Micron-sized Spheres Research Articles

Complex Contact-Based Dynamics of Microsphere Monolayers Revealed by Resonant Attenuation of Surface Acoustic Waves.

Contact-based vibrations play an essential role in the dynamics of granular materials. Significant insights into vibrational granular dynamics have previously been obtained with reduced-dimensional systems containing macroscale particles. We study contact-based vibrations of a two-dimensional monolayer of micron-sized spheres on a solid substrate that forms a microscale granular crystal. Measurements of the resonant attenuation of laser-generated surface acoustic waves reveal three collective vibrational modes that involve displacements and rotations of the microspheres, as well as interparticle and particle-substrate interactions. To identify the modes, we tune the interparticle stiffness, which shifts the frequency of the horizontal-rotational resonances while leaving the vertical resonance unaffected. From the measured contact resonance frequencies we determine both particle-substrate and interparticle contact stiffnesses and find that the former is an order of magnitude larger than the latter. This study paves the way for investigating complex contact-based dynamics of microscale granular crystals and yields a new approach to studying micro- to nanoscale contact mechanics in multiparticle networks.

Lithium insertion properties of mesoporous nanocrystalline TiO2 and TiO2–V2O5 microspheres prepared by non-hydrolytic sol–gel

Mesoporous nanocrystalline TiO2 and TiO2–V2O5 microspheres were prepared by non-hydrolytic sol–gel from TiCl4, VOCl3, and iPr2O at 110 °C without any solvent or additives. The samples were characterized by elemental analysis, X-ray diffraction, Raman spectroscopy, scanning electron microscopy, nitrogen physisorption, and impedance measurements. At low vanadium loadings, only TiO2 anatase was detected, and V2O5 scherbinaite was also detected at high vanadium loadings. The texture of the samples depended on the V loading, but all the samples appeared built of primary nanoparticles (≈10–20 nm in size) that aggregate to form mesoporous micron-sized spheres. The lithium insertion properties of these materials were evaluated by galvanostatic measurements taken using coin-type cells, in view of their application as electrode for rechargeable Li-ion batteries. The mesoporous TiO2 microspheres showed good performances, with a specific reversible capacity of 145 and 128 mAh g−1 at C/2 and C, respectively (C = 335.6 mA g−1), good coulombic efficiency, and a moderate capacity fade (6 %) from the 2nd to the 20th cycle at C/20. Although the addition of V effectively increased the electronic conductivity of the powders, the specific reversible capacity and cycling performances of the TiO2–V2O5 samples were only minimally improved for a 5 at% V loading and were lower at higher V loading.

Resonant attenuation of surface acoustic waves by a disordered monolayer of microspheres

Attenuation of surface acoustic waves (SAWs) by a disordered monolayer of polystyrene microspheres is investigated. Surface acoustic wave packets are generated by a pair of crossed laser pulses in a glass substrate coated with a thin aluminum film and detected via the diffraction of a probe laser beam. When a 170 μm-wide strip of micron-sized spheres is placed on the substrate between the excitation and detection spots, strong resonant attenuation of SAWs near 240 MHz is observed. The attenuation is caused by the interaction of SAWs with a contact resonance of the microspheres, as confirmed by acoustic dispersion measurements on the microsphere-coated area. Frequency-selective attenuation of SAWs by such a locally resonant metamaterial may lead to reconfigurable SAW devices and sensors, which can be easily manufactured via self-assembly techniques.

Transverse Magnetic Tweezers Allowing Coincident Epifluorescence Microscopy on Horizontally Extended DNA.

Longitudinal magnetic tweezers (L-MT) have seen wide-scale adoption as the tool-of-choice for stretching and twisting a single DNA molecule. They are also used to probe topological changes in DNA as a result of protein binding and enzymatic activity. However, in the longitudinal configuration, the DNA molecule is extended perpendicular to the imaging plane. As a result, it is only possible to infer biological activity from the motion of the tethered superparamagnetic microsphere. Described here is a "transverse" magnetic tweezers (T-MT) geometry featuring simultaneous control of DNA extension and spatially coincident video-rate epifluorescence imaging. Unlike in L-MT, DNA tethers in T-MT are extended parallel to the imaging plane between two micron-sized spheres, and importantly protein targets on the DNA can be localized using fluorescent nanoparticles. The T-MT can manipulate a long DNA construct at molecular extensions approaching the contour length defined by B-DNA helical geometry, and the measured entropic elasticity agrees with the worm-like chain model (force < 35 pN). By incorporating a torsionally constrained DNA tether, the T-MT would allow both the relative extension and twist of the tether to be manipulated, while viewing far-red emitting fluorophore-labeled targets. This T-MT design has the potential to enable the study of DNA binding and remodeling processes under conditions of constant force and defined torsional stress.

