Research Article www.acsami.org
Remote Manipulation of a Microdroplet in Water by Near-Infrared Laser Ying Chu, Fatang Liu, Liming Qin, and Qinmin Pan* State Key Laboratory of Robotics and Systems, School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, P. R. China S Supporting Information *
ABSTRACT: Facile manipulation of a tiny liquid droplet is an important but challenging issue for many miniaturized chemical and biological systems. Here we report that a microdroplet can be readily and remotely manipulated in aqueous environments under ambient conditions. The droplet is encapsulated with photothermal nanoparticles to form a liquid marble, and subsequently irradiated with a near-infrared (NIR) laser. The marble is able to ascend, shuttle, horizontally move, and even suspend in water by simply controlling the laser irradiation. Moreover, filling and draining of the marble can also be conducted on the water surface for the first time. This facile manipulation strategy does not use complicated nanostructures or sophisticated equipment, so it has potential applications for channel-free microfluidics, smart microreators, microengines, microrobots, and so on. KEYWORDS: remote manipulation, liquid droplet, photothermal nanoparticles, liquid marble, NIR laser
1. INTRODUCTION Controlled manipulation of a tiny liquid droplet has attracted increasing attention because of its potential applications in many fields,1 including miniaturized chemical or biological systems,2−5 patterning,6 microfluidics,7 and printing,8 and so forth. Until now liquid droplets have been mainly manipulated in channels9−11 or on nanostructured surfaces,12−16 which often required complicated nanostructures or precision instruments like pumps, valves, and microfluidic devices. Therefore, it is highly desirable but challenging to develop a facile, effective, and versatile strategy for manipulating a small volume of liquids. Liquid marble is a liquid droplet encapsulated with hydrophobic or hydrophilic particles.17−20 Because it is almost nonsticking to solid surfaces, a liquid marble can be easily manipulated on a solid (or water) surface and even in water.21−25 These features make liquid marble an ideal platform for channel-free manipulation of a droplet. From the standpoint of practical application and environmental friendliness, manipulating a liquid marble in aqueous conditions is attractive for many miniaturized systems and processes. However, a big challenge remains in precisely controlling the motion and position of a liquid marble in aqueous environments, such as launch, stop, direction of movement, and suspending. Moreover, rare studies remotely controlled the movement of a liquid marble through a simple but powerful physical trigger. Taking a tiny CHCl3 droplet as an example, here we report that its movement in water can be readily controlled by nearinfrared (NIR) laser at ambient temperature (Scheme 1). The © 2015 American Chemical Society
goal was achieved by encapsulating the droplet with core−shell Fe3O4@polydopamine nanoparticles to form a liquid marble. The resulting marble was able to ascend, shuttle, and even suspend in water by simply controlling the laser irradiation. Interestingly, filling and draining of the marble could also be conducted on the water surface. Although NIR laser was recently used for photothermal therapy,26,27 it has rarely been applied to manipulate a microdroplet. Considering easy synthesis of polydopamine-based nanoparticles and simple operation of laser irradiation, the present strategy offers a facile route for remotely manipulating a tiny liquid droplet in aqueous environments. Potential application of the strategy might be found in smart microreactors, microengines, microrobots, and so on.
2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Iron(III) chloride hexahydrate (FeCl3·6H2O), ethylene glycol, sodium acetate trihydrate (NaAc· 3H2O), polyethylene glycol, ethanol (95%), tris(hydroxymethyl) aminomethane hydrochloride (Tris-HCl), dopamine hydrochloride (DA), chloroform (CHCl3), and carbon tetrachloride (CCl4) were provided by Tianjin Kermel Chemical Reagent Co., Ltd. (China). 2.2. Instruments. Near-infrared laser (808 nm, 200 mW) was purchased from Shenzhen Fuzhe Technology Co., Ltd. (China). X-ray diffraction (XRD) analysis was performed by using a Shimadzu XRD6000 instrument. Transmission electron microscopy (TEM) images Received: October 19, 2015 Accepted: December 29, 2015 Published: December 29, 2015 1273
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ACS Applied Materials & Interfaces Scheme 1. Illustration of Remote Manipulation of a CHCl3 Microdroplet in Water at Room Temperature
Figure 1. (a) TEM image of the Fe3O4@PDA nanoparticles. (b) Underwater contact angle of a CHCl3 droplet on the nanoparticles. (c) Underwater adhesion force of the nanoparticles to a CHCl3 droplet (3 μL). (d,e) Infrared thermographs of the Fe3O4@PDA nanoparticles immersed in CHCl3 after laser irradiation for (d) 1 and (e) 10 min. (f) Effect of irradiation time on the temperature of the nanoparticles immersed in CHCl3 and water. Pure water and CHCl3 were used as a control. 2.6. Manipulation of a CHCl3 Marble in Water. A droplet of CHCl3 (6 μL) was gently rolled on the Fe3O4@PDA nanoparticles placed at the bottom of the water.25 After the nanoparticles completely encapsulated the droplet, a black and spherical marble was constructed in the water. Then the marble was irradiated with the NIR laser at room temperature. The distance between the laser source and the irradiated site was ∼3.0 cm. By focusing the laser spot on its surface for ∼17 min, the marble left the bottom and ascended in the water at a rate of ∼0.7 mm s−1. The marble could also shuttle and suspend in the water by elaborately controlling the irradiation intervals. 2.7. Measurement of Adhesion Force. First, Fe3O4@PDA nanoparticles were homogeneously attached to a glass slide with double-faced adhesive tape. The resulting slide was immersed in water. A CHCl3 droplet (3 μL) was fixed to a copper ring with a diameter of ∼1 mm. The ring was fixed to the balance of a microelectromechanical balance (DCAT-21, Dataphysics). The oil droplet was squeezed against the nanoparticles at a rate of 50 μm s−1 to reach a preset value of 0.5 mm. Then the droplet was relaxed by moving the slide downward. The adhesion force was measured according to the force− distance curves recorded by the balance.
