Micrometer-sized Silica Spheres Research Articles

Microrobots powered by concentration polarization electrophoresis (CPEP)

Second-order electrokinetic flow around colloidal particles caused by concentration polarization electro-osmosis (CPEO) can result in a phoretic motion of asymmetric particle dimers in a homogeneous AC electrical field, which we refer to as concentration polarization electro-phoresis (CPEP). To demonstrate this actuation mechanism, we created particle dimers from micron-sized silica spheres with sizes 1.0 μm and 2.1 μm by connecting them with DNA linker molecules. The dimers can be steered along arbitrarily chosen paths within a 2D plane by controlling the orientation of the AC electric field in a fluidic chamber with the joystick of a gamepad. Further utilizing induced dipole-dipole interactions, we demonstrate that particle dimers can be used to controllably pick up monomeric particles and release them at any desired position, and also to assemble several particles into groups. Systematic experiments exploring the dependence of the dimer migration speed on the electric field strength, frequency, and buffer composition align with the theoretical framework of CPEO and provide parameter ranges for the operation of our microrobots. Furthermore, experiments with a variety of asymmetric particles, such as fragmented ceramic, borosilicate glass, acrylic glass, agarose gel, and ground coffee particles, as well as yeast cells, demonstrate that CPEP is a generic phenomenon that can be expected for all charged dielectric particles.

Brownian translation and rotation from the ballistic to the diffusive limit and derivation of the physical properties of dust agglomerates.

We analyzed the translational and rotational Brownian motion of aggregates of micrometer-sized silica spheres under microgravity conditions and in rarefied gas. The experimental data was collected in the form of high-speed recordings using a long-distance microscope as part of the ICAPS (Interactions in Cosmic and Atmospheric Particle Systems) experiment on board of the sounding rocket flight Texus-56. Our data analysis shows that the translational Brownian motion can be used to determine the mass and translational response time of each individual dust aggregate. The rotational Brownian motion additionally provides the moment of inertia and the rotational response time. A shallow positive correlation between mass and response time was found as predicted for aggregate structures with low fractal dimensions. Translational and rotational response times are roughly in agreement. Using the mass and the moment of inertia of each aggregate, we determined the fractal dimension of the aggregate ensemble. Slight deviations from the pure Gaussian one-dimensional displacement statistics were found in the ballistic limit for both the translational and rotational Brownian motion.

Uniform formation of mesoporous silica shell on micron-sized cores in the presence of hydrocarbon used as a swelling agent

Hydrolysis and condensation of tetraethyl orthosilicate (TEOS) with cetyltrimethylammonium bromide (CTAB) used as a template for mesopores were conducted to form mesoporous silica shell onto micron-sized silica spheres with low polydispersity. The concentration of TEOS suitable for uniform formation of the mesoporous shell was examined in the reactions in the presence or absence of nonpolar hydrocarbon that was used as a swelling agent to expand the size of CTAB micelles. Employment of cyclohexane (CyH) with a molecular weight lower than a conventional swelling agent of 1,3,5-trimethylbenzene could effectively expand mesopores in the shell uniformly covering the micron-sized spheres. SEM images of the core-shell particles in the sol–gel reaction reveled that a web-like surface feature on the core was observed in an early stage of shell formation, whereas uniform mesoporous shell was finally obtained on the core. The extension of mixing time for the swelling agent of CyH to be diffused into CTAB micelles resulted in an increase in uniformity in thickness of the mesoporous shell with the large sizes of mesopores.

Spiral diffusion of self-assembled dimers of Janus spheres

ABSTRACTJanus spheres, micron-sized silica spheres half-coated with platinum, move rectilinearly away from the platinum side in aqueous hydrogen peroxide. Upon self-assembling, these colloidal particles can form dimers with different conformations that exhibit both rectilinear and rotational modes of motion depending upon the relative orientation of each Janus sphere. At the micron length-scale, stochastic rotational Brownian dynamics is of the order of deterministic dynamics, and their coupling results in effective diffusion, in addition to passive translational diffusion. For dimers with rotary motion, the dynamic coupling leads to spiral trajectories for an ensemble average of the displacement vector.

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).

Preparation of single-hole hollow polymer nanospheres by raspberry-like template method

Single-hole hollow polymer nanospheres were fabricated by raspberry-like template method using “graft-from” strategy through atom transfer radical polymerization (ATRP). Nanometer-sized silica spheres were covalently attached onto the surfaces of micrometer-sized silica spheres. Crosslinked polymer shells on the nano-sized spheres outside the attached area were formed by “graft-from” strategy through ATRP. After removal of the silica cores, single-hole hollow crosslinked polymer nanospheres were obtained. In this strategy, most of ATRP monomers may be used and thus many functional groups can be easily incorporated into the single-hole hollow crosslinked polymer nanospheres.