Interaction dynamics of two diffusing particles: contact times and influence of nearby surfaces.

Interactions of diffusing particles are governed by hydrodynamics on different length and timescales. The local hydrodynamics can be influenced substantially by simple interfaces. Here, we investigate the interaction dynamics of two micron-sized spheres close to plane interfaces to mimic more complex biological systems or microfluidic environments. Using scanned line optical tweezers and fast 3D interferometric particle tracking, we are able to track the motion of each bead with precisions of a few nanometers and at a rate of 10 kilohertz. From the recorded trajectories, all spatial and temporal information is accessible. This way, we measure diffusion coefficients for two coupling particles at varying distances h to one or two glass interfaces. We analyze their coupling strength and length by cross-correlation analysis relative to h and find a significant decrease in the coupling length when a second particle diffuses nearby. By analysing the times the particles are in close contact, we find that the influence of nearby surfaces and interaction potentials reduce the diffusivity strongly, although we found that the diffusivity hardly affects the contact times and the binding probability between the particles. All experimental results are compared to a theoretical model, which is based on the number of possible diffusion paths following the Catalan numbers and a diffusion probability, which is biased by the spheres' surface potential. The theoretical and experimental results agree very well and therefore enable a better understanding of hydrodynamically coupled interaction processes.

Microbiota and food residues including possible evidence of pre-mammalian hair in Upper Permian coprolites from Russia

Coprolites (fossil faeces) provide direct evidence on the diet of its producer and unique insights on ancient food webs and ecosystems. We describe the contents of seven coprolites, collected from the Late Permian Vyazniki site of the European part of Russia. Two coprolite morphotypes (A, B) contain remains of putative bacteria, cyanobacteria, fungi, protists, invertebrate eggs, arthropod elements, undigested bone and tooth fragments, fish scales and elongated hair-like structures with hollow interiors. Content, size and shape of the coprolites together with the associated body fossil record suggest that the most probable scat-producers were carnivorous tetrapods; the bone-rich morphotype A reveals short food retention time and a fast metabolism and is therefore assigned to therapsid carnivores whereas morphotype B with rarer and degraded bones are assigned to archosauromorphs or other non-therapsid carnivores. The general coprolite matrix contains abundant micron-sized spheres and thin-walled vesicles which are interpreted as oxide and phosphatic pseudomorphs after microbial cells. From analyses of the undigested bones, we infer that they represent remains of actinopterygian fish, a therapsid and unrecognizable parts of amphibians and/or reptiles. Additionally, hair-like structures found in one coprolite specimen occur as diagenetically altered (oxide-replaced) structures and moulds (or partly as pseudomorphs) in a microcrystalline carbonate-fluoride-bearing calcium phosphate. This suggests that the latest Permian therapsids probably were equipped with hair-like integument or hairsuit. If true, this is by far the oldest evidence of this mammalian character in the stem group of mammals.

Enhanced visible-light-driven photocatalytic inactivation of Escherichia coli using g-C3N4/TiO2 hybrid photocatalyst synthesized using a hydrothermal-calcination approach

