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Hydrochloric acid-impregnated paper for gallium-based liquid metal microfluidics
Sensors and Actuators B 207 (2015) 199–205
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Hydrochloric acid-impregnated paper for gallium-based liquid metal microfluidics Daeyoung Kim a,b,∗ , Yunho Lee c , Dong-Weon Lee d , Wonjae Choi c , Koangki Yoo e , Jeong-Bong (JB) Lee a a
Department of Electrical Engineering, The University of Texas, Dallas, TX, USA Department of Electrical Engineering, Korea Army Academy, Yeong-Cheon, South Korea c Department of Mechanical Engineering, The University of Texas, Dallas, TX, USA d School of Mechanical Systems Engineering, Chonnam National University, Gwangju, South Korea e Department of Information and Communication Engineering, Hanbat National University, Daejeon, South Korea b
a r t i c l e
i n f o
Article history: Received 2 May 2014 Received in revised form 13 September 2014 Accepted 29 September 2014 Keywords: Liquid metal Galinstan® HCl-impregnation Super-lyophobicity Microfluidic platform
a b s t r a c t We report a simple hydrochloric acid (HCl) impregnation method to substantially improve the lyophobicity of a paper against gallium-based liquid metal. Based on the HCl-impregnated paper, we also propose an extremely simple fabrication method of microfluidic channel for gallium-based liquid metal, Galinstan® . Due to its low cost, easy fabrication, and flexibility, recently paper has drawn attention as microfluidic platforms for various applications. We have treated two different types of paper (paper towel and printing paper) with various treatment methods such as laser printer flattening, fluorocarbon polymer coating, HCl-impregnation, and combination of these methods. We then studied their lyophobicity characteristics by measuring static and dynamic contact angles as well as bouncing experiment. We found that HCl-impregnation is a simple yet powerful method to engineer certain types of papers to make them super-lyophobic substrates against gallium-based liquid metals and effective for more than 30days after impregnation. To show the feasibility, we demonstrated manipulation of a Galinstan® droplet along microfluidic channels formed on the HCl-impregnated paper. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Paper is abundant and extremely cheap cellulosic material. Untreated paper is highly porous and thus paper can easily absorb most liquids. Due to these properties, paper can transport fluid only using capillary force without any active pumping system [1]. Great potential of paper as disposable microfluidic platforms has only recently unleashed and there have been demonstrations on the paper-based microfluidics for analytical chemistry [2], health diagnostics [3,4], and environmental monitoring [5] for the past few years. In these work, microfluidic channels were typically fabricated by patterning hydrophobic regions on super-hydrophilic papers. Chitnis et al. reported a method of converting paper’s hydrophilicity into hydrophobicity using a CO2 laser [6]. Balu et al. demonstrated that super-hydrophobicity can be created on a paper surface via a combination of etching by an oxygen
∗ Corresponding author at: The University of Texas at Dallas, Department of Electrical Engineering, Richardson, United States. Tel.: +1 9726930988. E-mail address: [email protected] (D. Kim). http://dx.doi.org/10.1016/j.snb.2014.09.108 0925-4005/© 2014 Elsevier B.V. All rights reserved.
plasma and deposition of a fluorocarbon film [7]. In addition, Bruzewicz et al. used polydimethylsiloxane (PDMS) to define hydrophobic microfluidic channels in a paper [8]. Based on these ease fabrication merit of paper-based microfluidic devices, various researches use expensive reagents as the working media [9,10] because paper offers a wide range of other advantages, such that a paper is flexible, does not easily break, portable, liquid-wicking as received, and so forth. Therefore, although reagents or other liquids add significant amount of cost to the final product, this character of paper allows the paper-based microfluidic devices to be fabricated in countries with very poor infrastructure, and thus the paper still has a merit even if it loses its low cost benefit. As an alternative to toxic mercury, non-toxic gallium-based liquid metal alloy such as EGaIn (a binary gallium and indium alloy) [11] and Galinstan® (a ternary alloy of gallium, indium and tin) [12] have been recently studied. Based on its favorable properties such as higher thermal and electrical conductivity compared to mercury, low melting point and low toxicity, gallium-based liquid metals have been investigated for a variety of applications including heat transfer [13], stretchable microfluidic antenna [14], hyperelastic pressure sensor [15], microfluidic wireless strain
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sensor [16] and tunable frequency selective surface [17]. However, gallium is readily oxidized in air, forming a gallium oxide layer at the outer skin of the gallium-based liquid metal alloys. This causes the gallium-based liquid metals to be viscoelastic and to adhere to almost any surface [18], which is an extremely challeging ptoblem to solve. In microfluidic devices such as a stretchable microfluidic antenna, a hyperelastic pressure sensor, and a strain sensor, liquid metal fills the entire microfluidic channel and the oxide layer does not compromise the performance of the device rather enhances the adhesion between the liquid metal and the surface of channel. However, for applications such as heat transfer and a microfluidicsbased switch in which it is essential to have a dynamic movement of the meniscus of liquid metal, the viscoelastic oxide layer is severely problematic. There have been several efforts to overcome this issue by using non-wettable surface against oxidized gallium-based liquid metal. It was reported that HCl solution or vapor can chemically remove the oxide skin from gallium-based liquid metal [19–21]. Rather than removing oxide skin, it was also reported that hierarchical micro-/nano-scale structured surface is super-lyophobic and oxidized Galinstan® does not adhere to the engineered surface [22]. In this paper, we report lyophobicity of (i) a paper towel and a printing paper, (ii) papers treated with laser printer flattening, fluorocarbon polymer coating, and HCl-impregnation, as well as their combinations. We characterized the lyophobicity by measuring static and dynamic contact angles and observing bouncing patterns of naturally oxidized Galinstan® droplets on various paper-based substrates. In addition, we demonstrated Galinstan® droplet manipulation on a HCl-impregnated paper as a substrate.
2. Method of experiment 2.1. Materials In our study, we chose two different papers: a conventional paper towel (Uline S-7127, Uline, Pleasant Prairie, WI, USA) and a printing paper (Office Depot #348037, letter-sized, Boca Raton, FL,
USA) which are widely used in our daily life. Conventional paper towel is extremely hydrophilic, and readily absorbs water due to its chemical preference and porous structure; its cellulose which has hydroxyl group prefers to have hydrogen bonding with water molecules and its cellulous fibers formed spatial solid network at the paper making process. The printing paper used in this experiment was for the inkjet printing, and thus, the surface of the paper was extremely smooth and easily absorbs water-based ink. However, if the printing paper has super-hydrophilicity, the water can spread to nearby regions and distort the printed image. Therefore, it was chemically treated with starch to have some degree of hydrophobicity [23,24]. In addition, calcium carbonate and titanium dioxide were deposited to modify the brightness and optical property of the paper [25,26]. Fig. 1 shows top-view scanning electron microscopy (SEM) images of a conventional paper towel and a printing paper. Both papers show micro-/nano-scale randomly distributed cellulose fibers. As shown in the SEM images, the porosity of the paper towel is larger than that of the printing paper as the surface of the printing paper was chemically treated with starch, calcium carbonate, and titanium dioxide (Fig. 1d). Numerically, we calculated density of the papers used in this experiment based on the given information from the company. They were turned out to be 0.150 g/cm3 for the paper towel and 0.735 g/cm3 for the printing paper.