High strain rate deformation of layered nanocomposites

Insight into the mechanical behaviour of nanomaterials under the extreme condition of very high deformation rates and to very large strains is needed to provide improved understanding for the development of new protective materials. Applications include protection against bullets for body armour, micrometeorites for satellites, and high-speed particle impact for jet engine turbine blades. Here we use a microscopic ballistic test to report the responses of periodic glassy-rubbery layered block-copolymer nanostructures to impact from hypervelocity micron-sized silica spheres. Entire deformation fields are experimentally visualized at an exceptionally high resolution (below 10 nm) and we discover how the microstructure dissipates the impact energy via layer kinking, layer compression, extreme chain conformational flattening, domain fragmentation and segmental mixing to form a liquid phase. Orientation-dependent experiments show that the dissipation can be enhanced by 30% by proper orientation of the layers.

Electrochemical study of o-toluidine blue impregnated in mesoporous silica channels

This paper reports an electrochemical study of ordered mesoporous silica impregnated with a cationic dye (o-toluidine blue), prepared with a non-ionic surfactant as the structure-directing agent. O-toluidine blue was chosen because of its utility as an electron transfer mediator (redox catalyst). O-toluidine blue impregnated mesoporous silica was characterized by nitrogen sorption porosimetry, small angle X-ray scattering, infrared spectroscopy, scanning electron microscopy and cyclic voltammetry using chemically modified carbon paste electrodes. Results indicate that the dye is located within the ordered mesopores of micron-sized silica spheres.

Fast adsorption and separation of bovine serum albumin and lysozyme using micrometer‐sized macromesoporous silica spheres

Micrometer-sized silica spheres with bimodal macromesoporous structures are synthesized continuously by a modified gelation technology, and a novel coaxial microfluidic device is applied to control the size of the silica spheres. These spheres are used as a new protein adsorbent to realize fast adsorption and separation of protein mixture. BSA and lysozyme (LYS) are as model proteins with different sizes and pIs. High protein adsorption capacity and rapid adsorption rate have been achieved. Fast separation of a binary mixture of BSA and LYS through a short packed column (50 mm x 4.6 mm id) has also been successfully realized. LYS was selectively adsorbed while BSA flowed through the column in 12 s at a flow rate of up to 5.0 mL/min. As a result, an ultrafast adsorption and separation of BSA and LYS was obtained, showing great potential for applications in fast, large-scale protein separation processes of these monodispersed silica spheres.

Preparation and functionalization of mesoporous silica spheres as packing materials for HPLC

This paper presents the integrated results of a series of new methods for preparing mesoporous silica spheres as high-performance liquid chromatography (HPLC) packing materials. The separation performance of the mesoporous spheres materials has also been determined. Micrometer-sized silica spheres with uniform spherical morphologies and ordered mesostructures were first successfully synthesized by the method employing a water-soluble polymer-assisted assembly. Then the templates for getting ordered mesoporous materials with high-density silanol groups were removed by using hydrothermal oxidation. Finally the silica spheres were functionalized with C8 alkyl groups by surface modification under hydrothermal conditions. The resultant functionalized silica spheres were demonstrated to be excellent packing materials for HPLC.

Preparation of High Performance Adsorbents by Functionalizing Mesostructured Silica Spheres for Selective Adsorption of Organosulfur Compounds

Micrometer-sized silica spheres with uniform spherical morphology and highly ordered mesostructures were synthesized by a temperature-induced and polyethylene glycol (PEG)-assisted assembly method. A modified and convenient incipient wetness impregnation method was then presented, and high performance adsorbent with silver nitrate (AgNO3) supported on micrometer-sized mesoporous silica spheres was prepared. The characteristics of adsorbing benzothiophene (BT), dibenzothiophene (DBT), and 4,6-dimethyldibenzothiophene (4,6-Me2DBT) from n-octane solution on the adsorbent were studied, and the equilibrium absorption data were described with the Freundlich equation and the Langmuir equation separately. The experiment of adsorptive desulfurization was also carried out in fixed-bed, and the results showed that the adsorption capacity reached as high as 20.5 mg S/g of adsorbent for DBT.

A new ‘pH-induced rapid colloid aggregation’ method to prepare micrometer-sized spheres of mesostructured silica in water-in-oil emulsion

Micrometer-sized silica spheres with mesoporous structures were prepared by a new pH-induced rapid colloid aggregation method in water-in-oil (W/O) emulsion with PEG (poly(ethylene glycol)) or the mixtures of PEG and sodium dodecylsulphate (SDS) as templates. The effect of the reactant composition and the operation conditions on the particle morphology was investigated by scanning electron microscopy (SEM) observation, including the molecular weight of PEG, the amount of the reactants, stirring speed, and the continuous phase characteristic of the emulsion. The prepared mesoporous silica spheres, characterized by nitrogen adsorption and desorption and transmission electron microscopy (TEM), exhibited high surface areas (∼700–980 m 2/g) and large pore volumes (∼0.68–1.48 mL/g). And the spheres exhibiting narrow pore size distributions were obtained by optimizing the composition of the reactants. Finally, the mechanism of this synthetic route was discussed.

