Andrew D. Hollingsworth

Colloids with valence and specific directional bonding

The ability to design and assemble three-dimensional structures from colloidal particles is limited by the absence of specific directional bonds. As a result, complex or low-coordination structures, common in atomic and molecular systems, are rare in the colloidal domain. Here we demonstrate a general method for creating the colloidal analogues of atoms with valence: colloidal particles with chemically distinct surface patches that imitate hybridized atomic orbitals, including sp, sp2, sp3, sp3d, sp3d2 and sp3d3. Functionalized with DNA with single-stranded sticky ends, patches on different particles can form highly directional bonds through programmable, specific and reversible DNA hybridization. These features allow the particles to self-assemble into ‘colloidal molecules’ with triangular, tetrahedral and other bonding symmetries, and should also give access to a rich variety of new microstructured colloidal materials.

Electrostatics at the oil–water interface, stability, and order in emulsions and colloids

Oil–water mixtures are ubiquitous in nature and are particularly important in biology and industry. Usually additives are used to prevent the liquid droplets from coalescing. Here, we show that stabilization can also be obtained from electrostatics, because of the well known remarkable properties of water. Preferential ion uptake leads to a tunable droplet charge and surprisingly stable, additive-free, water-in-oil emulsions that can crystallize. For particle-stabilized (“Pickering”) emulsions we find that even extremely hydrophobic, nonwetting particles can be strongly bound to (like-charged) oil–water interfaces because of image charge effects. These basic insights are important for emulsion production, encapsulation, and (self-)assembly, as we demonstrate by fabricating a diversity of structures in bulk, on surfaces, and in confined geometries.

Theoretical prediction of collision efficiency between adhesion-deficient bacteria and sediment grain surface

Abstract Our earlier results concerning bacterial transport of an adhesion-deficient strain Comamonas sp. (DA001) in intact sediment cores from near South Oyster, VA demonstrated that grain size is the principle factor controlling bacterial retention, and that Fe and Al hydroxide mineral coatings are of secondary importance. The experimentally determined collision efficiency ( α ) was in the range of 0.003–0.026 and did not correlate with the Fe and Al concentration. This study attempts to theoretically predict α , and identifies factors responsible for the observed low α . The modified Derjaguin–Landau–Verwey–Overbeek (DLVO) theory was used to calculate the total intersurface potential energy as a function of separation distance between bacterial and sediment surfaces and to provide insights into the relative importance of bacterial and sediment grain surface properties in controlling magnitude of α . Different models for calculating theoretical α were developed and compared. By comparing theoretical α values from different models with previously published experimental α values, it is possible to identify a suitable model for predicting α . When DA001 bacteria interact with quartz surfaces, the theoretical α best predicts experimental α when DA001 cells are reversibly attached to the secondary minimum of the energy interaction curve and α depends on the probability of escape from that energy well. No energy barrier opposes bacterial attachment to clean iron oxide surface of positive charge at sub-neutral pH, thus the model predicts α of unity. When the iron oxide is equilibrated with natural groundwater containing 5–10 ppm of dissolved organic carbon (DOC), its surface charge reverses, and the model predicts α to be on the order of 0.2. The theoretical α for DA001 in the natural sediments from South Oyster, VA was estimated by representing the surface potential of the sediment as a patch-wise binary mixture of negatively charged quartz ( ζ =−60 mV) and organic carbon coated Fe–Al hydroxides ( ζ =−2 mV). Such a binary mixing approach generates α that closely matches the experimental α . This study demonstrates that it is possible to predict α from known bacterial and grain surface properties.

Patchy Particle Self-Assembly via Metal Coordination

Colloids with high-symmetry patches are functionalized with metal-coordination-based recognition units and assembled into larger chain architectures, demonstrating for the first time the use of metal coordination as a specific force in colloidal self-assembly. The cross-linked poly(styrene)-based patchy particles are fabricated by encapsulation of colloidal clusters following a two-stage swelling and polymerization methodology. The particle patches, containing carboxylic acid groups, are site-specifically functionalized either with a triblock copolymer (TBC), bearing primary alcohols, alkyl chains, and palladated pincer receptors, synthesized by ring-opening metathesis polymerization, or with a small molecule bearing a pyridine headgroup. Functionalizing with a TBC provides design flexibility for independently setting the range of the interaction and the recognition motif.

Revisiting the synthesis of a well-known comb-graft copolymer stabilizer and its application to the dispersion polymerization of poly(methyl methacrylate) in organic media.

Polymeric stabilizers are an essential ingredient for the dispersion polymerization of poly(methyl methacrylate) (PMMA) in nonpolar media. In this contribution, we focus on the synthesis of an amphipathic copolymer consisting of pendant poly(12-hydroxystearic acid) (PHS) chains grafted to an insoluble PMMA backbone. This type of steric stabilizer is well established and capable of producing spherically shaped, monodisperse PMMA colloids. Unfortunately, the comb-graft copolymer is not available commercially; furthermore, the multistep synthesis of the desired stabilizer has proven challenging to reproduce. We discuss the practical matter of preparing PHS-graft-PMMA, and report specific techniques developed over several years in our lab. Gel permeation chromatography, mass spectroscopy, and end group analysis of the stabilizer and the precursor macromonomer reveal important, previously unreported details about the chemical synthesis. Our protocol is reproducible and resulted in the production of low polydispersity PMMA particles.

