Fluidic assembly and packing of microspheres in confined channels

We study fluidic assembly and packing of spherical particles in rectilinear microchannels that are terminated by a flow constriction. First, we introduce a method for active assembly of particles in the confined microchannels by triggering a local constriction in the fluid channel using a partially closed membrane valve. This microfluidic valve allows active, on-demand particle assembly as opposed to previous passive assembly methods based on terminal channels and weirs. Second, we study the three-dimensional assembly and packing of particles against a weir in confined rectilinear microchannels. The packings result in achiral particle chains with alternating (zigzag) structure. This structure is characterized by a single, repeated bond angle whose components projected into the frame of the channel are quantified by confocal microscopy and image processing. Brownian dynamics simulation of the packing comprehensively delineates the range of bond angles possible in narrow, rectilinear microchannels as well as the complex dependence of these angles on the relative dimensions of the channel and particles. The simulations of the three-dimensional packings are accurately modeled by a compact theory based on trigonometric relationships. The experimentally measured bond angles show excellent agreement with the simulations, thereby validating the functional dependence of the achiral packing bond angles on channel dimensions. This functional relationship is immediately useful for the design of anisotropic particles by microfluidic synthesis.     

Determination of static microstructure of dilute and concentrated suspensions of anisotropic particles by ultra-small-angle X-ray scattering

Ultra-small-angle X-ray scattering was performed on suspensions of anisotropic polystyrene particles of varying degrees of anisotropy. The wave vector dependence of particle form factors is well described by a model developed by Debye for the scattering from fused spheres. As volume fraction is raised, all suspensions undergo a disorder/order phase transition. The scattering from disordered and ordered suspensions of anisotropic particles is the same as that of spheres up to volume fractions of 0.45, suggesting that, in the dilute crystalline phase, the anisotropic particles order into a rotator or plastic crystal phase, where the particle centers of mass are ordered, but the particle directors are randomly distributed. Further increase in particle volume fraction leads to differences in scattering between homonuclear dicolloids and spheres, implying that the homonuclear dicolloids form a body-centered tetragonal phase with both positional and directional order. This conclusion is supported by real-space imaging of dried films of the particles.     

Stimuli‐Responsive Shape Switching of Polymer Colloids by Temperature‐Sensitive Absorption of Solvent

The dynamic manipulation of colloidal particle shape offers a novel design mechanism for the creation of advanced responsive materials. To this end, we introduce a versatile new strategy for shape control of anisotropic polymeric colloidal particles. The concept utilizes temperature‐sensitive absorption of a suitable solvent from a binary mixture. Specifically, increasing the temperature in the vicinity of the demixing transition of a binary mixture causes more solvent to be absorbed into the polymeric colloidal particle, which, in turn, lowers the glass transition temperature of the polymer inside the particle, with a concomitant decrease in viscosity. The balance between the internal viscosity and surface tension of the particle is thus disrupted, and the anisotropic shape of the particle shifts to become more spherical. Subsequent rapid temperature quenching can halt the process, leaving the particle with an intermediate anisotropy. The resultant shape anisotropy control provides new routes for studies of the phase transitions of anisotropic colloids and enables the fabrication of unique particles for materials applications.     

Diffusion coefficient of Brownian particle in rough micro-channel

This study examines the feasibility of using of the lattice Boltzmann method to determine how the surface roughness of a quadrate channel affects the diffusion coefficient of Brownian particle(s). The surface was represented by a regular array of spheres. Surface roughness reduced the diffusion coefficient of the Brownian particle(s) because of a change in the velocity autocorrelation function decay and in pressure. Additionally, the neighboring particles increased the diffusion coefficient of Brownian particle.     