were recorded on an H-7650 (Hitachi) apparatus. X-ray photoelectron spectroscopy (XPS) was conducted on a PHI-5700 ESCA system. An infrared thermal imager (FLIR T360) was used to record the temperature change of the Fe3O4@PDA nanoparticles during the NIR laser irradiation. Contact angle measurements were conducted on an OCA-20 (DataPhysics Instruments GmbH) instrument. The gas components of the “airbag” were identified via gas chromatography− mass spectrometry (GC-MS, Agilent Technologies, 5973N). 2.3. Synthesis of Fe3O4 Nanoparticles. Fe3O4 nanoparticles with a diameter of 200−300 nm were synthesized according to a previously reported procedure.28 Typically, FeCl3·6H2O (1.0 g) was dissolved in 30 mL of ethylene glycol by stirring. Then 2.7 g of NaAc·3H2O and 0.75 g of polyethylene glycol were added to the above solution. The resulting mixture was sealed in a Teflon-lined stainless steel autoclave and subsequently heated at 200 °C for 48 h. After the autoclave was cooled to room temperature, black powder was collected with a magnet bar and successively washed with ethanol and deionized water. 2.4. Synthesis of Core−Shell Fe3O4@PDA Nanoparticles. Fe3O4 nanoparticles (50 mg) were dispersed in 10 mL of Tris-HCl solution (10 mM) by ultrasonication. Then 50 mg of dopamine hydrochloride was added to the above dispersion.29 After stirring at room temperature for 24 h, Fe3O4@PDA nanoparticles were collected with a magnet bar and washed with deionized water five times. 2.5. Measurement of the Photothermal Performance. Fe3O4@PDA nanoparticles (0.6 mg) were immersed in 0.3 mL of deionized water (or CHCl3) placed in a cuvette and then irradiated with a NIR laser (808 nm, 200 mW) for 10 min. The distances between the laser source and irradiated sites were about 2.7, 3.0, 3.5, 4.0, and 5.5 cm, respectively. The temperature of the irradiated sites was recorded with an infrared thermal imager (FLIR T360).
3. RESULTS AND DISCUSSION First, Fe3O4 nanoparticles with a diameter of 200−300 nm were synthesized through a solvothermal process (Figure S1, Supporting Information).28 Next, their surfaces were coated with a layer of polydopamine (PDA) via a mussel-inspired method29,30 to obtain core−shell structured Fe3O4@PDA nanoparticles. The presence of PDA was confirmed by X-ray photoelectron spectroscopy (XPS; Figure S2, Supporting 1274
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Figure 2. (a) The marble ascended in water after laser irradiation at room temperature. Inset is the bubble formed on the irradiated marble. (b) Effect of irradiation time on the volume of the bubble. Scale bar: 4 mm.
