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Reversible Shape Transformation of Ultrathin Polydopamine-stabilized Droplet Hiroya Abe, Tomokazu Matsue, and Hiroshi Yabu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01355 • Publication Date (Web): 31 May 2017 Downloaded from http://pubs.acs.org on June 10, 2017
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Title
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Reversible Shape Transformation of Ultrathin Polydopamine-stabilized Droplet
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Authors
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Hiroya ABE† *, Tomokazu MATSUE†,‡, Hiroshi YABU‡*
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Affiliations
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†
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Aza-Aoba, Aoba-Ku, Sendai 980-0845, Japan
Graduate School of Environmental Science, Tohoku University, 468-1, Aramaki,
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‡
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Aoba-Ku, Sendai 980-8577, Japan
Advanced Institute for Materials Research (AIMR), Tohoku University, 2-1-1, Katahira,
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KEYWORDS
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Thin film, Polydopamine, Water droplet flattening, Wrinkles, Self-organization
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Abstract
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Here we report on the flattening of water droplets using an ultrathin membrane of
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auto-polymerized polydopamine at the air/water interface. This has only been previously
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reported with the use of synthetic or extracted peptides, two-dimensional designed synthetic
21
peptide thin films with thicknesses of several tens of nanometers. However, in the previous
22
study, the shape of the water droplet was changed irreversibly and the phenomenon was
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observed only at the air/water interface. In the present study, an ultrathin polydopamine
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membrane-stabilized droplet induced the flattening of a water droplet at the air/liquid and
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liquid/liquid interfaces because a polydopamine membrane was spontaneously formed at
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these interfaces. Furthermore, a reversible transformation of the droplet to flat and dome
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shape droplets were discovered at the liquid/liquid interface. These are a completely new
28
system because the polydopamine membrane is dynamically synthesized at the interface and
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the formation speed of the polydopamine membrane overcomes the flattening timescale.
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These results will provide new insight into physical control of the interfacial shapes of
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droplets.
32 33 34
Introduction
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Liquid droplets stabilized with thin films or solid particles have received
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considerable interest because they can maintain and formulate their shapes into various
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morphologies depending on the external environment and the nature of the stabilizing thin
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films or particles. For example, a liquid droplet covered with hydrophobic particles, referred
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to as a liquid marble, can maintain its shape after deformation by external forces1,2,3,4. 2 ACS Paragon Plus Environment
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Capillary origami is another example, which is a morphologically controlled liquid droplet
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that includes a triangular pyramid and Mylar balloon encapsulated by a structured thin film5,6.
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Two-dimensional thin films, several tens of nanometers thick, composed of designed
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synthetic peptides7,8 and hydrophobin (HFBI)9,10,11 , have also been recently reported to form
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at the air/liquid interfaces of their solutions and cause flattening of dome-shaped solution
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droplets. Jang and colleagues have proposed a mechanism for this spontaneous
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morphological change in the case of designed synthetic peptides solution as follows7:
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peptides float up to the air/water interface, and they assemble and form peptide rafts at
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various locations at the interface. The peptide rafts gradually migrate forwards to the top of
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the droplet, which results in the growth of a thin film and subsequent facet formation. A
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similar phenomenon was also observed in particle stabilized droplets12,13 ; however, the
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materials that form the ultrathin molecular layer, which exhibits facet formation, have been
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limited to a few types of peptides, and the process is irreversible in that it was difficult to
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recover the dome shape once the facet formed. Thus, to the best of our knowledge, a
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molecular system that exhibits reversible facet formation has not yet been reported.
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Polydopamine, which is easily formed by the auto-oxidative polymerization of
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dopamine, is expected to be used in materials science due to its strong adhesion properties to
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universal surfaces14,15. Hollow particles16, nanofilms17, and fibers18 of polydopamine can be
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formed after oxidative polymerization of dopamine on sacrificial templates such as solid
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substrates, micelles, and oil droplets in an emulsion, followed by removal of the sacrificial
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templates. Polydopamine thin films can also be spontaneously formed at the air/liquid
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interface under a non-stirred condition19. Free-standing polydopamine thin films can be
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prepared by crosslinking with polyethyleneimine20, and the yielded membranes can be
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applicable to actuators that respond to external stimuli21.
