Assembly of colloidal particles by evaporation on surfaces with patterned hydrophobicity

Drops containing suspended particles are placed on surfaces of patterned wettability created using soft lithography; the drop diameter is large compared to the dimensions of the patterns on the substrate. As the three-phase contact line of the drop recedes, spontaneous dewetting of the hydrophobic domains and flow into the hydrophilic domains create discrete fluid elements with peripheries that can mimic the underlying surface topography. Suspended particles are carried with the fluid into the wetted regions and deposit there as the discrete fluid domains evaporate. If particle volume fractions are sufficiently high, the entire wetted domain can be covered with colloidal crystals. At lower volume fractions, flow within the evaporating fluid element can direct the deposition of colloidal particles at the peripheries of the domains. High-resolution arrays of particles were obtained with a variety of features depending upon the relative size of the wetting regions to the particles. When the wetting region is larger than the particles, three-dimensional and two-dimensional arrays of ordered particles mimicking the shape of the wetting pattern form, depending on the particle volume fraction. For lower volume fractions, one-dimensional (1-D) arrays along the wet/non-wet boundaries form. When the particle size is similar to the height of fluid on the wetted domain, zero-dimensional distributions of single particles centered in the wet regions can form for wetted squares or 1-D distributions (stripes) form along the axis of striped domains. Finally, when the wetting region is smaller than the particle size, the particles do not deposit within the features but are drawn backward with the receding drop. These results indicate that evaporation on surfaces of patterned wetting provides a highly parallelizable means of tailoring the geometry of particle distributions to create patterned media.     

Responsive colloidal systems: reversible aggregation and fabrication of superhydrophobic surfaces

We report on a method of fabricating stimuli-responsive core-shell nanoparticles using block copolymers covalently bound to a silica nanoparticle surface. We used the "grafting to" approach to graft amphiphilic block copolymer brushes of poly(styrene-b-2-vinylpyridine-b-ethylene oxide) and poly(styrene-b-4-vinylpyridine) onto silica nanoparticles with two different diameters: colloidal silica 200 nm in diameter and fumed silica 15 nm in diameter. We used the pH-responsive properties of the grafted brush to regulate the interactions between the particles, and between the particles and their environment. We show that this behavior can be applied for a reversible formation of particle aggregates, and can be used to tune and stabilize the secondary aggregates of particles of the appropriate size and morphology in an aqueous environment. The suspensions of the particles form a textured hydrophilic coating on various substrates upon casting and the evaporation of water. Heating above the polymer's glass transition temperature or treatment in acidic water result in back and forth switching between superhydrophobic and hydrophilic surfaces, respectively.     

Parameters influencing the templated growth of colloidal crystals on chemically patterned surfaces

The influence of various experimental parameters on the vertical deposition and structure formation of colloidal crystals on chemically patterned surfaces, with hydrophilic and hydrophobic areas, was investigated. The pattern dimensions range from about 4 to 400 microm, which is much larger than the individual particle size (255 nm), to control the microscopic crystal shape rather than influencing the crystal lattice geometry (as achieved in colloidal epitaxy). The deposition resolution and selectivity were tested by varying the particle concentration in the suspension, the substrate withdrawing speed, pattern size and orientation, and wetting contrast between the hydrophilic and hydrophobic regions. The evolution of colloidal crystal thickness with respect to the pattern dimensions and deposition parameters was further studied. Our results show that the pattern size has a rather strong influence on the deposited number of colloid layers and on the crystal quality. Better results are obtained when the lines of a stripe pattern are oriented parallel to the withdrawing direction rather than perpendicular. The deposition resolution (defined as the minimum feature size on which particles can be deposited) depends on the wetting contrast and increases with lower average hydrophobicity of the substrate.     