2.2. Static/dynamic contact angle measurement We investigated wetting characteristics of papers against naturally oxidized Galinstan® by measuring static and dynamic contact angles. In order to increase the lyophobicity of two papers, we modified the paper with three different methods and their combinations: flattening the paper towel by running through a laser printer fuser, fluorocarbon (FC) polymer coating (∼20 nm) by applying inductively coupled plasma power with a mixture of fluorocarbon precursors (C4 F8 ) and Ar gas, and HCl (37 wt%, 7 L) impregnation. From the combinations of three methods above, eight different types of paper were made and tested: non-treated paper towel
Fig. 1. SEM images of (a and b) a paper towel and (c and d) a printing paper; (b) is close-up view of (a) showing highly porous, randomly oriented micro-/nano-scale cellulose fiber structures; (d) is close-up view of (c) showing chemical treatment.
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and printing paper; flattened paper towel; FC polymer-coated paper towel and printing paper; FC polymer-coated flattened paper towel; HCl-impregnated flattened paper towel and printing paper. On these substrates, ∼7.8 L Galinstan® droplet was dropped using a pipette in air environment. Therefore, the surface of the Galinstan® droplet was instantly oxidized and formed oxide skin. Static contact angle was measured with a sessile droplet using a goniometer (Ramé-hart 260-F4, Ramé-hart Instrument, Succasunna, NJ, USA). For dynamic contact angle measurement, we increased and decreased the volume of the Galinstan® droplet on the various paper-based substrates using a pipette with Tefloncoated tip (inner diameter of 0.3 mm) while the side view of the droplet was recorded by a charge-coupled device (CCD) camera at 70 frames per second. The advancing and receding contact angles were obtained from the frame of the recorded movie just before contact line was changed. The static and dynamic contact angle measurements for HCl-impregnated papers were carried out right after the HCl-impregnation. 3. Lyophobicity of papers 3.1. Static and dynamic contact angle measurements Fig. 2 shows static contact angles of the oxidized Galinstan® droplet on paper towel and printing paper with and without various treatments. The shapes of Galinstan® droplets on the various substrates except HCl-impregnated papers are not spherical, as the viscoelastic oxide skin layer causes the droplets to retain the deformed shape during the pipette dispensing process. The static contact angles of droplets on non-treated paper towel and printing paper were found to be 143.1 ± 5.4◦ and 149.6 ± 3.8◦ , respectively. These high static contact angles indicate that the paper itself shows lyophobicity against gallium-based liquid metals. Based on the various treatments, the contact angles of treatedpapers were enhanced compared to those of non-treated papers indicating the improvement of lyophobicity. By coating surface of the papers with low energy FC polymer, the static contact angles of FC-coated papers were increased in comparison to those of nontreated papers. Among various treated papers, the contact angle of the FC polymer-coated printing paper was the largest (155.3 ± 0.5◦ ). The static contact angles of HCl-impregnated flattened paper towel and printing paper were 149.9 ± 0.6◦ and 149.8 ± 1.8◦ , respectively. Due to chemical reaction with HCl, oxide skin layer of Galinstan® droplets were removed and thus their shapes were spherical as
Fig. 2. Static contact angles of ∼7.8 L Galinstan® droplets on papers with and without various treatments [22].
Fig. 3. (a) Advancing and receding contact angles and (b) contact angle hysteresis for papers with various treatments. Inset images in (a) show images of advancing and receding angles of Galinstan® droplets on the HCl-impregnated flattened paper which shows the lowest contact angle hysteresis and the non-treated printing paper which shows highest contact angle hysteresis. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
they were not affected by the viscoelastic behavior [20]. Therefore, the measured contact angles were relatively consistent. Values of static contact angles are, however, not the most important indicator of lyophobicity. The droplets on the FC-coated printing paper did not readily roll-off the surface, implying that the contact angle hysteresis was significant. We thus measured the dynamic contact angles for variously treated-paper towels and printing papers along with non-treated paper towel and printing paper, as shown in Fig. 3. Red-colored circle dot indicates advancing contact angles and blue-colored square dot indicates receding contact angles, respectively. The advancing contact angles of all variously treated-papers are very close to or higher than 150◦ . The receding contact angle, however, greatly changes by the treatment. The non-treated paper towel and printing paper show the receding contact angle of 97.5◦ and 41◦ , respectively. As a result, the contact angle hysteresis (advancing angle − receding angle) of non-treated paper towel is lower than that of the non-treated printing paper. This is attributed to the less contact area originating from the fact that the porosity of the paper towel is much larger than that of the printing paper. FC polymer-coated paper towel and printing
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Fig. 4. A series of time-lapse images of a Galinstan® droplet falling from 3 cm above the surface on (a) the flattened paper towel and (b) the FC polymer-coated paper towel, (c) the HCl-impregnated flattened paper towel, (d) the printing paper, (e) the FC polymer-coated printing paper, and (f) HCl-impregnated printing paper.
paper exhibit smaller contact angle hysteresis compared to those of the non-treated counterparts; smaller hysteresis indicates the enhancement of lyophobicity even when the static contact angles are more or less the same. Among them, the HCl-impregnated paper towel and printing paper show the lowest contact angle hysteresis of 8.8◦ and 11◦ , respectively. We believe that this is because the oxidized Galinstan® reacts with the impregnated HCl on the paper resulting in the removal of viscoelastic oxide layer. This low contact angle hysteresis shows that that the HCl-impregnation is the optimum way to modify the paper for manipulating oxidized gallium-based liquid metal droplets. 3.2. Bouncing tests The dynamic contact angle and contact angle hysteresis of the Galinstan® droplet on the variously treated-papers showed that the HCl impregnation in the paper is a simple yet powerful method to prepare a non-wetting substrate for gallium-based liquid metal microfluidics. To further verify the lyophobicity of the surface, we carried out bouncing experiments of the oxidized Galinstan® droplet (∼7.8 L) on six variations of paper treatments. Each
Galinstan® droplet was dropped from 3 cm above the surface and impacted the paper substrates while the movement of the droplet was being captured by a high-speed camera (Photron SA4, Photron America, San Diego, CA, USA) with 1000 frames per second. Note that the oxidation of Galinstan® is nearly instant and the droplet becomes fully oxidized by the moment of impact. Fig. 4 shows a series of time-lapse images of Galinstan® droplet onto the six different substrates. The shape of oxidized Galinstan® droplets in each bouncing experiment was not spherical before hitting the paper because the droplet was immediately oxidized. For those substrates except HCl-impregnated papers, after the droplet hit the surface, the Galinstan® droplet spread on the surface and never bounced back. However, the droplet readily bounced off from the surface of the HCl-impregnated flattened paper and printing paper without leaving any residue on the substrate (Fig. 4c and f). These results clearly demonstrated that HCl-impregnation can achieve the non-wetting characteristics of the paper against naturally oxidized gallium-based liquid metal. It is believed that the oxide skin of the Galinstan® droplet is reacted with HCl from the HCl impregnated in the papers [20] while falling and non-wetting characteristic was recovered at the moment of the impact at the paper surface.
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Fig. 5. Optical images of the shape change of the liquid metal droplet after 8 and 60 min of dry on the HCl-impregnated (a and b) paper towel and (c and d) printing paper. The time (seconds) in the figure indicates the required time to change the droplet shape based on the chemical reaction.
