Monolayer Of Microspheres Research Articles

Optical radiation from shock-compressed epoxy with glass microspheres

Hot-spot temperature in shock compression of a mixture of epoxy resins with glass hollow microspheres is estimated. The obtained hot-spot temperature of 3200 K at a shock pressure of 20 GPa significantly exceeds the temperature of shock-loaded homogeneous epoxy resin, which at this pressure is in the range of 1100–1400 K. The time characteristics of the radiation generated during viscoplastic compression of a monolayer of hollow microspheres in an epoxy matrix at a pressure of 9, 20, and 29 GPa were determined by an optical pyrometer. Experimental data are shown to be in good agreement with estimates based on the viscoplastic deformation model.

Microcavity substrates casted from self-assembled microsphere monolayers for spheroid cell culture.

Multicellular spheroids are an important 3-dimensional cell culture model that reflects many key aspects of in vivo microenvironments. This paper presents a scalable, self-assembly based approach for fabricating microcavity substrates for multicellular spheroid cell culture. Hydrophobic glass microbeads were self-assembled into a tightly packed monolayer through the combined actions of surface tension, gravity, and lateral capillary forces at the water-air interface of a polymer solution. The packed bead monolayer was subsequently embedded in the dried polymer layer. The surface was used as a template for replicating microcavity substrates with perfect spherical shapes. We demonstrated the use of the substrate in monitoring the formation process of tumor spheroids, a proof-of-concept scale-up fabrication procedure into standard microplate formats, and its application in testing cancer drug responses in the context of bone marrow stromal cells. The presented technique offers a simple and effective way of forming high-density uniformly-sized spheroids without microfabrication equipment for biological and drug screening applications.

Large-Area Self-Assembled Monolayer of Silica Microspheres by Convective Assembly

We reported here a convective assembly process for the formation of large-area self-assembled monolayers of silica microspheres on silicon and glass substrates. Uniformly coated monolayers of silica spheres were achieved on silicon wafers with and without coated SiN2 of 3 inch of diameter and large glass substrate of 6 × 6 in2 in size. The coating of large-area uniform monolayers of silica microspheres was characterized with scanning electron microscopy and optical microscopy. The mechanism of the convective assembly has been explained by the convective flux that is generated by capillary immersion force caused from the solution evaporation and hydrodynamic drag force. The patterns of silica microspheres were transferred to the silicon substrates using a deep reactive ion etching technique. It is found that textured silicon reduced the reflectance of silicon substrate from 52.2% to 33.2% around 400 nm and from 33.9% to 19.5% around 1,100 nm. The rapid self-assembled monolayer with silica microspheres provided a glimpse at the wide range of coating and photonic device applications where convective assembly can be used.

Perfect narrow-band absorber based on a monolayer of metallodielectric microspheres

We have studied how two-dimensional arrays of metallodielectric core-shell microspheres on a metal substrate can efficiently absorb infrared electromagnetic radiation in a narrow wavelength range under normal incidence. Our simulations indicate that perfect absorption efficiencies can be achieved for resonance wavelengths. The influence of core-shell microspheres geometry and lattice geometry is studied on absorption properties. For wavelength from 1.2 μm to 2.6 μm, an optimal combination of sphere and core radius was obtained to provide perfect absorption which can be wavelength adjusted.

Colloidal particle lens arrays-assisted nano-patterning by harmonics of a femtosecond laser

We consider nanopatterning of dielectric substrates by harmonics of single powerful femtosecond pulses from a Ti:Sapphire laser. The nanopatterning is mediated by closely packed monolayers of polystyrene microspheres that act as microlenses at the surface. Observed modification of the material proceeds via ionization. By our theory, the second harmonic is more effective in multi-photon ionization and is better focused than the fundamental frequency which is effective in multiplying of the amount of free electrons via impact ionization. Experiments show that conversion of a part of the pulse energy into the second harmonic decreases the modification threshold and improves the localization of the structures. Optimization of the time offset between the harmonics could further improve the efficiency and quality of nanostructuring.