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We investigated the effect of a polydopamine membrane self-organized at the
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air/water interface of a dopamine solution droplet on the flattening of the solution droplet.
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The polydopamine thin film was also formed at the liquid/liquid interface between the
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solution droplet and organic solvent, as well at the air/water interface of the solution droplet,
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and the shape of the interface became dome-like to flat. Moreover, the polydopamine thin
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film formed at the solution/organic solvent interface altered the interfacial shapes reversibly
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from dome to flat or from flat to dome by extracting or increasing the solution volume with a
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syringe. The reversible change of the interfacial droplet shape cannot be explained by the
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conventional mechanism proposed in the literature10, in which HFBI membranes have
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irreversibly formed at the air/solution interface. Herein, we propose a new mechanism to
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explaining the flattening of a water droplet.
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Experimental methods
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Chemicals. Dopamine hydrochloride and tris(hydroxymethyl) aminomethane (Tris-HCl)
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were purchased from Sigma-aldrichAldrich, St. Louis. Hexane and dichloromethane were
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purchased from Wako Pure Chemical Industris, Ltd., Tokyo. All chemicals were used as
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received.
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Flattening of dopamine solution. Dopamine hydrochloride was dissolved in Tris buffer (50
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mM, pH = 8.9) to prepare 10 mg/mL solution. A cup of 96 well polystyrene plate (Product
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No.: 2593, Corning) was overfilled with 475 µL of dopamine solution, and a droplet (c.a. 75 4 ACS Paragon Plus Environment
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µL) was formed on the top of it. In the case of the air/water interface, the flattening of
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dopamine solution occurred spontaneously. In the case of the hexane/water interface, the cup
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overfilled the dopamine solution was placed in a square glass bottle filled hexane, and the
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flattening of interface was induced by sucking the solution with a syringe (Figure S1). In the
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case of the dichloromethane/water interface, a glass capillary (an inner diameter is 1.1 mm,
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Product No.: PG10165-4, World Precision Instruments) filled with dopamine solution was
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placed in the square glass bottle filled with dichloromethane, and flattening was induced by
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same way.
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Characterizations: Shape changes of solution droplets were observed by using a CCD
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camera attached with contact angle analyzer (DM-300, Kyowa Interface Science Co.). A
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polydopamine membrane formed at the air/water interface was transferred to a Si substrate
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with the Langmuir-Schaeffer technique. UV-vis absorption spectra of the polydopamine
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membranes were acquired using a UV-vis spectrometer (V-670, Jasco). The average thickness
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was estimated by using an atomic force microscope (SPA 400, Seiko Instruments Inc.).
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Interfacial tensions at the air/liquid interface and the dopamine solution/hydrophobic solvent
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(hexane or dichloromethane) interface were estimated by a pendant drop method (DM-300,
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Kyowa Interface Science Co.).
