Charge-Powered Electrotaxis for Versatile Droplet Manipulation

Abstract

Taxis is an instinctive behavior of living organisms to external dangers or benefits. Here, we report a taxis-like behavior associated with liquid droplets on charged substrates in response to the external stimuli, referred to as droplet electrotaxis. Such droplet electrotaxis enables us to use either solid or liquid (such as water) matter, even a human finger, as stimuli to spatiotemporal precisely manipulate the liquid droplets of various physicochemical properties, including water, ethanol with low surface tension, viscous oil, and so on. Droplet electrotaxis also features a flexible configuration that even can manifest in the presence of an additional layer, such as the ceramic with a thickness of ∼10 mm. More importantly, superior to existing electricity-based strategies, droplet electrotaxis can harness the charges generated from diverse manners, including pyroelectricity, triboelectricity, piezoelectricity, and so on. These properties dramatically increase the application scenarios of droplet electrotaxis, such as cell labeling and droplet information recording.

Multifunctional Electrostatic Droplet Manipulation on the Femtosecond Laser-Prepared Slippery Surfaces

“…The nonuniform distribution of charges causes this droplet to be subjected to electrostatic force ( F e ) under the electrostatic field. The electrostatic force is the resultant force of the Maxwell stress acting on the surface of the droplet, which can be expressed as , F e = prefix∮ bold-italicT normale · n d S where T e , n , and S are the Maxwell stress exerted on the droplet, surface unit normal, and droplet surface area, respectively. The Maxwell stress can be calculated in tensor form , T normale , italicij = ε 0 ( E i E j − δ ij 2 E 2 ) , goodbreak0em1em⁣ i , j = x , y , z where δ ij is the Kronecker delta function, and E is the magnitude of the electric field intensity.…”

“…The development of a droplet movement method with a controllable direction and unrestricted distance is of great significance for droplet manipulation applications. In recent years, light, , electricity, , magnetism, , and heat − have been developed to intelligently manipulate droplets. Among them, static electricity has undoubtedly the greatest potential due to its simplicity and practicality.…”

Triboelectric ‘electrostatic tweezers’ for manipulating droplets on lubricated slippery surfaces prepared by femtosecond laser processing

“…where ε is the permittivity of the droplet (ε = ε r ε 0 , ε r is the relative permittivity of the liquid, and ε 0 is the permittivity of air), E is the electric field intensity, n is the surface unit normal, and S is the surface area of the droplet. The Coulomb force can be quantified by the following formula in tensor form [37,65,66]:…”

Antidurotaxis Droplet Motion onto Gradient Brush Substrates

“…Wettability gradients have also been exploited to steer the motion of fluids, − while the long-range transport of fluids can be realized by using electrostatic , or triboelectric charges . In contrast, the use of external fields, such as in the case of electrotaxis, requires energy supply by an external source, as, also, in the case of thermotaxis to sustain a temperature gradient . Other options requiring external sources may include the use of electrical current, − charge, − or even simple stretching, as well as chemically driven droplets, , droplets on vibrated substrates − or wettability ratchets. − …”