Molybdenum-based nanoparticles are often used as oil additives to enhance a material's tribological performance. Here, we present a highly efficient synthetic route for the bulk production of two types of MoS2 nanostructures: multi-wall nanotubes and fullerene-like nanostructures. The presented two-step synthesis involves the transformation of ammonium heptamolybdate tetrahydrate and aniline into precursor nanowires, which are later transformed into MoS2 through heating in a H2S, H2, and argon atmosphere to approximately 800 degrees C. Depending on the heating rate, we successfully grew MoS2 layered compounds in various shapes and sizes. The resulting structures and compositions were characterised by X-ray powder diffraction, Raman spectroscopy, energy-dispersive X-ray spectroscopy, and electron microscopy. To assess the application potential of these MoS2 compounds, they were dispersed in polyalphaolefin (PAO 6) oil. Improved tribological properties were observed compared to typically used transition metal dichalcogenides.

We report on observations of optical switching in bent-core liquid crystals possessing azocinnamoyl groups in both elongated side arms. UV-activated switching was observed in two of the synthesised materials, both of which were studied optically and with dielectric spectroscopy. Polarising micro-scopy was used to measure the changes in optical transmittance, and while no illumination effect was seen in the nematic phase, visible changes were observed in the smectic phase, most pronounced in the close vicinity of the smectic to nematic-phase transition. Switching dynamics was characterised, and we found that the average switching time varies strongly with temperature. The relaxation process is faster (below 200 ms) and exhibits no significant temperature dependence. Dielectric measurements have shown that the observed smectic C phases were not polar with the molecules possessing a rather small electric dipole moment. Upon illumination, small differences in the dielectric permittivity were observed only in the smectic phase, suggesting that the illumination effect can be attributed to changes in the molecular conformational and orientational order. (GRAPHICS)

Ferromagnetic ferrofluids are synthetic materials consisting of magnetic nanoplatelets dispersed in an isotropic fluid. Their main characteristics are the formation of stable magnetic domains and the presence of macroscopic magnetization even in the absence of a magnetic field. Here, the authors report on the experimental observation of spontaneous stripe formation in a ferromagnetic ferrofluid in the presence of an oscillating external magnetic field. The striped structure is identified as elongated magnetic domains, which exhibit reorientation upon reversal of the magnetic field. The stripes are oriented perpendicular to the magnetic field and are separated by alternating flow lanes. The velocity profile is measured using a space-time correlation technique that follows the motion of the thermally excited fluctuations in the sample. The highest velocities are found in the depleted regions between individual domains and reach values up to several µm s-1 . The fluid in adjacent lanes moves in the opposite directions despite the applied magnetic field being uniform. The formation of bidirectional flow lanes can be explained by alternating rotation of magnetic nanoparticles in neighboring stripes, which indicates spontaneous breaking of the chiral symmetry in the sample.

Although liquid crystal elastomers (LCE) are a fascinating class of materials with interesting thermomechanical properties of their own, the aim is to enhance their performance or add new features, e.g. response to external stimuli. The generally weak response of organic materials can be significantly intensified by mixing nano-sized magnetic particles into the host polymer matrix. An alternative approach is chemically coupling the nanoparticles to the elastomer. We achieved this by bonding functionalised magnetic nanoplatelets to the backbone of a main-chain LCE and obtained polydomain magnetic liquid crystal elastomers. We measured the magnetisation curves in samples, which were exposed to either small or large magnetic fields - their response being a consequence of partial particle reorientation or magnetic moment flipping. In contrast to the samples, which were exposed to small magnetic field and in which the remanent magnetisation can be reset to zero by heating the sample, the samples with flipped magnetisation within the platelets cannot be reversed into the original state. Coupling of magnetic and mechanical properties shows a slight magneto-elastic response at elevated temperatures and a significant inverse magneto-elastic effect: the magnetisation caused by mechanical stretching is almost equal to the magnetisation caused by an external magnetic field.

High specific surface area makes carbon nanofibres suitable for catalyst support. Here we report on optimization of carbon nanofibre (CNF) growth on molybdenum carbide nanowires (MoCNW) by direct carburization of Mo 6 S 2 I 8 nanowire bundles. Typical CNFs obtained by this method are several hundreds of nanometres long at a diameter of 10–20 nm. We show that nanofibre growth does not depend on the initial morphology of the nanowires: nanofibres grow on individual bundles of MoCNW, on dense networks of nanowires deposited on silicon substrate, and on free-standing nanowire foils. We find that carbon nanofibres remain firmly attached to the nanowires even if they are modified into Mo 2 C and further into Mo S 2 nanowires. The method thus enables production of a novel hybrid material composed of Mo S 2 nanowires densely covered with carbon nanofibres. We have additionally shown that the obtained CNFs can easily be self-decorated with platinum nanoparticles with diameters of several nanometres directly from water solution at room temperature without reducing agents. Such efficient synthesis and decoration process yield hybrid platinum/CNF/molybdenum-based NW materials, which are a promising material for a wide range of possible future applications, including sensitive sensorics and improved catalysis.