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Results and discussion
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Dopamine is oxidized in the presence of oxygen under alkaline conditions to form
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polydopamine, which is a black insoluble melanin-like compound, by auto-oxidation
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polymerization. Under non-stirring conditions, polydopamine membranes form at the 5 ACS Paragon Plus Environment
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air/water interface of the aqueous dopamine solution. Polydopamine membrane at the
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air/water interface was characterized by UV-vis adsorption spectra (Figure S1). The peak at
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280 nm in an air/water spectrum was attributed to phenolic groups22. The formation of
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polydopamine at a liquid/solid interface has been reported 23. The spectrum of polydopamine
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formed at the air/water interface is similar to the spectrum of polydopamine formed at
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water/silicon interface (Figure S1), which suggests that polydopamine membrane formed at
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the air/water interface. Figure 1(a) shows the deformation process of a water droplet
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containing dopamine (10 mg/mL, see also Movie S1). The droplet has a dome-like shape in
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the early stage (Figure 1(a)(i)), and then the top of the droplet gradually flattens with the
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evaporation of water (Figures 1(a)(ii) and (iii)). The top of the flattened droplet forms
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wrinkles (Figure 1(b)) and an area of the facet is gradually expanded. This wrinkle formation
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implies the presence of solid polydopamine films at the air/solution interface. The average
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wavenumber of the wrinkles measured from optical microscope images was 6.4±3.0 µm
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(Figure 1(c)). Microscale wrinkles are also observed when an ultrathin polystyrene film on
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water24,25 and a polydimethylsiloxane film26 are compressed. In a previous report on a peptide
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system, the peptide membrane formed only at the top of the solution droplet11. However, in
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the present case, polydopamine membranes were formed both at the top and at the sides of
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the droplet (Figure 1(d)), which indicates that flattening of the droplet is induced by
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formation of the membranes covering the entire solution droplet. The surface areas of
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spherical droplet (Figure S2a), S0, and flattened droplet (Figure S2b and c), S1, are given by
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ܵ = 2ߨ ݎଶ
(1)
and ܵଵ = ߨ( ݎଶ + 2ݎℎ − ℎଶ ),
(2) 6 ACS Paragon Plus Environment
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where r is radius of droplets and h were height of flattened droplets, respectively. From the
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equation, S0 is also decrease to S1 with decreasing of the droplet’s volume by solution
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evaporation or extraction. However, the surface area of polydopamine membrane formed at
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the interfaces is constant since polydopamine membrane cannot resolve into the solution.
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Therefore, to match the areas of the polydopamine membrane and the flattened droplet, an
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apparent area of the polydopamine membrane might decrease by formation of wrinkles. To
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measure the thickness of the polydopamine film, the membrane was transferred from the
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solution to a piece of silicon wafer using the Langmuir−Schaeffer technique, and the
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transferred film was observed using atomic force microscopy (AFM; Figures 1(e) and (f)).
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From the AFM image and the cross-sectional profile of the film, the average thickness of the
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polydopamine membranes was determined to be 38±23 nm.
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According to the previous studies16,27, the polydopamine is able to be formed at the
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organic solvent/solution interface. In order to form the polydopamine membrane at the
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hexane/water interface, a solution of dopamine was prepared and placed under hexane. We
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also confirmed that the dopamine was insoluble in hexane and dichloromethane
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prior to the experiments. Oxygen in a hexane solution also plays an important role as a
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source of oxidation of the dopamine solution. Since a concentration of dissolved oxygen (0.2
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mM) was lower than a concentration of dopamine (54 mM) in the water solution, the
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dissolved oxygen in the water solution was not enough to oxidize the dopamine. Therefore, it
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is expected that dopamine has been actively oxidized at the interface, and polydopamine
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membrane has been formed at the interfaces. However, the interfacial shape between the
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solution droplet and hexane did not change after 30 min. On the other hand, the flattening of
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water droplets at the hexane/water interface was observed by reducing the volume of the 7 ACS Paragon Plus Environment
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droplet with a syringe (Figure 2(a), Figure S3 and Movie S2). In the case of the air/water
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interface, the volume of droplets was decreased by the evaporation of water. In contrast, at
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the hexane/solution interface, it was necessary to reduce the volume of the droplets with
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using a syringe to deform the interface because the evaporation of water from the solution
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droplet was prevented by the hexane covering. These results indicate that a reduction in the
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volume of the droplet is required to induce flattening at the liquid/liquid interface. It is
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noteworthy that the flattened interface recovered to the original spherical dome shape by
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addition of the solution to the droplet (Figure 2(a)). This result indicates that dome-flat or
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flat-dome transformation performed reversibly. The reversibility of the dome-flat
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transformation (Figure 2(b)) was also evaluated. The width of the flattened surface was
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measured with 6 cycles of extraction or addition of the solution in the droplet. The results
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indicate that the dome shape was completely recovered at each cycle and this process has
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high reproducibility. Extractions and additions were operated within few seconds, and the
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operations were faster than the evaporation rate even though we cannot calculate the exact
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flow rate since we changed the volume of droplets by hand.
