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Transfer of Materials from Water to Solid Surfaces Using Liquid Marbles Hisato Kawashima,†,# Maxime Paven,∥,# Hiroyuki Mayama,*,⊥ Hans-Jürgen Butt,∥ Yoshinobu Nakamura,‡,§ and Syuji Fujii*,‡,§ †
Division of Applied Chemistry, Graduate School of Engineering, ‡Department of Applied Chemistry, Faculty of Engineering, and Nanomaterials Microdevices Research Center, Osaka Institute of Technology, 5-16-1 Omiya, Asahi-ku, Osaka 535-8585, Japan ∥ Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany ⊥ Department of Chemistry, Asahikawa Medical University, 2-1-1-1 Midorigaoka-Higashi, Asahikawa 078-8510, Japan §
S Supporting Information *
ABSTRACT: Remotely controlling the movement of small objects is desirable, especially for the transportation and selection of materials. Transfer of objects between liquid and solid surfaces and triggering their release would allow for development of novel material transportation technology. Here, we describe the remote transport of a material from a water film surface to a solid surface using quasispherical liquid marbles (LMs). A light-induced Marangoni flow or an air stream is used to propel the LMs on water. As the LMs approach the rim of the water film, gravity forces them to slide down the water rim and roll onto the solid surface. Through this method, LMs can be efficiently moved on water and placed on a solid surface. The materials encapsulated within LMs can be released at a specific time by an external stimulus. We analyzed the velocity, acceleration, and force of the LMs on the liquid and solid surfaces. On water, the sliding friction due to the drag force resists the movement of the LMs. On a solid surface, the rolling distance is affected by the surface roughness of the LMs. KEYWORDS: liquid marble, remote control, transfer, delivery, release
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INTRODUCTION Instead of moving through water, some insects walk and jump on the water surface.1−3 Supported by the surface tension of water, water striders4 and Stenus beetles5−7 rapidly move on the air−water interface. These insects can move more quickly on the water surface than through the bulk water phase because most of their body is in the air, which has a lower viscosity than that of water. Therefore, these insects experience less frictional resistance. Interestingly, the Stenus beetle can escape from predators using the power of Marangoni propulsion.7 By ejecting a hormone from its rear end, the Stenus beetle generates a surface tension gradient on the water surface and consequently can quickly move over the water surface at a velocity of a tenth of a centimeter per second. At some point, the beetle reaches the rim of the water surface and can then step onto solid ground to continue its escape. The behavior of these insects inspired our new approach to move small objects on water and to transport a cargo onto a solid surface. The purpose of this study is connected to material transport and the analysis of small amounts of liquids in a specific place, for example, on chips distributed over an analysis board. As a transportation carrier, liquid marbles (LMs) are ideal candidates.8−13 These marbles can move on water14−17 and solid surfaces,8,18,19 similarly to the Stenus beetle. LMs are millimeter-sized liquid droplets that are covered with hydrophobic solid particles.8−13 The particles are adsorbed at the © 2017 American Chemical Society
air−liquid interface and act as a shield protecting the inner liquid from wetting the underlying substrate, which can be either a solid or liquid. Indeed, LMs are promising carriers to move and transport materials, and several approaches (gravity,8,18 electrostatics,19 magnets,20−23 acoustic levitation,24,25 and Marangoni propulsion14−17) have been presented to move them on liquid or solid surfaces and in the gas phase. Marangoni flow is caused by a surface tension gradient. Gradients in surface tension can be generated by temperature gradients or concentration gradients of the surface-active substances. To decrease the total surface energy, the surface tension gradient equilibrates via liquid flow from low-surfacetension regions to high-surface-tension regions. This flow leads to efficient mass transfer along an interface. Floating objects that are close to the flow or even that cause the surface tension gradient can be powerfully propelled over a water surface.26,27 Examples of the Marangoni propulsion include the Stenus beetle,5−7 camphor crystals,28 soap boats,29 depolymerizable plastics,30 and organic solvent-loaded objects,31,32 all of which move quickly over the water surface by ejecting surface-active compounds that expand over the water surface. Recently, lightReceived: July 31, 2017 Accepted: September 7, 2017 Published: September 7, 2017 33351
DOI: 10.1021/acsami.7b11375 ACS Appl. Mater. Interfaces 2017, 9, 33351−33359
Research Article
ACS Applied Materials & Interfaces