4. Characterization of HCl-impregnated papers There are three potential mechanisms that lead to the nonwettability of a HCl-impregnated paper: HCl vapor, HCl liquid, and HCl monolayer on the surface of each microfibers on the paper surface. Unless the paper is encased in a closed package, the first two components will disappear and only a thin monolayer of HCl remains. Therefore, in order to understand the effect of wetting characteristics against the dry time of the impregnated HCl on papers, we placed oxidized Galinstan® droplet on the HClimpregnated paper after certain amount of dry time (8 min and 60 min of dry after HCl impregnation in ambient air at room temperature). Then, we recorded videos to measure the time required for changing from irregular shape (which means the Galinstan® droplet surface is oxidized) to spherical shape (which means the Galinstan® droplet surface is oxide-free). Fig. 5a shows Galinstan® droplet placed on the HCl impregnated paper towel after 8 min of dry. It shows that the Galinstan® droplet has irregular shape on the left side as it is oxidized as soon as it is placed on the paper towel surface. It takes 1.7 s to be oxide-free (right side). In comparison, the same experiment is carried out after 60 min of dry and it is found that reaction time is 890 s (Fig. 5b). Similarly, results on the printing paper show 0.5 s reaction time for the 8 min dried paper surface and 210 s reaction time for the 60 min dried paper surface (Fig. 5c and d). This result suggests that HCl-impregnated paper is an effective means of preparing non-wetting surface for gallium-based liquid alloy manipulation. Although it takes some time (e.g., 890 s for the
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60 min dried paper towel) to make the Galinstan® droplet oxidefree, it is not readily re-oxidized as we reported earlier. Based on these results, we believe that HCl-impregnated paper is an excellent disposable microfluidic platform for Galinstan® droplet manipulation. Additionally, to find how long the HCl-impregnated paper works as anti-wetting surface in ambient air environment against oxidized gallium-based liquid metal, we carried out bouncing tests in atmospheric environment with relative humidity of 43.0 ± 0.2% and temperature of 21.6 ± 0.1 ◦ C. HCl impregnation on papers was achieved by simply dropping HCl pendant droplet (7 L, 37 wt%) onto surface of papers. When it was dropped HCl droplet on the flattened paper towel was impregnated and spread out to circular area of ∼1.8 cm2 as soon as being dropped, but the HCl droplet on the printing paper was not readily impregnated right after being dropped and gradually spread out to the area of ∼0.8 cm2 up to 20 min after impregnation. We believe that this absorption difference due to different porosity and chemical treatment on the surface as described above. After HCl solution was completely impregnated on papers, papers were placed on ambient air environment and we repeated bouncing test long-term to find out effect of natural evaporation of HCl from the papers. However, we found that even after 30 days of testing, Galinstan® droplet was readily bounced off from both HCl-impregnated paper towel and HCl-impregnated printing paper. This testing result confirms that HCl-impregnation would be a simple but very practical method for converting regular paper towels and inkjet printing papers as microfluidic platforms against natively oxidized gallium-based liquid metal alloy applications. Since paper-based microfluidic devices are inexpensive, it could be designed as disposable devices. Therefore, more than 30-days of HCl-impregnation effect should be more than enough for most applications. 5. HCl-impregnated paper-based microfluidic channel Based on the HCl impregnation on the flattened paper towel, we fabricated HCl-impregnated paper-based microfluidic channels [27]. The fabrication process is illustrated in Fig. 6a. The grid-shape 1/16 in. (1.57 mm) diameter stainless rods were placed on the HCl-impregnated flattened paper, which in turn was placed onto a deformable material. Micro imprinted grid of microfluidic channels with a channel depth of approximately 1.2 mm was created by applying the pressure through a top plate. We placed an oxidized Galinstan® droplet by a pipette (Fig. 6b), and mobilized it by controlling N2 gas through the PTFE tube. Due to the imprinted channel, the droplet well followed inside the channel with an applied
Fig. 6. (a) A schematic of paper-based microfluidic channel fabrication using a deformable substrate, (b) a schematic of N2 gas driven liquid metal droplet mobilization on the fabricated paper-based channel and inset image showing optical image of ∼7.8 L Galinstan® droplet placed on the paper microfluidic channel, and (c) a series of still images taken from a real-time video of a moving Galinstan® droplet on a HCl-impregnated flattened paper.
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pressure. Fig. 6c shows a series of still images from a real-time video of a moving oxidized Galinstan® droplet on the HCl-impregnated flattened paper. There is no trace of oxidized Galinstan® on the HClimpregnated flattened paper after the droplet was mobilized. This clearly shows that this HCl-impregnated paper-based substrate is suitable for gallium-based liquid metal microfluidic applications. 6. Conclusion In this paper, we studied lyophobicity of papers with various treatments against oxidized Galinstan® droplets by measuring static and dynamic contact angles along with bouncing experiments. It was demonstrated that HCl-impregnation is a simple but powerful way to create a super-lyophobic substrate for galliumbased liquid metal alloy manipulation. On the HCl-impregnated flattened paper and printing paper, the spherical shape of sessile droplets was formed and small contact angle hystereses (8–11◦ ) were achieved. The super-lyophobic property of HCl-impregnated papers was confirmed by the bouncing experiments. To show the long-term feasibility of the HCl-impregnated paper as a microfluidic platform, we performed repeated long-term Galinstan® bouncing tests in ambient air environment and found that the HCl-impregnated paper is operational as super-lyophobic galliumbased liquid metal microfluidic substrate even after 30 days of testing. Finally, we demonstrated HCl-impregnated paper-based channel as a suitable microfluidic platform for the manipulation of oxidized Galinstan® droplets. We believe that this work has a great potential to solve one of the most challenging obstacles in the gallium-based liquid metal based microfluidics. Acknowledgements This work was supported in part by World Class University (WCU) project (R32-2009-000-20087-0) and also supported by MKE (The Ministry of Knowledge Economy) under the Brain Scouting Program (HB606-12-2001) supervised by the NIPA (National IT Promotion Agency), Korea. The authors would like to thank Republic of Korea (ROK) Army for financial support.
[10] Ali Kemal Yetisen, Muhammad Safwan Akram, Christopher R. Lowe, Paperbased microfluidic point-of-care diagnostic devices, Lab Chip 13 (2013) 2210–2251. [11] M.D. Dickey, R.C. Chiechi, R.J. Larsen, E.A. Weiss, D.A. Weitz, G.M. Whitesides, Eutectic gallium–indium (EGaIn): a liquid metal alloy for the formation of stable structures in microchannels at room temperature, Adv. Funct. Mater. 18 (2008) 1097–1104. [12] T. Liu, P. Sen, C.-J. Kim, Characterization of nontoxic liquid–metal alloy galinstan for applications in microdevices, J. Microelectromech. Syst. 21 (2012) 443–450. [13] K.-Q. Ma, J. Liu, Heat-driven liquid metal cooling device for the thermal management of a computer chip, J. Phys. D: Appl. Phys. 40 (2007) 4722. [14] M. Kubo, X. Li, C. Kim, M. Hashimoto, B.J. Wiley, D. Ham, G.M. Whitesides, Stretchable microfluidic radiofrequency antennas, Adv. Mater. 22 (2010) 2749–2752. [15] Y.-L. Park, C. Majidi, R. Kramer, P. Bérard, R.J. Wood, Hyperelastic pressure sensing with a liquid-embedded elastomer, J. Micromech. Microeng. 20 (2010) 125029. [16] S. Cheng, Z. Wu, A microfluidic, reversibly stretchable, large-area wireless strain sensor, Adv. Funct. Mater. 21 (2011) 2282–2290. [17] L. Meng, Y. Bin, N. Behdad, Liquid-tunable frequency selective surfaces, IEEE Microwave Wireless Compon. Lett. 20 (2010) 423–425. [18] F. Scharmann, G. Cherkashinin, V. Breternitz, C. Knedlik, G. Hartung, T. Weber, J.A. Schaefer, Viscosity effect on GaInSn studied by XPS, Surf. Interface Anal. 36 (2004) 981–985. [19] D. Zrnic, D.S. Swatik, On the resistivity and surface tension of the eutectic alloy of gallium and indium, J. Less-Common Met. 18 (1969) 67–68. [20] D. Kim, P. Thissen, G. Viner, D.-W. Lee, W. Choi, Y.J. Chabal, J.-B. Lee, Recovery of nonwetting characteristics by surface modification of gallium-based liquid metal droplets using hydrochloric acid vapor, ACS Appl. Mater. Interfaces 5 (2013) 179–185. [21] G. Li, M. Parmar, D. Kim, J.-B. Lee, D.-W. Lee, PDMS based coplanar microfluidic channels for the surface reduction of oxidized Galinstan, Lab Chip 14 (2014) 200–209. [22] D. Kim, D.W. Lee, W. Choi, J.B. Lee, A super-lyophobic 3-D PDMS channel as a novel microfluidic platform to manipulate oxidized Galinstan, J. Microelectromech. Syst. 22 (6) (2013) 1267–1275. [23] H.L. Lee, J.Y. Shin, C. Koh, H. Ryu, D. Lee, C. Sohn, Surface sizing with cationic starch: its effect on paper quality and the papermaking process, Tappi J. 1 (2002) 34–40. [24] Weyerhaeuser Writing and Printing Paper Material Safety Data Sheet [Online]. Available: http://www.officedepot.com/pdf/msds/348037.pdf. [25] Y. Zhao, Z. Hu, A. Ragauskas, Y. Deng, Improvement of paper properties using starch-modified precipitated calcium carbonate filler, Tappi J. 4 (2005) 3–7. [26] P.A.A.P. Marques, T. Trindade, C.P. Neto, Titanium dioxide/cellulose nanocomposites prepared by a controlled hydrolysis method, Compos. Sci. Technol. 66 (2006) 1038–1044. [27] D. Kim, Y. Lee, D.-W. Lee, W. Choi, J.-B. Lee, Hydrochloric acid-impregnated paper for liquid metal microfluidics, in: Solid-State Sensors, Actuators and Microsystems (Transducers & Eurosensors XXVII), 2013 Transducers & Eurosensors XXVII: The 17th International Conference on, 2013, pp. 2620–2623.
