Coalescence of droplets on liquid-infused surfaces has been extensively investigated for isotropic lubricants, where interfacial and hydrodynamic responses are well described by geometry-based and mass-spring models. However, the corresponding dynamics on anisotropic lubricating films, such as liquid crystals (LCs), remain largely unexplored. In this work, we report the use of high-speed imaging to study the attraction and coalescence of millimetre-sized water droplets on two classes of substrates, covered with a thin LC overlayer: LC-infused textured surfaces (LCITS) and LC-infused porous surfaces (LCIPS). On both substrates, the droplets coalesce in three stages over approximately one minute: long-range capillary-mediated attraction, drainage of the lubricant within the wetting ridge, and final merging accompanied by in-plane oscillations of the formed droplet. On LCITS, the initial approach velocities and post-merging dynamics are broadly consistent with the geometry-based mass-spring model developed for oil-impregnated surfaces of a similar type. However, on LCIPS, where a thicker lubricating film produces a larger wetting ridge, we observe substantially reduced attraction and merging velocities, no oscillations were resolved within our temporal resolution at the first velocity peak, and drainage times strongly influenced by evaporation. In the final stage, the peak velocity mainly depends on the LC mesophase and is nearly independent of droplet size, while the oscillation period scales approximately with the square root of the droplet radius. These results clarify how the porous LC scaffold and enlarged wetting ridge alter droplet-droplet interactions and coalescence dynamics relative to textured silicone substrates.

Topological defects in liquid crystals (LCs) emerge from elastic distortions of the director field under confinement, but the influence of geometric confinement in open microstructured systems remains poorly understood. Here, we both experimentally and computationally investigate how pillar spacing and spatial distribution govern defect formation in nematic LCs confined within micropillar arrays. By varying the lattice spacing from 10 to 60 µm and modulating interstitial space, we decouple local anchoring effects from global geometric constraints. We identify distinct regimes of defect behavior. Specifically, strong confinement suppresses isolated point defects and favors disclination lines, whereas weak confinement yields independent pillar-bound -1 defects. At intermediate spacings, defects delocalize from pillars and stabilize within interstitial regions, leading to the formation of both +1 and -1 defects. These states are not present in uniformly distributed arrays with identical spacing, highlighting the importance of free volume connectivity. These findings establish geometric confinement as a key parameter for programming topological states in LCs.

Understanding the dynamics of topological defects in liquid crystals is essential for optimizing their performance in adaptive optics, responsive surfaces, and advanced display technologies. Here, we investigate the dynamics of disclination loops enclosing an escaped structure in a nematic liquid crystal, known as dowser domains, within microfluidic channels of various geometries. Through a combination of experiments and numerical simulations, we demonstrate that fluid flow, dictated by the channel geometry alone, governs the dynamics, shape, and size of these domains. We find that channel constrictions extend the lifetime of dowser domains by accelerating their growth, while channel expansions slow down their dynamics and shorten their lifetime. In addition, manipulating the flow paths of dowser domains through serpentine microchannels can further influence their shape and lifespan. We also demonstrate domain splitting in a T-junction microchannel. These findings pave the way for the design of hierarchical networks that can manipulate dowser domains in high-throughput parallel channel systems. Taken together, the results presented here improve our understanding of defect loop dynamics in soft materials and advance the development of flow-based liquid crystal devices and applications.

The ability to manipulate polar entities with multiple external fields could enable functionalities and applications in spin systems, photonics, metamaterials and soft matter. Liquid crystals that exhibit both a crystalline structure and liquid fluidity represent a promising platform for manipulating phases with polar molecular order, notably ferroelectric ones. However, achieving a polar symmetry is challenging with rod-shaped liquid crystal molecules, which form predominantly apolar nematic phases. Here we report an approach in which a geometric lattice confinement of nematic liquid crystals is used to induce planar polar order on the scale of a mesoscopic metamaterial. We confine the nematic liquid crystal in a micropillar array, forming topological defect-pillar pairs of elastic dipoles with a free top interface in contact with an immiscible fluid. The resulting dipole lattice configurations can be programmed rheologically by flowing the top fluid and maintained even after flow cessation, a phenomenon attributed to orientational multistability of the dipoles. This multimemory effect enables the encoding and reconfiguration of directional information. Overall, these results advance our understanding of topological dipoles under confinement and shear flow, enabling the detection, tracking and recording of flow profiles and could facilitate the development of stimuli-responsive materials.

