Dudley A. Saville

Field-Induced Layering of Colloidal Crystals

An electrohydrodynamic methodology has been developed that makes possible the precise assembly of two- and three-dimensional colloidal crystals on electrode surfaces. Electrophoretically deposited colloidal particles were observed to move toward one another over very large distances (greater than five particle diameters) to form two-dimensional colloidal crystals for both micrometer- and nanometer-size particles. This coalescence of particles with the same charge is opposite to what is expected from electrostatic considerations and appears to result from electrohydrodynamic fluid flow arising from an ionic current flowing through the solution. The ability to modulate this “lateral attraction” between particles, by adjusting field strength or frequency, facilitates the reversible formation of two-dimensional fluid and crystalline colloidal states on the electrode surface. Further manipulation allows controlled structures to be assembled.

Electrophoresis of spherical polymer-coated colloidal particles

Abstract The motion of a spherical polymer-coated colloidal particle in a steady electric field is studied via “exact” numerical solutions of the electrokinetic equations. The hydrodynamic influence of the polymer is represented by a distribution of Stokes resistance centers. With neutral polymer, charge resides on the underlying bare particle, whereas for polyelectrolytes, the charge is distributed throughout the coating. The coatings may be brush-like or have long tails. As expected, neutral coatings lower the mobility because of increased drag and a decrease in the effective charge behind the shear surface. For polyelectrolyte coatings, the behavior is more complex. For example, the mobility becomes independent of the ionic strength and particle size when Donnan equilibrium prevails inside the coating and the coating is thick relative to the Brinkman screening length (square root of the coating permeability). In this limit, the mobility follows from a simple balance of forces within the coating and, therefore, becomes proportional to the fixed charge density and the coating permeability. If the permeability is sufficiently high, the mobility of a polyelectrolyte-coated particle may exceed that of its bare counterpart with the same net charge. In general, the effects of polarization and relaxation are as important for coated particles as they are for bare particles.

Bending Properties of Single Functionalized Graphene Sheets Probed by Atomic Force Microscopy

We probe the bending characteristics of functionalized graphene sheets with the tip of an atomic force microscope. Individual sheets are transformed from a flat into a folded configuration. Sheets can be reversibly folded and unfolded multiple times, and the folding always occurs at the same location. This observation suggests that the folding and bending behavior of the sheets is dominated by pre-existing kink (or even fault) lines consisting of defects and/or functional groups.

The electrohydrodynamic deformation of drops suspended in liquids in steady and oscillatory electric fields

When an electric field is applied to a drop suspended in another liquid the drop deforms. The relation between the applied field and the mode and magnitude of the deformation have been studied extensively. Nevertheless, Torza, Cox & Mason (1971) found that quantitative agreement between the leaky dielectric theory (Taylor 1966) and experiment is quite poor. Here we describe results from a new series of experiments. Drops suspended in weakly conducting liquids were deformed into spheroids with both steady and oscillatory fields. Drop deformation, interfacial tension, and the electrical properties of the fluids were measured for each system to provide a definitive test of the theory. The agreement between the leaky dielectric model and our results for drop deformations in steady fields is much improved over previous results, although discrepancies remain for some systems. Drop deformations in oscillatory fields consist of steady and oscillatory parts because of the quadratic dependence on the field strength. Measurements of the steady part at 60 Hz, where the oscillatory deformation is negligible, are in excellent agreement with the theory. The effects of frequency on the steady deformation were studied by measuring oblate deformations at a series of frequencies and field strengths; the agreement with theory is good. Finally, the time-dependent total deformation was measured under conditions where both parts of the deformation are commensurate. Good agreement was found between the measured and predicted maximum and minimum deformations. Nevertheless, only a small range of fluid properties could be studied owing to the need to avoid droplet sedimentation.

Scaling laws for pulsed electrohydrodynamic drop formation

A pulsed electrohydrodynamic jet can produce on-demand drops much smaller than the delivery nozzle. This letter describes an experimentally validated model for electrically pulsed jets. Viscous drag in a thin nozzle limits the flow rate and leads to intrinsic pulsations of the cone jet. A scale analysis for intrinsic cone-jet pulsations is derived to establish the operating regime for drop deployment. The scaling laws are applicable to similar electrohydrodynamic processes in miniaturized electrospraying systems.

