Polarity dependent electrowetting for directional transport of water through patterned superhydrophobic laser induced graphene fibers
Graphical abstract
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.
Section snippets
LIG fabrication
Unless otherwise indicated, polyimide sheets (PI, 0.005 inches) were used as received. A CO2 laser cutter system (UniversalX-660 laser cutter platform) with a 10.6 μm laser wavelength was used for laser scribing on polymer sheets. The size of the laser beam is about 100 mm. Laser power at 10 W was maintained during laser scribing. The laser system also offers an option to set the pulses per inch (PPI) with a range of 10–1000 PPI. We set the PPI value of 500 in our experiment. All laser
Results and discussion
Irradiation of CO2 laser on a commercial polyimide film in ambient air converted the PI film into a porous graphene structure. With software controlled laser writing, LIG can be readily patterned into different morphologies as shown in the SEM images in Fig. 1. The appearance of walls and valley-like structures can be seen where walls are predominantly the porous 3D graphene and the valleys in-between the walls are the untreated PI film. The cross-sectional SEM images of LIG film (inset I of
Conclusion
In conclusion, we have demonstrated the voltage polarity dependent directionally controlled transport of liquid through the laser-induced graphene surface, where a threshold bias is required to activate the transport. The strong voltage polarity dependent wetting of LIG, PLIG, and LIG200 suggests that the attached oxygen-containing functional groups and high interfacial capacitance (CEDL) causes the observed transition from nonwetting to wetting state when these films are acting as the anode.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
As a part of the University of Alberta’s Future Energy Systems research initiative, this research was made possible in part thanks to funding from the Canada First Research Excellence Fund (Project # T06P06). Also SD and PRW thank the Natural Science and Engineering Research Council (NSERC) for the financial support in the form of Grant No. RGPIN-2015-06542.
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