Droplet transportation by surface charge density gradient

The rapid advancement in the field of biomedical engineering and clinical diagnosis has created an increasing demand to precisely manoeuvre liquids in the form of a droplet inside Lab-on-a-chip or µ-TAS devices.  However, the predominance of the forces due to interfacial tension over body forces makes the liquid manipulation to be difficult in smaller scales. In microscale, droplets can be conveniently translated by manipulating the capillary forces by external parameters like surface wettability gradient, thermal gradient, sound wave, external electric field, etc. Among the other techniques, the use of surface wettability gradient grabs considerable attention for preferential droplet transport due to its easy fabrication, low cost, and precision. In this technique, the directional motion of the droplet is achieved by creating asymmetry in the droplet menisci by the modulation of the chemical affinity or the morphology of the surface.

Despite significant advancement, the droplet motion by wettability gradient is restricted to low droplet velocity or transport distance. Recently, an article entitled “Surface charge printing for programmed droplet transport” by Qiangqiang Sun et al. demonstrated a novel method of directional drop motion by creating a charge density gradient over a superamphiphobic surface overcoming the limitations of conventional wettability gradient. They produced the superamphiphobic surface by depositing a nano-porous silicon dioxide layer on a glass substrate. The surface charge density gradient is created by contact electrification due to modulated drop impact on the substrate. The drop placed on the surface experiences a net driving force towards the increasing charge density, which imparts a motion to the droplet. This novel method not only produces higher actuation force compared to the conventional wettability induced motion but also attains a very low energy dissipation at the surface owing to the high repellency of the superamphiphobic surface. (The paper is available at https://www.nature.com/articles/s41563-019-0440-2)