispersed in a nematic liquid crystal. Using elastic interaction of small colloidal particles with the disclination lines we succeed to demonstrate how one can decorate with small particles a topological matrix of defect rings and loops form...
Article
Full-text available
We present experimental and theoretical study of colloidal interactions in quadrupolar nematic liquid crystal colloids, confined to a thin planar nematic cell. Using the laser tweezers, the particles have been positioned in the vicinity of other colloidal particles and their interactions have been determined using particle tracking video microscopy...
Article
Colloidal structures assembled in confined nematic liquid crystals are examined. Theoretical predictions based on Landau-type approaches are complemented with the latest studies of laser assisted colloidal assembling. Effective colloidal interactions are particularly sensitive to the confinement and external fields. Their complexity leads to numero...
Article
Full-text available
It has been predicted, but never confirmed, that colloidal particles in a nematic liquid crystal could be self-assembled by delocalized topological defects and entangled disclinations. We show experimentally and theoretically that colloidal dimers and 1D structures bound by entangled topological defect loops can indeed be created by locally thermal...
Article
Full-text available
We study the interactions and directed assembly of dipolar nematic colloidal particles in planar nematic cells using laser tweezers. The binding energies for two stable configurations of a colloidal pair with homeotropic surface alignment are determined. It is shown that the orientation of the dipolar colloidal particle can efficiently be controlle...
Article
Full-text available
In a thin planar nematic cell, the application of an AC electric field induces a macroscopic transport of micrometer-sized colloidal particles along the nematic director. We have analyzed the dependence of particle velocities on the electric-field amplitude and frequency and found that it decreases exponentially with increasing frequency. Using spe...
Conference Paper
The ability to generate regular spatial arrangements of particles on different length scales is one of the central issues of the "bottom-up" approach in nanotechnology. Current techniques rely on single atom or molecule manipulation by the STM, colloidal particle manipulation by laser or optoelectronic tweezers, microfluidics, optofluidics, microma...
Article
Full-text available
We present a 27 Al NMR study of three giant-unit-cell complex metallic compounds, -Al 3 Mg 2 , the "Bergman-phase" Mg 32 Al, Zn 49 , and -Al-Pd-Mn, which contain some hundreds up to more than a thousand atoms in the unit cell. The NMR spectra of monocrystalline samples are strongly inhomogeneously broadened by the electric quadrupole interaction an...
Article
We present a 27Al NMR study of three giant-unit-cell complex metallic compounds, β-Al3Mg2, the “Bergman-phase” Mg32(Al,Zn)49, and ξ′-Al-Pd-Mn, which contain some hundreds up to more than a thousand atoms in the unit cell. The NMR spectra of monocrystalline samples are strongly inhomogeneously broadened by the electric quadrupole interaction and the...
Article
Full-text available
The ability to generate regular spatial arrangements of particles is an important technological and fundamental aspect of colloidal science. We showed that colloidal particles confined to a few-micrometer-thick layer of a nematic liquid crystal form two-dimensional crystal structures that are bound by topological defects. Two basic crystalline stru...

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