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2014.09.108. References [1] X. Li, D.R. Ballerini, W. Shen, A perspective on paper-based microfluidics: current status and future trends, Biomicrofluidics 6 (2012) 011301. [2] Z. Nie, C.A. Nijhuis, J. Gong, X. Chen, A. Kumachev, A.W. Martinez, M. Narovlyansky, G.M. Whitesides, Electrochemical sensing in paper-based microfluidic devices, Lab Chip 10 (2010) 477–483. [3] A.W. Martinez, S.T. Phillips, G.M. Whitesides, E. Carrilho, Diagnostics for the developing world: microfluidic paper-based analytical devices, Anal. Chem. 82 (2009) 3–10, 2010/01/01. [4] A.W. Martinez, S.T. Phillips, Z. Nie, C.-M. Cheng, E. Carrilho, B.J. Wiley, G.M. Whitesides, Programmable diagnostic devices made from paper and tape, Lab Chip 10 (2010) 2499–2504. [5] C.-z. Li, K. Vandenberg, S. Prabhulkar, X. Zhu, L. Schneper, K. Methee, C.J. Rosser, E. Almeide, Paper based point-of-care testing disc for multiplex whole cell bacteria analysis, Biosens. Bioelectron. 26 (2011) 4342–4348. [6] G. Chitnis, Z. Ding, C.-L. Chang, C.A. Savran, B. Ziaie, Laser-treated hydrophobic paper: an inexpensive microfluidic platform, Lab Chip 11 (2011) 1161–1165. ¨ [7] B. Balu, V. Breedveld, D.W. Hess, Fabrication of “Roll-offand “Sticky¨superhydrophobic cellulose surfaces via plasma processing, Langmuir 24 (2008) 4785–4790, 2008/05/01. [8] D.A. Bruzewicz, M. Reches, G.M. Whitesides, Low-cost printing of poly(dimethylsiloxane) barriers to define microchannels in paper, Anal. Chem. 80 (2008) 3387–3392, 2008/05/01. [9] Chao-Min Cheng, Andres W. Martinez, Jinlong Gong, Charles R. Mace, Scott T. Phillips, Emanuel Carrilho, Katherine A. Mirica, George M. Whitesides, Paperbased ELISA, Angew. Chem. 122 (2010) 4881–4884.
Biographies
Daeyoung Kim received B.S. degree in Electrical Engineering from Korea Military Academy, Seoul, Korea, in 2003. As a signal officer, he has served the Korean army since 2003. He received the M.S. degree in Electrical Engineering from Wayne State University, Detroit, MI in 2007. Since that time, he worked as a full time instructor at KAAY (Korea Army Academy at Yeong-Cheon). He is currently pursuing the Ph.D. degree at The University of Texas at Dallas, Richardson, Texas. His research focuses on the lyophobic surfaces, microfluidics, and frequency selective surfaces using liquid metal.
Yunho Lee received a B.S. degree in Chemistry from Korea Military Academy, Seoul, Korea, in 2003. Since 2003, he has served the Korean Army as a field artillery officer. In 2013, he received the M.S. degree in Mechanical Engineering from University of Texas at Dallas (UTD). He joined UTD Micro/Nano Devices and Systems (MiNDS) Lab in 2012 and had worked on microfluidics using liquid metals.
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Dong-Weon Lee received the Ph.D. degree in Mechatronics Engineering from Tohoku University, Sendai, Japan, in 2001. He has been a Professor of Mechanical Systems Engineering at Chonnam National University (CNU), South Korea since March of 2004. Previously, he was with the IBM Zurich Research Laboratory in Switzerland, working mainly on microcantilever devices for chemical AFM applications. At CNU, his research interests include smart cantilever devices, miniaturized energy harvester, smart structures, and nanoscale transducers. Dr. Lee is a member of the technical program committee of IEEE Sensors Conference, Transducers, and Microprocesses and Nanotechnology Conference (MNC). Wonjae Choi received the B.S. and M.S. degrees in mechanical engineering from Seoul National University in Korea, and obtained the Ph.D. degree in mechanical engineering from the Massachusetts Institute of Technology in 2009. Since then he worked as a postdoctoral research fellow at Harvard University, until he joined the University of Texas at Dallas in 2011. He is currently an Assistant Professor in the Department of Mechanical Engineering in the same university. His current research interests include microfluidics, multi-phase fluid mechanics, as well as surface wettability.
Koangki Yoo received the B.S., M.S. and Ph.D. degrees in electronics engineering from Hanyang University, Seoul, Korea, in 1986, 1988 and 2000, respectively. He then worked as a research engineer at the Electronics and Telecommunications Research Institute, South Korea during 2000–2002. He joined Hanbat National University as a faculty member in 2003. His research interests include design and verification of system-on-chip, hardwaresoftware co-design, and design of multimedia codec.
