The dynamics of droplet detachment in reversed electrowetting (REW)

Trampolining of Droplets on Hydrophobic Surfaces Using Electrowetting

Droplet detachment from solid surfaces is an essential part of many industrial processes. Electrowetting is a versatile tool for handling droplets in digital microfluidics, not only on plain surface but also in 3-D manner. Here, we report for the first time droplet trampolining using electrowetting. With the information collected by the real-time capacitor sensing system, we are able to synchronize the actuation signal with the spreading of the droplet upon impacting. Since electrowetting is applied each time the droplet impacts the substrate and switched off during recoiling of the droplet, the droplet gains additional momentum upon each impact and is able to jump higher during successive detachment. We have modelled the droplet trampolining behavior with a periodically driven harmonic oscillator, and the experiments showed sound agreement with theoretical predictions. The findings from this study will offer valuable insights to applications that demands vertical transportation of the droplets between chips arranged in parallel, or detachment of droplets from solid surfaces.

Manipulation of a Nonconductive Droplet in an Aqueous Fluid with AC Electric Fields: Droplet Dewetting, Oscillation, and Detachment

Electrowetting (EW) is an effective method for droplet manipulation in microfluidics. In traditional EW, a conductive droplet is actuated, which spreads on a solid substrate. Recently, we considered an opposite phenomenon of droplet actuation in EW: inducing nonconductive droplet dewetting and detaching from the substrate. An oil/water system is used in which the oil droplet (nonconductive) is actuated on a flat substrate in surrounding water (conductive) by EW. In this work, alternating current (AC) electric fields are applied to EW, and the transient dynamics of droplet dewetting, oscillation, and detachment with the AC signals are investigated. The droplet is not in contact with electrodes, and it dances freely on the substrate. Experiments are performed in a wide range of voltages and AC frequencies. To demonstrate the droplet dynamics, we divide the full process of droplet manipulation into three distinguishable periods, that is, an initiating period, a steady oscillation period, and a detaching condition. Transient droplet dewetting is considered in the initiating period, and we obtain the distribution of the contact line friction factor. In steady oscillation, the oscillation resonance is verified from the oscillating amplitude of the contact line. Different periodical features are found for the droplet dancing at the resonance frequencies and departure from resonance. The droplet is detached at high voltages, and we provide a map for the detachable and nondetachable zones. The voltage is the dominant factor determining the droplet detachment; however, the AC frequency has notable influences on the critical voltage. The detachment is promoted when the AC frequency is within the region of the oscillation resonance (e.g., 20 < f < 75 Hz). In this region, the detaching process is not monotonic but instead, the droplet rebounds by several times before it is completely detached.

Modelling of Electrowetting-Induced Droplet Detachment and Jumping over Topographically Micro-Structured Surfaces

Detachment and jumping of liquid droplets over solid surfaces under electrowetting actuation are of fundamental interest in many microfluidic and heat transfer applications. In this study we demonstrate the potential capabilities of our continuum-level, sharp-interface modelling approach, which overcomes some important limitations of convectional hydrodynamic models, when simulating droplet detachment and jumping dynamics over flat and micro-structured surfaces. Preliminary calculations reveal a considerable connection between substrate micro-topography and energy efficiency of the process. The latter results could be extended to the optimal design of micro-structured solid surfaces for electrowetting-induced droplet removal in ambient conditions.