Nonspherical Colloidal Particles Research Articles

Precise measurements in digital holographic microscopy by modeling the optical train

In the past few years, the venerable field of holographic microscopy has been revitalized by computational data analysis. It is now possible to fit a generative (forward) model of scattering directly to experimentally obtained holograms of complex microscopic objects. This approach enables precision measurements: it allows the motion of colloidal particles and biological organisms to be tracked with nanometer-scale precision and their optical properties to be inferred particle by particle. In this Perspective, we discuss how the model-based inference approach to holographic microscopy has opened up new applications. We also discuss how it must evolve to meet the needs of emerging applications that demand lower systematic uncertainties and higher precision. In this context, we present some new results on how modeling the optical train of the microscope can enable better measurements of the positions of spherical and nonspherical colloidal particles. Finally, we discuss how machine learning might play a role in future advances. Though we do not exhaustively catalog all the developments in this field, we show a few examples and some new results that spotlight open questions and opportunities.

Chiral motion in colloidal electrophoresis.

Asymmetrically charged, nonspherical colloidal particles in general perform complex rotations and oblique motions under an electric field. The interplay of electrostatic and hydrodynamic forces complicates the prediction of these motions. We demonstrate a method of calculating the body tensors that dictate translational and rotational velocity vectors arising from an external electric field. We treat insulating rigid bodies in the linear-response regime, with indefinitely small electrostatic screening length. The method represents the body as an assembly of point sources of both hydrodynamic drag and surface electric field. We demonstrate agreement with predicted electrophoretic mobility to within a few percent for several shapes with uniform and nonuniform charges. We show that even symmetric shapes can have strong chiral twisting motions. The method applies more generally to active colloidal swimmers.

Intrinsic viscosities of non-spherical colloids by Brownian dynamics simulations.

A numerical study is presented on the intrinsic viscosities of sheared dilute suspensions of nonspherical Brownian colloidal particles. The simulations confirm theoretical predictions on the intrinsic viscosities of highly oblate and highly prolate spheroids in the limits of weak and strong Brownian noise (i.e., for low and high Péclet numbers). Numerical data and fit functions are provided covering the entire shear-thinning regime, for spheroids ranging from highly oblate to highly prolate. The tumbling motion and intrinsic viscosities of a hemispherical cap and a helix are briefly discussed.

Controllable synthesis of monodisperse nonspherical colloidal particles with cavity structures

ABSTRACTThis article reports a facile controllable approach to prepare monodisperse nonspherical colloidal particles with cavity structures by one‐pot soap‐free emulsion polymerization of styrene (St), 3‐(trimethoxysilyl)propyl methacrylate (MPS), and acrylic acid (AA). In our strategy, only by varying the feeding time of AA to the as‐polymerized St and MPS, the nonspherical latex particles with single cavity of different surface roughness and multicavity structures could be successfully synthesized. The depth and width of the cavity can be also easily controlled by adjusting the amount of MPS and AA. A possible formation mechanism is proposed on the basis of experimental results. These nonspherical colloidal particles, which have controllable cavity structures, are good building blocks or templates for the construction of functional coating and complex colloidal architectures. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1645–1652

Transition from Linear to Circular Motion in Active Spherical-Cap Colloids.

Nonspherical self-propelling colloidal particles offer many possibilities for creating a variety of active motions. In this work, we report on the transition from linear to circular motion of active spherical-cap particles near a substrate. Self-propulsion is induced by self-diffusiophoresis by catalytic decomposition of hydrogen peroxide (H2O2) on one side of the particle. Asymmetric distribution of reaction products combined with the asymmetric shape of the particle gives rise to two types of motions depending upon the relative orientation of the particle with respect to the underlying substrate. At a low concentration of H2O2, linear active motion is observed, whereas increasing the H2O2 concentration leads to persistent circular motion. However, the speed of self-propulsion is nearly independent of the size of the particle. The study demonstrates the use of nonspherical particles to create linear and circular motion by varying the fuel concentration.

