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Self-propelled motion of monodisperse underwater oil droplets formed by a microfluidic device Naoko Ueno, Taisuke Banno, Arisa Asami, Yuki Kazayama, Yuya Morimoto, Toshihisa Osaki, Shoji Takeuchi, Hiroyuki Kitahata, and Taro Toyota Langmuir, Just Accepted Manuscript • Publication Date (Web): 14 May 2017 Downloaded from http://pubs.acs.org on May 17, 2017

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Self-propelled motion of monodisperse underwater oil droplets formed by a microfluidic device Naoko Ueno,† Taisuke Banno,‡ Arisa Asami,† Yuki Kazayama,† Yuya Morimoto,§ Toshihisa Osaki,§,ǁ Shoji Takeuchi,§ Hiroyuki Kitahata,# and Taro Toyota*,† † Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan. ‡ Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-141 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan. § Institute of Industrial Science (IIS), The University of Tokyo, 4-6-1 Komaba, Meguro, Tokyo 153-8505, Japan. ǁ KAST R&D, Kanagawa Institute of Industrial Science and Technology, 3-2-1 Sakado, Takatsuku, Kawasaki City, Kanagawa 213-0012, Japan. # Department of Physics, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan.

KEYWORDS Surfactant, Self-propelled motion, Oil droplet, Microfluidic device, Spontaneous emulsification.

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ABSTRACT

We evaluated the speed profile of self-propelled underwater oil droplets comprising a hydrophobic aldehyde derivative in terms of their diameter and the surrounding surfactant concentration using a microfluidic device. We found that the speed of the oil droplets is dependent on not only the surfactant concentration but also droplet size in a certain range of the surfactant concentration. This tendency is interpreted in terms of combination of the oil and surfactant affording spontaneous emulsification in addition to Marangoni effect.

INTRODUCTION Underwater self-propelled micro-objects that convert chemical energy to kinetic energy have recently received much attention as micrometer-sized carriers and/or probes in environmental or biological systems.1-3 In previous reports, they were prepared using colloidal polymer particles, whose surface was modified with a chemically active thin film, molecular aggregates including liquid droplets, vesicles, and nanotubes. Applications of such self-propelled micro-objects include a self-cleaning system using a self-propelled micro-object carrying a catalyst, as exhibited by Schmidt’s group.4 A micrometer-sized rocket-type vehicle for targetdriven drug delivery has also been developed.5 Some self-propelled micro-objects are transformable during their self-propelled motion and consequently have been highlighted as mobile microbe models.3 Among them, micrometer-sized liquid droplets exhibiting selfpropelled motion without assistance from solid-liquid or air-liquid interfaces in the presence of surfactant have attracted attention.6-8 Our group recently reported the self-propelled motion of


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underwater micrometer-sized oil droplets comprising a hydrophobic aldehyde derivative in a cationic surfactant solution, with their behavior controlled by the chemical conversion of oil and surfactant molecules.9,10 To successfully apply the self-propelled oil droplets as micrometersized carriers and/or probes, it is essential that we control the mechanism that causes the oil droplet locomotion. Even though the component molecules in these oil-in-water emulsion systems are simple, the mechanism that drives the self-propelled oil droplet when the Reynolds number is low is not fully clarified. Yoshinaga et al. proposed constituent equations that describe the oil droplet motion on the basis of hydrodynamics and the Marangoni effect, i.e., advection and mass transfer of surfactant molecules adsorbed at an oil/water interface.11,12 Several research groups have independently introduced mathematical models to the study of such systems.13-15 Matsuno et al. used numerical simulations to develop a model describing the sustainment of self-propelled oil droplet motion.16 The self-propelled motion of oil droplets is predominantly attributed to the Marangoni effect. However, the origin of the driving force is still unclear due to difficulties in quantitatively measuring certain parameters, especially the dynamic interfacial tension of the self-propelled oil droplets, which is related to the self-propelled motion of oil droplets that are poly-disperse in a surfactant solution (Figure S1). In addition, preparing the emulsions using simple mechanical agitation did not enable us to observe the beginning of the self-propelled motion. Here we overcame these limitations by preparing monodisperse oil droplets using a microfluidic device and analyzed their speed as a function of both their diameter and the surfactant concentration, which is associated with the Marangoni effect. MATERIALS AND METHODS