Synthesis of micrometer-sized hard silica spheres with uniform mesopore size and textural pores

Micrometer-sized silica spheres were prepared using a new pH-induced rapid colloid aggregation method in water-in-oil (W/O) emulsion separately with F127 and the mixture of Pluronic triblock copolymer (F127, P123, or P105) and PEG20000 as templates. All the mesoporous silica spheres exhibited high surface areas (657–1145 m 2/g) and large pore volumes (0.46–2.16 ml/g). Through optimizing the synthetic conditions, hard silica spheres with narrow particle size distribution, uniform pore size, and textural pores were obtained. Finally, the mechanism of this synthetic route is discussed.

Colloidal stabilization via nanoparticle halo formation

We present a detailed numerical study of effective interactions between micrometer-sized silica spheres, induced by highly charged zirconia nanoparticles. It is demonstrated that the effective interactions are consistent with a recently discovered mechanism for colloidal stabilization. In accordance with the experimental observations, small nanoparticle concentrations induce an effective repulsion that counteracts the intrinsic van der Waals attraction between the colloids and thus stabilizes the suspension. At higher nanoparticle concentrations an attractive potential is recovered, resulting in reentrant gelation. Monte Carlo simulations of this highly size-asymmetric mixture are made possible by means of a geometric cluster Monte Carlo algorithm. A comparison is made to results obtained from the Ornstein-Zernike equations with the hypernetted-chain closure.

PROGRA 2 experiment: New results for dust clouds and regoliths analogs

With the PROGRA 2 experience, linear polarization of scattered light is measured on various types of dust clouds lifted by microgravity, or by an air-draught. The aim is to compare the phase curves for dust analogs with those obtained in the Solar System (cometary comae, and solid particles in planetary atmospheres) by remote-sensing and in situ techniques. Measurements are also performed on layers of particles (on the ground) and compared with remote measurements on asteroidal regoliths and planetary surfaces. New phase curves have been obtained, e.g., for quartz samples, crystals, fluffy mixtures of silica and carbon blacks and a high porosity regolith analog made of micron-sized silica spheres. This work will contribute to the choice of the samples to be studied with the ICAPS experiment onboard the ISS and on the precursor experiment.

Study of Composite System: Silica Colloidal Particles in Nematic Liquid Crystal

We investigate a composite system formed by micrometer-sized silica spheres and a nematic liquid crystal with positive dielectric anisotropy. The surface of the spheres has been treated to obtain homeotropic alignment either by grafting molecules of n-Octadecyltrimetoxysilane or by using lecithin.In a glass cell with planar anchoring conditions, we have experimentally observed the dipole and the Saturn-ring configurations. Due to the planar treatment of the glass substrates the particles align and can form chains. We studied the effect of an electric field applied parallel to the direction of the rubbing, in the plane of the cell.

Measurement of long-range steric repulsions between microspheres due to an adsorbed polymer.

We have measured the interparticle potential between pairs of micron-sized silica spheres induced by adsorbed polyethylene oxide polymer using a line-scanned optical tweezer. We found this long-range steric repulsion to be exponential over the range of energies (0.1k(B)T-5k(B)T) and polymer molecular weights (452,000-1,580,000) studied, and that the potential scaled with the polymer's radius of gyration R(G). The potential's exponential decay length was about 0.6R(G) and its range was about 4R(G), although both parameters varied significantly from one pair of spheres to another. The potential's exponential prefactor was greater than mean-field predictions.

Direct measurements of slip in sheared polymer solutions

We report direct measurements of slip flow during and after plane-Couette shearing of poly(styrene) solutions between glass surfaces. Slip is studied by visualizing the microscopic motions of micron-sized silica spheres suspended in the solutions. Slip velocities as large as 4 μm s−1 are observed during shear, and recoil displacements of as much as 80 μm are observed at the glass surfaces after the driving surface stops moving. Significant slip is observed when the Weissenberg number $\lambda\dot{\gamma}$ exceeds 0.5 or so, where λ is the relaxation time and $\dot{\gamma}$ the shear rate. The magnitude of the slip correlates strongly with the fluid's birefringence, indicating that slip is induced by polymer stress or orientation. Away from the centre of the sample, toward the free edges, a secondary flow is observed, which appears to be driven by a normal-stress imbalance at these edges.