Molybdenum nitrides, which are used as hard coatings or corrosion and abrasion resistant layers, are in general considered as having good chemical resistance. We find, however, that molybdenum nitride Mo2N nanowires promptly react with lead nitrate in water at room temperature, resulting in formation of lead molybdate nanoparticles. Depending on the initial concentrations, the resulting material are either hybrid nanowires decorated with individual lead molybdate nanoparticles, or solely lead molybdate nanoparticles. The synthesised nanoparticles are fairly uniform in size, with diameters of up to a hundred nanometres. We find that the particle size is practically independent of the initial concentrations that we used. The performed one-step reaction, which is almost immediate and requires no additional reagents, clearly shows chemical reactivity of molybdenum nitrides and opens a new use for molybdenum nitrides as starting materials in a variety of chemical reactions. At the same time, the demonstrated reaction presents a new method for synthesis of lead molybdate nanoparticles or hybrid materials using molybdenum nitride nanowires as the starting material.

We present an experimental realisation of two new artificial microswimmers that swim at low Reynolds number. The swimmers are externally driven with a periodically modulated magnetic field that induces an alternating attractive/repulsive interaction between the swimmer parts. The field sequence also modulates the drag on the swimmer components, making the working cycle non-reciprocal. The resulting net translational displacement leads to velocities of up to 2 micrometers per second. The swimmers can be made omnidirectional, meaning that the same magnetic field sequence can drive swimmers in any direction in the sample plane. Although the direction of their swimming is determined by the momentary orientation of the swimmer, their motion can be guided by solid boundaries. We demonstrate their omnidirectionality by letting them travel through a circular microfluidic channel. We use simple scaling arguments as well as more detailed numerical simulations to explain the measured velocity as a function of the actuation frequency.

Here we report on decoration of MoS2 nanotubes with platinum nanoparticles and partial encapsulation of platinum during the transformation of decorated Mo6SyIz (8.2 <y+z< 10) nanowires into MoS2 nanotubes. The proposed method enables direct decoration of both nanotubes and nanowires directly from aqueous solution of Na2PtCl4 at room temperature without reducing agents. A stable and uniform decoration is observed with average particle diameters of around 2 nm and a controllable surface density, covering up to 80% of the nanowire surface. Sulphurisation of decorated nanowires was performed, during which platinum nanoparticles aggregated into larger formations, and partial encapsulation of elongated platinum nanorods in the obtained multi-wall MoS2 nanotubes is observed. (C) 2015 Elsevier B.V. All rights reserved.

Light scattering observation of nematic director fluctuations in confined geometries can be used to obtain interaction parameters of liquid crystals with surfaces. We present the basics of the method and some examples of the results in planar and cylindrical geometries. These results were obtained after neglecting the coupling of the director motion to flow. We give analytical and numerical results of flow effects on director fluctuations in a slab. The backflow contribution to the effective viscosity is strongly suppressed so that the results for the anchoring energy remain valid. Modal dispersion relations show an interesting behaviour of avoiding crossings.

Nanowires and nanotubes decorated with gold nanoparticles are known for their excellent sensing and catalytic properties. However, the decoration of transition–metal dichalcogenide nanotubes can be very complex. Here we report on a simple procedure that enables efficient production and purification of thin bundles of Mo6S y I z (8.2 ≤ y + z ≤ 10) nanowires decorated with gold nanoparticles and their transformation to gold-decorated MoS2 nanotubes. We isolated several hundred milligrams of nanowire bundles that were several microns long with average diameters of around 40 nm, and formed a stable dispersion in water without added surfactants. Gold nanoparticles were directly deposited on the nanowire bundles either in a solution or on a substrate at room temperature in a single-step reaction without any additional reducing reagents. The number of gold nanoparticles on a nanowire bundle is controlled by changing the concentration of chloroauric acid HAuCl4·3H2O in the solution. Since the nanowires can serve as precursor crystals for fabrication of nanotubes, we were able to transform gold-decorated nanowires and produce gold-decorated MoS2 nanotubes.