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Materials. 4-Heptyloxybenzaldehyde, 4-butyloxybenzaldehyde, octylphenylether, hexadecyltrimethylammonium bromide, octyltrimethylammonium bromide, and hexatrimethylammonium bromide were purchased from Tokyo Chemical Industry Co. Ltd., Japan. Photo-curing resins SU-8 and SU-100 were provided by Microchem (Westborough, MA, USA). Polydimethylsiloxane (SILPOT 184) was supplied by Dow Corning Toray Co., Ltd., Japan. All reagents were used without further purification. Water was purified and de-ionized by a Millipore reagent water system (Bedford, MA, USA). Microfluidic device. First, the patterned glass mask was prepared using mask-less exposure apparatus (Nano System Solutions, Inc., D-light DLS-50, Japan). Second, a silicon wafer (diameter = 50.0 mm, Matsuzaki Seisakusyo Co., Ltd., Japan) was baked and dehydrated on a 200 oC hotplate (DP-1M, AS ONE Co., Japan) for 15 min. The dust on the wafer was then removed by blowing with nitrogen gas and the wafer was set on a spin-coater (Opticoat MSA100, MIKASA Co., Ltd., Japan). A photo-curing resin (SU-8, MicroChem) was layered on the wafer surface and spun at 2000 rpm (SU-8 100), 1100 rpm (SU-8 100), and 2000 rpm (SU-8 50) for 30 s, resulting in resin layers thicknesses of 112, 255, and 54 µm, respectively. The resin was cured on a hotplate at 65oC for 30 min and subsequently at 95oC for 60 min, and slowly cooled at room temperature (23 − 25oC). Further curing was performed using mask alignment apparatus (MA-10, MIKASA Co., Ltd., Japan) with the glass mask for 100 s. The wafer was heated at 65oC for 3 min and then 95oC for 12 min, followed by cooling at room temperature. The unreacted resin was removed by an SU-8 developer (MicroChem) for 10 min. The wafer was then rinsed with an SU8 developer for 1 min and followed by rinsing with 2-propanol (Wako, Japan) twice (for 1 min each). Third, polydimethylsiloxane (PDMS, Dow Corning Toray Co., Ltd., SILPOT184, Japan) and a crosslinker (Dow Corning Toray Co., Ltd., SILPOT184, Japan)


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were mixed in a chamber (AR-100, THINKY Co., Japan) at a weight ratio of 10:1. The mixture was poured into a plastic dish (90 mm) and the SU-8 mold wafer was sunk in the mixture. After removing the air bubbles in vacuum, the dish was incubated on a hotplate at 75oC for 90 min. The cured PDMS was separated from the dish and the SU-8 mold wafer. After shaping and making holes for inlets and outlet, the cured PDMS was irradiated by O2 plasma (FA-1, SAMCO Inc., Japan) under 25 W and 20 mL/min for 5 s. A glass plate (Matsunami Glass Ind., 0.25–0.35 mm) that was also irradiated by O2 plasma for 65 s was attached to the cured PDMS and set at 75oC for 1 h. The as-prepared microfluidic device was stored in a dry cabinet. Microscopy observation and speed analysis. The HBA oil droplets prepared in the microfluidic device were transferred through an ETFE tube (36 cm, JR-T-082-M10, SHIMADZU GLC Ltd., Japan) and a six-way valve (Model 7000, Rheodyne, Japan) to the aqueous HTAB solutions (0 – 50 mM) in a chamber composed of two-sided tape (flame sealed chamber, 15×15×0.29 mm3, BIO-RAD) and two pieces of cover glass. Immediately after ~10 µL of the oil droplet dispersion had been transferred and the sample was observed under a phase contrast microscope (IX71, Olympus, Japan) equipped with a CCD camera (WAT-01U2, WatecCameraViewer, Watec Co., Ltd, Japan) at room temperature (23 − 25 oC). By analyzing the captured movies using Image J (NIH, USA), the center of mass of the self-propelled oil droplets was evaluated and the mean value of the speed was calculated over 5 seconds. The flow around the oil droplets was observed by fluorescent microspheres (Fluoresbrite Polychromatic Red Microspheres 0.5 µm, Polysciences, Inc.) at a density of 3.6×1018 L-1 and observing them using a florescence microscope (IX71, Olympus, Japan). The images were


