Physics of soft matter, surfaces, and nanostructures

Light control by topological solitons in chiral nematics

Topological solitons appearing in different areas of physics are fascinating localized perturbations of ordering fields enjoying topological protection.  We demonstrate refraction, reflection, and lensing of weak laser beams by various topological solitons in frustrated chiral nematic liquid crystals. In particular we show how interactions of light with such topological solitons are well described using a generalized Snell’s law and ray-tracing models. These may lead to new means for controlling flow of light for use in optics and photonics. The study was done by group members from Jozef Stefan Institute and Faculty of Mathematics & Physics at University of Ljubljana in collaboration with the group of Ivan Smalyukh from University of Colorado in Boulder. The paper (Physical Review X, 2020, DOI: 10.1103/PhysRevX.10.031042) was highlighted in APS Physics Focus by the editorial article “Liquid-Crystal Vortices Focus Light”.

Fig. 1: Simulated green light beam lensing from a toron in the unwounded cholesteric layer. Intensity of green color corresponds to the intensity of light. White lines are obtained by simple ray tracing. The 100 mm toron with the director field illustrated in the inset is visualized with a white light polarization image.

Chirality-enhanced periodic self-focusing of light in frustrated chiral nematics

In achiral liquid crystals, the reorientation of molecules around laser beams with appropriate power is responsible for the generation of spatial optical solitons called “nematicons”. We present numerical, experimental, and theoretical evidence of a strong link between chirality and the nonlinear optical response of frustrated chiral liquid crystal systems. In unwounded chiral nematics, the frustration caused by a confinement incompatible with their lowest-energy states allows a relatively weak light to locally destabilize the initially uniform orientational fields and thus boost their nonlinear optical responses. This yields self-focusing and formation of similaritons. In short, our study opens, at a fundamental level, a new research paradigm of chirality-enhanced nonlinear optical effects, thus suggesting a range of self-focusing based applications such as low power light self-actuated flat lenses or all-optical information processing based on spatial optical solitons interacting with chiral topological solitons. The study was done in collaboration from Jozef Stefan Institute and Faculty of Mathematics & Physics at University of Ljubljana with the group of Ivan Smalyukh from University of Colorado in Boulder (Physical Review Letters, 2020, DOI: 10.1103/PhysRevLett.125.077801).

Fig. 2: Propagation of a tilted Gaussian light beam inside the unwounded cholesteric layer in the linear (a) and nonlinear – similariton (b) optical regime. The white bars represent 10 mm.

Wrinkling instability in 3D active nematics

In this experimental and theoretical study, we focus on a 3D active nematic consisting of microtubules, kinesin motors, and a depleting agent. It shows a rich dynamics evolving from a nematically ordered space-filling distribution of microtubule bundles toward a flattened and contracted 2D ribbon that undergoes a wrinkling instability and subsequently transitions into a 3D active turbulent state. The interplay between depletion forces and kinesin motors thus leads to both a contractile stress that compresses the ribbon and an extensile stress that leads to the wrinkling instability. At the macroscopic scale, we used a continuum theory to explain the instability and predict the wavelength of the wrinkles that form. We supplemented the results with a detailed simulation at the molecular scale (using the Cytosim package) that reproduced the dynamics based on the properties of single kinesin motors. We found that the produced forces nearly cancel out, but a small asymmetry in the distribution leads to the buildup of the extensile stress. Whereas the wrinkle wavelength strongly correlates with their formation time, as predicted by the continuum theory, it is independent of the ATP concentration. The 3D to quasi-2D and back to 3D transition in our system represents a novel path of self-organization in active soft matter. The study was mostly performed at Max Planck Institute for Dynamics and Self-Organization in Göttingen (Nano Letters, 2020, DOI:10.1021/acs.nanolett.0c01546).

Fig. 3: Nematically aligned bundles of microtubules (green) first condense vertically into a ribbon that subsequently forms wrinkles due to active extensile stress.

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 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. The work was  performed as a collaboration between the University of Warwick (Prof. G. Alexander),  the  Faculty of Mathematics and Physics at the University of Ljubljana and Department of Condensed Matter Physics at the Jozef Stefan Institute (Physical Review Letters, 2020, DOI: 10.1103/PhysRevLett.124.088001)

Fig. 4: Flows and dynamics of different active nematic defect loops with zero topological charge.