Biohazards are widely present in wastewater, and contaminated water can arouse various waterborne diseases. Therefore, effectively removing biohazards from water is a worldwide need. In this study, a novel visible-light-driven (VLD) graphitic carbon nitride (g-C3N4)/TiO2 hybrid photocatalyst with high photocatalytic bacterial inactivation activity was successfully synthesized using a facile hydrothermal-calcination approach. The optimum synthesized hybrid photocatalyst is composed of micron-sized TiO2 spheres (average diameter: ca. 2 μm) and wrapped with lamellar g-C3N4 (thickness: ca. 2 nm), with narrowing bandgap (ca. 2.48 eV), leading to a significant improvement of visible light (VL) absorption and effective separation of photo-generated electron–hole pairs. This greatly enhances VL photocatalytic inactivation activity towards bacteria in water. Using this hybrid photocatalyst, 107 cfu mL−1 of Escherichia coli K-12 could be completely inactivated within 180 min under VL irradiation. SEM images indicate that bacterial cells were greatly damaged, leading to a severe leakage of intracellular components during photocatalytic inactivation processes. The study concludes that bacterial cell destruction and water disinfection can be achieved using this newly fabricated VLD hybrid photocatalyst.

Sound velocity and attenuation coefficient of hard and hollow microparticle suspensions observed by ultrasound spectroscopy

Size and elastic properties of micro-particles suspended in liquid can be acoustically determined by ultrasound attenuation and velocity measurements with the aid of elastic scattering theories and a dispersion relation. While quantitative evaluation for hard micron-sized spheres using the theories is available in literature, that for hollow particles is not yet achieved. In this study, we show that the shell thickness and the elastic modulus of hollow particles can be quantitatively evaluated by ultrasound spectroscopy. Several kinds of microparticles including polystyrene rigid particles, polydivinylbenzene rigid particles, borosilicate hollow particles, and phenolic-resin hollow particles were examined as a function of the particle concentration.

Effects of shear and walls on the diffusion of colloids in microchannels

Colloidal suspensions flowing through microchannels were studied for the effects of both the shear flow and the proximity of walls on the particles' self-diffusion. Use of hydrostatic pressure to pump micron-sized silica spheres dispersed in water-glycerol mixture through poly(dimethylsiloxane) channels with a cross section of 30×24μm(2), allowed variation in the local Peclet number (Pe) from 0.01 to 50. To obtain the diffusion coefficients, image-time series from a confocal scanning laser microscope were analyzed with a method that, after finding particle trajectories, subtracts the instantaneous advective displacements and subsequently measures the slopes of the mean squared displacement in the flow (x) and shear (y) directions. For dilute suspensions, the thus obtained diffusion coefficients (D(x) and D(y)) are close to the free diffusion coefficient at all shear rates. In concentrated suspensions, a clear increase with the Peclet number (for Pe > 10) is found, that is stronger for D(x) than for D(y). This effect of shear-induced collisions is counteracted by the contribution of walls, which cause a strong local reduction in D(x) and D(y).