V prispevku so predstavljeni rezultati raziskav o kemijski aktivnosti mikrolitrskih kapljic vode na spolzkih poroznih površinah, ki temeljijo na anizotropnih lastnostih tekočih kristalov. Spolzke, s tekočinami impregnirane mikroporozne površine, ki odbijajo tako vodo kot olje, so zadnje desetletje v ospredju interdisciplinarnih raziskav [1-3]. Zaradi izjemnih fizikalnih lastnosti, kot so odpornost na zmrzal, samo-celjenje, optična prosojnost in funkcionalnost pri visokih tlakih, take površine obetajo številne možnosti uporabe, med drugim v medicini, pri dezinfekciji in čiščenju embalaže, prevlekah površin in pri preciznem zaznavanju kemikalij. Lani so bili izdelani prvi primerki takih površin na osnovi tekočih kristalov [4, 5], ki omogočajo selektivno spreminjanje kemijske sestave kapljic vode glede na temperaturno nastavljivo mezofazo. Izkaže se, da na drsenje kapljic prvenstveno vpliva pozicijski red tekočekristalnih molekul ob stični plasti, medtem ko je prenos kemikalij med nemešljivima tekočinama odvisen od orientacijske urejenosti mezofaze. Polzeče kapljice vode na takih površinah lahko izjemno učinkovito odstranjujejo ione težkih kovin, kar nakazuje možnosti uporabe pri varovanju okolja, v biomedicinski diagnostiki in kemijski sintezi.

V prispevku so predstavljene aktualne raziskave na področju mikrofluidike tekočih kristalov, ki se v zadnjem času povezujejo z optotermično manipulacijo. Tokovne režime nematskega tekočega kristala raziskujemo v kanalčkih s pravokotnim površinskim sidranjem in ugotavljamo (meta)stabilnost posameznih orientacijskih stanj v odvisnosti od hitrosti toka. Uporaba laserske pincete nam omogoča kontrolirano tvorjenje mikroskopskih domen s polarnim redom, ki jih lahko transportiramo in prilagajamo z reguliranjem pretoka. Dinamika fazne meje, ki jo določa topološka defektna zanka, je izjemno občutljiva na gradiente v molekularnem in hitrostnem polju in zato zanimiva za senzorske aplikacije.

Magnetocontrollable droplet mobility on surfaces of both solids and simple fluids have been widely used in a wide range of applications. However, little is understood about the effect of the magnetic field on the wettability and mobility of droplets on structured fluids. Here, we report the manipulation of the dynamic behaviors of water droplets on a film of thermotropic liquid crystals (LCs). We find that the static wetting behavior and static friction of water droplets on a 4′-octyl-4-biphenylcarbonitrile (8CB) film strongly depend on the LC mesophases, and that a magnetic field caused no measurable change to these properties. However, we find that the droplet dynamics can be affected by a magnetic field as it slides on a nematic 8CB film, but not on isotropic 8CB, and is dependent on both the direction and strength of the magnetic field. By measuring the dynamic friction of a droplet sliding on a nematic 8CB film, we find that a magnetic field alters the internal orientational ordering of the 8CB which in turn affects its viscosity. We support this interpretation with a scaling argument using the LC magnetic coherence length that includes (i) the elastic energy from the long-range orientational ordering of 8CB and (ii) the free energy from the interaction between 8CB and a magnetic field. Overall, these results advance our understanding of droplet mobility on LC films and enable new designs for responsive surfaces that can manipulate the mobility of water droplets.

Liquid crystal surfaces enable manipulation of liquid mobility and cargo release via temperature, electrolytes, and light.

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.

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 in periodically arranged micrometer scale air pillars. The anisotropic fluid arranges into chiral domains that depend on the arrangement and spacing of the pillars. We characterize the resulting domains by polarized optical microscopy, support their reconstruction by numerical calculations, and extend the findings with experiments, which include chiral dopants. Well-controlled and addressed chiral structures will be useful in potential applications like programmable scaffolds for living liquid crystals and as sensors for detecting chirality at the molecular level.

