Polarity dependent electrowetting for directional transport of water through patterned superhydrophobic laser induced graphene fibers

Abstract

The possibilities of the precise control of wetting properties of a series of laser-induced graphene (LIG) films consisting of microscale air pockets on top of nano-scale surface roughness using electrowetting are demonstrated. By application of a marginal DC bias (∼2 V), water can efficiently wet as well as can be pumped through the superhydrophobic LIG substrates. Interestingly, the electrowetting phenomenon is strongly dependent on the applied voltage polarity and it causes an abrupt wetting transition from superhydrophobic (contact angle ∼152°) Cassie state to superhydrophilic (contact angle ∼7°) Wenzel state on the LIG films. By analyzing the voltage polarity dependent electrowetting results with an equivalent electrical circuit model at the solid-liquid interface, and considering the hierarchical dual surface roughness (micro-nano scale), the transition between the “slippy” Cassie state and the “sticky” Wenzel states is explained. Furthermore, we demonstrate that the unique structural characteristics of the custom-designed micropatterned LIGs, with precisely tailored surface energy by simple post-annealing treatment, enable easy preparation of superhydrophobic LIG films. The approach to prepare stable superhydrophobic LIG with voltage polarity dependent wetting mode transition is used here to controllably transport of water through 3D porous LIG surfaces.

Introduction

In recent years, wettability control and capillary transport of water through carbon membranes have generated considerable interest, especially in the biomedical field where water flows through biological water channels [1,2]. In general, the wettability of the solid surface is strongly prejudiced by either altering the surface chemistry or by introducing multiscale physical roughness [3]. Several experimental or theoretical studies have been adopted to exploit the surface roughness (both microscale and nanoscale roughness features) to engineer superhydrophobicity (Cassie state) or superhydrophilicity (Wenzel state) [3]. These artificial synthesized surfaces maintain their wettability characteristics over a period of time but do not actively regulate their wettability after being prepared. It has been a real challenge to tune the wettability on demand, which can be solved by mimicking the electrowetting scenario where an external perturbation in the form of an electric field is accountable for the wettability variation from superhydrophobic to superhydrophilic state [4,5]. The ability to real-time droplet actuation on a solid substrate using an electric field makes electrowetting suitable for many optical, biomedical, and electronics applications that have led to the commercially available liquid lenses and digital microfluidics diagnostics kits [[6], [7], [8], [9], [10]]. There is, therefore, renewed interest to synthesize a new material with a single-step process that is not only accessible for variable wettability but also adequate for electrowetting.

Graphene, the thinnest nanomaterial known so far, where carbon atoms packed into a 2D honeycomb lattice has gained huge recognition due to its fascinating electrical, thermal, and mechanical properties [[11], [12], [13], [14]]. An often desired alteration of graphene wettability in the range of superhydrophobic to superhydrophilic includes specific approaches, such as controlling the relative proportion of electrolytes in graphene dispersed solvent, preparing graphene/carbon nanotube hybrids, introducing fluorine groups in the graphene sheet, functionalizing the graphene nanostructures [3,[15], [16], [17], [18], [19]]. Yet, in all the reported methods, multiple steps are involved to yield the desired surface functionalities and all the modifications are done in post-graphene formation. Moreover, the lack of large-scale production methods and translating these properties into an ordered architecture has been the main hurdle to use it in a practical application.

Recently the graphene-based 3D porous nanomaterial called laser-induced graphene (LIG), prepared from flexible polyimide (PI) sheets has been investigated worldwide due to its easy and scalable synthesis [[20], [21], [22], [23]]. Its physical properties can be controlled by varying experimental conditions such as laser pulse parameters, laser power, and lasing atmosphere [24,25]. Applications of LIG for microsupercapacitors, electrochemical sensors, electrochemical water splitting, electrocatalysis, water treatment have been demonstrated where wettability control was the prime factor for improved device performances [[20], [21], [22], [23],26]. LIG has now emerged as a smart material in which many commercial applications can be envisioned. Reports are available demonstrating that the wetting properties of LIG can be altered by preparing it under a controlled gas atmosphere [24,25]. However the method requires an additional controlled atmosphere chamber which reducing the advantage of LIG formation of being a simple and rapid process. Alternatively, a simple and superior electrowetting approach, by means of an external electric field can modulate the wettability of carbon-based films from superhydrophobic to superhydrophilic state [27,28]. In general, electrowetting refers to electrowetting on dielectrics (EWOD) where the electrowetting phenomenon is independent of the applied voltage polarity and contact angle responses with voltage are symmetrical in both positive and negative applied bias [4]. However, recently few studies have described the dielectric-free approach to electrowetting on the basal plane of conductive graphite surface where specific interactions at the water-graphite interface and graphite crystallinity are said to be responsible for electrowetting on conductive graphite and the phenomena is also referred to as electrically driven flow [[29], [30], [31]]. But surprisingly, until now, single-step synthesis of micropatterned LIG has not been investigated and no reports are available showing their electrowetting and corresponding wetting properties. Liquid repellency and electrowetting characteristics of hierarchical structured LIG fibers are still unexplored. It is well known that the introduction of hierarchical roughness is beneficial to produce stable hydrophobic surfaces and as compared with 2D and 3D carbon geometry, carbon fibers with their flexible geometry are more suitable for multifunctional application [32,33]. Therefore, we believe the development of electrowettable superhydrophobic micropatterned LIG fibers in terms of the cost-effective simple production process could be of great significance in the field of surface science and engineering.

Aiming to expand the range of properties for LIG, in particular, to tune the underlying wettability of LIG surface, and eventually to further broaden the LIGs field of applications using electrowetting, here we presented polarity dependent low voltage electrowetting for electrically driven transport of water and precise actuation of droplets on LIG surfaces which are made of densely packed vertically aligned carbon nanofibers. Moreover, we report here a method to prepare LIG fiber and patterned LIG (PLIG) fiber under an ambient atmosphere where superhydrophobic LIG and PLIG fibers are obtained by post-annealing treatment and superhydrophilic LIG and PLIG fibers are obtained by mimicking the electrowetting scenario. Here we developed graphene walls where sequential air pockets are regularly distributed on the PLIG surface. Such a system can be considered as a two-scale structure; the regularly distributed micron size air pockets represent microscale surface roughness which is coupled with the nanoscale surface roughness associated with the sharp fiber tips. The influence of micropatterning, surface chemical functionalities, microstructure on the variable wettability, and corresponding electrowetting is investigated using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and Raman spectroscopy. Finally, polarity dependent electrowetting characteristics are explained by analyzing the electrowetting behavior at the electrode/electrolyte interface with an equivalent circuit model (electrochemical impedance spectroscopy) and by taking into account the role of interfacial capacitance using cyclic voltammetry method.