Microfluidic synthesis of highly shape-anisotropic particles from liquid crystalline elastomers with defined director field configurations

In this article, we present the synthesis of highly shape-anisotropic, micrometer-sized particles from liquid crystalline elastomers, which have the ability to reversibly change their shape in response to a certain external stimulus. For their preparation, we utilized a microfluidic setup. We succeeded in preparing sets of particles with differing degrees of shape anisotropy in their ground state including highly anisotropic fiber-like objects. All samples produced movement during the phase transition from the nematic to the isotropic phase of the liquid crystal. Depending on the direction of this shape change, we classified the samples in two groups. One type showed a contraction, while the other showed an expansion during the actuation, generating displacements of 60% and 80%, respectively. Using X-ray diffraction experiments, we could show that the different actuation properties arise from different director patterns of the liquid crystalline moieties in the microparticles. While the weakly shape-anisotropic microparticles possess a concentric director field (director perpendicular to the symmetry axis), the highly anisotropic fiber-like particles show an alignment of the director along the fiber axis. We present an explanation, claiming that this is the result of two different orientation mechanisms involving elongational flow on the one side and "log-rolling" on the other.     

Influence of surface topography on the interactions between nanostructured hydrophobic surfaces

Nanostructured particle coated surfaces, with hydrophobized particles arranged in close to hexagonal order and of specific diameters ranging from 30 nm up to 800 nm, were prepared by Langmuir-Blodgett deposition followed by silanization. These surfaces have been used to study interactions between hydrophobic surfaces and a hydrophobic probe using the AFM colloidal probe technique. The different particle coated surfaces exhibit similar water contact angles, independent of particle size, which facilitates studies of how the roughness length scale affects capillary forces (previously often referred to as "hydrophobic interactions") in aqueous solutions. For surfaces with smaller particles (diameter < 200 nm), an increase in roughness length scale is accompanied by a decrease in adhesion force and bubble rupture distance. It is suggested that this is caused by energy barriers that prevent the motion of the three-phase (vapor/liquid/solid) line over the surface features, which counteracts capillary growth. Some of the measured force curves display extremely long-range interaction behavior with rupture distances of several micrometers and capillary growth with an increase in volume during retraction. This is thought to be a consequence of nanobubbles resting on top of the surface features and an influx of air from the crevices between the particles on the surface.     

Double emulsions with controlled morphology by microgel scaffolding

Double emulsions are valuable structures that consist of drops nested inside bigger drops; they can be formed with exquisite control through the use of droplet-based microfluidics, allowing their size, composition, and monodispersity to be tailored. However, only little control can be exerted on the morphology of double emulsions in their equilibrium state, because they are deformable and subject to thermal fluctuations. To introduce such control, we use droplet-based microfluidics to form oil-in-water-in-oil double emulsion drops and arrest their shape by loading them with monodisperse microgel particles. These particles push the inner oil drop to the edge of the aqueous shell drop such that the double emulsions adopt a uniform arrested, anisotropic shape. This approach circumvents the need for ultrafast polymerization or geometric confinement to lock such non-spherical and anisotropic droplet morphologies. To demonstrate the utility of this technique, we apply it to synthesize anisotropic and non-spherical polyacrylate-polyacrylamide microparticles with controlled size and shape.     

Acoustic control of suspended particles in micro fluidic chips

A method to separate suspended particles from their medium in a continuous mode at microchip level is described. The method combines an ultrasonic standing wave field with the extreme laminar flow properties obtained in a silicon micro channel. The channel was 750 microm wide and 250 microm deep with vertical side walls defined by anisotropic wet etching. The suspension comprised "Orgasol 5 microm" polyamide spheres and distilled water. The channel was perfused by applying an under pressure (suction) to the outlets. The channel was ultrasonically actuated from the back side of the chip by a piezoceramic plate. When operating the acoustic separator at the fundamental resonance frequency the acoustic forces were not strong enough to focus the particles into a well defined single band in the centre of the channel. The frequency was therefore changed to about 2 MHz, the first harmonic with two pressure nodes in the standing wave, and consequently two lines of particles were formed which were collected via the side outlets. Two different microchip separator designs were investigated with exit channels branching off from the separation channel at angles of 90 degrees and 45 degrees respectively. The 45 degrees separator displayed the most optimal fluid dynamic properties and 90% of the particles were gathered in 2/3 of the original fluid volume.     