Fabrication of shape-tunable macroparticles by seeded polymerization of styrene using non-cross-linked starch-based seed

Nonspherical colloidal particles with various geometries and different compositions have attracted tremendous attention and been widely researched. The preparation of polymer colloidal particles with controlled shapes by seeded polymerization is recognized as the most promising technique owing to the precise control of various morphologies and using non-cross-linked seed particles are of particular interest. Seeds particles derived from natural biopolymers are seldom applied. Hence, non-cross-linked starch-based seed could be used to fabricate the anisotropic particles by soap-free seed polymerization.Non-cross-linked starch-based seed particles were prepared by a nanoprecipitation method. Starch/polystyrene composite colloidal particles with shape-tunable were fabricated by soap-free seeded polymerization using starch-based seed. The effect of the polymerization time, monomer feed ratio and seed type were investigated.The seed particles with a single- or multi-hole structure were obtained after swelling with styrene. The resulting particles including golf-like, raspberry-like, octahedron-like and snowman-like structures, was fabricated on the polymerization process. This study firstly reports that the morphology of composite particles from golf-like to snowman-like at high monomer feed ratio using starch-based seed. At low monomer feed ratio, raspberry-like particles were obtained by surface nucleation increasing process. In addition, seed type also effect the morphology of composite particles.

Effect of mixing ratio of non-spherical particles in colloidal silica slurry on oxide CMP

Colloidal silica and fumed silica are common slurry materials for oxide chemical mechanical polishing (CMP). Non-spherical colloidal silica particles are manufactured via a multi-step feeding method to compensate for the material removal rate of colloidal silica slurry, which is lower than that of fumed silica slurry. Additionally, mixed abrasive slurry has been used by combining non-spherical and spherical colloidal silica particles. Experiments were conducted on a 4-inch wafer with silicon dioxide film (SiO 2 , 1-μm thick) and KOH-based colloidal silica slurry. The slurry had spherical colloidal silica particles with the size of 30 nm and 70 nm and non-spherical colloidal silica particles with the size of 70 nm. Effects of different colloidal particles in the oxide CMP were observed under the same pH condition in order to achieve high material removal rate. The mixing ratios were 2:1, 1:1, and 1:2 respectively. The analysis of the particle combinations showed that the mixing of particles with different sizes caused agglomeration and increased the material removal rate. The relatively smaller spherical particles improved the surface roughness and overall performance, especially when mixed with non-spherical particles. Such mixtures of particles with different sizes and shapes mixed at an appropriate ratio can outperform the fumed silica slurry.

General Potential for Anisotropic Colloid-Surface Interactions.

A general closed-form, analytical potential is developed for the interaction of planar surfaces with superellipsoidal particles (which includes shapes such as spheres, ellipsoids, cylinders, polygons, superspheres, etc.). The Derjaguin approximation is used with DLVO half-space interactions (e.g., electrostatics and van der Waals) to yield potentials for arbitrary particle-wall separation and orientation. The resulting potential is a function of the minimum distance between surfaces and the particle's local Gaussian curvature at the minimum distance position. The validity of the solution is reported in terms of the local Gaussian curvature (Γ) and characteristic interaction range (e.g., Debye length, κ-1, for electrostatics) based on the limits of the Derjaguin approximation. This solution is limited for superellipsoids with convex shapes and orientations where the condition κ/Γ1/2 > 2 is satisfied. The potentials reported in this work should be useful for modeling a wide range of natural and synthetic nonspherical and anisotropic colloidal particles in environmental, biological, and advanced material applications.

Zeta potential measurements for non-spherical colloidal particles – Practical issues of characterisation of interfacial properties of nanoparticles

The optimisation of new particle based products with respect to their stability and physico-chemical behaviour requires the characterisation of the interfacial properties. Such a characterisation may be achieved by means of the zeta potential, which is related to the surface charge and electric double layer (EDL) of colloidal particles. Furthermore, the zeta potential is a powerful parameter of the EDL and an appropriate way of predicting the suspension stability. This study discusses the performance of two techniques for the measurement of zeta potential value and its comparability of zeta potential values for pyrogenic materials. This measurement encounters several experimental challenges, because each measurement technique possesses specific limits with regard to particle size, concentration and morphology. Furthermore, this study examines the comparability of zeta potential measurements by variation of mentioned specific limits taking the conservation of the chemical equilibrium into account and also understanding the impact of aggregate morphology on the electrokinetic mobility.

The attachment of colloidal particles to environmentally relevant surfaces and the effect of particle shape

Despite the prevalence of nonspherical colloidal particles, the role of particle shape in the transport of colloids is largely understudied. This study investigates the attachment of colloidal particles onto environmentally relevant surfaces while varying particle shape and ionic strength. Using quartz crystal microbalance and atomic force microscopy measurements, the role of particle shape was elucidated and possible mechanisms discussed. The attachment of both spherical and stretched polystyrene colloidal particles onto a smooth alginate-coated silica surface showed qualitative agreement with DLVO theory. Attachment onto a Harpeth humic acid (HHA) surface, however, significantly deviated from DLVO theory due to its high surface heterogeneity and extended confirmation from the silica surface. This extended confirmation provided increased potential for spherical particle entanglement, while the enlarged major axis of the stretched particles hindered their ability to attach. As ionic strength increased, the HHA layer condensed and provided less potential for spherical particle entanglement and therefore the selectivity for spherical particle attachment vanished. The findings presented in this study suggest that colloidal particle shape may play a complex and important role in predicting the transport of colloidal particles, especially in the presence of natural organic matter-coated surfaces.