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captured in video mode and the location and speed of each observed microsphere for 1 – 3 seconds within the first ten minutes were analyzed by ImageJ (NIH, USA). RESULTS AND DISCUSSION Microfluidic devices have been widely used to prepare monodisperse oil droplets under a bright field microscope. Thus we constructed a microfluidic device by bonding a polydimethylsiloxane microfluidic channel which was produced by such techniques and a glass substrate (see MATERIALS AND METHODS). The microfluidic channel consisted of three parts: an orifice for formation of oil droplets using a continuous phase of surfactant solution, a meandering section for stabilization of the oil surface, and a gentle current section for measuring the droplet sizes (Figure 1). The width of the orifice was 60 µm and the channel depths were set to 255, 112, and 54 µm, respectively, to afford the different sizes of droplets.17-19 An aqueous solution of hexadecyltrimethylammonium bromide (HTAB, 10 mM) was introduced at Inlet 1 as the continuous phase and 4-heptyloxybenzaldehyde (HBA) was introduced at Inlet 2 as the inner oil phase. The oil droplets were thus collected from the outlet through a six-way valve to an observation chamber (15×15×0.29 mm3) filled with HTAB solution (0 – 50 mM) and imaged using a phase contrast microscope (IX71, Olympus) at room temperature (23 − 25 oC). The diameters of the oil droplets collected in the observation chamber were 220 µm (coefficient of variance (CV) = 2.1%), 148 µm (1.7%), 120 µm (13%), and 103 µm (20%) at varying the flow rates of both HTAB solution and HBA and flow channel depths (Table S1). The corresponding diameters of the oil droplets in the microfluidic device were 232 µm (CV = 2.2%), 170 µm (1.5%), 146 µm (2.7%), and 155 µm (4.7%), respectively (Figure S2). The slight difference among the diameters and their CV values in the chamber compared with those in the


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microfluidic device is due to the smaller depth of the channel in the device compared with that of the observation chamber, the wetness difference of PDMS and glass substrates, and the fusion/fission/shrinkage of oil droplets inside the collecting tube in the presence of the surfactant.

Figure 1. Schematic of the microfluidic device and phase contrast microscopy images of oil droplet formation. The oil droplets were conveyed to the collection tube and transferred into an observation chamber. Scale bars = 500 µm.

Figure 2 demonstrates the typical two-dimensional trajectories of the monodispersed selfpropelled oil droplets in the observation chamber and Figure 3 shows their speed profiles as a function of the mean diameter and HTAB concentration. The speed was measured in a twodimensional manner for 1 − 3 min (Number of investigated oil droplets = 20 for each batch). Note that we excluded the data on oil droplets that came near each other within the range of their


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diameters, changed their direction frequently or, appeared in the vicinity of the chamber wall, since such oil droplets tended to slow down. We measured the speed of the oil droplets that sustained movement in a fixed focal plane (optical thickness of focal plane ~ 10 µm) during observation; thus, the standard deviation of the speed was greater for smaller oil droplets than larger ones. Series of observations using monodisperse droplets revealed that oil droplet speed peaked at a particular HTAB concentration. Moreover, the speed displayed a remarkable dependency on diameter when the HTAB concentration was < 15 mM, while no size dependency was observed when the HTAB concentration was > 35 mM. Since the droplet sizes and surfactant concentrations were highly reproducible in the chamber, we attributed the observed speed profiles to the interaction between the oil and surfactant molecules under given conditions.

