Microdroplet Actuation via Light Line Optoelectrowetting (LL-OEW).

Christoph Doering,Henning Fouckhardt,Johannes Strassner,Chanbasha Basheer

doi:10.1155/2021/3402411

Christoph Doering, Henning Fouckhardt + Show 2 more

Open Access

https://doi.org/10.1155/2021/3402411

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Abstract

Meanwhile, electrowetting-on-dielectric (EWOD) is a well-known phenomenon, even often exploited in active micro-optics to change the curvature of microdroplet lenses or in analytical chemistry with digital microfluidics (DMF, lab on a chip 2.0) to move/actuate microdroplets. Optoelectrowetting (OEW) can bring more flexibility to DMF because in OEW, the operating point of the lab chip is locally controlled by a beam of light, usually impinging onto the chip perpendicularly. As opposed to pure EWOD, for OEW, none of the electrodes has to be structured, which makes the chip design and production technology simpler; the path of any actuated droplet is determined by the movement of the light spot. However, for applications in analytical chemistry, it would be helpful if the space both below as well as that above the lab chip were not obstructed by any optical components and light sources. Here, we report on the possibility to actuate droplets by laser light beams, which traverse the setup parallel to the chip surface and inside the OEW layer sequence. Since microdroplets are grabbed by this surface-parallel, nondiverging, and nonexpanded light beam, we call this principle “light line OEW” (LL-OEW).

Highlights

Electrowetting-on-dielectric (EWOD) is a phenomenon [1,2,3,4,5,6,7,8,9,10,11,12,13], which has been widely used to change microdroplet curvature
For EWOD, the droplets are usually made of ionized water or an aqueous solution and sandwiched between a top electrode and a dielectric layer above a bottom electrode. (Often, for both the substrate as well as for the superstrate, glass wafers with indium tin oxide (ITO) coatings as electrode layers are used, with the ITO films pointing inwards.) Attached to a voltage source, this setup constitutes a loaded capacitor. e stored electrostatic energy modifies the surface energy of the droplet on the dielectric layer such that the droplet’s contact angle and curvature are reduced
If the bottom electrode is structured and initially only part of the droplet is above the electrode structure, the change of the contact angle will be local. is way, a net force arises parallel to the substrate surface, which drags/pulls the droplet onto the electrode [19,20,21,22]. us, EWOD can be used to move/actuate droplets, allowing for splitting and merging/fusion of droplets. is is beneficial for digital microfluidics (DMF) in analytical chemistry, i.e., in lab-ona-chip applications with droplets [23,24,25,26,27,28,29,30,31,32,33]

Summary

Introduction

Electrowetting-on-dielectric (EWOD) is a phenomenon [1,2,3,4,5,6,7,8,9,10,11,12,13], which has been widely used to change microdroplet curvature. For OEW, the alternating voltage is necessary because the interplay of the complex impedances of the dielectric layer, the photoconductive layer, and the path resistance of droplet and contact layers allows for the manipulation of the operating point of the OEW chip. 1 + c/3cla􏼁cos θ0 with Cdpc as the capacitance of the double-layer (dielectric plus photoconductive layer) and Vdpc as the voltage drop across the double-layer Both the denominator as well as the parameter c in the above equation are related to the Laplace pressure and its case-dependent significance, respectively [52, 53], which get strong for microdroplets with volumes well below 1 μl; the Laplace pressure (related to the strong surface curvature of small droplets) works against the reduction of surface tension.

Droplet Movement with Usual OEW

Findings

Conclusions