Electric-field-induced deformation, yielding, and crumpling of jammed particle shells formed on non-spherical Pickering droplets.

Droplets covered with densely packed solid particles, often called Pickering droplets, are used in a variety of fundamental studies and practical applications. For many applications, it is essential to understand the mechanics of such particle-laden droplets subjected to external stresses. Several research groups have studied theoretically and experimentally the deformation, relaxation, rotation, and stability of Pickering droplets. Most of the research concerns spherical Pickering droplets. However, little is known about non-spherical Pickering droplets with arrested particle shells subjected to compressive stress. The experimental results presented here contribute to filling this gap in research. We deform arrested non-spherical Pickering droplets by subjecting them to electric fields, and study the effect of droplet geometry and size, as well as particle size and electric field strength, on the deformation and yielding of arrested non-spherical Pickering droplets. We explain why a more aspherical droplet and/or a droplet covered with a shell made of larger particles required higher electric stress to deform and yield. We also show that an armored droplet can absorb the electric stress differently (i.e., through either in-plane or out-of-plane particle rearrangements) depending on the strength of the applied electric field. Furthermore, we demonstrate that particle shells may fail through various crumpling instabilities, including ridge formation, folding, and wrinkling, as well as inward indentation.

Electrorotation of particle-coated droplets: from fundamentals to applications.

Electrically insulating objects immersed in a weakly conducting liquid may Quincke rotate when subjected to an electric field. Experimental and theoretical investigations of this type of electrorotation typically concern rigid particles and particle-free droplets. This work provides the basic features of electric field-induced rotation of particle-covered droplets that expand the current knowledge in this area. Compared to pure droplets, we show that adding particles to the droplet interface considerably changes the parameters of electrorotation. We study in detail deformation magnitude (D), orientation (ß) and rotation rate (ω) of a droplet subjected to a DC E-field. Our experimental results reveal that both the critical electric field (for electrorotation) and the rotational rate depend on droplet size, particle shell morphology (smooth vs. brush-like), and composition (loose vs. locked particles). We also demonstrate the importance of the electrical parameters of the surface particles by comparing the behavior of droplets covered by (insulating) polymeric particles and droplets covered by (non-ohmic) clay mineral particles. The knowledge acquired from the electrorotation experiments is directly translated into practical applications: (i) to form arrested droplets with shells of different permeability; (ii) to study solid-to-liquid transition of particle shells; (iii) to mix particles on shells; and (iv) to increase the formation efficiency of Pickering emulsions.

Particle-covered drops in electric fields: drop deformation and surface particle organization.

Drops covered by adsorbed particles are a prominent research topic because they hold promise for a variety of practical applications. Unlocking the enormous potential of particle-laden drops in new material fabrication, for instance, requires understanding how surface particles affect the electrical and deformation properties of drops, as well as developing new routes for particle manipulation at the interface of drops. In this study, we utilized electric fields to experimentally investigate the mechanics of particle-covered silicone oil drops suspended in castor oil, as well as particle assembly at drop surfaces. We used particles with electrical conductivities ranging from insulating polystyrene to highly conductive silver. When subjected to electric fields, drops can change shape, rotate, or break apart. In the first part of this work, we demonstrate how the deformation magnitude and shape of drops, as well as their electrical properties, are affected by electric field strength, particle size, conductivity, and coverage. We also discuss the role of electrohydrodynamic flows on drop deformation. In the second part, we present the electric field-directed assembly and organization of particles at drop surfaces. In this regard, we studied various parameters in detail, including electric field strength, particle size, coverage, and electrical conductivity. Finally, we present a novel method for controlling the local particle coverage and packing of particles on drop surfaces by simply tuning the frequency of the applied electric field. This approach is expected to find uses in optical materials and applications.