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Jeong-Bong (JB) Lee received the Ph.D. degree in electrical engineering from the Georgia Institute of Technology, Atlanta, in 1997. During 1997–1998, he was a research engineer at Georgia Tech. During 1999–2001, he was an assistant professor at the Louisiana State University, Baton Rouge, Louisiana. He then moved to UT Dallas in May, 2001 and he is currently a Full Professor in the Department of Electrical Engineering, The University of Texas at Dallas, Richardson, Texas. He is the director of the Micro/Nano Devices and Systems Laboratory. His current research interests include microelectromechanical systems and nanophotonics. Dr. Lee was a recipient of a National Science Foundation CAREER Award in 2001. In 2007, Dr. Lee served as a member of the external review panel for the Microsystems Division, Sandia National Laboratories. Dr. Lee serves as an editorial board member for Micromachines, The World Scientific Journal and Journal of nanotechnology and Smart Materials. Dr. Lee also served as an Executive Program Subcommittee Member for the Transducers 2011 and a Program Committee Member for the Transducers 2013 and IEEE Sensors 2013.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Hydrochloric acid-impregnated paper for gallium-based liquid metal microfluidics Daeyoung Kim a,b,∗ , Yunho Lee c , Dong-Weon Lee d , Wonjae Choi c , Koangki Yoo e , Jeong-Bong (JB) Lee a a
Department of Electrical Engineering, The University of Texas, Dallas, TX, USA Department of Electrical Engineering, Korea Army Academy, Yeong-Cheon, South Korea c Department of Mechanical Engineering, The University of Texas, Dallas, TX, USA d School of Mechanical Systems Engineering, Chonnam National University, Gwangju, South Korea e Department of Information and Communication Engineering, Hanbat National University, Daejeon, South Korea b
a r t i c l e
i n f o
Article history: Received 2 May 2014 Received in revised form 13 September 2014 Accepted 29 September 2014 Keywords: Liquid metal Galinstan® HCl-impregnation Super-lyophobicity Microfluidic platform
a b s t r a c t We report a simple hydrochloric acid (HCl) impregnation method to substantially improve the lyophobicity of a paper against gallium-based liquid metal. Based on the HCl-impregnated paper, we also propose an extremely simple fabrication method of microfluidic channel for gallium-based liquid metal, Galinstan® . Due to its low cost, easy fabrication, and flexibility, recently paper has drawn attention as microfluidic platforms for various applications. We have treated two different types of paper (paper towel and printing paper) with various treatment methods such as laser printer flattening, fluorocarbon polymer coating, HCl-impregnation, and combination of these methods. We then studied their lyophobicity characteristics by measuring static and dynamic contact angles as well as bouncing experiment. We found that HCl-impregnation is a simple yet powerful method to engineer certain types of papers to make them super-lyophobic substrates against gallium-based liquid metals and effective for more than 30days after impregnation. To show the feasibility, we demonstrated manipulation of a Galinstan® droplet along microfluidic channels formed on the HCl-impregnated paper. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Paper is abundant and extremely cheap cellulosic material. Untreated paper is highly porous and thus paper can easily absorb most liquids. Due to these properties, paper can transport fluid only using capillary force without any active pumping system [1]. Great potential of paper as disposable microfluidic platforms has only recently unleashed and there have been demonstrations on the paper-based microfluidics for analytical chemistry [2], health diagnostics [3,4], and environmental monitoring [5] for the past few years. In these work, microfluidic channels were typically fabricated by patterning hydrophobic regions on super-hydrophilic papers. Chitnis et al. reported a method of converting paper’s hydrophilicity into hydrophobicity using a CO2 laser [6]. Balu et al. demonstrated that super-hydrophobicity can be created on a paper surface via a combination of etching by an oxygen
∗ Corresponding author at: The University of Texas at Dallas, Department of Electrical Engineering, Richardson, United States. Tel.: +1 9726930988. E-mail address: [email protected] (D. Kim). http://dx.doi.org/10.1016/j.snb.2014.09.108 0925-4005/© 2014 Elsevier B.V. All rights reserved.
plasma and deposition of a fluorocarbon film [7]. In addition, Bruzewicz et al. used polydimethylsiloxane (PDMS) to define hydrophobic microfluidic channels in a paper [8]. Based on these ease fabrication merit of paper-based microfluidic devices, various researches use expensive reagents as the working media [9,10] because paper offers a wide range of other advantages, such that a paper is flexible, does not easily break, portable, liquid-wicking as received, and so forth. Therefore, although reagents or other liquids add significant amount of cost to the final product, this character of paper allows the paper-based microfluidic devices to be fabricated in countries with very poor infrastructure, and thus the paper still has a merit even if it loses its low cost benefit. As an alternative to toxic mercury, non-toxic gallium-based liquid metal alloy such as EGaIn (a binary gallium and indium alloy) [11] and Galinstan® (a ternary alloy of gallium, indium and tin) [12] have been recently studied. Based on its favorable properties such as higher thermal and electrical conductivity compared to mercury, low melting point and low toxicity, gallium-based liquid metals have been investigated for a variety of applications including heat transfer [13], stretchable microfluidic antenna [14], hyperelastic pressure sensor [15], microfluidic wireless strain
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sensor [16] and tunable frequency selective surface [17]. However, gallium is readily oxidized in air, forming a gallium oxide layer at the outer skin of the gallium-based liquid metal alloys. This causes the gallium-based liquid metals to be viscoelastic and to adhere to almost any surface [18], which is an extremely challeging ptoblem to solve. In microfluidic devices such as a stretchable microfluidic antenna, a hyperelastic pressure sensor, and a strain sensor, liquid metal fills the entire microfluidic channel and the oxide layer does not compromise the performance of the device rather enhances the adhesion between the liquid metal and the surface of channel. However, for applications such as heat transfer and a microfluidicsbased switch in which it is essential to have a dynamic movement of the meniscus of liquid metal, the viscoelastic oxide layer is severely problematic. There have been several efforts to overcome this issue by using non-wettable surface against oxidized gallium-based liquid metal. It was reported that HCl solution or vapor can chemically remove the oxide skin from gallium-based liquid metal [19–21]. Rather than removing oxide skin, it was also reported that hierarchical micro-/nano-scale structured surface is super-lyophobic and oxidized Galinstan® does not adhere to the engineered surface [22]. In this paper, we report lyophobicity of (i) a paper towel and a printing paper, (ii) papers treated with laser printer flattening, fluorocarbon polymer coating, and HCl-impregnation, as well as their combinations. We characterized the lyophobicity by measuring static and dynamic contact angles and observing bouncing patterns of naturally oxidized Galinstan® droplets on various paper-based substrates. In addition, we demonstrated Galinstan® droplet manipulation on a HCl-impregnated paper as a substrate.
2. Method of experiment 2.1. Materials In our study, we chose two different papers: a conventional paper towel (Uline S-7127, Uline, Pleasant Prairie, WI, USA) and a printing paper (Office Depot #348037, letter-sized, Boca Raton, FL,
USA) which are widely used in our daily life. Conventional paper towel is extremely hydrophilic, and readily absorbs water due to its chemical preference and porous structure; its cellulose which has hydroxyl group prefers to have hydrogen bonding with water molecules and its cellulous fibers formed spatial solid network at the paper making process. The printing paper used in this experiment was for the inkjet printing, and thus, the surface of the paper was extremely smooth and easily absorbs water-based ink. However, if the printing paper has super-hydrophilicity, the water can spread to nearby regions and distort the printed image. Therefore, it was chemically treated with starch to have some degree of hydrophobicity [23,24]. In addition, calcium carbonate and titanium dioxide were deposited to modify the brightness and optical property of the paper [25,26]. Fig. 1 shows top-view scanning electron microscopy (SEM) images of a conventional paper towel and a printing paper. Both papers show micro-/nano-scale randomly distributed cellulose fibers. As shown in the SEM images, the porosity of the paper towel is larger than that of the printing paper as the surface of the printing paper was chemically treated with starch, calcium carbonate, and titanium dioxide (Fig. 1d). Numerically, we calculated density of the papers used in this experiment based on the given information from the company. They were turned out to be 0.150 g/cm3 for the paper towel and 0.735 g/cm3 for the printing paper.