Activated Li2s As a High-Performance Cathode for Rechargeable Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are considered to be a promising alternative to lithium-ion (Li-ion) batteries as the next-generation rechargeable battery system. Despite significant progress in recent years, degradation of the lithium-metal anode remains a persistent challenge for Li-S batteries, resulting in safety concern for Li-S batteries and hindering their practical utility. One possible strategy to circumvent the problems associated with using metallic lithium is to use alternative, lithium-free anodes coupled with a lithium-containing sulfur-based cathode. Accordingly, Li2S has attracted much attention as a prospective cathode material. However, Li2S is highly insulating; either intricate synthesis to produce small Li2S particles or a high initial charging-cut-off voltage to activate Li2S bulk particles is required. However, these traditional methods invariably lead to increased manufacturing cost and/or instabilities in the ether-based electrolytes in Li-S batteries. We demonstrate here that the Li2S bulk particles can be effectively activated through surface interaction with P2S5. Furthermore, the effective activation and the facilitated oxidation of Li2S into polysulfides are believed to be due to the formation of shallow sulfur- and phosphorus-containing species at the surface of Li2S particle. In contrast to the pristine Li2S, the P2S5-activated Li2S cathodes demonstrate lower initial charging voltage plateaus. Furthermore, the lowest charging voltage plateau appears when the molar ratio between Li2S and P2S5 is 7:1, resulting in an activation of ~ 80 % of the theoretical capacity of Li2S below 3.0 V (Figure 1a). Considering that Li2S or P2S5 alone lacks electrochemical activity before charging above 3.0 V, we believe the accessible charge capacity below 3.0 V is attributed to the interaction between P2S5 and Li2S, leading to Li2S activation. However, further increase in the concentration of P2S5 produces a gradual increase in the charging voltage plateau, likely due to the increased cell resistance. Scanning electron microscopy images reveal that the typical Li2S particles are micron-sized spheres when the molar ratio between Li2S and P2S5 is higher than 5:1. Core-shell-like morphology appears when the molar ratio between Li2S and P2S5 is 5:1. In addition, X-ray photoelectron spectroscopy and Raman spectroscopy results confirm the formation of sulfur- and phosphorus-containing species at the activated Li2S surface. Especially, when the molar ratio between Li2S and P2S5 is 10:1, S 2p 3/2 peaks center at 159.8, 161.0, and 162.6 eV. The 159.8 eV peak is a characteristic peak of Li2S. The 159.8 eV Li2S peak is still present when the Li2S: P2S5 ratio is 7:1, but disappears when the ratio reaches 5:1, suggesting the formation of a thick surface layer. Reversible discharge capacities of ~ 800 mAh g (Li2S)–1 are obtained and the capacity retention is as high as 83 % after 80 cycles (Figure 1b). Coulombic efficiency remains near 100 % upon long-term cycling. This is in contrast to the pristine Li2S that delivers limited reactivity and inferior capacity due to the incomplete oxidation reaction. The superior cycling performance of the activated Li2S is due to the enhanced surface charge transfer reducing the electrochemically inaccessible Li2S and ensuring cathode conductivity. This facile activation enables the direct use of Li2S bulk particles as high-capacity cathode materials for Li-S batteries, significantly decreasing the manufacturing cost of Li-S batteries. We believe this strategy is of great significance for the safe and effective use of Li2S as a cathode material, and is a promising step toward the low-cost fabrication of metallic lithium-free Li-S batteries. Figure 1

Stiffness of sphere-plate contacts at MHz frequencies: dependence on normal load, oscillation amplitude, and ambient medium.

The stiffness of micron-sized sphere–plate contacts was studied by employing high frequency, tangential excitation of variable amplitude (0–20 nm). The contacts were established between glass spheres and the surface of a quartz crystal microbalance (QCM), where the resonator surface had been coated with either sputtered SiO2 or a spin-cast layer of poly(methyl methacrylate) (PMMA). The results from experiments undertaken in the dry state and in water are compared. Building on the shifts in the resonance frequency and resonance bandwidth, the instrument determines the real and the imaginary part of the contact stiffness, where the imaginary part quantifies dissipative processes. The method is closely analogous to related procedures in AFM-based metrology. The real part of the contact stiffness as a function of normal load can be fitted with the Johnson–Kendall–Roberts (JKR) model. The contact stiffness was found to increase in the presence of liquid water. This finding is tentatively explained by the rocking motion of the spheres, which couples to a squeeze flow of the water close to the contact. The loss tangent of the contact stiffness is on the order of 0.1, where the energy losses are associated with interfacial processes. At high amplitudes partial slip was found to occur. The apparent contact stiffness at large amplitude depends linearly on the amplitude, as predicted by the Cattaneo–Mindlin model. This finding is remarkable insofar, as the Cattaneo–Mindlin model assumes Coulomb friction inside the sliding region. Coulomb friction is typically viewed as a macroscopic concept, related to surface roughness. An alternative model (formulated by Savkoor), which assumes a constant frictional stress in the sliding zone independent of the normal pressure, is inconsistent with the experimental data. The apparent friction coefficients slightly increase with normal force, which can be explained by nanoroughness. In other words, contact splitting (i.e., a transport of shear stress across many small contacts, rather than a few large ones) can be exploited to reduce partial slip.

Measurement of single leukemia cell's density and mass using optically induced electric field in a microfluidics chip.

