Electrically Controllable Microparticle Synthesis and Digital Microfluidic Manipulation by Electric-Field-Induced Droplet Dispensing into Immiscible Fluids.

Taewoong Um,In Seok Kang,Jiwoo Hong,Do Jin Im,Sang Joon Lee

doi:10.1038/srep31901

Taewoong Um, In Seok Kang + Show 3 more

Open Access

https://doi.org/10.1038/srep31901

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Abstract

The dispensing of tiny droplets is a basic and crucial process in a myriad of applications, such as DNA/protein microarray, cell cultures, chemical synthesis of microparticles, and digital microfluidics. This work systematically demonstrates droplet dispensing into immiscible fluids through electric charge concentration (ECC) method. It exhibits three main modes (i.e., attaching, uniform, and bursting modes) as a function of flow rates, applied voltages, and gap distances between the nozzle and the oil surface. Through a conventional nozzle with diameter of a few millimeters, charged droplets with volumes ranging from a few μL to a few tens of nL can be uniformly dispensed into the oil chamber without reduction in nozzle size. Based on the features of the proposed method (e.g., formation of droplets with controllable polarity and amount of electric charge in water and oil system), a simple and straightforward method is developed for microparticle synthesis, including preparation of colloidosomes and fabrication of Janus microparticles with anisotropic internal structures. Finally, a combined system consisting of ECC-induced droplet dispensing and electrophoresis of charged droplet (ECD)-driven manipulation systems is constructed. This integrated platform will provide increased utility and flexibility in microfluidic applications because a charged droplet can be delivered toward the intended position by programmable electric control.

Highlights

The EHD jetting method, which is atomization of a liquid by an applied electric field between the nozzle and the bottom electrode, has attracted significant attention because it can generate droplets with sizes ranging from millimeters to micrometers without reducing the size of a capillary nozzle[16,17]
This paper presents the first study to systematically investigate the droplet dispensing into oil by electric charge concentration (ECC) method (Fig. 1) and the applicability of the proposed dispensing method to various applications, including synthesis of microparticles and digital microfluidics
The dispensing behavior of droplets induced by ECC method is consecutively observed when different voltages are increasingly applied to the nozzle under fixed flow rate and fixed gap distance conditions (Fig. 2)

Summary

Introduction

The EHD jetting method, which is atomization of a liquid by an applied electric field between the nozzle and the bottom electrode, has attracted significant attention because it can generate droplets with sizes ranging from millimeters to micrometers without reducing the size of a capillary nozzle[16,17] This method can resolve the technical problems of conventional inkjet printing, such as clogging from the use of chemical or biological samples and consequential damage of inkjet nozzle[16,17]. The electric charge concentration (ECC) method has been proposed to address these problems This method can dispense droplets by using the electric attraction between charges on the droplet surface and the induced counter charges on the surface of the target substrate, when the electric potential is applied to the conducting nozzle[18,19]. The ECC-induced droplet dispensing and manipulation systems are integrated by adopting electrophoresis of charged droplet (ECD), which can provide increased utility and flexibility in digital microfluidics (i.e., microfluidic technology to electrically manipulate discrete droplets on an array of electrodes) and their practical applications

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