Drop formation via breakup of a liquid bridge in an AC electric field

doi:10.1016/j.jcis.2006.05.060

Journal of Colloid and Interface Science

Volume 302, Issue 1, 1 October 2006, Pages 294-307

https://doi.org/10.1016/j.jcis.2006.05.060 Get rights and content

Abstract

Experimental results are presented for the study of drop formation mechanism in a newly proposed electrohydrodynamic (EHD) method of drop generation in an AC electric field. In the method, a small drop is generated in two stages. A pendant drop is elongated with large oscillation by an electric force in the first stage. Then, it undergoes formation and breakup of a liquid bridge between the upper nozzle and the insulator-coated lower flat plate in the second stage. It is found that there exists a resonant frequency for maximum oscillation, which leads to an efficient drop formation in the latter stage. It is also found that breakup of liquid bridge is accelerated by the electrowetting tension acting on the drop perimeter contacting the insulator-coated flat plate. Thus the whole procedure of drop formation depends heavily on the frequency of AC field and the properties of the insulator such as hydrophilicity, thickness, and the dielectric constant. It is demonstrated that a wide range of drop size, from picoliter to nanoliter, can be obtained by controlling such key parameters without changing the nozzle diameter.

Graphical abstract

Mechanism in a newly proposed EHD method of drop formation in an AC electric field is studied and it is divided into two stages, such as drop elongation and breakup of a liquid bridge.

Introduction

Drop-on-demand (DOD) printing method has been a big issue due to its great potential applicability in diverse areas such as ink-jet printing [1] and DNA microarraying [2], [3]. Study of the literature on DOD drop formation reveals that there are not many methods for decreasing the drop volume without reducing the nozzle size except a few recent works [4], [5]. Chen and Basaran [4] reported that an order of magnitude reduction in the drop volume could be attained by using the same nozzle with the control of the capillary, viscous, and inertial time scales in the voltage waveforms used to drive a piezoelectric transducer. Yogi et al. [5] studied an electrohydrodynamic (EHD) method and reported that it could produce drops from picoliter to femtoliter with several orders of magnitude reduction in drop volume. Here, a new EHD method is presented as a DOD printing method with the discussion in its mechanism. The key idea of the method is to form a drop via breakup of a liquid bridge in an AC electric field.

Dynamics of a liquid bridge [6], [7], [8], [9], [10] has been of great interest because its stability and breakup are important in the process of drop formation. However, despite numerous theoretical and experimental researches on the dynamics, direct utilization as a drop formation method has received less attention. If an electric field is applied to a pendant drop hanging on a cylindrical nozzle and the distance between the nozzle (the upper electrode) and the horizontal flat plate (the lower electrode) is sufficiently small, a liquid bridge is formed and breaks up by the electric stress. In this way, a drop can be generated on the lower electrode.

As a driving force to generate a drop or a bubble in aforementioned EHD system, an external electric field has been used [5], [9], [10], [11], [12], [13], [14], [15]. Examination of previous studies elucidates that the dynamics governing drop formation depends highly not only on the electric field strength, but also on the type of electric field such as DC or AC electric field. If the liquid is highly conducting, it is difficult to control the volume of generated drop in a narrow size distribution when a DC electric field is used [12]. Sato investigated the effect of an AC electric field superimposed on a DC electric field [13]. At a certain voltage the dripping frequency is observed to be equal to the applied AC frequency, resulting in uniform size. In the case of pure AC electric field, Yeo et al. [14] found that, unlike the DC electric field, drops are ejected from a resonating meniscus of a pendant drop. So the oscillation of a pendant drop under AC electric field shows resonance and it has the corresponding resonant frequency. For the case of bubble, Kweon et al. [15] performed experimental visualization on deformation and departure of the bubble attached to a wall. They reported that bubble departure occurs most effectively near the resonant frequency.

In this study, we focus our attention to the dynamics of a pendant water drop and the mechanism of drop formation in an AC electric field. The drop formation mechanism can be divided into two stages: the stage of drop elongation by the electric force and the stage of breakup of a liquid bridge with the assistance of electrowetting tension. In the first part of this study, the two-stage mechanism is discussed in detail by experimental results. Next, we discuss the effects of key parameters on the drop formation procedure from the viewpoint of drop oscillation, the electrode distance, and the insulating materials. As will be seen later, the proposed EHD method has controllability of drop volume ranging from nanoliter to picoliter without changing the nozzle diameter.

Section snippets

Apparatus

The schematic experimental setup is shown in Fig. 1. Key components of the system are the electrodes; the upper and the lower electrodes. The upper electrode is a syringe nozzle of 26G (OD 0.46 mm, ID 0.26 mm, Hamilton) and the lower electrode is an insulator-coated flat plate of stainless steel (SUS, $22 \times 22 \times 0.1 mm$ ). These electrodes are installed on a high-precision motorized Z-stage which can control the distance between the upper and the lower electrodes with the accuracy of 10 μm (Sigma

Two-stage falling mechanism

The dynamic falling behavior of a pendant drop is studied by experiment to see how it is disintegrated onto the lower electrode when an AC electric field is applied. Fig. 3, Fig. 4 show the continuous falling behaviors of all types of initial drops with different initial shapes on the different insulator-coated lower electrodes (5 μm-thick PMMA for Fig. 3 and 50 μm-thick PE for Fig. 4). All of these drops have a common falling behavior that can be divided into two stages. The first stage

Summary

A new method of drop formation under an AC electric field has been presented with the experimental results on the falling mechanism. From the results, we have reached the following conclusions.

(1)
The drop formation mechanism can be divided into two stages. In the first stage, drop elongation is induced by the electric force and the drop undergoes oscillation during the elongation until a liquid bridge is formed. In the second stage, breakup of the liquid bridge occurs. During the breakup process

Acknowledgements

This research has been supported by Biochip Technology Group in SAIT, by BK21 program of Ministry of Education of Korea, by the grant R01-2004-000-10838-0 from KOSEF, and by Center for Ultramicrochemical Process Systems (CUPS) sponsored by KOSEF. The authors greatly acknowledge the financial supports.

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Drop formation via breakup of a liquid bridge in an AC electric field

Abstract

Graphical abstract

Introduction

Section snippets

Apparatus

Two-stage falling mechanism

Summary

Acknowledgements

Trends Biotechnol.

Biosens. Bioelectron.

J. Colloid Interface Sci.

J. Electrostat.

Int. J. Multiphase Flow

J. Colloid Interface Sci.

Philips Tech. Rev.

Phys. Fluids

Anal. Chem.

Phys. Fluids A