We present a method capable of rapidly (∼20 s) determining the density and mass of a single leukemic cell using an optically induced electrokinetics (OEK) platform. Our team had reported recently on a technique that combines sedimentation theory, computer vision, and micro particle manipulation techniques on an OEK microfluidic platform to determine the mass and density of micron-scale entities in a fluidic medium; the mass and density of yeast cells were accurately determined in that prior work. In the work reported in this paper, we further refined the technique by performing significantly more experiments to determine a universal correction factor to Stokes' equation in expressing the drag force on a microparticle as it falls towards an infinite plane. Specifically, a theoretical model for micron-sized spheres settling towards an infinite plane in a microfluidic environment is presented, and which was validated experimentally using five different sizes of micro polystyrene beads. The same sedimentation process was applied to two kinds of leukemic cancer cells with similar sizes in an OEK platform, and their density and mass were determined accordingly. Our tests on mouse lymphocytic leukemia cells (L1210) and human leukemic cells (HL-60) have verified the practical viability of this method. Potentially, this new method provides a new way of measuring the volume, density, and mass of a single cell in an accurate, selective, and repeatable manner.

Testing the Maxwell-Boltzmann distribution using Brownian particles.

We report on shot-noise limited measurements of the instantaneous velocity distribution of a Brownian particle. Our system consists of a single micron-sized glass sphere held in an optical tweezer in a liquid in equilibrium at room temperature. We provide a direct verification of a modified Maxwell-Boltzmann velocity distribution and modified energy equipartition theorem that account for the kinetic energy of the liquid displaced by the particle. Our measurements confirm the distribution over a dynamic range of more than six orders of magnitude in count-rate and five standard deviations in velocity.

The fabrication and photocatalytic performances of flower-like Ag nanoparticles/ZnO nanosheets-assembled microspheres

A new micro/nanostructure photocatalyst, Ag nanoparticles decorated ZnO nanosheets-assembled microspheres (Ag-NPs/ZnOs), was synthesised by a two-step method. The flower-like micron-sized ZnO spheres assembled with ∼25nm thick ZnO nanosheets were initially fabricated via a facile solvothermal method. Then, highly dispersed Ag nanoparticles (Ag-NPs) with dimension ranging from 15 to 50nm were anchored onto the surface of the each ZnO nanosheet by the Sn(II) ion activation method. The as-prepared Ag-NPs/ZnOs demonstrated enhanced photocatalytic performance in eliminating methylene blue and methyl orange aqueous solutions under UV irradiation, showing twice faster reaction rate than the bare ZnOs. The enhanced photocatalytic activity was due to the suppression of electron/hole pair recombination and the acceleration of surface charge transfer induced by the highly dispersive Ag-NPs, which was further demonstrated by the cyclic voltammetry and impedance spectra measurements.

Stability of micron-sized spheres formed by pulsed laser ablation of metals in superfluid helium and water

It has been shown that micrometer-sized balls, resulting from the condensation of products of laser ablation of fusible metals in both superfluid helium and water, are in the state of strong internal tension counterbalanced by external compression. By radiation-induced or chemical damage to the integrity of their surface, they break up, ejecting a plurality of nanoparticles. The empty shells of the microspheres, which nonetheless remain intact, are identical to the “hollow spheres” of unclear origin that have been observed previously under laser ablation in usual liquids. The metastability of the microparticles produced by ablation in a liquid should be taken into consideration in their use in engineering and medicine.

Near-field nanoimprinting using colloidal monolayers

We experimentally and theoretically explore near-field nanopatterning obtained by irradiation of hexagonal monolayers of micron-sized polystyrene spheres on photosensitive Ge(2)Sb(5)Te(5) (GST) films. The imprinted patterns are strongly sensitive to the illumination conditions, as well as the size of the spheres and the orientation of the monolayer, which we change to demonstrate control over the resulting structures. We show that the presence of multiple scattering effects cannot be neglected to describe the resulting pattern. The experimental patterns imprinted are shown to be robust to small displacements and structural defects of the monolayer. Our method enables the design and experimental verification of patterns with multiple focii per particle and complex shapes, which can be directly implemented for large scale fabrication on different substrates.