2.2. Static/dynamic contact angle measurement We investigated wetting characteristics of papers against naturally oxidized Galinstan® by measuring static and dynamic contact angles. In order to increase the lyophobicity of two papers, we modified the paper with three different methods and their combinations: flattening the paper towel by running through a laser printer fuser, fluorocarbon (FC) polymer coating (∼20 nm) by applying inductively coupled plasma power with a mixture of fluorocarbon precursors (C4 F8 ) and Ar gas, and HCl (37 wt%, 7 L) impregnation. From the combinations of three methods above, eight different types of paper were made and tested: non-treated paper towel
Fig. 1. SEM images of (a and b) a paper towel and (c and d) a printing paper; (b) is close-up view of (a) showing highly porous, randomly oriented micro-/nano-scale cellulose fiber structures; (d) is close-up view of (c) showing chemical treatment.
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and printing paper; flattened paper towel; FC polymer-coated paper towel and printing paper; FC polymer-coated flattened paper towel; HCl-impregnated flattened paper towel and printing paper. On these substrates, ∼7.8 L Galinstan® droplet was dropped using a pipette in air environment. Therefore, the surface of the Galinstan® droplet was instantly oxidized and formed oxide skin. Static contact angle was measured with a sessile droplet using a goniometer (Ramé-hart 260-F4, Ramé-hart Instrument, Succasunna, NJ, USA). For dynamic contact angle measurement, we increased and decreased the volume of the Galinstan® droplet on the various paper-based substrates using a pipette with Tefloncoated tip (inner diameter of 0.3 mm) while the side view of the droplet was recorded by a charge-coupled device (CCD) camera at 70 frames per second. The advancing and receding contact angles were obtained from the frame of the recorded movie just before contact line was changed. The static and dynamic contact angle measurements for HCl-impregnated papers were carried out right after the HCl-impregnation. 3. Lyophobicity of papers 3.1. Static and dynamic contact angle measurements Fig. 2 shows static contact angles of the oxidized Galinstan® droplet on paper towel and printing paper with and without various treatments. The shapes of Galinstan® droplets on the various substrates except HCl-impregnated papers are not spherical, as the viscoelastic oxide skin layer causes the droplets to retain the deformed shape during the pipette dispensing process. The static contact angles of droplets on non-treated paper towel and printing paper were found to be 143.1 ± 5.4◦ and 149.6 ± 3.8◦ , respectively. These high static contact angles indicate that the paper itself shows lyophobicity against gallium-based liquid metals. Based on the various treatments, the contact angles of treatedpapers were enhanced compared to those of non-treated papers indicating the improvement of lyophobicity. By coating surface of the papers with low energy FC polymer, the static contact angles of FC-coated papers were increased in comparison to those of nontreated papers. Among various treated papers, the contact angle of the FC polymer-coated printing paper was the largest (155.3 ± 0.5◦ ). The static contact angles of HCl-impregnated flattened paper towel and printing paper were 149.9 ± 0.6◦ and 149.8 ± 1.8◦ , respectively. Due to chemical reaction with HCl, oxide skin layer of Galinstan® droplets were removed and thus their shapes were spherical as
Fig. 2. Static contact angles of ∼7.8 L Galinstan® droplets on papers with and without various treatments [22].
Fig. 3. (a) Advancing and receding contact angles and (b) contact angle hysteresis for papers with various treatments. Inset images in (a) show images of advancing and receding angles of Galinstan® droplets on the HCl-impregnated flattened paper which shows the lowest contact angle hysteresis and the non-treated printing paper which shows highest contact angle hysteresis. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
they were not affected by the viscoelastic behavior [20]. Therefore, the measured contact angles were relatively consistent. Values of static contact angles are, however, not the most important indicator of lyophobicity. The droplets on the FC-coated printing paper did not readily roll-off the surface, implying that the contact angle hysteresis was significant. We thus measured the dynamic contact angles for variously treated-paper towels and printing papers along with non-treated paper towel and printing paper, as shown in Fig. 3. Red-colored circle dot indicates advancing contact angles and blue-colored square dot indicates receding contact angles, respectively. The advancing contact angles of all variously treated-papers are very close to or higher than 150◦ . The receding contact angle, however, greatly changes by the treatment. The non-treated paper towel and printing paper show the receding contact angle of 97.5◦ and 41◦ , respectively. As a result, the contact angle hysteresis (advancing angle − receding angle) of non-treated paper towel is lower than that of the non-treated printing paper. This is attributed to the less contact area originating from the fact that the porosity of the paper towel is much larger than that of the printing paper. FC polymer-coated paper towel and printing
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Fig. 4. A series of time-lapse images of a Galinstan® droplet falling from 3 cm above the surface on (a) the flattened paper towel and (b) the FC polymer-coated paper towel, (c) the HCl-impregnated flattened paper towel, (d) the printing paper, (e) the FC polymer-coated printing paper, and (f) HCl-impregnated printing paper.
paper exhibit smaller contact angle hysteresis compared to those of the non-treated counterparts; smaller hysteresis indicates the enhancement of lyophobicity even when the static contact angles are more or less the same. Among them, the HCl-impregnated paper towel and printing paper show the lowest contact angle hysteresis of 8.8◦ and 11◦ , respectively. We believe that this is because the oxidized Galinstan® reacts with the impregnated HCl on the paper resulting in the removal of viscoelastic oxide layer. This low contact angle hysteresis shows that that the HCl-impregnation is the optimum way to modify the paper for manipulating oxidized gallium-based liquid metal droplets. 3.2. Bouncing tests The dynamic contact angle and contact angle hysteresis of the Galinstan® droplet on the variously treated-papers showed that the HCl impregnation in the paper is a simple yet powerful method to prepare a non-wetting substrate for gallium-based liquid metal microfluidics. To further verify the lyophobicity of the surface, we carried out bouncing experiments of the oxidized Galinstan® droplet (∼7.8 L) on six variations of paper treatments. Each
Galinstan® droplet was dropped from 3 cm above the surface and impacted the paper substrates while the movement of the droplet was being captured by a high-speed camera (Photron SA4, Photron America, San Diego, CA, USA) with 1000 frames per second. Note that the oxidation of Galinstan® is nearly instant and the droplet becomes fully oxidized by the moment of impact. Fig. 4 shows a series of time-lapse images of Galinstan® droplet onto the six different substrates. The shape of oxidized Galinstan® droplets in each bouncing experiment was not spherical before hitting the paper because the droplet was immediately oxidized. For those substrates except HCl-impregnated papers, after the droplet hit the surface, the Galinstan® droplet spread on the surface and never bounced back. However, the droplet readily bounced off from the surface of the HCl-impregnated flattened paper and printing paper without leaving any residue on the substrate (Fig. 4c and f). These results clearly demonstrated that HCl-impregnation can achieve the non-wetting characteristics of the paper against naturally oxidized gallium-based liquid metal. It is believed that the oxide skin of the Galinstan® droplet is reacted with HCl from the HCl impregnated in the papers [20] while falling and non-wetting characteristic was recovered at the moment of the impact at the paper surface.
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Fig. 5. Optical images of the shape change of the liquid metal droplet after 8 and 60 min of dry on the HCl-impregnated (a and b) paper towel and (c and d) printing paper. The time (seconds) in the figure indicates the required time to change the droplet shape based on the chemical reaction.
