Ultra-low voltage electrowetting using graphite surfaces.

Deborah J Lomax,Anne Juel,Hollie V Patten,Pallav Kant,Yuqin Zou,Robert A W Dryfe,Aled T Williams

doi:10.1039/c6sm01565d

Abstract

The control of wetting behaviour underpins a variety of important applications from lubrication to microdroplet manipulation. Electrowetting is a powerful method to achieve external wetting control, by exploiting the potential-dependence of the liquid contact angle with respect to a solid substrate. Addition of a dielectric film to the surface of the substrate, which insulates the electrode from the liquid thereby suppressing electrolysis, has led to technological advances such as variable focal-length liquid lenses, electronic paper and the actuation of droplets in lab-on-a-chip devices. The presence of the dielectric, however, necessitates the use of large bias voltages (frequently in the 10-100 V range). Here we describe a simple, dielectric-free approach to electrowetting using the basal plane of graphite as the conducting substrate: unprecedented changes in contact angle for ultra-low voltages are seen below the electrolysis threshold (50° with 1 V for a droplet in air, and 100° with 1.5 V for a droplet immersed in hexadecane), which are shown to be reproducible, stable over 100 s of cycles and free of hysteresis. Our results dispel conventional wisdom that reversible, hysteresis-free electrowetting can only be achieved on solid substrates with the use of a dielectric. This work paves the way for the development of a new generation of efficient electrowetting devices using advanced materials such as graphene and monolayer MoS2.

Highlights

We return to the canonical droplet configuration pioneered by Frumkin to demonstrate a robust and versatile approach to reversible and hysteresis-free electrowetting on a conductor’’ (EWOC), see Fig. 1(a), which does not require the application of an alternating voltage or voltage pulses to overcome hysteresis
Note that the potential of zero charge of the 6 M LiCl system shown in Fig. 1 is Epzc = À0.6 V vs. the Pt pseudo-reference electrode (RE), for which the equilibrium contact angle (CA) is identical to the value with no applied bias
EWOC is not restricted to the case of an aqueous electrolyte surrounded by air: Fig. 1(c) shows the evolution of CA with E for an aqueous 6 M LiCl droplet immersed in hexadecane

Summary

Introduction

We return to the canonical droplet configuration pioneered by Frumkin to demonstrate a robust and versatile approach to reversible and hysteresis-free EWOC, see Fig. 1(a), which does not require the application of an alternating voltage or voltage pulses to overcome hysteresis. Reversible wetting is shown to occur on a laminar conductor, the basal plane of highly oriented pyrolytic graphite (HOPG). HOPG can be readily refreshed by mechanical cleavage, it possesses macroscopic (mm scale) lateral domains containing only microscopic (sub-micron scale) steps[21] and a high equilibrium contact angle (CA) for aqueous droplets 641 for water in air, with considerably higher values for aqueous solutions immersed in organic phases),[22,23] from which EWOC can be performed. On ion-intercalation, i.e. wetting involved graphitic edges (as opposed to the basal-plane), the notable contact angle hysteresis observed

Experimental

Results & discussion

Dynamics of EWOC

Conclusion