Multi-Functionality of Macroporous TiO2 Spheres in Dye-Sensitized and Hybrid Heterojunction Solar Cells

Micron-sized macroporous TiO2 spheres (MAC-TiO2) were synthesized using a colloidal templating process inside emulsions, which were then coated on a nanocrystalline TiO2 light absorption film to prepare a bilayered photoanode for liquid-based dye-sensitized solar cells (DSSC) and hybrid heterojunction solid-state solar cells. MAC-TiO2 layers can enhance light scattering as well as absorption, because their pore size and periodicity are comparable to light wavelength for unique multiple scattering and a porous surface can load dye more. Moreover, due to the bicontinuous nature of macropores and TiO2 walls, electrolyte could be transported much faster in between the TiO2 spheres rather than within the small TiO2 nonporous architectures. Electron transport was also facilitated along the interconnected TiO2 walls. In DSSCs with these MAC-TiO2 scattering layers, efficiency was higher than conventional DSSCs incorporating a commercial scattering layer. The unique geometry of MAC-TiO2 results in strong improvements in light scattering and infiltration of hole-transporting materials, thereby the MAC-TiO2-based solid-state device showed comparatively higher efficiency than the device with conventional nanocrystalline TiO2.

Facile Synthesis of Micron-Sized Hollow Silver Spheres as Substrates for Surface-Enhanced Raman Scattering

A well-designed type of micron-sized hollow silver sphere was successfully synthesized by a simple hard-template method to be used as substrates for surface-enhanced Raman scattering. 4 Å molecular sieves were employed as a removable solid template. [Ag(NH3)2]+was absorbed as the precursor on the surface of the molecular sieve. Formaldehyde was selected as a reducing agent to reduce [Ag(NH3)2]+, resulting in the formation of a micron-sized silver shell on the surface of the 4 Å molecular sieves. The micron-sized hollow silver spheres were obtained by removing the molecular sieve template. SEM and XRD were used to characterize the structure of the micron-sized hollow silver spheres. The as-prepared micro-silver spheres exhibited robust SERS activity in the presence of adsorbed 4-mercaptobenzoic acid (4-MBA) with excitation at 632.8 nm, and the enhancement factor reached ~1.5 × 106. This synthetic process represents a promising method for preparing various hollow metal nanoparticles.

Investigating Mechanical Properties of Short Polymers with Optical Tweezers

Studying mechanical response of biological molecules at a microscopic level is crucial for better understanding their structure-function relationship and how it relates to their physiological performance. Our group uses optical tweezers to study mechanical properties of structural proteins such as collagen. Collagen is an extracellular matrix protein that serves as the main foundation of connective tissues with a major role in the structural support of cells. Being the most abundant protein in the body, collagen is important for development, tissue regeneration and repair. After decades of study, the flexibility of collagen remains unresolved, with estimates of persistence length ranging over an order of magnitude (15-160nm), and how and at what forces it denatures under applied tension revealed only indirectly. Direct measurements of its force-extension relationship, and their dependence on composition and chemical environment, will help to resolve these structural questions.The contour length of collagen (∼300nm) is short with respect to the micron-sized spheres used in our optical tweezers experiments. Because of this, factors negligible in the studies of longer polymers, such as lambda-DNA, become important here, including geometric offsets associated with binding and stretching directions. In order to characterize and eliminate these offsets in our stretching experiments, we adapt a method from surface-based assays [Seol et al. Biophys J, 93: 4360 (2007)] to our pipette-based manipulation experiments. By stretching at different offset positions perpendicular to the stretching axis, we find that the geometric pulling offset can be minimized. Applying this method to force-extension curves of short DNA, we find that this approach improves the confidence in worm-like chain fitting parameters. We then apply this method to characterizing the flexibility of type III collagen molecules, whose contour length is comparable to the 1kbp DNA used in our control experiments.

Photovoltaic absorption enhancement in thin-film solar cells by non-resonant beam collimation by submicron dielectric particles

We propose the enhancement of the photovoltaic absorption in thin-film solar cells using densely packed arrays (not obviously regular) of non-absorbing submicron or micron-sized dielectric spheres located on top of the cell. The spheres can decrease reflection forming an effective blooming layer. Simultaneously, they can suppress the transmission through the photovoltaic layer transforming the incident radiation into a set of collimated beams. The focusing of the light inside the photovoltaic layer allows enhanced absorption in it leading to the increase of the photovoltaic current. Every sphere focuses the incident wave separately—this mechanism does not require collective effects or resonances and therefore takes place in a wide spectral range. Since the fabrication of such the coating is easy, our light-trapping structure may be cheaper than previously known light-trapping ones and perhaps even than flat anti-reflecting coatings.