Controlled particle placement through convective and capillary assembly

A wide variety of methods are now available for the synthesis of colloidal particle having controlled shapes, structures, and dimensions. One of the main challenges in the development of devices that utilize micro- and nanoparticles is still particle placement and integration on surfaces. Required are engineering approaches to control the assembly of these building blocks at accurate positions and at high yield. Here, we investigate two complementary methods to create particle assemblies ranging from full layers to sparse arrays of single particles starting from colloidal suspensions of gold and polystyrene particles. Convective assembly was performed on hydrophilic substrates to create crystalline mono- or multilayers using the convective flow of nanoparticles induced by the evaporation of solvent at the three-phase contact line of a solution. On hydrophobic surfaces, capillary assembly was investigated to create sparse arrays and complex three-dimensional structures using capillary forces to trap and organize particles in the recessed regions of a template. In both methods, the hydrodynamic drag exerted on the particle in the suspension plays a key role in the assembly process. We demonstrate for the first time that the velocity and direction of particles in the suspension can be controlled to perform assembly or disassembly of particles. This is achieved by setting the temperature of the colloidal suspension above or below the dew point. The influence of other parameters, such as substrate velocity, wetting properties, and pattern geometry, is also investigated. For the particular case of capillary assembly, we propose a mechanism that takes into account the relative influences of these parameters on the motion of particles and that describes the influence of temperature on the assembly efficiency.     

Elaboration of hydrophilic aminodextran containing submicron magnetic latex particles

Aminodextran containing submicron magnetic latex particles were prepared in two steps: (a) transformation of oil-in-water magnetic emulsion into structured magnetic latex particles via combination of seed and miniemulsion-like polymerization process and (b) immobilization (adsorption and chemical grafting) of prepared aminodextran onto negatively charged seed magnetic latex particles. The elaborated magnetic latex particles were characterized in terms of particle size, size distribution, morphology, surface charge density, chemical composition, magnetic properties, and also colloidal stability. The results showed that the morphology of the prepared seed magnetic latex is core–shell like and the cationic latex particles are hydrophilic and of high colloidal stability, irrespective of the aminodextran immobilization process.     

Physical and Chemical Properties of Magnetite and Magnetite-Polymer Nanoparticles and Their Colloidal Dispersions

The properties of polymer-coated magnetite nanoparticles, which have the potential to be used as effective magnetic resonance contrast agents, have been studied. The magnetite particles were synthesized by using continuous synthesis in an aqueous solution. The polymer-coated magnetite nanoparticles were synthesized by seed precipitation polymerization of methacrylic acid and hydroxyethyl methacrylate in the presence of the magnetite nanoparticles. The particle size was measured by laser light scattering. It was shown that the particle size, variance, magnetic properties, and stability of aqueous magnetite colloidal dispersion strictly depend on the nature of the stabilizing agent. The average hydrodynamic radius of the magnetite particles was found to be 5.7 nm in the stable aqueous colloidal dispersion. An inclusion of the magnetite particle into a hydrophilic polymeric shell increases the stability of the dispersion and decreases the influence of the stabilizing agent on the magnetic and structural properties of the magnetite particles as was shown by X-ray diffraction and M?ssbauer and IR spectroscopy, as well as by vibrating sample magnetometry. The variation in the polymeric shell size and the polymer net density can be useful tools for evaluation of the polymer-coated magnetite particles as effective contrast agents. Copyright 1999 Academic Press.     

Collodial cluster arrays by electrohydrodynamic printing

A "stable" electrohydrodynamic jet is used to print arrays of colloidal suspensions on hydrophobic surfaces. Printed lines break up into sessile drops, and capillary forces guide the self-assembly of colloidal particles during the evaporation of the liquid, resulting in arrays of colloidal single particles or particle clusters depending on the concentration of the suspensions. The clusters differ from those formed in the absence of a substrate when the number of particles is larger than three. Multiple structures are found for the same number of particles.     