A Model for Calculating Electrostatic Interactions between Colloidal Particles of Arbitrary Surface Topology

A numerical model for calculating the electrostatic interaction between two particles of arbitrary shape and topology is described. A key feature of the model is a generalized discretization program, capable of simulating any desired analytical shape as a set of flat, triangular elements. The relative sizes of the elements are adjusted using a density function to better match the desired shape and the spatial variation of the electrical surface properties on each particle. The distribution of either surface potential or surface charge density is then calculated using a boundary element approach to solve the linearized Poisson-Boltzmann equation. Example interaction energy profiles are calculated for three different types of roughness-bumps, pits, and surface waves. It is found that the interaction energy between rough particles remains different from that between two equivalent smooth spheres at all separations, even for gap widths much larger than either the solution Debye length or the characteristic roughness size. This behavior at large gap widths arises from the nature of the decay of the electric potential away from each particle. In addition, the magnitude of the roughness effect is found to depend greatly on the size and shape of the nonuniformity as well as the electrostatic boundary conditions. For example, for a sphere containing asperities of height equal to 0.2 times the particle radius, the interaction energy can be as much as 50% greater than that between two equivalent spheres under the condition of constant surface potential. At constant surface charge density, the ratio of the interaction energies between rough and smooth spheres was found to either diverge or become zero as contact between the two particles is approached, depending on the nature of the roughness. Changes of this magnitude could clearly have a substantial impact on the stability behavior of a dispersion of such particles. Copyright 2001 Academic Press.     

A hierarchic structure in zirconium butoxide complexes in <Emphasis Type="Italic">n</Emphasis>-butanol solutions

The structure of particles in zirconium n-butoxide solutions in n-butyl alcohol is determined by means of EXAFS, SAXS, and molecular mechanics modeling. Zirconium atoms are found to be bonded to each other via the oxygen atom and to form large anisotropic particles in the solution. Primary particles have a shape close to spherical; their diameter together with the solvate shell is 28.9 Å. These particles then aggregate into anisotropic structures. During solution aging under normal conditions without contact with the atmosphere, the particle anisotropy increases because of the aggregation of complexes. When the solution concentration decreases, the particles are divided into primary spherical particles with a characteristic size of 28.9 Å. The described changes are confirmed by a decrease in the number of Zr-Zr distances of 4.8 Å and 5.1 Å, which according to the EXAFS data, correspond to the bonds between the primary particles. The characteristic maximum sizes of particles in solutions with concentrations from 0.1 g to 0.003 g ZrO₂/ml are 160–80 Å.     

A single-site anisotropic soft-core model for the study of phase behavior of soft rodlike particles

A novel mesoscopic simulation model is proposed to study the liquid crystal phase behavior of the anisotropic rodlike particles with a soft repulsive interaction,which possesses a modified anisotropic conservative force type used in dissipative particle dynamics.The influences of the repulsion strength and the particle shape on the phase behavior of soft rodlike particles are examined.In the simulations,we observe the formation of the nematic phase and smectic-A phase from the initially isotropic phase.More...     

Monodisperse conducting colloidal dipoles with symmetric dimer structure for enhancing electrorheology properties

This study introduces an electrorheological (ER) approach that allows us to obtain remarkably enhanced ER properties by using monodisperse colloidal dimer particles. Two sets of colloidal particles, which are spheres and symmetric dimers, were synthesized employing the seeded polymerization technique. The aspect ratio of dimer particles was ~1.43. Then, the surface of the particles was coated with polyaniline by using the chemically oxidative polymerization method. After preparation of the particle suspensions having the same particle volume and concentration, their ER behavior was investigated with changing the electric field strength. At the same experimental condition, both shear stress and shear yield stress of the dimer particle suspension remarkably increased, compared with those of the spherical particle suspension. This attributes to the fact that the shape anisotropy of suspending particles effectively led to increase in the dipole moment under the electric field, thus resulting in formation of a well-structured colloidal chains between the electrodes.     

