Peter M. Ireland

The role of changing contact in sliding triboelectrification

Sliding contact usually seems to promote triboelectric charge transfer between solids, but the reasons for this are not always clear. Existing theories propose a number of explanations, including an increase in contact area due to interfacial deformation, frictional heating and direct material transfer. In the absence of these mechanisms, sliding may still promote charging by occasioning a changing contact pattern. This changing contact results in a greater cumulative surface area available for charging than static contact. A theoretical structure is introduced to characterize the effect of changing contact on perfectly insulating surfaces with instantaneous contact charging. This is extended to include non-instantaneous charging by means of a simple charging-time model.

Electrostatic formation of liquid marbles and agglomerates

We report observations of a sudden, explosive release of electrostatically charged 100 μm glass beads from a particle bed. These cross an air gap of several millimeters, are engulfed by an approaching pendant water drop, and form a metastable spherical agglomerate on the bed surface. The stability transition of the particle bed is explained by promotion of internal friction by in-plane electrostatic stresses. The novel agglomerates formed this way resemble the “liquid marbles” formed by coating a drop with hydrophobic particles. Complex multi-layered agglomerates may also be produced by this method, with potential industrial, pharmaceutical, environmental, and biological applications.

Giant Pickering Droplets: Effect of Nanoparticle Size and Morphology on Stability

The interaction between a pair of millimeter-sized nanoparticle-stabilized n-dodecane droplets was analyzed using a high-speed video camera. The droplets were grown in the presence of either poly(glycerol monomethacrylate)-poly(benzyl methacrylate) (PGMA-PBzMA) diblock copolymer spheres or poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate)-poly(benzyl methacrylate) (PGMA-PHPMA-PBzMA) triblock copolymer worms prepared by polymerization-induced self-assembly. The effect of nanoparticle morphology on droplet coalescence was analyzed by comparing 22 nm spheres to highly anisotropic worms with a mean worm width of 26 nm and comparable particle contact angle. Both morphologies lowered the interfacial tension, providing direct evidence for nanoparticle adsorption at the oil-water interface. At 0.03 w/v % copolymer, an aging time of at least 90 s was required to stabilize the n-dodecane droplets in the presence of the worms, whereas no aging was required to produce stable droplets when using the spheres, suggesting faster diffusion of the latter to the surface of the oil droplets. The enhanced stability of the sphere-coated droplets is consistent with the higher capillary pressure in this system as the planar interfaces approach. However, the more strongly adsorbing worms ultimately also confer stability. At lower copolymer concentrations (≤0.01 w/v %), worm adsorption promoted droplet stability, whereas the spheres were unable to stabilize droplets even after longer aging times. The effect of mean sphere diameter on droplet stability was also assessed while maintaining an approximately constant particle contact angle. Small spheres of either 22 or 41 nm stabilized n-dodecane droplets, whereas larger spheres of either 60 or 91 nm were unable to prevent coalescence when the two droplets were brought into contact. These observations are consistent with the greater capillary pressure stabilizing the oil-water interfaces coated with the smaller spheres. Addition of an oil-soluble polymeric diisocyanate cross-linker to either the 60 or the 91 nm spheres produced highly stable colloidosomes, thus confirming adsorption of these nanoparticles.

Some comments on contact charge relaxation

The established contact charge relaxation scheme of Matsuyama and Yamamoto [J. Phys. D 28, 2418 (1995); 30, 2170 (1997)] is reassessed in light of the observations of Horn and Smith [Science 256, 362 (1992)] and Horn et al. [Nature (London) 366, 442 (1993)] of charge relaxation between separated charged surfaces. The multiple partial discharges observed in these studies are inconsistent with a scheme where onset and extinction are both described by a single Paschen curve. A modified version of the established scheme is therefore proposed.