Nanoparticle fabrication by geometrically confined nanosphere lithography

Arrays of metal nanoparticles, typically gold or silver, exhibit localized surface plasmon resonance, a phenomenon that has many applications, such as chemical and biological sensing. However, fabrication of metal nanoparticle arrays with high uniformity and repeatability, at a reasonable cost, is difficult. Nanosphere lithography (NSL) has been used before to produce inexpensive nanoparticle arrays through the use of monolayers of self-assembled microspheres as a deposition mask. However, control over the size and location of the arrays, as well as uniformity over large areas is poor, thus limiting its use to research purposes. In this paper, a new NSL method, called here geometrically confined NSL (GCNSL), is presented. In GCNSL, microsphere assembly is confined to geometric patterns defined in photoresist, allowing high-precision and large-scale nanoparticle patterning while still remaining low cost. Using this new method, it is demonstrated that 400 nm polystyrene microspheres can be assembled inside of large arrays of photoresist patterns. Results show that optimal microsphere assembly is achieved with long and narrow rectangular photoresist patterns. The combination of microsphere monolayers and photoresist patterns is then used as a deposition mask to produce silver nanoparticles at precise locations on the substrate with high uniformity, repeatability, and quality.

Experimental exploration of the fabrication of GaN microdome arrays based on a self-assembled approach

The formation of large scale, highly uniform and controllable GaN microdome arrays based on a self-assembled low cost method was investigated. The deposition of a large area, hexagonally close-packed SiO2 microsphere monolayer on top of the III-nitride semiconductor using the dip-coating method was optimized, which leads to surface coverage of 87% of SiO2 on GaN (ideal close-packed microsphere surface coverage is 90.7%). Reactive ion etching was used to simultaneously etch both SiO2 microspheres and GaN substrate to form GaN microdomes. Experiments show that GaN microdomes with controllable size, shape, and aspect ratio are achievable through controlling the plasma etching conditions.

Fabrication of Combined One-Dimensional and Three-Dimensional Structure of Potassium Tungstate Crystal Layers by Spray Deposition with Polystyrene Colloidal Crystal Templates

Well-aligned honeycomb-designed layers of potassium tungstate (K0.33WO3.165) were successfully fabricated by spray deposition and microsphere lithography using polystyrene (PS) colloidal crystals as templates. A densely packed monolayer of monodisperse PS microspheres was formed on a silica glass surface, using the Langmuir–Blodgett thin film technique. Subsequently, a (NH4)10W12O41·5H2O-KCl aqueous solution was sprayed as a micromist on the PS templates. While the K0.33WO3.165 crystal layers were synthesized by chemical reaction between W and K sources during the heating, the PS templates were thermally decomposed, generating an inverse opal surface structure where the interstitial space of the densely hexagonal packed PS microspheres had been. Numerous hexagonal rodlike crystals were grown on the honeycomb-designed layer of K0.33WO3.165. X-ray diffraction analysis, energy-dispersive X-ray spectroscopy analyses, and X-ray photoelectron spectroscopy were performed to analyze the combined one-dimensional and three-dimensional honeycomb-designed structure.

Hybrid Plasmonic–Photonic Nanostructures: Gold Nanocrescents Over Opals

The interaction of localized surface plasmon resonances (LSPRs) with the photonic modes in an opal photonic crystal is reported. Gold nanocrescents are evaporated at glancing angles over both microsphere monolayers and artificial opals. Microsphere monolayers allow the three localised plasmonic resonances observed in the nanocrescent optical spectra to be identified. By changing the microsphere diameter, the opal photonic stop band and a selected plasmonic resonance can be spectrally overlapped causing a dramatic change of the properties of both excitations. The creation of new modes is observed, whose spectral position and dispersion properties are a combination of those localised in nanoparticle plasmons and in the photonic crystal.

Altering Colloidal Surface Functionalization Using DNA Encapsulated Inside Monodisperse Gelatin Microsphere Templates

Soluble oligonucleotides are typically introduced to bulk solution to promote hybridization activity on DNA-functionalized surfaces. Here, an alternative approach is explored by encapsulating secondary target strands inside semipermeable colloidal satellite assemblies, then triggering their release at 37 °C for subsequent surface hybridization activity. To prepare DNA-loaded satellite assemblies, uniform gelatin microspheres are fabricated using microfluidics, loaded with 15 base-long secondary DNA targets, capped with a polyelectrolyte bilayer, and finally coated with a monolayer of polystyrene microspheres functionalized with duplexes comprised of immobilized probes and soluble, 13 base-long hybridization partner strands. Once warmed to 37 °C, secondary DNA targets are released from the gelatin template and then competitively displace the shorter, original hybridization partners on the polystyrene microspheres.