Figure 2. Two-dimensional trajectories (5 seconds) of the center-of-mass of self-propelled oil droplets with the average diameter of 220 µm in the observation chamber including the HTAB aqueous solutions of 1 (A), 15 (B), and 50 mM (C) (n = 5). Length unit for the axis in each graph is micrometer. The start point of each trajectory is set to zero point.


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Figure 3. Speed profile of self-propelled oil droplets of differing size (mean diameters are shown in the legend) as a function of the HTAB concentration (left) and the size dependency of the speed at a 10 mM HTAB concentration (right).

This speed profile is quite characteristic so that we need to interpret these data referring several factors. Since the size and spherical shape of oil droplets did not significantly change during the speed measurement under the microscope, according to the previous researches on the self-propelled oil droplets,6,10-12 we postulated that the oil droplets are primarily driven by not micellar solubilization but Marangoni effect. We thus evaluated the advection related to the selfpropelled motion of the oil droplets (Figure S3). Using a fluorescence microscope, we monitored the flow of fluorescent polystyrene beads (φ= 0.5 µm) floating in the immediate surroundings of the self-propelled oil droplets in the aqueous HTAB solution. The flow at the front side gradually increased while it decreased at the rear side and ceased far from the oil droplet. We dispersed another type of fluorescent polystyrene beads (φ= 1 µm) into HBA and observed the movement of a droplet of this HBA in the aqueous HTAB solution, the fluorescent beads flowed forwards at the center-of-mass of the self-propelled oil droplet and backwards near its surface. This


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advection pattern resembles that of the Marangoni-effect-driven locomotion of liquid droplets.1116

Marangoni effect resulting from surfactant adsorption at a liquid-liquid interface is induced by interfacial tension heterogeneity. Regarding each half-body of the self-propelled oil droplet, HTAB is more adsorbed at the front side than that at the rear side. It is necessary that the adsorbed HTAB is transferred from the oil droplet surface for sustaining the interfacial tension heterogeneity for continuous self-propelled motion. We then conceived the dissolution process of HTAB into HBA phase associated with the self-propelled motion. In order to examine this, we first studied a biphasic system composed of HBA and a 50 mM aqueous HTAB solution in a glass vial. The HBA layer became turbid immediately after addition of HBA on top of the aqueous HTAB solution without any agitation (Figure 4). A tangential flow in the vicinity of the interface was then observed by eye (Movie S1). This spontaneous emulsification was not observed when using 4-butyloxybenzaldehyde and octylphenylether instead of HBA. These oils scarcely afforded self-propelled oil droplets in the HTAB aqueous solution. Next, the HBAHTAB solution system was probed using in situ infrared (IR) absorbance spectroscopy (ReactIR, Mettler Toledo). An IR detection probe was introduced to a centimeter-sized stationary HBA droplet in an aqueous HTAB solution and the finger print region of the HBA IR spectrum was monitored over time (Figure 5). The peaks at 1350 and 1560 cm-1 which are assigned to benzaldehyde moiety gradually became smaller and a new peak at 1460 − 1480 cm-1, assigned to trimethylammonium moiety of HTAB, simultaneously appeared over 15 min. This indicates that the spontaneous emulsification involved the phase transfer of HTAB (and water) from the aqueous phase to the HBA phase due to strong attractive interaction.


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Taking these results into account, it is considered that the oil droplet obtains energy for self-propelled motion from the oil-surfactant complexation in the core and from surfactant adsorption due to the flow at the surface. The former is related to dissolution energy20 and dependent on the volume of the oil droplet, and the latter is to interfacial energy related to its surface area. On the other hand, as the oil droplets move in the HTAB aqueous solution, they dissipate energy due to the Stokes resistance. Since the Stokes resistance is dependent on radius r of oil droplet and its speed V, i.e. equals to 6πηrV (viscosity constant η), the energy dissipation rate is written as 6πηrV2.13 We measured the viscosity of the HTAB solution and confirmed only a slight change (~10%) as its concentration was varied between 0 – 50 mM (Figure S4). Therefore, we can interpret that the oil-surfactant complexation strongly enhances the role of the radius to the speed more than the surfactant adsorption in a certain range of the surfactant concentration. When the surfactant concentration is high, both the oil-surfactant complexation and the surfactant adsorption are enhanced, however the complexation of the oil and surfactant is equilibrated rapidly and the surfactant ions are largely adsorbed not only at the front side but also at the rear side. As a result, the size dependency of oil droplet speed is not observed and the speed itself decreases.

