Active structuring of a particle film on a droplet's surface and examples of its applications

Particle films have a broad range of applications, for instance in smart materials as constituent of strain sensors, actuators, or smart windows. Such films can be assembled and actively controlled by acoustic, magnetic, or electric fields. In this work, we used electric fields to actively compress and stretch particle film monolayers on a droplet interface. Particle manipulation was achieved through the synergetic action of electric field-induced droplet deformation and electrohydrodynamic flows. We studied in detail how droplet deformation and particle structuring are influenced by parameters such as electric field strength, ionic conductivity, and viscosity. We found that the time for particles to compress at the droplet interface was strongly influenced by the electric field, and that it scaled as E-⁴. Moreover, hysteresis effects were observed both in the magnitude of droplet deformation and in particle expansion and contraction. Looking towards practical applications, we also demonstrated how particles at droplet interfaces manipulated by electric fields could be used as an active optical lens, a tool for detecting of material encapsulated in particle-covered droplets, or for fabrication of porous materials used in biological applications.