Structural characterization of nanoparticles from thermoresponsive poly(N-isopropylacrylamide)-DNA conjugate

Poly(N-isopropylacrylamide) (PNIPAAm) grafted with single-stranded (ss) DNA conjugate (PNIPAAm-g-DNA) self-assembles above its lower critical solution temperature to form colloidal particles. When the ssDNA within the particle hybridizes with its complementary DNA, the particles aggregate above a certain threshold of salt concentration with drastically increased turbidity in solution. Detailed structural information of the particle was obtained mainly by small-angle X-ray scattering. The influence of copolymer composition on the morphology of particle and non-crosslinking aggregation was examined. The particle consists of hydrophobic PNIPAAm core surrounded by hydrophilic DNA strands. The increase in DNA fraction brought about a significant decrease in core size, whereas the shell thickness little changed and corresponded to the length of DNA. A structural model with a sticky potential was applied to the analysis of particle aggregate. This analysis provided that the particles aggregate while the coronal layers interpenetrate each other. The interaction between the particles was quantified in terms of the sticky potential and showed a trend to be influenced by the particle size rather than the graft density of DNA strands on the particle.     

Morphological and compositional evolution of polymeric colloidal monolayer during UV irradiation

In this study, we investigated the morphological and compositional evolution polymeric colloidal monolayer during UV irradiation. A PS colloidal monolayer with interparticle bridges was prepared and exposed to the UV light. As a consequence of photochemical reactions containing chain-scission, UV irradiation induced morphological changes in the monolayer surface including changes in the size, shape, and packing structure of PS particles. By manipulating the UV irradiation time, fine tuning of size and shape of the interstice in the monolayer was achieved. In these procedures, the interparticle bridges play an important role. The UV irradiation induced the formation of polar groups in the PS particle surface and thus the particle surface became highly hydrophilic.     

Deposition of magnetic colloidal particles on graphite and mica surfaces driven by solvent evaporation

The deposition of colloidal magnetite particles onto graphite and mica surfaces induced by solvent evaporation is studied using atomic force microscopy. After evaporation under ambient conditions we observe polydisperse beadlike aggregates; the mean aggregate diameter is larger on graphite than on mica. After evaporation at elevated temperatures we observe a variety of effects, including enhanced particle aggregation and spinodal-like deposition patterns. To explain these trends, we propose mechanisms involving the wetting properties of the solvent. We have also made a brief study of the effects of applied magnetic fields on the formation of aggregates. A field applied parallel to the surface enhances aggregation and favors deposition patterns characteristic of hole-nucleation processes. A perpendicular field leads to a reduction in aggregate size and favors a homogeneous distribution of particles on the surface. These effects are explained in terms of the likely orientation of the dipolar particles on the surface.     

Surfactant-induced Marangoni eddies alter the coffee-rings of evaporating colloidal drops

The influence of the small ionic surfactant sodium dodecyl sulfate (SDS) on the evaporation of drying colloidal droplets is quantitatively investigated. The addition of SDS leads to a significantly more uniform deposition of colloidal particles after evaporation (i.e., the so-called "coffee-ring effect" is dramatically altered). We understand this phenomenon in the context of circulating radial Marangoni flows induced by the variation of SDS concentration along the air-water interface. Video microscopy permits the direct visualization of the colloidal particles involved in these flows, revealing a surprisingly stable "Marangoni eddy" that prevents particle deposition at the drop perimeter.     

Effect of colloidal particle size on adsorbed monodisperse and bidisperse monolayers

Coating hydrogel films or microspheres by an adsorbed colloidal shell is one synthesis method for forming colloidosomes. The colloidal shell allows control of the release rate of encapsulated materials, as well as selective transport. Previous studies found that the packing density of self-assembled, adsorbed colloidal monolayers is independent of the colloidal particle size. In this paper we develop an equilibrium model that correlates the packing density of charged colloidal particles in an adsorbed shell to the particle dimensions in monodisperse and bidisperse systems. In systems where the molar concentration in solution is fixed, the increase in adsorption energy with increasing particle size leads to a monotonic increase in the monolayer packing density with particle radius. However, in systems where the mass fraction of the particles in the adsorbing solutions is fixed, increasing particle size also reduces the molar concentration of particles in solution, thereby reducing the probability of adsorption. The result is a nonmonotonic dependence of the packing density in the adsorbed layer on the particle radius. In bidisperse monolayers composed of two particle sizes, the packing density in the layer increases significantly with size asymmetry. These results may be utilized to design the properties of colloidal shells and coatings to achieve specific properties such as transport rate and selectivity.     

