Mechanical properties of particle-covered droplets probed by nonuniform electric field

Particle-covered droplets are used in several fundamental studies and practical applications. For many applications, it is important to understand the mechanics of these droplets subjected to induced external stresses, such as an electric field (E-field). Several research groups have studied the deformation and stability of droplets subjected to uniform E-fields. However, the behavior of particle-laden droplets in nonuniform E-fields is inadequately reported. In this study, we present the deformation of silicone oil droplets coated by an electrically insulating particle shell suspended in castor oil. Such droplets deform compressively under a direct current E-field. We create E-fields with different intensities and field gradients by changing the applied electric potential, the shape of a signal electrode, and its position relative to a stationary plate-shaped electrode. The experimental results of the droplet deformation are compared with the calculated values obtained through modeling the distribution of a nonuniform E-field around a cylindrical electrode using the finite element method. We quantitatively present how the electrode geometry, electric potential, and droplet size affect the magnitude of droplet deformation. Then we relate droplet deformation to the size of an opening in the particle shell created in the presence of an E-field. We show that this allows designing procedures for inserting a material into a droplet, as well as inspecting of the interior content of a particle-covered droplet without affecting its particle shell. Furthermore, we study the origin of liquid flows in the sample cell through two-dimensional time-averaged flow maps, finding that the E-field-induced flows have two origins. These findings may be relevant for heat-transfer enhancement.