Laser Confocal Microscopy. The surface roughness of the LMs was characterized using a laser scanning confocal microscope (VK9500, Keyence, Osaka, Japan) and software (VK-H1A9, Keyence, Osaka, Japan). Contact Angle Measurement. Pressed PPy pellets were prepared from dried PPy powders (pelletized at 300 kg/cm2 for 15 min using an SSP-IOA hand press, Shimadzu, Kyoto, Japan). Water drops (15 μL) were placed on the pellets, and the contact angles were measured after a resting time of 30 s using an SImage02 apparatus (Excimer Inc., Kanagawa, Japan) at 25 °C. Light-Controlled Marangoni Propulsion Using a NIR Laser. The laser light was manually directed sideways onto the LMs, aiming for the three-phase contact line for the LM, water, and air. The irradiation angle played a crucial role. A horizontal angle of up to 45° was found to be most efficient for propelling LMs. At 90° (vertical irradiation), the LMs were stationary. Continuous and accurate aiming of the irradiation was difficult because aiming was done manually. This difficulty explains the fluctuations in the velocity and acceleration of the LMs. The laser exposure time was less than a few hundred milliseconds. To facilitate the aiming, a NIR laser detection sheet was placed underneath the Petri dish and acrylic glass (IR sensor card 800−1600 nm, LDT-008, Laser Components GmbH, Olching, Germany). Notably, the visible laser dot stemmed from the laser detection sheet, which was under the water bath. When the laser light did not hit the LM and convert into heat, it was converted into visible light after hitting the laser detection sheet. The shift between the visible laser dot resulted from the height difference between the LM floating on the water bath and the laser detection sheet. For the transfer experiments of LMs from water to solid surfaces, water films (pink, dyed with alizarin blue black B, height = approximately 3 mm) were prepared on poly(methyl methacrylate) (PMMA) substrates. At the rim of the water film, a meniscus with a contact angle of 40 ± 3° formed. The LMs were placed on the air−water interface at a distance of approximately 3 cm from the rim of the water film, that is, the threephase contact line of the water, air, and PMMA glass. The motion of the LMs was then induced via NIR laser irradiation or a weak air stream. The air stream was made using a rubber air blower. A digital video camera (Sony Handycam HDR-CX270 V; 30× optical zoom lens, Sony Co., Tokyo, Japan) was used to image the LMs. Their motion was recorded using a digital camera (Ricoh G700SE; 5.0× optical zoom lens, Ricoh, Tokyo, Japan). The LMs remained nearspherical, and the effect of water evaporation from the LMs could be negligible on the experimental time scale. Thermographic Analysis. Thermographic analysis was conducted using an 890-2 Thermal Imager from Testo (Lenzkirch, Germany). Gel Trapping Experiments. The gel trapping technique is a method, which was invented by Paunov, to investigate the position of particles adsorbed at the air−water interface.38,39 An Agar gel aqueous solution (1 wt %) was prepared at 80−90 °C, and the gelling temperature was in the range of 40−50 °C. The LMs prepared using the Agar gel solution as the inner liquid phase were placed on a planar Agar gel aqueous solution surface at 60 °C. After the placement of the LMs, the system was cooled to room temperature and left for 60 min to allow the gel to set. After cooling to room temperature, a strong gel was formed, and the positions of the LMs with respect to the water interface were fixed. (Volume change % of Agar gel aqueous solution before and after gelation was measured to be 6%, which could be negligible.) The position of the LMs at the water surface was evaluated using a cross section of the gelled samples, cut with a utility knife. Software. The movies were analyzed using commercial software (Keyence VW-9000 MotionAnalyzer, Keyence, Osaka, Japan) to obtain the velocity, acceleration, and applied force. Using this software, we first determined the position of the center of the LMs in every frame, r(t). Next, the differences in the positions between subsequent images (displacement), Δr(t) from the time course of r(t), and velocity v(t) = (Δr(t))/Δt, where Δt is the video rate (1/30 s), were determined. From the finite difference of v(t), we obtained the acceleration, a(t) = (Δv(t))/Δt, and the force, F(t) = ma(t), where m is the mass of the LM.
induced surface tension gradients have been demonstrated to produce powerful propulsion forces to move small objects.33−35 In our previous study,16 we have used Marangoni flow to transport LMs over a water surface. Anisotropic heat distribution around the floating marble, which is necessary for Marangoni propulsion, was generated via irradiation of LMs with a photothermally responsive shell using a near-infrared (NIR) laser. On the basis of the temperature-induced Marangoni effect, LMs can be precisely and noninvasively maneuvered over long distances to a desired location. In this study, we attempted to use LMs to transport cargo by moving them on a water surface and transporting them onto a solid surface. We investigated how LMs move over water and solid surfaces and transition from a water surface onto a solid by overcoming a water meniscus. Once on a solid surface, the LMs were disrupted via an external stimulus to release their internal content. Through an analysis of the velocity, acceleration, generated force, velocity decay time, and friction of the moving LMs, we compared and evaluated two different LM shell materials varying in surface roughness and wettability.
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EXPERIMENTAL SECTION
Materials. Unless otherwise stated, all materials were reagent grade. Ferric chloride (FeCl3·6H2O), pyrrole (Py, 98%), heptadecafluorooctane sulfonic acid (C8F, 40 wt % aqueous solution), tridecafluorohexane-1-sulfonic acid potassium salt (C6F, ≥98.0%), Agar (ash 2.0−4.5%), and aluminum oxide (activated, basic, Brockmann 1, standard grade, ∼150 mesh, 58 Å) were obtained from Sigma-Aldrich (MO) and were used without further purification. Py was purified with a column of activated basic alumina and was stored at −15 °C before use. Hydrophobic CaCO3 particles (Dn: 80 nm, hydrophobized via a surface treatment with octadecanoic acid) were kindly donated by Shiraishi Kogyo Kaisha, Ltd. (Osaka, Japan). Deionized water (
