Electrostatic Transfer of Conductive Particles for the Formation of Liquid Marbles–Charge Transfer Behavior

Abstract

The electrostatic method for liquid marble formation has shown great promise in allowing contactless particle delivery to pendent liquid droplets. However, size has been shown to be a significant limitation in the selection of suitable particles for this transfer process. Here we overcome this size limitation using conductive copper particles as small as 6 μm diameter, delivering them to a 5 μL pendent water droplet at applied voltages between 0.5 and 3.0 kV. An electrometer in the grounding circuit enabled calculation of the average charge on an individual particle during the transfer process, accounting for the movement of particles within the bed during extraction utilizing the analytical sphere and plate model proposed by Morrison. A new constant, δ, is proposed to compensate for the dynamic charging that takes place during this process and nonuniformity in particle morphology. Using this constant a particle and mass transfer rate was calculated, demonstrating that smaller particles, once extracted from the bed, transfer at a faster rate than larger particles.

Formation of liquid marbles & aggregates: rolling and electrostatic formation using conductive hexagonal plates

“…5), due to their greater conductivity (Table 2); behaviour consistent with previous work examining the impact of particle conductivity on electrostatic particle extraction. 22,28 Both sets of doped particles demonstrate a tendency to arrive at the droplet edge first. Due to the non-uniform packing in the particle bed and high aspect ratio geometry of the platelet particles, their edges are likely to protrude slightly from the bed allowing for charge to concentrate here, resulting in the particles becoming aligned with their plane parallel to the electric field and maintaining this orientation throughout the extraction and transfer process.…”

“…Electrostatic particle coating of pendent water droplets was carried out following the same regime as previously reported. 22,23,25,28,35 An electrically-earthed 5 mL pendent tap water droplet was positioned above a stainless steel plate, upon which a microscope slide with concave glass inset and bed of PET plates were mounted. The bed was prepared by placing a small quantity of platelets on the slide.…”

“…In recent years an alternative method of liquid marble formation has been reported, utilising an electric field. [22][23][24] This process features contactless delivery of the particles to a pendent droplet and allows for substantial control of the droplet volume with no risk of coalescence. Furthermore, it allows for the use of hydrophilic particles to form particle-stabilised aggregates, where the particles are internalised in the droplet resulting in a metastable structure.…”

“…25,27 However, some of the limitations of this particular method are that the particles must overcome interparticle forces, such as cohesion and friction in order to be extracted to the pendent droplet, which may be aided by increased conductivity and particle size. 22,23,28 To this end, the viability of relatively large conductive platelets, compared to those previously studied, 22,23,28 to stabilise a droplet will be investigated.…”

“…21,34 These newly synthesised coatings will be investigated with respect to their viability as liquid marble stabilisers using both the traditional rolling 1,4,21 and the contactless electrostatic method of liquid marble formation, during which their electrostatic transfer behaviour will be investigated. 22,23,25,28,35…”