A process is described to produce single sheets of functionalized graphene through thermal exfoliation of graphite oxide. The process yields a wrinkled sheet structure resulting from reaction sites involved in oxidation and reduction processes. The topological features of single sheets, as measured by atomic force microscopy, closely match predictions of first-principles atomistic modeling. Although graphite oxide is an insulator, functionalized graphene produced by this method is electrically conducting.

This book covers the physical side of colloidal science from the individual forces acting between particles smaller than a micrometer that are suspended in a liquid, through the resulting equilibrium and dynamic properties. A variety of internal forces both attractive and repulsive act in conjunction with Brownian motion and the balance between them all decides the phase behaviour. On top of this various external fields, such as gravity or electromagnetic fields, diffusion and non-Newtonian rheology produce complex effects, each of which is of important scientific and technological interest. The authors aim to impart a sound, quantitative understanding based on fundamental theory and experiments with well-characterised model systems. This broad grasp of the fundamentals lends insight and helps to develop the intuitive sense needed to isolate essential features of the technological problems and design critical experiments. The main prerequisites for understanding the book are basic fluid mechanics, statistical mechanics and electromagnetism, though self contained reviews of each subject are provided at appropriate points. Some facility with differential equations is also necessary. Exercises are included at the end of each chapter, making the work suitable as a textbook for graduate courses in chemical engineering or applied mathematics. It will also be useful as a reference for individuals in academia or industry undertaking research in colloid science.

▪ Abstract  Electrohydrodynamics deals with fluid motion induced by electric fields. In the mid 1960s GI Taylor introduced the leaky dielectric model to explain the behavior of droplets deformed by a steady field, and JR Melcher used it extensively to develop electrohydrodynamics. This review deals with the foundations of the leaky dielectric model and experimental tests designed to probe its usefulness. Although the early experimental studies supported the qualitative features of the model, quantitative agreement was poor. Recent studies are in better agreement with the theory. Even though the model was originally intended to deal with sharp interfaces, contemporary studies with suspensions also agree with the theory. Clearly the leaky dielectric model is more general than originally envisioned.

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.

The production of materials with micrometre- and submicrometre-scale patterns is of importance in a range of applications, such as photonic materials, high-density magnetic data storage devices, microchip reactors and biosensors. One method of preparing such structures is through the assembly of colloidal particles. Micropatterned colloidal assemblies have been produced with lithographically patterned electrodes or micromoulds. Here we describe a different method that combines the well-known photochemical sensitivity of semiconductors with electric-field-induced assembly to create ordered arrays of micrometre-sized colloidal particles with tunable patterns. We show that light affects the assembly processes, and demonstrate how to produce patterns using electrophoretic deposition in the presence of an ultraviolet (UV) illumination motif. The distribution of current across an indium tin oxide (ITO) electrode can be altered by varying the illumination intensity: during the deposition process, this causes colloidal particles to be swept from darkened areas into lighted regions. Illumination also assists in immobilizing the particles on the electrode surface. Although the details of these processes are not well understood, the patterning effects of the UV light are discussed in terms of alterations in the current density that affects particle assembly on an ITO electrode.

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We describe a theory for a new type of colloid behavior whereby particles deposited on a surface by electrophoresis are manipulated to form two-dimensional crystals. Since the particles are equally charged, the clustering is opposite that expected from electrostatic considerations. Such behavior is consistent with migration due to electrohydrodynamic flows associated with polarization layers and ion currents. Provided colloid stability is maintained, the assembly processes take place with both dc and ac fields and may be modulated by adjusting the field strength or frequency. No migration is present at frequencies above 1 MHz. Two-dimensional fluid and crystalline states can be formed on the electrode surface. Experiments with patterned electrodes demonstrate the presence of the electrohydrodynamic flow. A mathematical model of the electrohydrodynamics provides insight into the assembly process.

Electric fields generate transverse flows near electrodes that sweep colloidal particles into densely packed assemblies. We interpret this behavior in terms of electrohydrodynamic motion stemming from distortions of the field by the particles that alter the body force distribution in the electrode charge polarization layer. A scaling analysis shows how the action of the applied electric field generates fluid motion that carries particles toward one another. The resulting fluid velocity is proportional to the square of the applied field and decreases inversely with frequency. Experimental measurements of the particle aggregation rate accord with the electrohydrodynamic theory over a wide range of voltages and frequencies.