EDITORIAL article Front. Phys., 11 September 2020Sec. Soft Matter Physics Volume 8 - 2020 | https://doi.org/10.3389/fphy.2020.00373

Topological quantum and classical materials can exhibit robust properties that are protected against disorder, for example for non-interacting particles and linear waves. In this colloquium, we review recent theoretical and experimental studies of topologically protected sound waves in mechanical metamaterials and active matter. Next, we demonstrate how to construct topologically protected states that arise from the combination of strong interactions and thermal fluctuations inherent to soft matter. Specifically, we consider fluctuating lines under tension (e.g. polymeric systems), subject to a class of spatially modulated substrate potentials. At equilibrium, the lines acquire a collective tilt proportional to an integer topological invariant called the Chern number. These results point to a new class of classical topological phenomena in which the topological signature manifests itself in an equilibrium structural property rather than a transport measurement. Bio: Vincenzo Vitelli has been a Professor in the Physics Department and the James Franck Institute at the University of Chicago since 2017. Previously he was a Professor in the Institute Lorentz for theoretical physics in Leiden University where he held the Chair of Condensed Matter Theory since 2015. Vitelli obtained a Bsc in theoretical physics from Imperial College London in 2000 and a Phd in Physics from Harvard University in 2006 working under the supervision of David Nelson. He was a post-doctoral fellow at Upenn from 2006 till 2009. He was awarded the H. Callan Prize in 2007, he received a Feinberg Foundation Fellowship (Weizmann Institute) in 2009 and was an invited Professor at ESPCI (Juliot Curie Chair) and Ecole Normale, Paris in 2013 and 2015 respectively. In 2018 he became a fellow of the American Physical Society (GSOFT) for theoretical contributions to topological mechanics. Topological Soft Matter

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.

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 assembled structures and demonstrate the correspondence of the knot invariants, constructed graphs, and surfaces associated with the disclination loop to the physically observable features specific to the geometry at hand. The nematic nature of the medium adds additional topological parameters to the conventional results of knot theory, which couple with the knot topology and introduce order into the phase diagram of possible structures. The crystalline order allows the simplified construction of the Jones polynomial and medial graphs, and the steps in the construction algorithm are mirrored in the physics of liquid crystals.

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 polarizing optical microscopy experiments and numerical solutions of the nematofluidic equations of motion. The relative stability of the regimes is related to the de Gennes characteristic shear-flow lengths e(1) and e(2), together with the channel's aspect ratio w/d. Finally, we show that the liquid crystalline microfluidic flow can be fully steered from left to right of a simple microchannel by applying transverse temperature gradients.

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 varied locally by laser tweezers and other external fields. We discuss the role of elasticity driven colloidal interactions and demonstrate the importance of precise optical manipulation of topological defects for a targeted design of entangled structures. We conclude that the interplay between particle and defect topologies in liquid crystals provides an exciting journey to the burgeoning area of applied topology and opens several new directions in advanced engineering of soft materials.

We have studied theoretically and experimentally the effects of various types of nanoparticles (NPs) on the temperature stability range \( \Delta\) 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 \( \Delta\) T BP(x) \( \propto\) 1/(1 - xb) , where x stands for the concentration of NPs and b is a constant. Our calculations suggest that the DCR mechanism is efficient if a local NP environment resembles the core structure of disclinations, which represent the characteristic property of BP structures. These predictions are in line with high-resolution ac calorimetry and optical polarising microscopy experiments using the CE8 LC and CdSe or aerosil NPs. In mixtures with CdSe NPs of 3.5nm diameter and hydrophobic coating the BPIII stability range has been extended up to 20K. On the contrary, the effect of aerosil silica nanoparticles of 7.0nm diameter and hydrophilic coating is very weak.

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 micromanipulation tool. All knots and links with up to six crossings, including the Hopf link, the Star of David and the Borromean rings are demonstrated, stabilizing colloidal particles into an unusual soft matter. The knots in chiral nematic colloids are classified by the quantized self-linking number, a direct measure of the geometric, or Berry's, phase. Forming arbitrary microscopic knots and links in chiral nematic colloids is a demonstration of how relevant the topology can be for the material engineering of soft matter.