Electrohydrodynamic flow around a colloidal particle near an electrode with an oscillating potential

Electrohydrodynamic (EHD) flow around a charged spherical colloid near an electrode was studied theoretically and experimentally to understand the nature of long-range particle–particle attraction near electrodes. Numerical computations for finite double-layer thicknesses confirmed the validity of an asymptotic methodology for thin layers. Then the electric potential around the particle was computed analytically in the limit of zero Peclet number and thin double layers for oscillatory electric fields at frequencies where Faradaic reactions are negligible. Streamfunctions for the steady component of the EHD flow were determined with an electro-osmotic slip boundary condition on the electrode surface. Accordingly, it was established how the axisymmetric flow along the electrode is related to the dipole coefficient of the colloidal particle. Under certain conditions, the flow is directed toward the particle and decays as r −4 , in accord with observations of long-range particle aggregation. To test the theory, particle-tracking experiments were performed with fluorescent 300 nm particles around 50μm particles over a wide range of electric field strengths and frequencies. Treating the particle surface conductivity as a fitting parameter yields velocities in excellent agreement with the theoretical predictions. The observed frequency dependence, however, differs from the model predictions, suggesting that the effect of convection on the charge distribution is not negligible as assumed in the zero Peclet number limit.

Dielectric spectroscopy of colloidal suspensions: I. The dielectric spectrometer

Abstract Four-electrode signal-processing techniques have a number of advantages over conventional bridge techniques in dielectric spectroscopy, but are not widely used because complex electronic circuits are required. A new design for a dielectric spectrometer of this type which circumvents these problems is described here. The device has been tested using electrical networks, pure electrolytes, and dilute colloidal suspensions. In experiments with a dilute, amphoteric latex, large suspension dielectric constants are found.

An extended Maxwell-Wagner theory for the electric birefringence of charged colloids

Recent frequency resolved electric birefringence experiments on dilute suspensions of charged, ellipsoidal particles reveal new relaxation features in the MHz frequency range. Such behavior can be explained in terms of surface transport processes. Here we develop a model for the electric polarizability of spheroidal particles by combining features of electrokinetic and Maxwell–Wagner theories. The model accurately depicts the high frequency features and relates them to the colloidal properties of the polyelectrolyte. Using the model, particle charge can be extracted from electric birefringence measurements.

The electrical conductivity of suspensions of charged particles in ionic solutions: the roles of added counterions and nonspecific adsorption

Abstract Previous theories for the electrical conductivity of dilute suspensions fail to take proper account of the effects of nonspecific adsorption, which alters the concentrations of ions in regions outside the double layers, and counterions derived from the particle charging processes. As a result, agreement with experimental data is poor. These effects are included in the theory developed here and a formula for the conductivity of a dilute suspension of particles with an arbitrarily large charge or ζ-potential is derived, along with an asymptotic expression for situations where the Debye-Huckel approximation is valid. The agreement with extant data is much improved over that obtained with earlier theories.

The dielectric response of polystyrene latexes: Effects of alterations in the structure of the particle surface

Abstract The presence of a “hairy” layer has been hypothesized to explain discrepancies between the electrokinetic theory and experiments with polystyrene latexes by Van der Put and Bijsterbosch (J. Colloid Interface Sci.92, 499 (1983)]. We tested the hypothesis by making electrokinetic measurements on latexes before and after they had been heated above the polystyrene glass transition temperature. This sort of treatment may smooth the particle surface by collapsing or removing the charged polymer filaments which comprise the layer. Low-frequency dielectric measurements reveal that heat treatment improves the agreement between experiment and theory. For an amphoteric latex, the dielectric constant and conductivity decrease with heat-treatment time, and the characteristic relaxation frequency increases. This is consistent with the hairy surface hypothesis, suggesting that heat treatment smooths the surface, creating particles which more closely conform to the assumptions of the classical theory. Experiments with an anionic latex further suggest the presence of surface structure, but discrepancies between experiment and theory persist; it appears that factors other than hairiness contribute to these discrepancies. The suspensions dielectric properties are evidently very sensitive to the state of the particle surface, and thus dielectric spectroscopy offers a promising means of studying the structure of colloidal interfaces.