Self-assembly of poly(N-isopropylacrylamide)-carrying microspheres into two-dimensional colloidal arrays

Droplets containing polymer particles were deposited on a substrate. Poly(N-isopropylacrylamide) (PNIPAM) hydrogel and particles with PNIPAM graft chains on the surface self-assembled into a two-dimensional (2-D) superlattice when their dilute dispersions were dried on substrates. The capillary force between the particles induced ordered array formation during water evaporation. The presence of a PNIPAM layer on the particle surface gave the particles steric stability during ordered array formation. By grafting PNIPAM chains on particle surfaces by living radical polymerization, we successfully controlled the structural patterns of the colloidal arrays. These, controllable, 2-D colloidal arrays were generated on various substrates upon air-drying.     

PS colloidal particles stabilized by graphene oxide

Exfoliated graphene oxide (GO) sheets with hydrophilic functional groups on the surface were prepared by the oxidation of graphite. Because of the hydrophilic groups on the sheets and the hydrophobic carbon surface, GO sheets were located at the oil-water interface and could be used as a stabilizer in Pickering emulsions. After the Pickering emulsion polymerization of styrene, PS colloidal particles with GO sheets on the surface were prepared. The size of the GO sheets exerts an important influence on the preparation of PS colloidal particles. Small GO sheets located at the liquid-liquid interface and GO-stabilized PS colloidal particles were prepared; however, for large GO sheets, smaller PS colloidal particles prepared on the GO surface were observed. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) were used to characterize the structure and morphology of the colloidal particles. TEM, SEM, and XPS results all suggest the successful preparation of GO-stabilized PS colloidal particles.     

Monte Carlo computer simulations and electron microscopy of colloidal cluster formation via emulsion droplet evaporation

We consider a theoretical model for a binary mixture of colloidal particles and spherical emulsion droplets. The hard sphere colloids interact via additional short-ranged attraction and long-ranged repulsion. The droplet-colloid interaction is an attractive well at the droplet surface, which induces the Pickering effect. The droplet-droplet interaction is a hard-core interaction. The droplets shrink in time, which models the evaporation of the dispersed (oil) phase, and we use Monte Carlo simulations for the dynamics. In the experiments, polystyrene particles were assembled using toluene droplets as templates. The arrangement of the particles on the surface of the droplets was analyzed with cryogenic field emission scanning electron microscopy. Before evaporation of the oil, the particle distribution on the droplet surface was found to be disordered in experiments, and the simulations reproduce this effect. After complete evaporation, ordered colloidal clusters are formed that are stable against thermal fluctuations. Both in the simulations and with field emission scanning electron microscopy, we find stable packings that range from doublets, triplets, and tetrahedra to complex polyhedra of colloids. The simulated cluster structures and size distribution agree well with the experimental results. We also simulate hierarchical assembly in a mixture of tetrahedral clusters and droplets, and find supercluster structures with morphologies that are more complex than those of clusters of single particles.     

Pyramidal assemblies of colloidal particles by micromolding underneath top-gathering pillar arrays

Assemblies of colloidal particles are known to have special photonic and optical properties. Periodic pyramidal assemblies of SiO2 particles with diameters of 0.5 and 1 microm were fabricated using top-gathering pillar arrays as a new template. These top-gathering pillar arrays consisted of four pillars gathered at the top, and were fabricated by photolithography of an organic-inorganic hybrid material. The top-gathering units were obtained by controlling both the capillary and restoring forces. When a colloidal water suspension was spread on the template and the water was allowed to evaporate, the SiO2 particles were molded under the top-gathering pillars according to particle size, resulting in pyramidal assembly arrays of the particles. From in situ observation during the evaporation of water, it was found that the particles were molded underneath the top-gathering pillars by flux generated by the evaporation and by the capillary force among the particles.     