Microfluidic control over topological states in nematic liquid crystal flow

An interesting sequence of phase transitions between topologically different states in a pressure-driven liquid crystal flow was first characterized in detail by the experimental group at the Faculty of Medicine, University of Ljubljana. This attracted us to a collaboration, where with a theoretical analysis we were able to provide understanding of the mechanisms that lead to complex dynamics in such systems. This collaborative study completes our decade-long research in the field of nematofluidics and promises the application of these concepts in lyotropic and active nematic liquids (Nature Communications, 2020, DOI: 10.1038/s41467- 019-13789-9). With the invitation of the editors, the paper has been presented in a popular form on the Nature Portfolio Device and Materials Engineering Community website.

Fig.5: Transitions in the intermediate and strong pressure driven nematic flows

Topological-Defect-Induced Surface Charge Heterogeneities in Nematic Electrolytes

We show that topological defects in an ion-doped nematic liquid crystal can be used to manipulate the surface charge distribution on chemically homogeneous, charge-regulating external surfaces, using a minimal theoretical model. In particular, the location and type of the defect encodes the precise distribution of surface charges and the effect is enhanced when the liquid crystal is flexoelectric. We demonstrate the principle for patterned surfaces and charged colloidal spheres. More generally, our results indicate an interesting approach to control surface charges on external surfaces without changing the surface chemistry (Physical Review Letters, 2020, DOI: 10.1103/PhysRevLett.125.037801).

Fig. 6: Surface induced electric charge variability caused by nematic defects.

Speckle-free liquid crystal microscopy with nanosecond illumination

We present a setup for the speckle-free, low-light microscopy imaging at 5 ns exposure times. The design is based on stroboscopic principle and uses fast and incoherent fluorescent emission from a solution of a fluorescent dye, excited by a picosecond laser pulse. The proposed solution of Rhodamine in ethanol put in a glass cuvette gives an excellent image quality with high contrast, excellent stability and tunable coherence. The setup was used for the photographic measurements with 5 ns exposure time, demonstrated in imaging of thermally quenched 5CB liquid crystal, what enables studies of Kibble-Zurek mechanism of topological defect nucleation and growth at sub-microsecond time resolution and extremely fast cooling rates of ~40,000 K/s. The research was conducted in collaboration with the Faculty of Mathematics and Physics and Faculty of Pharmacy at University of Ljubljana. (Liquid Crystals, DOI: 10.1080/02678292.2020.1790049).

Fig. 7: Thermal quenching of liquid crystal with 5 ns exposure time.

Bright flower-like domains in inverse nematic gels

We discovered that mixing the gelator 12-HSA with a nematic liquid crystal results in creation of localised flower-like domains with higher concentration of the gelator than the surrounding medium. The gelator in these domains forms fibers, which interact with the molecules of the liquid crystal and stabilise its director into a spiral, toron-like structure. We examined their structure by optical and confocal microscopy. The nematic liquid crystal in the volume around the domains is achiral, but all the domains exhibit the same handedness. The research was conducted in collaboration with the Faculty of Mathematics and Physics at University of Ljubljana and Raman Institute in Bangalore. (Soft matter, 2020, DOI: 10.1039/C9SM02547B).

Fig. 8: Bright flower-like domains in the inverse nematic gel.

Nematic liquid-crystal necklace structure made by microfluidic system

We succeeded to fabricate a necklace structures made of liquid crystal droplets that are tens of micrometers in diameter and are connected by birefringent micro-tethers made of PVA- liquid crystal composite. Micro-tethers can be elastically stretched by applying external force and the elastic constant of the tether was determined by using laser tweezers. The Whispering Gallery Modes circulating inside individual droplets in the necklace structure were also observed. Research was done in collaboration with AIST in Tsukuba (Langmuir, DOI: 10.1021/acs.langmuir.0c00101).

Fig. 9: Two necklaces made of LC droplets connected by micro-tether.

Electric-field-induced reorientation of ferroelectric platelets in liquid crystal

We demonstrated that ferroelectric micro-platelets can be reoriented in the nematic liquid crystal by the linear coupling to an external electric field. The electric dipole moment of platelets is perpendicular to the plane of platelets and provides torque that rotates the platelets. The experiments were made in dispersions of platelets in a zero dielectric anisotropy nematic LC, which excludes the reorientation of the dispersion via the dielectric coupling of a LC. LC molecules are reoriented only via the rotation of platelets (Liquid Crystals, DOI: 10.1080/02678292.2020.1785026).

