Shaping electrospray deposition profile by a quadrupole: From circular to elliptical patterns

Thin films deposited by conventional electrospray of volatile solutions often give rise to rough morphology consisting of “coffee rings” formed by individual sessile droplets. This is due to the long average inter-droplet distance and low merging probability of deposited droplets within the sprayed circular area. We present a quadrupole configuration to form a squeezing electric field that compresses the circular electrospray pattern into a much slenderer elliptical shape. The design is validated by tracking the trajectories of all sprayed droplets using the Lagrangian method. Moreover, the statistical study of the deposited droplet merging process and experimental results prove that the quadrupole configuration can dramatically decrease inter-droplet distance and increase the merging probability of deposited droplets. The simple apparatus offers precise controls of the aspect ratio of the pattern and provides a feasible approach to produce smooth thin films without pin holes.