Eric M. Furst

Directed Self-Assembly of Nanoparticles

Within the field of nanotechnology, nanoparticles are one of the most prominent and promising candidates for technological applications. Self-assembly of nanoparticles has been identified as an important process where the building blocks spontaneously organize into ordered structures by thermodynamic and other constraints. However, in order to successfully exploit nanoparticle self-assembly in technological applications and to ensure efficient scale-up, a high level of direction and control is required. The present review critically investigates to what extent self-assembly can be directed, enhanced, or controlled by either changing the energy or entropy landscapes, using templates or applying external fields.

Direct Measurements of the Effects of Salt and Surfactant on Interaction Forces between Colloidal Particles at Water−Oil Interfaces

The forces between colloidal particles at a decane-water interface, in the presence of low concentrations of a monovalent salt (NaCl) and the surfactant sodium dodecyl sulfate (SDS) in the aqueous subphase, have been studied using laser tweezers. In the absence of electrolyte and surfactant, particle interactions exhibit a long-range repulsion, yet the variation of the interaction for different particle pairs is found to be considerable. Averaging over several particle pairs was hence found to be necessary to obtain a reliable assessment of the effects of salt and surfactant. It has previously been suggested that the repulsion is consistent with electrostatic interactions between a small number of dissociated charges in the oil phase, leading to a decay with distance to the power -4 and an absence of any effect of electrolyte concentration. However, the present work demonstrates that increasing the electrolyte concentration does yield, on average, a reduction of the magnitude of the interaction force with electrolyte concentration. This implies that charges on the water side also contribute significantly to the electrostatic interactions. An increase in the concentration of SDS leads to a similar decrease of the interaction force. Moreover, the repulsion at fixed SDS concentrations decreases over longer times. Finally, measurements of three-body interactions provide insight into the anisotropic nature of the interactions. The unique time-dependent and anisotropic interactions between particles at the oil-water interface allow tailoring of the aggregation kinetics and structure of the suspension structure.

Assembly of Optical-Scale Dumbbells into Dense Photonic Crystals

We describe the self-assembly of nonspherical particles into crystals with novel structure and optical properties combining a partial photonic band gap with birefringence that can be modulated by an external field or quenched by solvent evaporation. Specifically, we study symmetric optical-scale polymer dumbbells with an aspect ratio of 1.58. Hard particles with this geometry have been predicted to crystallize in equilibrium at high concentrations. However, unlike spherical particles, which readily crystallize in the bulk, previous experiments have shown that these dumbbells crystallize only under strong confinement. Here, we demonstrate the use of an external electric field to align and assemble the dumbbells to make a birefringent suspension with structural color. When the electric field is turned off, the dumbbells rapidly lose their orientational order and the color and birefringence quickly go away. In this way, dumbbells combine the structural color of photonic crystals with the field addressability of liquid crystals. In addition, we find that if the solvent is removed in the presence of an electric field, the particles self-assemble into a novel, dense crystalline packing hundreds of particles thick. Analysis of the crystal structure indicates that the dumbbells have a packing fraction of 0.7862, higher than the densest known packings of spheres and ellipsoids. We perform numerical experiments to more generally demonstrate the importance of controlling the orientation of anisotropic particles during a concentration quench to achieve long-range order.

Laser tweezer microrheology of a colloidal suspension

The microrheology of a colloidal suspension is measured using laser tweezers. Suspensions of refractive index-matched fluorinated ethylene propylene (FEP) particles are seeded with index-mismatched polystyrene or silica probe particles. Laser trapped probes are then subjected to steady uniform flows, enabling measurements of the suspension microviscosity as a function of FEP volume fraction and flow velocity. The microrheology results agree with bulk rheology, and both exhibit the same volume fraction dependence of the Krieger-Dougherty relationship for hard spheres. As volume fraction increases, the microrheology more closely agrees with the infinite shear bulk viscosity. In this regime, measurements using small probes exhibit additional shear thinning. Using confocal microscopy and fluorescent poly(methylmethacrylate) dispersions, we demonstrate that the nonlinear microrheology is consistent with the development of an anisotropic nonequilibrium pair distribution function between the probe and bath parti...

Attractive interactions between colloids at the oil–water interface

The effects of salts and surfactants on the interaction force between colloidal polystyrene latex particles confined to a decane–water interface are measured directly using optical tweezers. After adding 0.25 M NaCl, 0.25 M NaCl and 0.1 mM sodium dodecyl sulfate (SDS) to the aqueous sub-phase, or 25 μM sorbitan monooleate (SPAN 80) to the decane super-phase, the strong repulsive force between particles is reduced and an attractive force becomes significant. The magnitude and dependence of the attraction on particle separation is consistent with a capillary quadrupole interaction. Similar interaction forces between polystyrene latex doublet particles at a pristine interface are measured, however, the anisotropic particles exhibit only a long-range attraction that is approximately two orders of magnitude stronger than spherical colloids. These results confirm the presence of long-range capillary attractions and provide a guide for manipulating colloidal interactions with additives or particle shape at fluid interfaces to control suspension structure and surface rheology.

