Electrically Controlled Localized Charge Trapping at Amorphous Fluoropolymer-Electrolyte Interfaces.

Hao Wu,Guofu Zhou,Ranabir Dey,Lingling Shui,Frieder Mugele,Dirk Van Den Ende,Igor Siretanu

doi:10.1002/smll.201905726

Abstract

Charge trapping is a long-standing problem in electrowetting on dielectric, causing reliability reduction and restricting its practical applications. Although this phenomenon is investigated macroscopically, the microscopic investigations are still lacking. In this work, the trapped charges are proven to be localized at the three-phase contact line (TPCL) region by using three detecting methods-local contact angle measurements, electrowetting (EW) probe, and Kelvin probe force microscopy. Moreover, it is demonstrated that this EW-assisted charge injection (EWCI) process can be utilized as a simple and low-cost method to deposit charges on fluoropolymer surfaces. Charge densities near the TPCL up to 0.46 mC m-2 and line widths of the deposited charge ranging from 20 to 300 µm are achieved by the proposed EWCI method. Particularly, negative charge densities do not degrade even after a "harsh" testing with a water droplet on top of the sample surfaces for 12 h, as well as after being treated by water vapor for 3 h. These findings provide an approach for applications which desire stable and controllable surface charges.

Highlights

This phenomenon is investigated macroscopically, the microscopic investigations are still lacking
Nano-sized Kelvin Probe Force Microscopy (KPFM) probe in ambient air and macroscopic EW-probed drops in ambient oil experience the same surface charge density σT, which could be obtained from the measured voltages using Equation (3) with UT = US. These results from three types of micro- and nanoscale measurements reveal and confirm that the charges are trapped at the AFP surfaces after EW process and accumulate at the three-phase contact line (TPCL) regions. These results indicate that for EW applications driven by DC voltage, failures such as charge trapping or film break-down are more likely to occur at the TPCL region
We conclusively show that, the deposition of surface charges from an aqueous drop on an electrically insulating fluoropolymer surface in an EW configuration at high voltage, preferentially occurs along the contact line of the drop, in accordance with the established EW theory but deviating from recent suggestions based on unconventional and/ or unstable EW systems[27,36,37] in which the maximum charge densities are reported accumulated in the center of the drop

Summary

Introduction

This phenomenon is investigated macroscopically, the microscopic investigations are still lacking. Negative charge densities do not degrade even after a “harsh” testing with a water droplet on top of the sample surfaces for 12 h, as well as are popular materials for various applications[1,2,3,4,5] because of the unique combination of favorable material properties such as chemical inertness, mechanical strength, water repellency, dielectric strength, optical transparency, and easy solution processability.[5,6] For these reasons, AFPs are predominantly used as insulating and hydrophobic layer in electrowetting (EW) devices.[6,7,8,9,10,11] EW, which is often denoted as “electrowetting on dielectric” (EWOD) to emphasize the relevance of the dielectric layer, relies after being treated by water vapor for 3 h.

Objectives

Results

Conclusion