Fig. 10: Micro-platelets are rotated in LC by the external electric field.

Graphene derivatives, liquid crystal, and CdS/TiO2 hybrid matrices: optoelectronic and biotechnological aspects  

Different assemblies of nanomaterial and related new switchable device technologies are reviewed. Complex systems consisting of graphene and its derivatives, hydrogen-bonded liquid crystals, and semiconducting nanoparticles or nano-wires are addressed. Seamless stable assemblies are enabled by relatively strong hydrogen bonds. Of particular interest are configurations that can undergo sensitive stimulus-induced electro-optical changes between states exhibiting significantly different effective properties. Such assemblies could be exploited in flexible electronics, high contrast ratio smart displays, optoelectronic devices, sensors (monitoring inflammability, explosive nature, or toxicity of chemicals), bio-sensing, and antimicrobial applications. Cost-effective technologies, enabling large scalability and seamless heterogeneous integration are presented (Critical Reviews in Solid State and Materials Sciences, 2020, DOI: 10.1080/10408436.2020.1805295).

Fig. 11: Polarizing microscopy texture of a hybrid composite.

Reconfigurable multistable topological defect patterns

We report on robust theoretical and experimental investigations in which an external electric field is used to switch between pre-determined stable chargeless disclination patterns in a nematic cell. 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 polarised 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= 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 rewireable nanowires. Research was performed in collaboration with Case Western Reserve University Cleveland (Physical Review Reserach, 2020, DOI:  10.1103/PhysRevResearch.2.013176).

 Fig. 12: a) Typical defect configuration, b) bottom plate enforced nematic pattern.

Emergent collective colloidal currents generated via exchange dynamics in a broken dimer state

We investigated the dynamics of paramagnetic colloidal particles confined between two planes. When subjected to a precessing magnetic field, they show a rich phase diagram depending on the rotation frequency, cell thickness, precession angle and particle density. We used linear stability analysis to theoretically predict the boundaries between the phases. They include single particles on a hexagonal lattice, synchronously and asynchronously rotating particle pairs. Between them, a particularly interesting phase emerges where dimers form and break transiently. As a consequence, an edge current emerges where particles at one boundary move in one direction and those at the opposite boundary in the opposite direction. These results demonstrate how similar physical phenomena can exist across a range of length scales. The research was conducted in the collaboration of researchers from University of Barcelona Max Planck Institute for Dynamics and Self-Organization in Göttingen (Science Advances, 2020, DOI:10.1126/sciadv.aaz2257).

Fig. 13: Trajectories of magnetic particles in a precessing field. Particles in the proximity of both planes move in opposite directions (blue and yellow traces).

Generation of microdroplets and microbeads with nanometer precision

We demonstrated that it is possible to produce small droplets and solid spheres of an unprecedented monodispersity of 1 nm and 20 nm, respectively. For the droplets the corresponding coefficient of variation (CV) is only 0.0042 %, which improves the size precision by three orders of magnitude compared to standard production methods such as reported in the microfluidics. Encoding of short words and numbers has been demonstrated by producing three beads with predefined sizes. The stored information has been read from the emitted spectrum. The potential to store information on such small scale, which can be read purely through the spectrum, has the potential for barcoding various products, for security applications and even for tagging and tracking of individual live cells. The article that was featured on the back cover of the journal Lab on a Chip was done in collaboration with the Harvard Medical School (Lab on a Chip, 2020, DOI: 10.1039/C9LC01034C).

Fig. 14:  Graphic illustration of four groups of microbeads, each with embedded information, which can be read by a laser

Blood flow limits endothelial cell extrusion in the zebrafish dorsal aorta

We studied the formation of the dorsal aorta, the main artery in the developing zebrafish embryo. We combined confocal imaging on live embryos with a theoretical prediction of stress distribution in the tissue surrounding the blood vessel. The latter is amplified in the proximity of another blood vessel, but reduced in the proximity of the stiff notochord. In particular, we show that cells migrate towards a direction that coincides with the maximum pulsatile stress during heartbeat. On the other hand, reduced blood flow facilitates cell extrusion, pointing to a mechanism that regulates the blood vessel diameter. Our study demonstrates how the formation and growth of blood vessels depend on an intricate interplay of chemical signals, tissue mechanics and fluid dynamics. The research was conducted in the collaboration of researchers from Université de Strasbourg and Max Planck Institute for Dynamics and Self-Organization in Göttingen (Cell Reports, 2020, DOI:10.1016/j.celrep.2020.03.069).