Comment on “A broad frequency range dielectric spectrometer for colloidal suspensions: cell design, calibration, and validation”

Recently, a simple analytical model was derived from the standard electrokinetic theory to deal with electrode polarization in low frequency dielectric measurements [Hollingsworth and Saville, J. Colloid Interface Sci. 257 (2003) 65-76]. Comparisons were made between dielectric constant-frequency relationships extracted from data on electrolyte solutions using the analytical model and the well-known RC-circuit analog. Electrolyte spectra interpreted with the analytical approach remained smooth (and constant) down to much lower frequencies than was the case with the RC model. It was also shown how the RC model arises naturally from the analytical model as an asymptotic expansion in inverse powers of the electrode spacing. Numerical calculations indicated substantial differences between the two models at low frequencies due to higher order terms omitted in the RC model. As it turns out, the comparison contained a numerical error. Here we revisit the methodology to show that although the two formulations disagree, they do so (in a numerical sense) only at much smaller electrode separations than those used in the aforementioned example. The purpose of this Letter is to correct the numerical error and show, explicitly, how the RC-circuit analog coefficients are related to the asymptotic expansion at low frequencies.

Patchy Particle Packing under Electric Fields

Colloidal particles equipped with two, three, or four negatively charged patches, which endow the particles with 2-fold, 3-fold, or tetrahedral symmetries, form 1D chains, 2D layers, and 3D packings when polarized by an AC electric field. Two-patch particles, with two patches on opposite sides of the particle (2-fold symmetry) pack into the cmm plane group and 3D packings with I4mm space group symmetry, in contrast to uncharged spherical or ellipsoidal colloids that typically crystallize into a face-centered ABC layer packing. Three-patch particles (3-fold symmetry) form chains having a 21 screw axis symmetry, but these chains pair in a manner such that each individual chain has one-fold symmetry but the pair has 21 screw axis symmetry, in an arrangement that aligns the patches that would favor Coulombic interactions along the chain. Surprisingly, some chain pairs form unanticipated double-helix regions that result from mutual twisting of the chains about each other, illustrating a kind of polymorphism that may be associated with nucleation from short chain pairs. Larger 2D domains of the three-patch particles crystallize in the p6m plane group with alignment (with respect to the field) and packing densities that suggest random disorder in the domains, whereas four-patch particles form 2D domains in which close-packed rows are aligned with the field.

Dislocation reactions, grain boundaries, and irreversibility in two-dimensional lattices using topological tweezers

Significance The properties of real-world materials are typically determined by small collections of defects within them. One common defect––a dislocation––results from removing half a row of atoms from a crystal, leaving behind the other half that now ends abruptly at a tip in the crystal. The termination point behaves like a point “particle” that carries a topological charge, obeys dynamical laws, and interacts with other such particles to create grain boundaries. We report a method to precisely manipulate dislocations by using specially designed light fields (“topological tweezers”) to massage an experimental model system consisting of microscopic colloids. We use this tool to study their interactions and complex collective dynamics that are not always as reversible as one might think. Dislocations, disclinations, and grain boundaries are topological excitations of crystals that play a key role in determining out-of-equilibrium material properties. In this article we study the kinetics, creation, and annihilation processes of these defects in a controllable way by applying “topological tweezers,” an array of weak optical tweezers which strain the lattice by weakly pulling on a collection of particles without grabbing them individually. We use topological tweezers to deterministically control individual dislocations and grain boundaries, and reversibly create and destroy dislocation pairs in a 2D crystal of charged colloids. Starting from a perfect lattice, we exert a torque on a finite region and follow the complete step-by-step creation of a disoriented grain, from the creation of dislocation pairs through their reactions to form a grain boundary and their reduction of elastic energy. However, when the grain is rotated back to its original orientation the dislocation reactions do not retrace. Rather, the process is irreversible; the grain boundary expands instead of collapsing.

Charged hydrophobic colloids at an oil–aqueous phase interface

Hydrophobic poly(methyl methacrylate) (PMMA) colloidal particles, when dispersed in oil with a relatively high dielectric constant, can become highly charged. In the presence of an interface with a conducting aqueous phase, image-charge effects lead to strong binding of colloidal particles to the interface, even though the particles are wetted very little by the aqueous phase. We study both the behavior of individual colloidal particles as they approach the interface and the interactions between particles that are already interfacially bound. We demonstrate that using particles which are minimally wetted by the aqueous phase allows us to isolate and study those interactions which are due solely to charging of the particle surface in oil. Finally, we show that these interactions can be understood by a simple image-charge model in which the particle charge q is the sole fitting parameter.

Freezing on a Sphere

The best understood crystal ordering transition is that of two-dimensional freezing, which proceeds by the rapid eradication of lattice defects as the temperature is lowered below a critical threshold. But crystals that assemble on closed surfaces are required by topology to have a minimum number of lattice defects, called disclinations, that act as conserved topological charges—consider the 12 pentagons on a football or the 12 pentamers on a viral capsid. Moreover, crystals assembled on curved surfaces can spontaneously develop additional lattice defects to alleviate the stress imposed by the curvature. It is therefore unclear how crystallization can proceed on a sphere, the simplest curved surface on which it is impossible to eliminate such defects. Here we show that freezing on the surface of a sphere proceeds by the formation of a single, encompassing crystalline ‘continent’, which forces defects into 12 isolated ‘seas’ with the same icosahedral symmetry as footballs and viruses. We use this broken symmetry—aligning the vertices of an icosahedron with the defect seas and unfolding the faces onto a plane—to construct a new order parameter that reveals the underlying long-range orientational order of the lattice. The effects of geometry on crystallization could be taken into account in the design of nanometre- and micrometre-scale structures in which mobile defects are sequestered into self-ordered arrays. Our results may also be relevant in understanding the properties and occurrence of natural icosahedral structures such as viruses.