Exploring the Impact of Particle Material Properties on Electrostatic Liquid Marble Formation

Abstract

Specific particle material properties such as conductivity, cohesion, and density have been neither directly nor thoroughly studied regarding particle behavior in an electrostatic field and the follow-on impact this has on the electrostatic formation of liquid marbles. In this method, an applied electric field drives the extraction of particles from a bed and their transport to a pendent, earthed water droplet. Herein, prior studies of electrostatic formation of particle-stabilized droplets and liquid marbles have been expanded to compare the impact of density using the spherical polystyrene (PS) latex and glass particles of similar shape and size. The addition of thin polymer shells to both samples, which increases the conductivity and cohesion, allows the interplay of these three properties to be examined systematically. Separation distances between the particle bed and the droplet from which particles can initially be extracted increase as the negative applied potential increases. Initial extraction distances of both core particles were found to be similar, ∼1.5 mm at 2.0 kV applied potential, despite the greater density, and thus mass of the glass particles. It is demonstrated that this is a result of competitive interactions between particle density, conductivity, and cohesion; PS is less conductive and more cohesive than glass. Introducing a polypyrrole shell increases the separation distance for extraction to approximately 4 mm for PS core particles but has little impact on glass core particles, demonstrating that for particles with constant conductivity and cohesion reducing the density facilitates extraction. Modeling and quantification of extraction threshold forces for each particle type were undertaken, utilizing the measurement of a radially symmetric area of the particle bed from which particles were observed in the initial extraction stages. This measurement highlighted that it is significantly easier to extract PS compared to glass, with particles extracted from a region in the bed up to 5 times the width in the PS case. Particle density is hypothesized to not be the determining factor in the stabilization of the coated liquid droplets; therefore, the interplay of a multitude of physical properties must be considered when determining the suitability of particulate materials for this electrostatic method.

Interparticle Repulsion of Microparticles Delivered to a Pendent Drop by an Electric Field

“…For example, previous work has clearly demonstrated that transferring unmodified glass resulted in net charge deposition 27 and is classified as conductive, while polystyrene particles transferred but did not deposit charge and are therefore deemed dielectric. 7,10 A similar interparticle repulsion has been reported in hydrophobic 3 μm diameter silanized silica microparticles at horizontal and vertical oil/water interfaces in the absence of an electric field. 28−31 These particles exhibited an interparticle spacing over nine times their diameter (up to 28 μm separation).…”

“…The slight increase in measured charge from the TCMS-coated glass and lycopodium was due to the induction of an image charge on the droplet as the drop-bed separation was reduced, thereby increasing the electric field strength and increasing the image charge. 8,10 The lack of measured charge transfer from the particles suggests that the transport of the particles for these samples was primarily due to dielectrophoresis (the induced motion of a neutral particle as a result of the polarization of the particle and gradient of a nonuniform electric field), and not due to a net charge on the particle. 10,49,50 This lack of charge is concordant with the observation of particle chains in the particle bed prior to, and on the droplet surface after, extraction and transfer due to the alignment of interparticle dipoles and interparticle electric field deformation in sets of particles (Figures 2 and 3 and Supporting Videos 1 and 2).…”

Liquid Marbles under Electric Fields: New Capabilities for Non-wetting Droplet Manipulation and Beyond

“…1.07 g cm −3 ) result in an increased mass per particle, which in turn leads to an expectation of a larger electrostatic force to extract them from the particle bed and transport them to the drop interface. 81 Surprisingly, for uncoated glass and PS particles, the separation distances for the initial particle extraction were similar, thus indicating particle extraction as a result of the competitive interaction of density, conductivity, and cohesion. The impact of density was further studied on particles of the same surface chemistry by coating PS and glass particles with increasingly conductive and cohesive PPy-Cl and PPy-dedoped coatings.…”

“…Spherical glass particles with a shape and size similar to those of PS latex particles but more than twice the density (2.31 g cm –3 , cf. 1.07 g cm –3 ) result in an increased mass per particle, which in turn leads to an expectation of a larger electrostatic force to extract them from the particle bed and transport them to the drop interface . Surprisingly, for uncoated glass and PS particles, the separation distances for the initial particle extraction were similar, thus indicating particle extraction as a result of the competitive interaction of density, conductivity, and cohesion.…”

Interfacial Crystallization within Janus Saline Marbles

“…This observation supports the idea that the interrelation Δ G het1 < Δ G het2 takes place, where Δ G het1 is the EBHN at the contact line and Δ G het2 is the EBHN at the saline/powder, respectively, regardless of the powder. In our future research, we plan to investigate an impact of electric field on interfacial crystallization taking place in Janus marbles …”