Fig. 15: Theoretical prediction of mechanical stress in the tissue surrounding the dorsal aorta (DA).

Control of viscosity in biopharmaceutical protein formulations

Controlling  the viscosity of concentrated protein solutions — usually reducing — is an open challenge, with major recent relevance in protein formulations for biopharmaceutical, medical, food, and other applications. It is of major importance to be able to establish control over the combination of viscosity-affecting additives and adequate protein stability, usually at high protein concentrations. Here, we demonstrate the control and manipulation of the viscosity profile of a selected protein solution (monoclonal antibody of immunoglobulin gamma type – IgG) of direct biopharmaceutical relevance, by identifying elementary viscosity contributions via selected additives that target different protein-protein interactions. Specifically, a combined study of viscosity control and protein aggregation is performed with viscosity characterized by microfluidic measurements and protein aggregation by size-exclusion chromatography, where aggregation data is further supplemented with conformational stability measurements via thermal and chemical protein denaturation. More generally, we show a control over the interplay of viscosity, potency and stability in a distinct protein system, as a general contribution to understanding the viscosity in different colloidal, biological, and  soft materials. The work was  performed as colaboration beween the Lek Pharmaceuticals d.d, part of Novartis,  the Faculty of Mathematics and Physics and the Biotechnical Faculty at the University of Ljubljana and Department of Condensed Matter Physics at the Jozef Stefan Institute (J. Coll. Int. Sci., 2020, DOI: 10.1016/j.jcis.2020.06.105)

Fig. 16: Control of viscosity in biopharmaceutical protein formulations via design of different protein interactions.

Antimicrobial coatings based on MoO₃ nanowires

A novel antimicrobial nanocomposite was designed from inert biocompatible PVDF-HFP and water-soluble PVP polymers with incorporated MoO₃ nanowires. Dissolving in water in concentration of 5mg/ml, it lowers the pH value to 4.6 in 5 min. Antimicrobial activity studied in collaboration with Biotechnical Faculty University in Ljubljana was explained by two-step action; in the first stage, MoO₃ dissolves, causing a drop in pH, which then triggers the hydrolysis of the PVP polymer and the release of the ammonium salt. Complete deactivation of Staphylococcus aureus, Listeria monocytogenes, Escherichia coli and Pseudomonas aueroginosa was achieved within 6 h, and deactivation of Penicilium verrucosum and Pichia anomala within 24 h (Surface Innovations 9(5) 2021, DOI: 10.1680/jsuin.20.00073 )

Fig. 17: Schematic illustration of antimicrobial activation of PVDF-HFP/PVP/MoO₃ nanowires composite.

New quasi-two-dimensional W_nO_3n−1 crystals

We synthesized new quasi-two-dimensional tungsten oxide crystals, which nucleate by epitaxial growth on the W₁₉O₅₅ nanowires.  In a single platelet, several stoichiometric phases were identified for the first time: W₁₈O₅₃, W₁₇O₅₀, W₁₆O₄₇, W₁₅O₄₄, W₁₄O₄₁, and W₉O₂₆. The structure was directly resolved from the high-resolution electron microscopy images and modelled using electron and X-ray diffraction data. These layered crystals show a new kind of polycrystallinity, where crystallographic shear planes accommodate oxygen deficiency and at the same time contribute to the stability of a particular phase. (Nanoscale, 2020, DOI: 10.1039/D0NR02014A)

Fig.18: HRTEM image and its simulation of the W₁₆O₄₇ phase with pentagonal and hexagonal tunnels at crystallographic shear planes.The scale bar is 1 nm

Orientational ordering of point dipoles on a sphere*

Arrangement of interacting particles on a sphere is a well-known problem, however, ordering of particles with anisotropic interaction, such as the dipole-dipole interaction, has remained unexplored. We calculate the equilibrium orientational ordering of point dipoles on a sphere with fixed positional order. With  a numerical minimization of interaction energy we analyze stable configurations depending on their symmetries and degrees of ordering. We find that a macro-vortex is a generic ground state for locally triangular spherical lattices with various discrete rotational symmetries for different system sizes. Higher-energy metastable states are similar but less ordered. We observe orientational phase transitions and hysteresis in response to changing external field both for the fixed sphere orientation with respect to the field as well as for a freely rotating sphere. In case of a freely rotating sphere, we observe changes in the symmetry axis with increasing field strength (Physical Review B, 2020, DOI: 10.1103/PhysRevB.102.075416).