4. Characterization of HCl-impregnated papers There are three potential mechanisms that lead to the nonwettability of a HCl-impregnated paper: HCl vapor, HCl liquid, and HCl monolayer on the surface of each microfibers on the paper surface. Unless the paper is encased in a closed package, the first two components will disappear and only a thin monolayer of HCl remains. Therefore, in order to understand the effect of wetting characteristics against the dry time of the impregnated HCl on papers, we placed oxidized Galinstan® droplet on the HClimpregnated paper after certain amount of dry time (8 min and 60 min of dry after HCl impregnation in ambient air at room temperature). Then, we recorded videos to measure the time required for changing from irregular shape (which means the Galinstan® droplet surface is oxidized) to spherical shape (which means the Galinstan® droplet surface is oxide-free). Fig. 5a shows Galinstan® droplet placed on the HCl impregnated paper towel after 8 min of dry. It shows that the Galinstan® droplet has irregular shape on the left side as it is oxidized as soon as it is placed on the paper towel surface. It takes 1.7 s to be oxide-free (right side). In comparison, the same experiment is carried out after 60 min of dry and it is found that reaction time is 890 s (Fig. 5b). Similarly, results on the printing paper show 0.5 s reaction time for the 8 min dried paper surface and 210 s reaction time for the 60 min dried paper surface (Fig. 5c and d). This result suggests that HCl-impregnated paper is an effective means of preparing non-wetting surface for gallium-based liquid alloy manipulation. Although it takes some time (e.g., 890 s for the
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60 min dried paper towel) to make the Galinstan® droplet oxidefree, it is not readily re-oxidized as we reported earlier. Based on these results, we believe that HCl-impregnated paper is an excellent disposable microfluidic platform for Galinstan® droplet manipulation. Additionally, to find how long the HCl-impregnated paper works as anti-wetting surface in ambient air environment against oxidized gallium-based liquid metal, we carried out bouncing tests in atmospheric environment with relative humidity of 43.0 ± 0.2% and temperature of 21.6 ± 0.1 ◦ C. HCl impregnation on papers was achieved by simply dropping HCl pendant droplet (7 L, 37 wt%) onto surface of papers. When it was dropped HCl droplet on the flattened paper towel was impregnated and spread out to circular area of ∼1.8 cm2 as soon as being dropped, but the HCl droplet on the printing paper was not readily impregnated right after being dropped and gradually spread out to the area of ∼0.8 cm2 up to 20 min after impregnation. We believe that this absorption difference due to different porosity and chemical treatment on the surface as described above. After HCl solution was completely impregnated on papers, papers were placed on ambient air environment and we repeated bouncing test long-term to find out effect of natural evaporation of HCl from the papers. However, we found that even after 30 days of testing, Galinstan® droplet was readily bounced off from both HCl-impregnated paper towel and HCl-impregnated printing paper. This testing result confirms that HCl-impregnation would be a simple but very practical method for converting regular paper towels and inkjet printing papers as microfluidic platforms against natively oxidized gallium-based liquid metal alloy applications. Since paper-based microfluidic devices are inexpensive, it could be designed as disposable devices. Therefore, more than 30-days of HCl-impregnation effect should be more than enough for most applications. 5. HCl-impregnated paper-based microfluidic channel Based on the HCl impregnation on the flattened paper towel, we fabricated HCl-impregnated paper-based microfluidic channels [27]. The fabrication process is illustrated in Fig. 6a. The grid-shape 1/16 in. (1.57 mm) diameter stainless rods were placed on the HCl-impregnated flattened paper, which in turn was placed onto a deformable material. Micro imprinted grid of microfluidic channels with a channel depth of approximately 1.2 mm was created by applying the pressure through a top plate. We placed an oxidized Galinstan® droplet by a pipette (Fig. 6b), and mobilized it by controlling N2 gas through the PTFE tube. Due to the imprinted channel, the droplet well followed inside the channel with an applied
Fig. 6. (a) A schematic of paper-based microfluidic channel fabrication using a deformable substrate, (b) a schematic of N2 gas driven liquid metal droplet mobilization on the fabricated paper-based channel and inset image showing optical image of ∼7.8 L Galinstan® droplet placed on the paper microfluidic channel, and (c) a series of still images taken from a real-time video of a moving Galinstan® droplet on a HCl-impregnated flattened paper.
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pressure. Fig. 6c shows a series of still images from a real-time video of a moving oxidized Galinstan® droplet on the HCl-impregnated flattened paper. There is no trace of oxidized Galinstan® on the HClimpregnated flattened paper after the droplet was mobilized. This clearly shows that this HCl-impregnated paper-based substrate is suitable for gallium-based liquid metal microfluidic applications. 6. Conclusion In this paper, we studied lyophobicity of papers with various treatments against oxidized Galinstan® droplets by measuring static and dynamic contact angles along with bouncing experiments. It was demonstrated that HCl-impregnation is a simple but powerful way to create a super-lyophobic substrate for galliumbased liquid metal alloy manipulation. On the HCl-impregnated flattened paper and printing paper, the spherical shape of sessile droplets was formed and small contact angle hystereses (8–11◦ ) were achieved. The super-lyophobic property of HCl-impregnated papers was confirmed by the bouncing experiments. To show the long-term feasibility of the HCl-impregnated paper as a microfluidic platform, we performed repeated long-term Galinstan® bouncing tests in ambient air environment and found that the HCl-impregnated paper is operational as super-lyophobic galliumbased liquid metal microfluidic substrate even after 30 days of testing. Finally, we demonstrated HCl-impregnated paper-based channel as a suitable microfluidic platform for the manipulation of oxidized Galinstan® droplets. We believe that this work has a great potential to solve one of the most challenging obstacles in the gallium-based liquid metal based microfluidics. Acknowledgements This work was supported in part by World Class University (WCU) project (R32-2009-000-20087-0) and also supported by MKE (The Ministry of Knowledge Economy) under the Brain Scouting Program (HB606-12-2001) supervised by the NIPA (National IT Promotion Agency), Korea. The authors would like to thank Republic of Korea (ROK) Army for financial support.
[10] Ali Kemal Yetisen, Muhammad Safwan Akram, Christopher R. Lowe, Paperbased microfluidic point-of-care diagnostic devices, Lab Chip 13 (2013) 2210–2251. [11] M.D. Dickey, R.C. Chiechi, R.J. Larsen, E.A. Weiss, D.A. Weitz, G.M. Whitesides, Eutectic gallium–indium (EGaIn): a liquid metal alloy for the formation of stable structures in microchannels at room temperature, Adv. Funct. Mater. 18 (2008) 1097–1104. [12] T. Liu, P. Sen, C.-J. Kim, Characterization of nontoxic liquid–metal alloy galinstan for applications in microdevices, J. Microelectromech. Syst. 21 (2012) 443–450. [13] K.-Q. Ma, J. Liu, Heat-driven liquid metal cooling device for the thermal management of a computer chip, J. Phys. D: Appl. Phys. 40 (2007) 4722. [14] M. Kubo, X. Li, C. Kim, M. Hashimoto, B.J. Wiley, D. Ham, G.M. Whitesides, Stretchable microfluidic radiofrequency antennas, Adv. Mater. 22 (2010) 2749–2752. [15] Y.-L. Park, C. Majidi, R. Kramer, P. Bérard, R.J. Wood, Hyperelastic pressure sensing with a liquid-embedded elastomer, J. Micromech. Microeng. 20 (2010) 125029. [16] S. Cheng, Z. Wu, A microfluidic, reversibly stretchable, large-area wireless strain sensor, Adv. Funct. Mater. 21 (2011) 2282–2290. [17] L. Meng, Y. Bin, N. Behdad, Liquid-tunable frequency selective surfaces, IEEE Microwave Wireless Compon. Lett. 20 (2010) 423–425. [18] F. Scharmann, G. Cherkashinin, V. Breternitz, C. Knedlik, G. Hartung, T. Weber, J.A. Schaefer, Viscosity effect on GaInSn studied by XPS, Surf. Interface Anal. 36 (2004) 981–985. [19] D. Zrnic, D.S. Swatik, On the resistivity and surface tension of the eutectic alloy of gallium and indium, J. Less-Common Met. 18 (1969) 67–68. [20] D. Kim, P. Thissen, G. Viner, D.-W. Lee, W. Choi, Y.J. Chabal, J.-B. Lee, Recovery of nonwetting characteristics by surface modification of gallium-based liquid metal droplets using hydrochloric acid vapor, ACS Appl. Mater. Interfaces 5 (2013) 179–185. [21] G. Li, M. Parmar, D. Kim, J.-B. Lee, D.-W. Lee, PDMS based coplanar microfluidic channels for the surface reduction of oxidized Galinstan, Lab Chip 14 (2014) 200–209. [22] D. Kim, D.W. Lee, W. Choi, J.B. Lee, A super-lyophobic 3-D PDMS channel as a novel microfluidic platform to manipulate oxidized Galinstan, J. Microelectromech. Syst. 22 (6) (2013) 1267–1275. [23] H.L. Lee, J.Y. Shin, C. Koh, H. Ryu, D. Lee, C. Sohn, Surface sizing with cationic starch: its effect on paper quality and the papermaking process, Tappi J. 1 (2002) 34–40. [24] Weyerhaeuser Writing and Printing Paper Material Safety Data Sheet [Online]. Available: http://www.officedepot.com/pdf/msds/348037.pdf. [25] Y. Zhao, Z. Hu, A. Ragauskas, Y. Deng, Improvement of paper properties using starch-modified precipitated calcium carbonate filler, Tappi J. 4 (2005) 3–7. [26] P.A.A.P. Marques, T. Trindade, C.P. Neto, Titanium dioxide/cellulose nanocomposites prepared by a controlled hydrolysis method, Compos. Sci. Technol. 66 (2006) 1038–1044. [27] D. Kim, Y. Lee, D.-W. Lee, W. Choi, J.-B. Lee, Hydrochloric acid-impregnated paper for liquid metal microfluidics, in: Solid-State Sensors, Actuators and Microsystems (Transducers & Eurosensors XXVII), 2013 Transducers & Eurosensors XXVII: The 17th International Conference on, 2013, pp. 2620–2623.
