Characterization of electrode alignment for optimal droplet charging and actuation in droplet‐based microfluidic system

Abstract

The actuation method using electric force as a driving force is utilized widely in droplet-based microfluidic systems. In this work, the effects of charging electrode alignment on direct charging of a droplet on electrified electrodes and a subsequent electrophoretic control of the droplet are investigated. The charging characteristics of a droplet according to different electrode alignments are quantitatively examined through experiments and systematic numerical simulations with varying distances and angles between the two electrodes. The droplet charge acquired from the electrified electrode is directly proportional to the distance and barely affected by the angle between the two electrodes. This implies that the primary consideration of electrode alignment in microfluidic devices is the distance between electrodes and the insignificant effect of angle provides a great degree of freedom in designing such devices. Not only the droplet charge acquired from the electrode but also the force exerted on the droplet is analyzed. Finally, the implications and design guidance for microfluidic systems are discussed with an electrophoresis of a charged droplet method-based digital microfluidic device.

Compact Three-Dimensional Digital Microfluidic Platforms with Programmable Contact Charge Electrophoresis Actuation

“…Furthermore, increasing the electrode distance increases the transport velocity in the presence of the same electric field. The reason is that the electric field becomes increasingly concentrated near the electrodes as the distance between the electrodes increases …”

“…Unlike other transport cases where the electrodes are arranged with the sides facing one another, the case of vertical transport via lifting scheme exhibits electrodes arranged with the tips facing one another. Thus, the dielectrophoretic force becomes increasingly strong near the tip of the lower electrode because of the nonuniform electrical field, possibly impeding droplet mobility . In addition, the fastest and slowest modes of droplet transport are shown to be vertical transport by lifting and horizontal transport, respectively.…”

Electrostatic Origins of the Positive and Negative Charging Difference in the Contact Charge Electrophoresis of a Water Droplet

“…Therefore, a numerical model for the simulation of electrostatics was developed and analyzed to explain the origin of the charging difference not only qualitatively but also quantitatively. According to the previous work, the electrical double layer inside an aqueous droplet and the charge leakage from a charged water droplet into silicone oil can be ignored. , Therefore, the electric potential (ϕ) distribution of the system can be described by the following Laplace equation: …”

“…According to the previous work, the electrical double layer inside an aqueous droplet and the charge leakage from a charged water droplet into silicone oil can be ignored. 32,43 Therefore, the electric potential (ϕ) distribution of the system can be described by the following Laplace equation: To include all of the electrostatic sources of the experimental system, a full 3D model was developed as shown in Figure 2. The Laplace equation was solved using the electrostatic module of a commercial numerical software COMSOL Multiphysics using appropriate boundary conditions: specific voltage at the electrodes, ground at the bottom metallic stage surface and Faraday cage (when it was used), and ∂ϕ/∂n = 0 at the other boundaries.…”

Fabrication and electrokinetic motion of electrically anisotropic Janus droplets in microchannels

“…Compared with other ways, this high energy mixing method has the advantage of being able to prepare a large number of Janus droplets within limited time. Recently, the droplet microfluidic system has been developed and widely used for generating and manipulating droplets , which provides a potential way to form Janus droplets. Based on the formation mechanism, the microfluidic method can be divided into three groups: breakup formation , evolution from core‐shell emulsion and phase separation .…”