Effect of non-spherical colloidal silica particles on removal rate in oxide CMP

Oxide CMP generally has used colloidal silica slurry and fumed silica slurry. To compensate the lower removal rate (RR) of colloidal silica slurry compared to the one of fumed silica slurry, the multi-step feeding method was used to produce a non-spherical silica particle, for which the existing spherical colloidal silica particle was combined with two to three particles. To improve the performances, the prepared non-spherical colloidal silica particles were assessed by both single and mixed abrasive slurry. The slurry has two different sizes, 30 nm and 70 nm, of spherical colloidal silica particles and 70nm of non-spherical colloidal silica particles. Through the experiments, it was found out that non-spherical colloidal silica slurry had the higher RR than spherical colloidal silica slurry, and its RR was similar to the one of fumed silica slurry. For mixed abrasive slurry, the combinations of spherical and spherical or spherical and non-spherical showed higher RR, which were 152.8% higher than single abrasive slurry and 19.3% higher than fumed silica slurry, and better surface roughness than the fumed silica slurry. Therefore, non-spherical colloidal silica particles are found to be efficient to gain a higher RR and a better surface roughness than the ones of fumed silica particles.

Non-spherical colloidal silica particles—Preparation, application and model

Non-spherical colloidal particles have shown their great potential in the field of chemical mechanical polishing (CMP) due to their high polishing rate. A new method named “cation induced method” has been developed and used to produce non-spherical colloidal silica particles. We have tested this kind of novel shape particles in the application of SiO2's CMP. The material removal rate of this kind of abrasive particles has been tested and verified to be much faster than traditional spherical silica particles. In addition, both the particles’ morphology and properties have been characterized by the scanning electron microscope (SEM), transmission electron microscope (TEM) and dynamic light scattering (DLS). Finally, a semiquantitative model is built to show the relationship between the shape of particles and the amount of inducing agents.

Using the discrete dipole approximation and holographic microscopy to measure rotational dynamics of non-spherical colloidal particles

We present a new, high-speed technique to track the three-dimensional translation and rotation of non-spherical colloidal particles. We capture digital holograms of micrometer-scale silica rods and sub-micrometer-scale Janus particles freely diffusing in water, and then fit numerical scattering models based on the discrete dipole approximation to the measured holograms. This inverse-scattering approach allows us to extract the position and orientation of the particles as a function of time, along with static parameters including the size, shape, and refractive index. The best-fit sizes and refractive indices of both particles agree well with expected values. The technique is able to track the center of mass of the rod to a precision of 35nm and its orientation to a precision of 1.5°, comparable to or better than the precision of other 3D diffusion measurements on non-spherical particles. Furthermore, the measured translational and rotational diffusion coefficients for the silica rods agree with hydrodynamic predictions for a spherocylinder to within 0.3%. We also show that although the Janus particles have only weak optical asymmetry, the technique can track their 2D translation and azimuthal rotation over a depth of field of several micrometers, yielding independent measurements of the effective hydrodynamic radius that agree to within 0.2%. The internal and external consistency of these measurements validate the technique. Because the discrete dipole approximation can model scattering from arbitrarily shaped particles, our technique could be used in a range of applications, including particle tracking, microrheology, and fundamental studies of colloidal self-assembly or microbial motion.

Towards the self-assembly of anisotropic colloids: Monodisperse oblate ellipsoids

We present a robust and straightforward method for producing colloidal particles of oblate ellipsoidal shape via thermo/mechanical stretching of elastomeric films with embedded spherical particles. Our method produces uniformly sized and shaped colloidal particles. The method can be used for producing biaxially stretched particles of different aspect ratios and volumes; moreover, the method has a higher yield and batch size than previously reported methods for producing non-spherical particles via film stretching. These particles are ideal model systems for studying the self-assembly and gel formation for systems with anisotropic shapes and interactions. We illustrate this by adding of a non-adsorbing polymer to the solvent, thereby inducing directional depletion interactions between the particles.

Chiral colloidal molecules and observation of the propeller effect.