One-step synthesis of highly monodisperse hybrid silica spheres in aqueous solution

An effective and reproducible method of preparing highly monodisperse organic-inorganic hybrid silica spheres was studied. One process, one precursor (organosilane) and one solvent (water) were used in our experiments. The size of hybrid silica spheres could be adjusted from 360 to 770 nm with relative standard deviation below 2% by controlling the concentration of the organosilane precursor and the ammonia catalyst. The increasing of the precursor concentration increases the particle size while the catalyst concentration has a reverse effect on the particle size. The concept of homogeneous nucleation and growth processes are introduced to explain the formation mechanism and the effect of reaction conditions. The scanning electron microscopy (SEM) images illustrate the copiousness in quantity and the uniformity in size/shape of the particles that could be routinely accomplished in this synthesis. Fourier transform infrared (FT-IR) and (29)Si nuclear magnetic resonance (NMR) spectra confirm the structure of vinyl hybrid silica spheres, where the vinyl group (-CH=CH(2)) exists and connects to the silicon atom. This method has also been extended to design and prepare other organic-inorganic hybrid materials especially in monodisperse surface-modified silica spheres.     

Synthesis of anisotropic PbS nanoparticles using heterocyclic dithiocarbamate complexes

We report the synthesis of lead piperidine and lead tetrahydroquinoline dithiocarbamate (DTC) complexes and their use as single source precursors for the preparation of anisotropic PbS nanoparticles. The complexes were thermolysed in coordinating solvents such hexadecylamime (HDA), tri-n-octylphosphine oxide (TOPO), oleylamine (OA) and decylamine (DA) at various reaction temperatures. The variation of the reaction conditions and precursors produced PbS particles with shapes ranging from spheres to cubes and rods. The size of the particles is generally larger than those synthesized by conventional precursor routes. The electron microscopy and X-ray diffraction data confirm the particles to be very crystalline with the dominant cubic rock salt phase present in all samples.     

Ordered packing of soft discoidal system

A novel mesoscopic simulation method is adopted to study the ordered packing of the anisotropic disklike particles with a soft repulsive interaction, which possesses a modified anisotropic conservative force type used in dissipative particle dynamics. We examine the influence of the shape of the particles, the angular width of the repulsion, and the strength of the repulsion on the packing structures. Specifically, an ordered hexagonal columnar structure is obtained in our simulations. Our study demonstrates that an anisotropic repulsive potential between soft discoidal particles is sufficient to produce a relatively ordered hexagonal columnar structure.     

Multi-gradient hydrogels produced layer by layer with capillary flow and crosslinking in open microchannels

This technical note describes a new bench-top method for producing anisotropic hydrogels composed of gradient layers of soluble factors, particles, polymer concentrations or material properties. Each gradient layer was produced by a previous gradient method in which a droplet of one precursor solution was added to a thin layer of a second solution. The ensuing rapid capillary flow along the open channel generated a gradient precursor solution, which was then crosslinked to form a gradient gel. Repeating these steps allowed a layered gel to be iteratively constructed with as many gradient layers as desired. This technique renders the synthesis of multi-layered gradient gels accessible to virtually any researcher and should help simplify the production of more biologically relevant cellular microenvironments.     