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If the solution was over added, in the previous reports about liquid marbles, the droplets were
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in a fluid state or a jamming state in which the particles on the droplet were non-packing state
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or packing state, respectively28. In our case, we considered that the droplet completely
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covered by the polydopamine membrane, which means the system was in jamming state.
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However, when the solution is over added to the droplet, the droplet is expected to be turned
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into the fluid state from the jamming state, and which results in formation of cracks of the
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polydopamine membrane.
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To reveal the driving force of polydopamine membrane formation, the interfacial tensions at
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the air/solution and the hexane/solution interfaces were evaluated using a pendant-drop
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method. In the case of the air/water interface, there was almost no change in the interfacial
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tension (Figure 3(b) and Figure S4). On the other hand, the interfacial tensions between air
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and the dopamine solution decreased after approximately 2 min (Figure 3(a), (b) and Figure
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S5). In the case of the hexane/water interface, the interfacial tension gradually decreased;
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however, the interfacial tension between hexane and the dopamine solution (Figure 3(c), (d)
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and Figure S7) rapidly decreased, unlike the case without dopamine (Figure S6). This result
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also implies formation of a polydopamine film at the interface. After 2 min, the interfacial
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tension became constant (32 mN/m) because the entire interface was covered with
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polydopamine and there was no evaporation of water from the pendant drop in the
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hexane/solution case. The same decrease in interfacial tension has also been observed in the
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cases of graphene oxide-polymer composites (10 ~ 20 mN/m)29 and HFBI (30 ~ 40 mN/m)30
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at the oil/water interface, and this trend indicates that the pendant-drop was covered by the
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polydopamine membrane. These results suggest that polydopamine membranes were
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spontaneously formed at the interfaces to reduce the interfacial tension.
193 194
Figure 4 shows the change in the interfacial shape at the dichloromethane/water interface.
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The interfacial tensions at a dichloromethane/water interface were also measured using the
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reverse pendant-drop method because the density of dichloromethane is higher than that of
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water. In the case of the dichloromethane/water interface, the interfacial tension gradually
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decreased; however, the interfacial tension between hexane and the dopamine solution
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(Figure 4(b) and Figure S9) rapidly decreased, unlike the case without dopamine (Figure S8). 9 ACS Paragon Plus Environment
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This also implies formation of a polydopamine film at the dichloromethane/water interface.
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After 15 min of standing, wrinkles were also observed at the interface of the solution, which
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also supported the formation of a polydopamine film at the interface (Figure 4(b) and (c)).
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Although the glass capillary was tilted 7°, it is noteworthy that the water droplets were
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flattened horizontally against a ground (Figure 4(a)). The diameter of droplet at the
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hexane or dichloromethane solution was approximately 6 mm or 1.2 mm,
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respectively. The flattening phenomenon was observed in both experiments,
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which suggested that the droplet size does not affect the flattening phenomenon.
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According to a previous report11, the top of the droplet flattened because a membrane formed
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at the top of droplet due to its buoyancy. However, the flattening was observed at the
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air/solution, hexane/solution and dichloromethane/solution interfaces. These results indicate
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that the driving force for membrane formation at the interface in the present case is a
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decrease in the interfacial tension, regardless of the buoyancy, because the membrane was
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formed for both the hexane/solution and dichloromethane/solution cases. Gravity or the
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buoyancy force applied to droplets determines the direction of flattening (air/solution and
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hexane/solution interfaces are downward, and the dichloromethane/solution interface is
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upward).
217 218
To compare with the previously reported mechanism (Scheme 1(a))10,11, we propose the
219
following formation process (Scheme 1(b)). i) Dopamine in the water drop is polymerized by
220
auto-oxidation; ii) the polydopamine membrane forms at the interfaces of the entire droplet to
221
reduce the interfacial tension; iii) the water droplet is flattened by water evaporation or
222
directorial extraction. The polydopamine membrane was formed over the entire interface; 10 ACS Paragon Plus Environment
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therefore, the driving force for flattening should be buckling. It has been reported that
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spherical membranes, such as with ping-pong balls31 and rubber balls32,33 are flattened by
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buckling when the internal air volume is reduced. Liquid marbles were also flattened via the
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freezing process34, and volume of the droplet was nearly constant. However, in our case,
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flattening phenomenon was observed via changing volume of droplet, and which was more
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similar to a ball buckling process than the case of freezing process. In the present case, the
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interface is covered with a polydopamine membrane and the polydopamine membrane is
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stiffer than typical polymeric thin films due to the high aromatic content and
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inter/intramolecular hydrogen bonding; therefore, the facet formed due to buckling after the
232
extraction of water to minimize the surface free energy.