Colloidal Interaction at the Air-Liquid Interface

In this paper, we propose a model to analyze the stability of colloidal particles at the air-liquid interface. The proposed model for the colloidal particle interaction considers DLVO interactions and capillary, hydrophobic, and dipolar interactions between the particles. Typical values from the literature were assigned to most parameters included in the model. Numerical computation revealed the most important parameter in determining the total interaction is the density of dipoles at the external surface of the particles. We have found significant differences for the pair potential between hydrophobic and hydrophilic particles. Hydrophobic particles must aggregate in a principal minimum of the interaction potential curve while hydrophilic particles aggregate in a secondary minimum. Copyright 2000 Academic Press.     

Fast evaporation of spreading droplets of colloidal suspensions

When a coffee droplet dries on a countertop, a dark ring of coffee solute is left behind, a phenomenon often referred to as the coffee-ring effect. A closely related yet less-well-explored phenomenon is the formation of a layer of particles, or skin, at the surface of the droplet during drying. In this work, we explore the behavior of a mathematical model that can qualitatively describe both phenomena. We consider a thin axisymmetric droplet of a colloidal suspension on a horizontal substrate undergoing spreading and evaporation. In contrast to prior work, precursor films (rather than pinned contact lines) are present at the droplet edge, and evaporation is assumed to be limited by how quickly molecules can transfer out of the liquid phase (rather than by how quickly they can diffuse through the gas phase). The lubrication approximation is applied to simplify the mass and momentum conservation equations, and the colloidal particles are allowed to influence the droplet rheology through their effect on the viscosity. By describing the transport of the colloidal particles with the full convection-diffusion equation, we are able to capture depthwise gradients in particle concentration and thus describe skin formation, a feature neglected in prior models of droplet evaporation. The highly coupled model equations are solved for a range of problem parameters using a finite-difference scheme based on a moving overset grid. The presence of evaporation and a large particle Peclet number leads to the accumulation of particles at the liquid-air interface. Whereas capillarity creates a flow that drives particles to the droplet edge to produce a coffee ring, Marangoni flows can compete with this and promote skin formation. Increases in viscosity due to particle concentration slow down droplet dynamics and can lead to a reduction in the spreading rate.     

Stabilization of oil-in-water emulsions by colloidal particles modified with short amphiphiles

Emulsions stabilized through the adsorption of colloidal particles at the liquid-liquid interface have long been used and investigated in a number of different applications. The interfacial adsorption of particles can be induced by adjusting the particle wetting behavior in the liquid media. Here, we report a new approach to prepare stable oil-in-water emulsions by tailoring the wetting behavior of colloidal particles in water using short amphiphilic molecules. We illustrate the method using hydrophilic metal oxide particles initially dispersed in the aqueous phase. The wettability of such particles in water is reduced by an in situ surface hydrophobization that induces particle adsorption at oil-water interfaces. We evaluate the conditions required for particle adsorption at the liquid-liquid interface and discuss the effect of the emulsion initial composition on the final microstructure of oil-water mixtures containing high concentrations of alumina particles modified with short carboxylic acids. This new approach for emulsion preparation can be easily applied to a variety of other metal oxide particles.     

Preparation of monodisperse ellipsoidal polystyrene particles

A method is described for the preparation of monodisperse ellipsoidal particles of polystyrene in the colloidal size range. Monodisperse polystyrene particles were dispersed in a solution of polyvinyl alcohol. This dispersion was then allowed to form, by evaporation, a thin film of polyvinyl alcohol containing spherical polystyrene particles. Strips of this film were clamped into a metal frame, heated rapidly in an oil bath to 200°C and stretched to a predetermined extent in order to convert the spherical particles into ellipsoids; the film was then cooled. A wide range of axial ratios for a variety of initial particle sizes was obtained by this method.