Phase diagrams of Wyoming Na-montmorillonite clay. Influence of particle anisotropy

Natural Na-Wyoming montmorillonite was size fractionated by successive centrifugation. Polydisperse particles with average sizes of 400, 290, and 75 nm were then obtained. As the structural charge of the particles belonging to three fractions (determined by cationic exchange capacity measurements) is the same, such a procedure allows studying the effect of particle anisotropy on the colloidal phase behavior of swelling clay particles. Osmotic stress experiments were carried out at different ionic strengths. The osmotic pressure curves display a plateau whose beginning systematically coincides with the sol/gel transition determined by oscillatory stress measurements. The concentration corresponding to the sol/gel transition increases linearly with particle anisotropy, which shows that the sol/gel transition is not directly related to an isotropic/nematic transition of individual clay particles. Indeed, a reverse evolution should be observed for an I/N transition involving the individual clay particles. Still, when observed between crossed polarizer and analyzer, the gel samples exhibit permanent birefringent textures, whereas in the "sol" region, transient birefringence is observed when the samples are sheared. This suggests that interacting clay particles are amenable to generate, at rest and/or under shear, large anisotropic particle associations.     

A modeling approach to describe the adhesion of rough, asymmetric particles to surfaces

A combined theoretical and experimental study of the adhesion of alumina particles and polystyrene latex spheres to silicon dioxide surfaces was performed. A boundary element technique was used to model electrostatic interactions between micron-scale particles and planar surfaces when the particles and surfaces were in contact. This method allows quantitative evaluation of the effects of particle geometry and surface roughness on the electrostatic interaction. The electrostatic interactions are combined with a previously developed model for van der Waals forces in particle adhesion. The combined model accounts for the effects of particle and substrate geometry, surface roughness and asperity deformation on the adhesion force. Predictions from the combined model are compared with experimental measurements made with an atomic force microscope. Measurements are made in aqueous solutions of varying ionic strength and solution pH. While van der Waals forces are generally dominant when particles are in contact with surfaces, results obtained here indicate that electrostatic interactions contribute to the overall adhesion force in certain cases. Specifically, alumina particles with complex geometries were found to adhere to surfaces due to both electrostatic and van der Waals interactions, while polystyrene latex spheres were not affected by electrostatic forces when in contact with various surfaces.     

Effect of trimethylamine on the formation of anatase titania nanoparticles by gel-sol method

The anatase titania particles with controlled size and shape were prepared in large amount in the presence of trimethylamine (TMA) from the hydrolysis of a Ti-triethanolamine (TEOA) complex by gel-sol method. In the absence of TMA, ellipsoidal particles were obtained due to the anisotropic growth caused by the specific adsorption of TEOA onto the crystal planes parallel to the c-axis of a titania particle. TMA acted as a complexing agent of Ti(IV) ion to promote the growth of ellipsoidal particles and then inhibited the anisotropic crystal growth to produce ellipsoids of a low aspect ratio, rather than a shape controller to produce ellipsoids with a high aspect ratio. This may be explained in terms of the weak complexing between TMA (a tertiary amine) and Ti(IV) ion. The particle size was also controlled by seeding of anatase titania. Moreover, the seeding suggested that the rate-determining step of the gel-sol process was not the dissolution of the hydroxide gel, but the deposition of the monomeric precursor from the solution phase. Some cationic surfactants also promoted the particle grown to produce particles with a high symmetry in a similar way to TMA. The text was submitted by the authors in English.     

A mathematical model for the interactions between non-identical rough spheres, liquid bridge and liquid vapor

Investigation of the interactions between the skeleton of an unsaturated particulate material and the contained liquid involves the general interaction model consisting of a liquid bridge in contact with two rigid smooth spherical particles of unequal size and dissimilar material, at a separation determined by their actual surface roughness, and surrounded by a gas with a vapor pressure at equilibrium with the liquid. The liquid retention and capillary force of the system are related to the capillary suction, liquid-solid contact angles, filling angles, roughness of the particles, and the ratio of particle radii in normalized terms by assuming a circular arc for the shape of the liquid profile. The normalized suction is also related to the corresponding relative humidity of the pore air. The calculated equilibrium relations are shown to possess non-uniqueness, which is interpreted in terms of mechanical properties of unsaturated particulate materials. The model is able to provide new insights into the behavior of an unsaturated particulate material.