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2014.09.108. References [1] X. Li, D.R. Ballerini, W. Shen, A perspective on paper-based microfluidics: current status and future trends, Biomicrofluidics 6 (2012) 011301. [2] Z. Nie, C.A. Nijhuis, J. Gong, X. Chen, A. Kumachev, A.W. Martinez, M. Narovlyansky, G.M. Whitesides, Electrochemical sensing in paper-based microfluidic devices, Lab Chip 10 (2010) 477–483. [3] A.W. Martinez, S.T. Phillips, G.M. Whitesides, E. Carrilho, Diagnostics for the developing world: microfluidic paper-based analytical devices, Anal. Chem. 82 (2009) 3–10, 2010/01/01. [4] A.W. Martinez, S.T. Phillips, Z. Nie, C.-M. Cheng, E. Carrilho, B.J. Wiley, G.M. Whitesides, Programmable diagnostic devices made from paper and tape, Lab Chip 10 (2010) 2499–2504. [5] C.-z. Li, K. Vandenberg, S. Prabhulkar, X. Zhu, L. Schneper, K. Methee, C.J. Rosser, E. Almeide, Paper based point-of-care testing disc for multiplex whole cell bacteria analysis, Biosens. Bioelectron. 26 (2011) 4342–4348. [6] G. Chitnis, Z. Ding, C.-L. Chang, C.A. Savran, B. Ziaie, Laser-treated hydrophobic paper: an inexpensive microfluidic platform, Lab Chip 11 (2011) 1161–1165. ¨ [7] B. Balu, V. Breedveld, D.W. Hess, Fabrication of “Roll-offand “Sticky¨superhydrophobic cellulose surfaces via plasma processing, Langmuir 24 (2008) 4785–4790, 2008/05/01. [8] D.A. Bruzewicz, M. Reches, G.M. Whitesides, Low-cost printing of poly(dimethylsiloxane) barriers to define microchannels in paper, Anal. Chem. 80 (2008) 3387–3392, 2008/05/01. [9] Chao-Min Cheng, Andres W. Martinez, Jinlong Gong, Charles R. Mace, Scott T. Phillips, Emanuel Carrilho, Katherine A. Mirica, George M. Whitesides, Paperbased ELISA, Angew. Chem. 122 (2010) 4881–4884.
Biographies
Daeyoung Kim received B.S. degree in Electrical Engineering from Korea Military Academy, Seoul, Korea, in 2003. As a signal officer, he has served the Korean army since 2003. He received the M.S. degree in Electrical Engineering from Wayne State University, Detroit, MI in 2007. Since that time, he worked as a full time instructor at KAAY (Korea Army Academy at Yeong-Cheon). He is currently pursuing the Ph.D. degree at The University of Texas at Dallas, Richardson, Texas. His research focuses on the lyophobic surfaces, microfluidics, and frequency selective surfaces using liquid metal.
Yunho Lee received a B.S. degree in Chemistry from Korea Military Academy, Seoul, Korea, in 2003. Since 2003, he has served the Korean Army as a field artillery officer. In 2013, he received the M.S. degree in Mechanical Engineering from University of Texas at Dallas (UTD). He joined UTD Micro/Nano Devices and Systems (MiNDS) Lab in 2012 and had worked on microfluidics using liquid metals.
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Dong-Weon Lee received the Ph.D. degree in Mechatronics Engineering from Tohoku University, Sendai, Japan, in 2001. He has been a Professor of Mechanical Systems Engineering at Chonnam National University (CNU), South Korea since March of 2004. Previously, he was with the IBM Zurich Research Laboratory in Switzerland, working mainly on microcantilever devices for chemical AFM applications. At CNU, his research interests include smart cantilever devices, miniaturized energy harvester, smart structures, and nanoscale transducers. Dr. Lee is a member of the technical program committee of IEEE Sensors Conference, Transducers, and Microprocesses and Nanotechnology Conference (MNC). Wonjae Choi received the B.S. and M.S. degrees in mechanical engineering from Seoul National University in Korea, and obtained the Ph.D. degree in mechanical engineering from the Massachusetts Institute of Technology in 2009. Since then he worked as a postdoctoral research fellow at Harvard University, until he joined the University of Texas at Dallas in 2011. He is currently an Assistant Professor in the Department of Mechanical Engineering in the same university. His current research interests include microfluidics, multi-phase fluid mechanics, as well as surface wettability.
Koangki Yoo received the B.S., M.S. and Ph.D. degrees in electronics engineering from Hanyang University, Seoul, Korea, in 1986, 1988 and 2000, respectively. He then worked as a research engineer at the Electronics and Telecommunications Research Institute, South Korea during 2000–2002. He joined Hanbat National University as a faculty member in 2003. His research interests include design and verification of system-on-chip, hardwaresoftware co-design, and design of multimedia codec.
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Jeong-Bong (JB) Lee received the Ph.D. degree in electrical engineering from the Georgia Institute of Technology, Atlanta, in 1997. During 1997–1998, he was a research engineer at Georgia Tech. During 1999–2001, he was an assistant professor at the Louisiana State University, Baton Rouge, Louisiana. He then moved to UT Dallas in May, 2001 and he is currently a Full Professor in the Department of Electrical Engineering, The University of Texas at Dallas, Richardson, Texas. He is the director of the Micro/Nano Devices and Systems Laboratory. His current research interests include microelectromechanical systems and nanophotonics. Dr. Lee was a recipient of a National Science Foundation CAREER Award in 2001. In 2007, Dr. Lee served as a member of the external review panel for the Microsystems Division, Sandia National Laboratories. Dr. Lee serves as an editorial board member for Micromachines, The World Scientific Journal and Journal of nanotechnology and Smart Materials. Dr. Lee also served as an Executive Program Subcommittee Member for the Transducers 2011 and a Program Committee Member for the Transducers 2013 and IEEE Sensors 2013.