Formation of Liquid Marbles Using pH-Responsive Particles: Rolling vs Electrostatic Methods

Aqueous dispersions of micrometer-sized, monodisperse polystyrene (PS) particles carrying pH-responsive poly[2-(diethylamino)ethyl methacrylate] (PDEA) colloidal stabilizer on their surfaces were dried under ambient conditions at pH 3.0 and 10.0. The resulting dried cake-like particulate materials were ground into powders and used as a stabilizer to fabricate liquid marbles (LMs) by rolling and electrostatic methods. The powder obtained from pH 3.0 aqueous dispersion consisted of polydisperse irregular-shaped colloidal crystal grains of densely packed colloids which had hydrophilic character. On the other hand, the powder obtained from pH 10.0 aqueous dispersion consisted of amorphous and disordered colloidal aggregate grains with random sizes and shapes, which had hydrophobic character. Reflecting the hydrophilic-hydrophobic balance of the dried PDEA-PS particle powders, stable LMs were fabricated with distilled water droplets by rolling on the powders prepared from pH 10.0, but the water droplets were adsorbed into the powders prepared from pH 3.0. In the electrostatic method, where an electric field assists transport of powders to a droplet surface, the PDEA-PS powders prepared from pH 3.0 jumped to an earthed pendant distilled water droplet to form a droplet of aqueous dispersion. Conversely the larger powder aggregates prepared from pH 10.0 did not jump due to cohesion between the hydrophobic PDEA chains on the PS particles, resulting in no LM formation.

Electrostatic Formation of Polymer Particle Stabilised Liquid Marbles and Metastable Droplets – Effect of Latex Shell Conductivity

HYPOTHESIS Particle cohesion and conductivity affects the electrostatically driven transport of particles to a suspended water droplet. The conditions at which liquid marbles and particle stabilised liquid droplets form are a function of these parameters. EXPERIMENT Particle beds placed below an earthed pendent water drop had a negative potential applied, thus inducing an opposing positive charge on the liquid, which results in particle transfer and eventual coating of the liquid drop. Experiments where both the particle bed was constantly moved slowly toward the droplet, and the particle bed remained at a fixed, small separation distance were completed. These enabled the investigation of a number of variables that influence successful aggregate formation, including separation distance between the droplet and particle bed, coating mechanism and kinetics of the transfer process. FINDINGS Monodisperse polystyrene core particles with polypyrrole shells of various cohesiveness and conductivity were observed to behave differently in the presence of the applied potential, where the least cohesive and conductive sample (polystyrene) required the smallest separation distance, i.e. the greatest field strength for particle transfer. Increasing conductivity of the particle shell decreases the field strength required for particle transfer, and thus an increase was observed in separation distance at which particles were observed to move to the air-water interface. The transfer kinetics followed the same trend where the least conductive and cohesive sample was the slowest to coat the air-water interface, and vice-versa. Since an increase in cohesion hinders particle transfer, it is concluded that particle conductivity is of greater importance in the electrostatic aggregation process.

pH-Responsive Particle-Liquid Aggregates—Electrostatic Formation Kinetics

Liquid-particle aggregates were formed electrostatically using pH-responsive poly[2-(diethylamino)ethyl methacrylate] (PDEA)-coated polystyrene particles. This novel non-contact electrostatic method has been used to assess the particle stimulus-responsive wettability in detail. Video footage and fractal analysis were used in conjunction with a two-stage model to characterize the kinetics of transfer of particles to a water droplet surface, and internalization of particles by the droplet. While no stable liquid marbles were formed, metastable marbles were manufactured, whose duration of stability depended strongly on drop pH. Both transfer and internalization were markedly faster for droplets at low pH, where the particles were expected to be hydrophilic, than at high pH where they were expected to be hydrophobic. Increasing the driving electrical potential produced greater transfer and internalization times. Possible reasons for this are discussed.

An Electrostatic Method for Manufacturing Liquid Marbles and Particle-Stabilized Aggregates

We have developed a method for transferring particles from a powder bed to a liquid droplet using an electric field. This process has been used to create liquid marbles with characteristics not normally found in those formed by direct contact methods such as rolling. It has also been used to manufacture hydrophilic particle-liquid aggregates and more complex layered aggregates incorporating both hydrophobic and hydrophilic particles. This article briefly outlines the electrostatic aggregation method itself, the materials used and structures formed thus far, and explores the rich fundamental physics and chemistry underpinning the process as they are understood at present.