A systematic investigation into the influence of the wettability of spherical, nanometer-sized silica particles on the type and stability of water-toluene emulsions is described. The particles range from hydrophilic to hydrophobic depending on the extent of chemisorbed silane on their surfaces. We show that predictions based on considerations of the energy of attachment of a single particle to the oil-water interface relate directly to the stability of emulsions. A combination of powder immersion, shelf stability, light diffraction, and microscopy measurements is used to characterize the systems. Emulsions stabilized by either very hydrophilic or very hydrophobic particles are large (>100 µm) and unstable to coalescence. Those with particles of intermediate hydrophobicity are submicrometer and stable to coalescence indefinitely. For these, catastrophic inversion of emulsions occurs upon increasing the volume fraction of water, φw, and their stability to sedimentation or creaming increases approaching inversion. The stability of emulsions to gravity-induced separation passes through a sharp maximum upon increasing the particle hydrophobicity, alongside a minimum in the average drop diameter. This is universal and independent of φw and the type of emulsion formed. In one system, an otherwise very stable water-in-oil emulsion is destabilized in an ultracentrifugal field leading to enhanced sedimentation and eventual coalescence.

A short review of the experimental findings concerning the stabilization of emulsions by solid particles is given. We then describe the preparation and properties of water-in-oil (w/o) emulsions stabilized by nanometer-sized hydrophobic silica particles alone. Emulsions of median diameter equal to 0.6 μm are completely stable to coalescence as a result of an adsorbed layer of particles at the oil−water interface. Their stability to sedimentation increases with particle concentration due to network formation of the particles in the continuous oil phase. The w/o emulsions catastrophically invert, without hysteresis, to oil-in-water (o/w) at volume fractions of water around 0.7, i.e., as soon as the drops begin to deform. The drops in o/w emulsions are larger (100 μm) and cream rapidly but remain stable to coalescence. We demonstrate that for emulsions stabilized by hydrophilic silica particles, phase inversion from o/w to w/o occurs at the same dispersed phase volume fraction as above. It is therefore suggested that the system hydrophile−lipophile balance is determined by the particle wettability. Comparisons with the behavior of surfactant-stabilized emulsions are given throughout.

The preparation, type, and stability of emulsions of oil and water stabilized solely by spherical, monodisperse polystyrene latex particles of different size is described. Two types of behavior occur depending on whether particles remain intact (in the case of cyclohexane) or dissolve to give free polymer chains (in the case of toluene). Emulsions formed with cyclohexane and either “hydrophilic” aldehyde/sulfate particles or “hydrophobic” sulfate particles are water-in-oil (w/o) over a wide range of salt concentrations and water volume fractions. Average emulsion drop diameters initially increase from 35 to 75 μm with increasing particle diameter and then remain constant. Although such emulsions sediment, there is no sign of coalescence for over 6 months. We show evidence of the transition from nonflocculated to flocculated emulsions upon increasing the water volume fraction, as predicted theoretically for charged drops in oil. By use of toluene and “hydrophilic” particles however, emulsions can be inverted from oil-in-water (o/w) to w/o with increasing salt concentration. The concentration of salt required to screen the repulsions between negatively charged adsorbed polymers increases with initial particle size as the average molecular weight also increases. Water-in-oil emulsions, of around 1 μm diameter, are stable to coalescence for long periods.

A new class of microwires can be assembled by dielectrophoresis from suspensions of metallic nanoparticles. The wires are formed in the gaps between planar electrodes and can grow faster than 50 micrometers per second to lengths exceeding 5 millimeters. They have good ohmic conductance and automatically form electrical connections to conductive islands or particles. The thickness and the fractal dimension of the wires can be controlled, and composite wires with a metallic core surrounded by a latex shell can be assembled. The simple assembly process and their high surface-to-volume ratio make these structures promising for wet electronic and bioelectronic circuits.

We describe the preparation and properties of oil-in-water emulsions stabilised by colloidal silica particles alone. The charge on the particles and their extent of Ñocculation, assessed via turbidity measurements, can be modiÐed by pH control and addition of simple electrolytes. The stability of emulsions to both creaming and coalescence is low in the absence of electrolyte, and the e †ects of adding salt are dependent on the type of salt. In systems containing NaCl, emulsions are less stable once the particles are Ñocculated. In the presence of either or tetraethylammonium bromide (TEAB), emulsion stability increases dramatically for conditions LaCl 3 where the silica particles are weakly Ñocculated ; extensive Ñocculation of the particles however leads to destabilisation of the emulsions. For TEAB, relatively large emulsions of diameter around 40 lm remain very stable for up to 3 months at salt concentrations corresponding to the onset of coagulation of the colloid. Such emulsions are themselves strongly Ñocculated.