Ultra-low voltage electrowetting using graphite surfaces

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 MoS.

Electrowetting on conductors: anatomy of the phenomenon

“…In spite of the presence of the larger particles, the surface roughness is still below the threshold of 100 nm previously observed to cause droplet pinning on graphite surfaces. 17 Raman spectroscopy is also presented in Fig. 4 and is consistent with previous reports for this material.…”

“…Where C(E) denotes the potential-dependent capacitance of the solid/liquid interface (HOPG/aqueous LiCl in this case), E pzc is the potential of zero charge, ߛ LV is the surface tension between the two fluid phases (aqueous/air here, taken to be 83.3 mN m −1 ) 17 , and η denotes the electrowetting number. The analytical form of the expression results when the capacitance is assumed to be constant, an approximation made widely in the literature.…”

“…A recent report from our laboratory has described reversible, hysteresis-free EWOC on basal plane graphite using concentrated aqueous electrolyte solutions, surrounded by air, organic solvents and exploiting the ITIES configuration proposed by Kornyshev et al Contact angle changes of 50°, over a 1 V potential range, and 100°, over a 1.5 V potential range, were seen for the liquid/air and liquid/liquid configurations, respectively. 17 The aim of the present contribution is to study this wetting on carbonaceous surfaces in more depth, noting that the interaction of water (and aqueous electrolyte solutions) with graphite, graphene and carbon nanotubes has been the focus of considerable activity over the past decade. This is driven by a desire to understand better the nature of the sp 2 carbon-water interaction, given that such surfaces are believed to facilitate the motion of aqueous droplets over them, and is given a further impetus by numerous applications (e.g.…”

Electrotunable liquid sulfur microdroplets

“…Reducing the thickness of the dielectric layer leads to a lower required voltage; e.g., reversible electrowetting was achieved at < 50 V on an amorphous fluoropolymer dielectric layer thinner than 1 μm 13 , while an applied voltage of 15 V was sufficient to electrowet a thin (70 nm) dielectric layer with a fluoropolymer coating 14 . Recently, ultra-low-voltage electrowetting systems based on the interface between two immiscible electrolytic solutions 15 or using graphite surfaces 16,17 have been reported, reducing the voltage requirement to < 2.0 V, showing their great promise. However, despite these efforts, realization of a low activating voltage requires an extremely thin dielectric layer, which is challenging.…”

Analytic model for the electrowetting properties of oil-water-solid systems

“…These results suggest that electrowetting on reactive substrates might access a much wider wetting configuration space. Recent experiments by Lomax et al [32] suggest that this might indeed be possible for specific graphitic substrates. One should take into account, however, that electrostatic interactions are always complemented by other interaction forces such as long-range van der Waals forces and short-range chemical (e.g., hydration) forces.…”