Induced flow inside a droplet by static electrical charge

Abstract

Introducing controlled fluid motion in a droplet turns out to be of outstanding scientific importance, hallmarked by a plethora of applications ranging from engineering to biology. While internal mechanisms such as interfacial tension or buoyancy-driven dynamics may trigger fascinating flow structures inside a droplet, controllability of the same without external forcing remains questionable. On the other hand, in an electrically forced environment, complex fabrication steps and special choices of the ionic liquid are often demanded. Circumventing these limits, here we bring out a new method of flow manipulation inside a sessile droplet by simply deploying a static charge produced by the triboelectric effect. This is physically actuated by charge transfer between the two lateral electrodes within which the droplet is entrained, triggering a strong ionized air current. The flow inside the droplet is generated due to the shear exerted at the interface by the charge-induced ionized airflow around the droplet, a paradigm that has hitherto remained unexplored. The strength of the fluid flow can be controlled by adjusting the supplied static charge. Such unique controllability without sacrificing the physical simplicity opens up new possibilities for flow manipulation in a multitude of applications ranging from droplet microreactors to digital microfluidics.