Fig. 19: (a) 3D visualization of the ground state of 60 dipoles shows a macro-vortex with the axis oriented along the blue arrow. (b) The vortex shown in the azimuthal projection. (c) The ground state of dipoles on the hexagonal lattice is also a macro-vortex.

Spatial ordering of the charge density waves in NbSe₃

The ordering of the two incommensurate charge density waves (CDW) in the quasi-one-dimensional NbSe₃ structure was studied using low-temperature scanning tunneling microscopy (PRB 102, 075442 (2020)). Larger (100) van der Waals surfaces were analyzed using one-dimensional Fourier transforms along the trigonal prismatic columns. The procedure enabled unambiguous differentiation between both CDWs modulating individual columns and allowed quantitative comparison of modulation amplitudes along different columns of the same type. The results suggest the formation of CDW nanodomains. The possibility of interchanging both CDWs along columns forming symmetry-related pairs results in a charge difference, which is supposed to be the possible origin of CDW sliding. The paper was published in collaboration with a researcher from Canada (Physical Review B, 2020, DOI: 10.1103/PhysRevB.102.075442)

Fig. 20: STM image of the surface of NbSe₃ shown with the uppermost Se atoms and a DFT simulated STM image

The ultra-high-TP charge-density wave in the monoclinic phase of NbS₃

We reported on high-temperature studies of electrical conductivity of the monoclinic phase of NbS₃ type-II (J. Alloys Compd 854, 157098 (2020)). The compound was shown to be stable up to temperature T ≈ 550 K. At T_P0 ≈ 450 – 475 K a step-like growth of conductivity was observed, a clear evidence that T_P0 is the temperature of a third, high-temperature CDW formation. The synchronization at moderate frequencies, 10 – 50 MHz, demonstrates coherent sliding of CDW-0. The charges condensed in this CDW show relatively high density and, at the same time, extremely low mobility. Their mobility appears low in the single-particle states as well, giving a plausible clue to the surprising dielectric-like temperature variation of σ above T_P0. The paper was published in collaboration with groups from Taiwan and Russia (Journal of Alloys and Compounds, 2020,  DOI: 10.1016/j.jallcom.2020.157098).

Fig. 21 Temperature dependence of I-V curves with and without the RF irradiation applied

Spontaneous Antiferromagnetic Ordering in a Single Layer of (BETS)₂GaCl₄ Organic Superconductor

The fabrication of well-defined and atomically clean interfaces between materials of different orders are of fundamental interest to engineer novel functionalities and to study emergent phenomena in condensed matter physics. Our study was focused on the interplay between electronic orders of hybrid mix of nested antiferromagnetic molecular chains and superconducting molecular stripes at the single layer limit. Results of low temperature scanning tunneling microscopy and spectroscopy have shown that low level vibronic and magnetic excitations that dominate the higher temperature phase are absent below T_c which point to their cooperative existence and possible renormalization to mediate superconductivity in such d-wave superconductors. The paper was published in collaboration with the group at Nihon University, Tokyo (Advanced Electronic Materials, 2020, DOI: 10.1002/aelm.202000461)

Fig. 22: Hybrid order of two ground states where molecular magnetic chains are nested within a single layer organic superconductor.

Air pollution by nanoparticles released during football match

We measured air pollution with nanoparticles during a derby between the football clubs NK Olimpija Ljubljana and NK Maribor on March 2019 in Stožice. The fan groups of both teams, Green Dragons and Viola Maribor, lit torches and pyrotechnics in support of their clubs, despite the ban. The number of nanoparticles ranging in size from 30 nm to 300 nm in the air when the torches were lit increased by 1.200 %, and the players inhaled 300 % more particles than in a low-pollution environment. Chemical analysis showed that in addition to carbon, there were also elements that were potentially toxic and used for colour effects and as fuel: strontium (red), barium (green), potassium, magnesium and chlorine (Atmospheric Environment 2020, DOI: 10.1016/j.atmosenv.2020.117567).

Fig. 23: Use of flares and pyrotechnics during the match caused up to 12-fold increase in number concentration of nanoparticles in air.