233 234
If mechanical stiffness of the polydopamine membrane is weak against the surface or
235
interfacial tension, it is expected that the membrane may form cracks with a deformation of
236
droplet. Therefore, the mechanical properties are necessary to reversibly deform membrane.
237
The Young’s modulus of polystyrene is known as 3.4 GPa35,25, and polystyrene nanosheets
238
have enough mechanical property to deform a droplet6,. On the other hand, according to the
239
literature36, the Young’s modulus of polydopamine membrane is usually 4.1–10.5 GPa, which
240
indicates that the polydopamine membrane also have enough mechanical property to deform
241
a droplet.
242 243
Furthermore, in the conventional mechanism, the two-dimensional thin film is irreversibly
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formed; therefore, no reversible dome-flat or flat-dome water droplets are observed. In this
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study, the water droplets were flattened by polydopamine membranes at the air/water, 11 ACS Paragon Plus Environment
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hexane/water and dichloromethane/water interfaces. In addition, reversible flattening of water
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droplets was observed at these liquid/liquid interfaces. This is a completely new system
248
because the polydopamine membrane is dynamically synthesized at the interface and the
249
formation speed of the polydopamine membrane overcomes the flattening timescale (several
250
seconds). The mechanism for membrane formation and flattening is expected to provide new
251
insights into physical control of the interfacial shapes of droplets.
252 253
Corresponding Author
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H. Abe (
[email protected] )
255
H. Yabu (
[email protected] )
256
*Give contact information for the author to whom correspondence should be addressed.
257 258 259
Acknowledgement
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This work was supported by a Grant-in-Aid for Fellows from the Japan Society for the
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Promotion of Science (JSPS) and a Grant-in-Aid from the Tohoku University Institute for
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International Advanced Research and Education. This work was also supported by a
263
Grant-in-Aid for Exploratory Research, MEXT, Japan (16K14071).
264 265 266
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Figure 1. Flattening of water droplets at the air/water interface. (a) Side view and (b) top
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view photographs of droplet flattening by polydopamine membranes at the air/water interface.
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Microscopic images of the polydopamine membrane formed at (c) a facet and at (d) a side
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surface. (e) AFM image and (f) cross-sectional profile.
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Figure 2. (a) Sequential video images of a water droplet covered with a polydopamine
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membrane during (i) extraction and (ii) addition of water in hexane. (b) Cycles of water
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droplet flattening at a hexane/water interface.
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Figure 3. Pendant-drop profile shapes of 10 mg/mL dopamine in Tris-HCl (pH 8.9) at the (a)
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air/water and (c) hexane/water interfaces. Changes in the interfacial tension of 0 mg/mL
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(open circles) and 10 mg/L (closed circles) at the (b) air/water and (d) hexane/water
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interfaces.
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Figure 4. (a) Flattening of water droplets at the polydopamine/dichloromethane/water
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interface. (b) Pendant-drop profile shapes of 10 mg/mL dopamine in Tris-HCl (pH 8.9) at the
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dichloromethane/water interface. Changes in the interfacial tension at the 0 mg/mL (open
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circles) and 10 mg/mL (closed circles) dichloromethane/water interface.
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Dopamine or dopamine oligomer
(a) Conventional mechanism (i)
Polydopamine membrane
( ii ) Evaporation
(b) This study (i)
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( ii )
( iii ) Evaporation
Extraction
( iii )
Addition
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Scheme 1. Mechanisms for the flattening of water droplets at interfaces. (a) Conventional
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mechanism for the flattening of water droplets. (b) New mechanism for the flattening of
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water droplets.
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