Figure 4. Time-course change of biphasic oil-surfactant solution systems, (A) 4butyloxybenzaldehyde, (B) 4-heptyloxybenzaldehyde, and (C) octylphenylether were layered on


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an aqueous HTAB solution (50 mM). Spontaneous tangential flow in the vicinity of the interface occurred only in (B).

Figure 5. (A) Photograph of the in situ IR absorbance spectroscopy setup. Detector (Attenuated Total Reflection (ATR) sensor) diameter = 10 mm, Volume of HBA droplet = 50 µL, and volume of HTAB aqueous solution (50 mM) = 50 mL. (B) Time-course change of in situ HBA IR absorbance spectra in the biphasic system constructed by subtracting the spectra of aqueous HTAB solution. The contour diagrams were colored with the absorbance intensity.

Our current argument involves two speculative estimations, especially regarding the relationship between the dissolution energy and interfacial tension heterogeneity. Several theoretical research groups have independently explained the self-propelled motion of liquid droplets under surfactant stimuli, though they did not explicitly refer to spontaneous


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emulsification.11-16 Maass’s group and other groups have proposed several mechanisms other than Marangoni effect for the driving force of droplet locomotion.13-15 Hence, we performed reference experiments on this self-propelled oil droplet system. We observed the oil droplets starting self-propelled motion in a HTAB concentration gradient (Figure S5). The oil droplets initially accelerated for several seconds in the low concentration region; however, the speed decreased when the oil droplet reached the region of high HTAB concentration. When using hexyltrimethylammonium bromide and octyltrimethylammonium bromide instead of HTAB, the HBA droplets did not exhibit self-propelled motion while their concentrations were varied in the range of 1 − 150 mM. Since these combinations did not afford spontaneous emulsification in the biphasic system, we imply that the complex formation during spontaneous emulsification is the predominant driving force as well as the Marangoni effect for the self-propelled motion of micrometer-sized oil droplets.

CONCLUSIONS The speed profile of self-propelled micrometer-sized HBA droplets in aqueous HTAB solutions was measured by using uniform droplet sizes via a microfluidic device. We proposed that the driving force for self-propelled motion is based on the Marangoni effect and the oilsurfactant interaction that affords spontaneous emulsification. The current findings contribute to the further development of micrometer-sized underwater chemical rovers and also the experimental models for studying mobile microbes. ASSOCIATED CONTENT


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Supporting Information. The following files are available free of charge. Table S1, Figures S1-S5 (PDF), and Movie S1 (AVI). AUTHOR INFORMATION Corresponding Author *TEL; +81.3.5465.7634, E-mail; [email protected]. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The authors acknowledge Ms Rie Kuroha, Mr Shingo Miura, and Mr Ryota Izako (The University of Tokyo, Japan) for supporting oil droplet observation experiments. This work was supported by the Platform for Dynamic Approaches to Living System (S.T., T.O., T.T.) from the Ministry of Education, Culture, Sports, Science and Technology, Japan and JSPS KAKENHI Grant Number 25103008 (H.K.), 25790033 (T.B.), 16H04032 (T.T.), and 25103009 (T.T.). REFERENCES (1) Wang, W.; Duan, W.; Ahmed, S.; Mallouk, T. E.; Sen, A. Small power: Autonomous nano-and micromotors propelled by self-generated gradients. Nano Today 2013, 8, 531554. (2) Sánchez, S.; Soler, L.; Katuri, J. Chemically Powered Micro- and Nanomotors. Angew. Chem. Int. Ed. 2015, 54, 1414-1444. (3) Elgeti, J.; Winkler, R. G.; Gompper, G. Physics of microswimmers - single particle motion and collective behavior: a review. Rep. Prog. Phys. 2015, 78, 056601.


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