Large area laser surface micro/nanopatterning by contact microsphere lens arrays

Laser surface micro/nanopatterning by particle lens arrays is a well-known technique. Enhanced optical fields can be achieved on a substrate when a laser beam passes through a self-assembled monolayer of silica microspheres placed on the substrate. This enhanced optical field is responsible for ablative material removal from the substrate resulting in a patterned surface. Because of the laser ablation, the microspheres are often ejected from the substrate during laser irradiation. This is a major issue impeding this technique to be used for large area texturing. We explored the possibility to retain the spheres on the substrate surface during laser irradiation. A picosecond laser system (wavelength of 515 nm, pulse duration 6.7 ps, repetition rate 400 kHz) was employed to write patterns through the lens array on a silicon substrate. In this experimental study, the pulse energy was found to be a key factor to realize surface patterning and retain the spheres during the process. When the laser pulse energy is set within the process window, the microspheres stay on the substrate during and after laser irradiation. Periodic patterns of nanoholes can be textured on the substrate surface. The spacing between the nanoholes is determined by the diameter of the microspheres. The depth of the nanoholes varies, depending on the number of laser pulses applied and pulse energy. Large area texturing can be made using overlapping pulses obtained through laser beam scanning. © 2013 Springer-Verlag Berlin Heidelberg.

The swapping-trajectory effect: lattice evolution and buckling transition in wall-bounded hydrodynamic crystals

We analyze novel structural transformations in perturbed periodic square monolayers of microspheres in parabolic flow between two parallel walls. We find that a perturbed monolayer is initially stabilized by the swapping-trajectory mechanism that causes the particles to fluctuate between faster and slower streamlines in such a way that particle collisions do not occur. The fluctuations slowly decay in time, and the array achieves nearly perfect rectangular order. Surprisingly, after the fluctuations have dissipated, the particle lattice undergoes a sudden buckling instability that produces coherent vertical displacements of particle rows oriented in the flow direction. The instability results in formation of a disordered phase in which particles are arranged into meandering strings, similar to the structures observed in recent experiments [2012 PNAS 109 63]. We show that the behavior of the system is controlled by the swapping-trajectory interactions at all stages of the evolution.

Site-controlled growth of InP/GaInP islands on periodic hole patterns in GaAs substrates produced by microsphere photolithography

We present microsphere photolithography in combination with wet chemical etching as a fast and low-cost method to produce regular hole arrays in a (100) GaAs surface, which are suitable for controlled nucleation of self-organized InP islands in a metal–organic vapor-phase epitaxy (MOVPE) system. A hexagonal close-packed monolayer of microspheres is used as an array of microlenses to focus UV-light on UV-sensitive photoresist. In this way, regular arrays of holes with 2μm spacing can be realized in the photoresist with controllable feature size in the sub-μm range, which are transferred to a GaAs buffer, using an isotropic etchant. These templates are used to study the site-controlled nucleation of InP islands on a Ga0.51In0.49P barrier. For this purpose the subsequent overgrowth of the templates with a GaAs buffer layer and the GaInP barrier is investigated previous to the additional deposition of the InP. The template quality is monitored during structuring and overgrowth experiments using atomic force microscopy (AFM). Preferred nucleation of InP islands inside the almost filled holes can be observed for uncapped samples. The correlation between the initial patterning and the optical signal of the InP islands is investigated by micro-photoluminescence (micro-PL) measurements. We observe site-controlled nucleation of large, but optically active, InP islands on these templates.

Laser surface micro-/nano-structuring by a simple transportable micro-sphere lens array

A micro-sphere array optic was employed for laser surface micro-structuring. This array optic consists of a hexagonally close-packed monolayer of silica micro-spheres. It was organized through a self-assembly process and held together on a glass support, without using any adhesives. The array assembly was then reversed, placed in direct contact with the substrate and exposed to 515 nm, 6.7 ps laser pulses. During the exposure, the silica spheres act as micro-lenses, which enhance the near-field light intensity underneath them. As the spheres are confined in the space between the substrate and glass support, they are not ejected during laser machining. Using this type of direct write laser machining, a large number of identical features (nano-holes) can be produced in parallel simultaneously. The holes drilled are a few hundred nanometres in diameter and the depth depends on the number of laser pulses applied. The impact of laser machining on the micro-spheres was also studied. The micro-spheres were contaminated or partially damaged after micro-structuring. Combination of a moderate laser pulse energy and multiple shots was found to ensure a good surface structuring quality and minimum damage to the spherical particles.