Biological cilia are found on surfaces of some microorganisms and on surfaces of many eukaryotic cells where they interact with the surrounding fluid. The periodic beating of the cilia is asymmetric, resulting in directed swimming of unicellular organisms or in generation of a fluid flow above a ciliated surface in multicellular ones. Following the biological example, externally driven artificial cilia have recently been successfully implemented as micropumps and mixers. However, biomimetic systems are useful not only in microfluidic applications, but can also serve as model systems for the study of fundamental hydrodynamic phenomena in biological samples. To gain insight into the basic principles governing propulsion and fluid pumping on a micron level, we investigated hydrodynamics around one beating artificial cilium. The cilium was composed of superparamagnetic particles and driven along a tilted cone by a varying external magnetic field. Nonmagnetic tracer particles were used for monitoring the fluid flow generated by the cilium. The average flow velocity in the pumping direction was obtained as a function of different parameters, such as the rotation frequency, the asymmetry of the beat pattern, and the cilium length. We also calculated the velocity field around the beating cilium by using the analytical far-field expansion. The measured average flow velocity and the theoretical prediction show an excellent agreement.

This seminar is about the movement of micro-organisms with flagella and cilia. They swim in a world of very low Reynolds numbers. As a consequence flagella and cilia need to move in a nonreciprocal way. Through their structure and function I explain how these small organisms swim. At the end I mention artificial cilia, which can be used in microfluidics. FLAGELLA AND CILIA: MOTILITY AT LOW REYNOLDS NUMBER

We observed and measured the fluid flow that was generated by an artificial cilium. The cilium was composed of superparamagnetic microspheres, in which magnetic dipole moments were induced by an external magnetic field. The interaction between the dipole moments resulted in formation of long chains-cilia, and the same external magnetic field was also used to drive the cilia in a periodic manner. Asymmetric periodic motion of the cilium resulted in generation of fluid flow and net pumping of the surrounding fluid. The flow and pumping performance were closely monitored by introducing small fluorescent tracer particles into the system. By detecting their motion, the fluid flow around an individual cilium was mapped and the flow velocities measured. We confirm that symmetric periodic beating of one cilium results in vortical motion only, whereas asymmetry is required for additional translational motion. We determine the effect of asymmetry on the pumping performance of a cilium, verify the theoretically predicted optimal pumping conditions, and determine the fluid behaviour around a linear array of three neighbouring cilia. In this case, the contributions of neighbouring cilia enhance the maximal flow velocity compared with a single cilium and contribute to a more uniform translational flow above the surface.

Due to their small dimensions, microfluidic devices operate in the low Reynolds number regime. In this case, the hydrodynamics is governed by the viscosity rather than inertia and special elements have to be introduced into the system for mixing and pumping of fluids. Here we report on the realization of an effective pumping device that mimics a ciliated surface and imitates its motion to generate fluid flow. The artificial biomimetic cilia are constructed as long chains of spherical superparamagnetic particles, which self-assemble in an external magnetic field. Magnetic field is also used to actuate the cilia in a simple nonreciprocal manner, resulting in a fluid flow. We prove the concept by measuring the velocity of a cilia-pumped fluid as a function of height above the ciliated surface and investigate the influence of the beating asymmetry on the pumping performance. A numerical simulation was carried out that successfully reproduced the experimentally obtained data.

We have developed a magneto-optic tweezers that offer new experimental possibilities when laser tweezers were traditionaly used. The magneto-optic tweezers combine a multi-trap optical tweezers based on acousto optic deflectors and homogeneous magnetic field which direction and magnitude can be time modulated in arbitrary fashion. Superparamagnetic beads that are readily available from several commercial sources are used as trap handles. They can be manipulated using optical tweezers in a well known way. By applying magnetic field additional repulsive or attractive interaction between the particles can be induced, giving rise to new micromanipulation possibilities. Several examples of how magneto-optic traps can be used in colloidal physics reasearch and potential applications in biophysics and microfluidic systems are presented.

We studied the confinement effect on the interaction force in nematic liquid crystal colloids with spherical particles inducing planar anchoring. Using magneto-optical tweezers, we measured the spatial dependence of the quadrupolar structural interparticle force over 4 orders of magnitude. For small separations, the interparticle potential follows the power law, whereas for separations larger than the sample thickness, it decreases exponentially with the decay length proportional to the sample thickness. Experimental results are reproduced by using the Landau-de Gennes free-energy minimization approach.

Magneto-optic tweezers were used for measurements of liquid-crystal-mediated forces between spherical beads with tangential anchoring in thin nematic samples. Repulsive force, which results from the quadrupolar symmetry of defects around the immersed beads, decreases proportionally to 1/x6, with x being the bead separation. The velocity with which the particles are pushed apart also follows the same separation dependence. We thus find the effective drag coefficient gamma(eff) independent of x for surface-to-surface distances as small as 10% of the bead diameter.