Chiral molecules play an important role in biological and chemical processes, but physical effects due to their symmetry-breaking are generally weak. Several physical chiral separation schemes which could potentially be useful, including the propeller effect, have therefore not yet been demonstrated at the molecular scale. However, it has been proposed that complex nonspherical colloidal particles could act as “colloidal molecules” in mesoscopic model systems to permit the visualization of molecular phenomena that are otherwise difficult to observe. Unfortunately, it is difficult to synthesize such colloids because surface minimization generally favors the growth of symmetric particles. Here we demonstrate the production of large numbers of complex colloids with glancing angle physical vapor deposition. We use chiral colloids to demonstrate the Baranova and Zel’dovich (BaranovaN. B.Zel’dovichB. Y.Chem. Phys. Lett.1978, 57, 435) propeller effect: the separation of a racemic mixture by application of a rotating field that couples to the dipole moment of the enantiomers and screw propels them in opposite directions. The handedness of the colloidal suspensions is monitored with circular differential light scattering. An exact solution for the colloid’s propulsion is derived, and comparisons between the colloidal system and the corresponding effect at the molecular scale are made.

Critical Casimir torques and forces acting on needles in two spatial dimensions

We investigate the universal orientation-dependent interactions between nonspherical colloidal particles immersed in a critical solvent by studying the instructive paradigm of a needle embedded in bounded two-dimensional Ising models at bulk criticality. For a needle in an Ising strip, the interaction on mesoscopic scales depends on the width of the strip and the length, position, and orientation of the needle. By lattice Monte Carlo simulations we evaluate the free-energy difference between needle configurations being parallel and perpendicular to the strip. We concentrate on small but nonetheless mesoscopic needle lengths for which analytic predictions are available for comparison. All combinations of boundary conditions for the needles and boundaries are considered which belong to either the "normal" or the "ordinary" surface universality class, i.e., which induce local order or disorder, respectively. We also derive exact results for needles of arbitrary mesoscopic length, in particular for needles embedded in a half plane and oriented perpendicularly to the corresponding boundary as well as for needles embedded at the center line of a symmetric strip with parallel orientation.

Syntheses and applications of concave and convex colloids with precisely controlled shapes

This article describes a facile and promising method to prepare uniform, non-spherical colloidal particles with shape and chemical anisotropies. We first synthesize polystyrene (PS) colloids with a concave multi-dimple structure, through a unique combination of two-stage dispersion polymerizations and a programmed feeding of cross-linker during the synthesis. These concave particles, after a seeded polymerization process, can be swelled out to precisely controlled shapes by varying the amounts of the swelling chemicals. A myriad of concave- and convex-shaped colloidal particles, including hemispheres, peanuts and bowls are obtained. In addition, we explore the potential applications of the synthesized anisotropic colloids by looking at the field-driven directional self-assembly, the optical hiding effect and their use as colloidal surfactants.

Directed Self-Assembly of Colloidal Dumbbells with an Electric Field

We demonstrate the assembly of colloidal particles with the shape of diatomic molecules ("dumbbells") into crystals that we study with confocal microscopy. The literature on the preparation of nonspherical colloidal particles has grown steadily. Assembly of these particles into regular three-dimensional crystalline lattices, however, is rarely, if ever, achieved and has not yet been studied quantitatively in 3D real space. We find that, by application of an electric field, such particles assemble quite readily. By varying the particle aspect ratio, range of interactions, and electric field strength, we find several different crystal structures of which three have never before been observed. Moreover, the electric field can be used to switch between different structures and manipulate/switch the photonic properties. Moreover, our work sheds light on fundamental questions related to the self-assembly of nonspherical particles.

Scalable, shape-specific, top-down fabrication methods for the synthesis of engineered colloidal particles.

The search for a method to fabricate nonspherical colloidal particles from a variety of materials is of growing interest. As the commercialization of nanotechnology continues to expand, the ability to translate particle-fabrication methods from a laboratory to an industrial scale is of increasing significance. In this feature article, we examine several of the most readily scalable top-down methods for the fabrication of such shape-specific particles and compare their capabilities with respect to particle composition, size, shape, and complexity as well as the scalability of the method. We offer an extensive examination of particle replication in nonwetting templates (PRINT) with regard to the versatility and scalability of this technique. We also detail the specific methods used in PRINT particle fabrication, including harvesting, purification, and surface-modification techniques, with an examination of both past and current methods.

Critical adsorption on nonspherical colloidal particles

We consider a nonspherical colloidal particle immersed in a fluid close to its critical point. The temperature dependence of the corresponding order parameter profile is calculated explicitly. We perform a systematic expansion of the order parameter profile in powers of the local curvatures of the surface of the colloidal particle. This curvature expansion reduces to the short distance expansion of the order parameter profile in the case that the solvent is at the critical composition.