Rheological characterization of mechanical properties of chemically crosslinked microspheres

AbstractThis work presents a rheological method to characterize degradation of microspheres made of chemically crosslinked hydrogels. Conditions to measure rheological properties of microspheres, remaining as individual microspheres during measurement, were established. Relevant and reproducible measurements could be obtained with a rheometer equipped with a parallel plate geometry in which a homogenous and continuous monolayer of microspheres was inserted in a gap between the plates. The storage modulus of microspheres was determined under an imposed strain of 0.04% with an oscillatory measurement mode at various frequencies. The microspheres showed almost purely elastic behavior while their storage modulus was affected by the degree of crosslinking. In the second part of the work, the rheological method was applied to investigate the degradation of microspheres made of a hydrolyzable crosslinked hydrogel. The method was found suitable by measuring the storage modulus over time. A good correlation was identified between acidification of the incubation medium because of the release of degradation products and the decrease of the storage modulus of the microspheres indicating a reduction in the crosslinking of the hydrogel resulting from the degradation process. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Laser Synthesis and Characterization of Nitrogen-Doped TiO2 Vertically Aligned Columnar Array Photocatalysts

Nitrogen-doped TiO2 vertically aligned columnar arrays were grown by reactive pulsed laser deposition using self-assembled polystyrene (PS) microsphere monolayers as templates and TiO2 hemispherica...

Fabrication and characterization of a multilayered optical tissue model with embedded scattering microspheres in polymeric materials

We report on a novel fabrication approach to build multilayered optical tissue phantoms that serve as independently validated test targets for axial resolution and contrast in scattering measurements by depth-resolving optical coherent tomography (OCT) with general applicability to a variety of three-dimensional optical sectioning platforms. We implement a combinatorial bottom-up approach to prepare monolayers of light-scattering microspheres with interspersed layers of transparent polymer. A dense monolayer assembly of monodispersed microspheres is achieved via a combined methodology of polyelectrolyte multilayers (PEMs) for particle-substrate binding and convective particle flux for two-dimensional crystal array formation on a glass substrate. Modifications of key parameters in the layer-by-layer polyelectrolyte deposition approach are applied to optimize particle monolayer transfer from a glass substrate into an elastomer while preserving the relative axial positioning in the particle monolayer. Varying the dimensions of the scattering microspheres and the thickness of the intervening transparent polymer layers enables different spatial frequencies to be realized in the transverse dimension of the solid phantoms. Step-wise determination of the phantom dimensions is performed independently of the optical system under test to enable precise spatial calibration, independent validation, and quantitative dimensional measurements.

High-Throughput Generation of Micropatterns of Dye-Containing Capsules Embedded in Transparent Elastomeric Monoliths by Inkjet Printing

We report the high-throughput fabrication of transparent elastomeric monoliths containing customized micropatterns of microcapsules, which might be used as highly flexible identity tags, sensor elements, and photochromic, photonic, or phononic systems. High-throughput replication molding of microsphere monolayers used as sacrificial primary templates via negative secondary replicas and positive tertiary replicas yielded elastomeric specimens containing dense hexagonal arrays of open spherical microcavities. In a subsequent inkjet printing step, some of the open-spherical microcavities were filled with functional materials such as dyes. The subsequent addition of an elastomeric cover layer led to encapsulation of the dyes trapped in the printed microcavities, while empty microcavities were filled with elastomer and vanished.

Short pulse laser interaction with micro-structured targets: simulations of laser absorption and ion acceleration

The interaction of an ultrashort intense laser pulse with thin foil targets is accompanied by the acceleration of ions from the target surface. To make this ion source suitable for application, it is of particular importance to increase the efficiency of laser energy transformation into accelerated ions and the maximum ion energy. This can be achieved by using a thin foil target with a microscopic structure on the front, laser-irradiated surface. The influence of the microscopic structure on the target surface on the laser target interaction and subsequent ion acceleration is studied here using numerical simulations. The influence of the shape and size of the microstructure, the density profile and the laser pulse incidence angle is also studied. Based on the simulation results, we propose to construct the target for ion acceleration experiments by depositing a monolayer of polystyrene microspheres of a size similar to the laser wavelength on the front surface of a thin foil.

Close-packed hemispherical microlens arrays for light extraction enhancement in organic light-emitting devices

Close-packed hemispherical microlens arrays have been fabricated using a soft lithography method and used to enhance the light extraction efficiency in organic light-emitting devices (OLEDs). A close-packed monolayer of polystyrene microspheres was used to produce a soft template of polydimethylsiloxane for fabricating microlens arrays from a photopolymerizable optical adhesive. The microlens contact angle and array fill-factor increase with the polystyrene microsphere size, reaching (85°±5°) and (85±3)% for 100μm size microspheres, respectively. Attaching such a large-area (2cm in diameter) microlens array on the glass substrate of an OLED leads to up to 70% efficiency enhancement, which also shows dependencies on the area and detailed structure of the OLED.