Polarization and interactions of colloidal particles in ac electric fields

Micrometer-sized polystyrene particles form two-dimensional crystals in alternating current (ac) electric fields. The induced dipole-dipole interaction is the dominant force that drives this assembly. We report measurements of forces between colloidal particles in ac electric fields using optical tweezers and find good agreement with the point dipole model. The magnitude of the pair interaction forces depends strongly on the bulk solution conductivity and decreases as the ionic strength increases. The forces also decrease with increasing field frequency. The salt and frequency dependences are consistent with double layer polarization with a characteristic relaxation frequency omega(CD) approximately a(2)/D, where a is the particle radius and D is the ion diffusivity. This enables us to reinterpret the order-disorder transition reported for micrometer-sized polystyrene particles [Lumsdon et al., Langmuir 20, 2108 (2004)], including the dependence on particle size, frequency, and ionic strength. These results provide a rational framework for identifying assembly conditions of colloidal particles in ac fields over a wide range of parameters.

Heterogeneity of the electrostatic repulsion between colloids at the oil–water interface

The pairwise and multi-body interaction forces between polystyrene particles at an oil–water interface are measured. The electrostatic repulsive force has the expected dependence on particle separation for a dipole–dipole interaction, Frep ∼ r−4, but exhibits a distribution of magnitudes in which the force depends on the particle pairs tested and sample preparation method. A gamma distribution accurately models this variation in the repulsion between pairs of particles. Despite this heterogeneity, the multibody interactions measured in small ensembles are pairwise additive. Good agreement is found for the two-dimensional equilibrium suspension structure between experiments and Monte Carlo simulations when a heterogeneous interaction potential is implemented in the latter. The heterogeneity and long-range of the repulsive interaction accounts for the lower apparent pair interaction potential derived from the suspension radial distribution function at dilute, but finite, surface concentrations when compared to the direct pair interaction measurements made with laser tweezers at nearly infinite dilution.

Directed colloidal self-assembly in toggled magnetic fields

Suspensions of paramagnetic colloids are driven to phase separate and self-assemble by a toggled magnetic field. Initially, all suspensions form network structures that span the sample cell. When the magnetic field is toggled, this network structure coarsens diffusively for a time that scales exponentially with frequency. Beyond this break through time, suspensions cease diffusive coarsening and undergo an apparent instability. The magnetic field drives suspensions to condense into dispersed, domains of bodycentered tetragonal crystals. Within these domains the crystalline order depends on the pulse frequency. Because the scaling of the break through time with respect to frequency is exponential, the steady state limit corresponding to an infinite pulse frequency is kinetically arrested and the equilibrium state is unreachable. These experiments show that there is an out-of-equilibrium pathway that can be used to escape a kinetically arrested state as well as a diverging time scale for phase separation as the critical frequency for condensation is approached. Rather than fine tuning the strength of the interactions among particles, a simple annealing scheme - toggling of the magnetic field - is used to create a broad envelope for assembly of ordered particle structures.

Active microrheology of a colloidal suspension in the direct collision limit

The single-point active nonlinear microrheology of a colloidal suspension is measured using laser tweezers in the limit that the diameter of the probe particle approaches the diameter of the bath suspension particles. The microviscosity thins as the probe velocity (and corresponding microrheological Peclet number) increases. This thinning behavior correlates with the development of a nonequilibrium suspension microstructure surrounding the probe particle, in which a boundary layer forms on the upstream face of the probe and a wake depleted of bath particles trails the probe. The magnitude of the microviscosities and the thinning behavior are in good agreement with Brownian dynamics simulations reported by Carpen and Brady [J. Rheol. 49, 1483 (2005)]. The microviscosity increment collapses onto a single curve for all volume fractions when scaled by the contact distribution of bath particles around the probe. Scaling the microviscosity increment yields values lower than the dilute theory; furthermore, it pl...

Fabrication of unusual asymmetric colloids at an oil-water interface.

We present a novel method for creating asymmetrical particles with unusual, flattened shapes from colloidal latex microspheres pinned at an oil-water interface. The shape and degree of asymmetry are controlled by incubating particles for minutes to tens of minutes at an elevated temperature. Estimates of the surface energy and work account for the shape-change mechanism in which heated particles deform as they spread at the oil-water interface to minimize the contact between these immiscible phases.