Activating Bubble’s Escape, Coalescence, and Departure under an Electric Field Effect

Abstract

In this paper, we present the results of applying an electric field to activate bubbles' escape, coalescence, and departure. A simple electrowetting-ondielectric device was utilized in this bubble dynamics study. When a copper electrode wire inserted into deionized water was positioned on one side of single or multiple bubbles, the bubble tended to continuously escape from its initial position as the voltage was turned on. Contact angle imbalance at different sides of the bubble was observed, which further promoted the bubble's escape. An analysis model with an electromechanical framework was developed to study the charging time difference on two sides of the bubble, which generated a wettability gradient and capillary force to propel it away from the electrode. Sine, ramp, and square alternating current waveforms with 60 V amplitude and 2 Hz frequency were tested for comparison. It was shown that all waveforms promoted the bubble's escape; the square wave shape manifested the farthest escape capability, followed by sine and ramp waves. An upper view of several bubbles aligning in triangle, square, pentagon, and hexagon shapes demonstrated that the bubbles tended to move outward when the electrode is placed at the geometric centers. Experiments with an electrode on one side and several bubbles positioned in a line were conducted. In these cases, the bubbles closer to the electrode reacted faster than those farther from the electrode, resulting in coalescence. Once the bubble size became larger, it departed either by overcoming the disjoining pressure in a thin film of water or via the buoyancy force in a thick film of water. Controlling bubble dynamics by the electric field, including escape, coalescence, and departure provides an active and reversible approach to move bubbles or increase departure frequency in many fluid mechanics and heat transfer studies.

Directional Transport of Underwater Bubbles on Solid Substrates: Principles and Applications

“…By remotely loading and releasing unilateral near-infrared stimulation, the movement direction and state of underwater bubbles can be controlled (Figure 11a). Yan et al 65 used the mechanism of electrowetting-on-dielectric, that is, by changing the voltage to adjust the wettability of the surface of the dielectric layer, to activate the movement of bubbles. The device consists of a conductive substrate electrode and a copper wire electrode vertically inserted into the water.…”

“…Based on the bubble-driven force, the transportation on the substrate can be classified as buoyancy-driven (Section ), Laplace-pressure-difference-driven (Section ), and external-force-driven (Section ). The buoyancy force can be easily regulated by the inclined angle of the substrate. − Laplace pressure difference can be achieved by adjacent bubbles, , shape gradient, ,,− and wettability gradient. − The external fields were used to regulate the drag force of buoyance-driven bubbles (summarized in Section ) and the wettability of substrates for Laplace-pressure-difference-driven bubbles (summarized in Section ). It can also provide an additional driven force to the bubble with the assistant of other kinds medium such as pusher, steel ball, ferromagnetic fluid, and microcilia (summarized in Section ).…”

“…In addition, when the substrate is responsive to an external field, the driven force and drag force of the bubble can be controlled by adjusting the surface structure and properties of the substrate via mechanical stretching, photothermal field, magnetic field, electric field, and chemical substances. A real-time bubble motion control can be realized when an external-field-responsive substrate is employed or an external force is introduced. − ,− Furthermore, current methods to achieve complex transportation routes are discussed herein. When the bubble slides on a smooth isotropic plane by buoyancy, it can only slide straight up.…”

Review of Bubble Applications in Microrobotics: Propulsion, Manipulation, and Assembly

“…To improve the controllability of the manipulation, double bubbles were used to form a flow field with a fixed direction; this could be used to realize the directional transport of fish eggs. Recently, Yan et al induced the escape, coalescence, and departure of bubbles activated by an electric field by using a simple EWOD device [ 186 ]. When the copper electrode wire (inserted into deionized water) was placed on one side of the bubbles, the bubbles escaped from their initial positions with the opening of the voltage.…”

Bubble Manipulation Driven by Alternating Current Electrowetting: Oscillation Modes and Surface Detachment

“…The study of bubble dynamics is helpful to understand its role in a variety of technical applications such as microfluidic mixing, , macromolecular delivery, − biological cell manipulation, boiling transmission, and micromotor driving. , In the past few decades, various methods of bubble actuation have been implemented, including the use of acoustic, optical, and electric fields, , where the driving forces couple directly to the bubble interface. Electrowetting on dielectric (EWOD) is a mechanism of changing the wettability of a dielectric solid surface with the electric field. − Application of AC driving conditions on EWOD can provide dynamic and adjustable wettability by changing the driving voltage amplitude or frequency. , Bubble actuation by EWOD provokes stable oscillations of the bubble shape by periodic electric–mechanical forces that are concentrated on the three-phase contact line (TCL). , EWOD applied to a sessile bubble has proven that the deformation of the bubble increases with electric field strength. , This concept can be applied to splitting, transporting, merging, and detachment of bubbles. ,− An interesting application arising from bubble manipulation by alternating current (AC) electric fields is the production of a synthetic jet of an electrolyte generated by an oscillating bubble …”