surface temperature of the irradiated nanoparticles exceeded the measured value. This is because the infrared light emitted from the irradiated site was partly absorbed by the water and glass vial before it reached the thermal imager. Owing to their strong absorption in the NIR region, the Fe3O4@PDA nanoparticles exhibited a photothermal conversion efficiency of 33.9% (Figure S5, Supporting Information). The nanoparticles were used to construct a liquid marble, and its movement in water was investigated. The marble was constructed by gently rolling a droplet of CHCl3 (6 μL) on the nanoparticles placed in deionized water.25 The nanoparticles spontaneously encapsulated the droplet due to the minimization of the free energy of the CHCl3/water interface.17−20,32 Then we tried to manipulate the marble in water at room temperature (24.5 °C) by focusing a laser spot on its surface. The marble remained still and its appearance unchanged in the initial irradiation stage (e.g., 1 s). Subsequently, a black “airbag” started to grow on the top of the marble, which finally lifted the marble at a rate of ∼0.7 mm s−1 after irradiation for 17 min (Figure 2a). The marble stopped ascending but fell on the bottom once the laser was removed from its surface. In contrast, a bare CHCl3 droplet remained still at the bottom of water even after irradiation for 30 min, as almost no “airbag” was observed on its top (Figure S6, Supporting Information). To directly observe the inner structure of the “airbag”, we opened the marble via a magnetic bar beneath it.21 A bubble was formed and then grew on the top of the CHCl3 droplet during the irradiation (Figure 2a and Supporting Information Figure S7). The growth of the bubble was dependent on the water temperature, the time and distance of laser irradiation (Figure 2b, Supporting Information Figures S8 and S9). The components in the bubble were identified by gas chromatography−mass spectrometry (GC-MS), which confirmed the presence of CHCl3 vapor, air, and steam (Figure S10, Supporting Information). The gaseous CHCl3 resulted from the part vaporization of the encapsulated CHCl3 droplet, while the steam might come from the trace water that absorbed on the syringe during the measurement. These gaseous components were stored at the top of the droplet and constituted the inner structure of the “airbag”. Clearly, this “airbag” produced an extra buoyant force to lift the marble in water. To further understand the formation mechanism of the “airbag”, we simulated the temperature and flow distribution of the marble and surrounding liquids by using COMSOL multiphysics software.34 Our simulation considered heat conduction and convection in this study, and employed the following heat transfer equation
Information). Further transmission electron microscopy (TEM) observation revealed that the surface of Fe 3O4 nanoparticles is homogeneously wrapped with a rimlike layer of ∼30 nm in thickness (Figure 1a). These polydopamine shells prevent Fe3O4 cores from severe aggregation to some extent, as well as provide hydrophilic groups like − OH and − NH2 for the Fe3O4@PDA nanoparticles (Figure S2, Supporting Information). The wettability of the Fe3O4@PDA nanoparticles was investigated by contact angle (CA) measurements. The nanoparticles exhibited superhydrophilicity in air (with a water CA of ∼0°) because a water droplet (3 μL) quickly spread on their surface within 1 s. The superhydrophilicity is related to the hydrophilic groups (like −OH and −NH2) existing on PDA shells. However, these nanoparticles showed high repellency to a CHCl3 droplet when they were immersed in water. They displayed an underwater CA of 156.9 ± 1° to the CHCl3 droplet (Figure 1b). The underwater superoleophobicity originates from hydrophilic PDA shells and the interstice between adjacent Fe3O4@PDA nanoparticles. Both factors allow the nanoparticles to trap a large amount of water, and thus greatly decrease the contact area between them and the oil droplet.31 To further understand the oil repellency of the nanoparticles, we measured their adhesion force to a CHCl3 droplet under water by using a microelectromechanical balance (DCAT-21, Dataphysics). An adhesion force of ∼13.91 μN was measured to the oil droplet in aqueous conditions (Figure 1c). The underwater superoleophobicity and low adhesion allow the Fe3O4@PDA nanoparticles to encapsulate an oil droplet in water without contaminating it. Then the nanoparticles were immersed in CHCl3 (or water) and irradiated with a near-infrared laser (808 nm, 200 mW) to study their photothermal performance. Figure 1d and e shows typical thermographs recorded with an infrared thermal imager (FLIR T360), which shows that temperature is highly localized around the irradiated nanoparticles. The temperature quickly increases to 42.0 °C in the initial irradiation stage and then slowly reaches 48.1 °C (Figure 1f). Similar temperature increase (47.3 °C) was also detected for the nanoparticles immersed in deionized water after the same irradiation time (Figure 1f and Supporting Information Figure S3). In contrast, the temperature of pure water and CHCl3 is increased by only 1.9 and 0.2 °C under the same conditions, respectively (Figure 1f). Although the maximal temperature was only 48.1 °C, it was still able to vaporize liquid CHCl3 because small bubbles were observed near the irradiated site (Figure S4, Supporting Information). Similar laser-induced vaporization was also found for dopamine-melanin nanospheres dispersed in deionized water.26 The vaporization indicates that the real 1275
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Figure 3. Temperature distribution of the marble after irradiation for (a) 1 s, (b) 15 s, and (c) 17 min. Flow distribution (d−f) inside the marble and (g−i) at the interface of the marble and water after irradiation for (d, g) 1 s, (e, h) 15 s, and (f, i) 17 min.
ρc p
⇀ ∂T + ρc pu ·∇T = ∇·(ki∇T ) + Q ∂t
control the volume of the “airbag” and thus the movement of the marble in water. Indeed, the marble could shuttle in the water by simply switching the laser on-demand. As mentioned above, the marble would descend in the water once the laser was turned off, indicating a decrease in the buoyant force produced by the marble. The decreased buoyant force was related to a smaller “airbag” caused by the heat transfer from the marble to the water. However, the marble restarted to ascend in the water when it was irradiated by the laser again. The irradiation time for the second transport was shortened to 1−2 s. By simply turning the laser on or off, the marble was able to shuttle in water for three cycles until its “airbag” burst (Figure 4a). The burst of the “airbag” resulted from continuous accumulation of CHCl3 vapor during the irradiation process. Then the remaining marble fell on the bottom to form a smaller marble. Likewise, the smaller marble also shuttled in water for two cycles by carefully controlling the irradiation interval. In this study, a marble with a diameter of 2 mm continuously shuttled for 6 cycles until it failed to form an “airbag” (Movie 1, Supporting Information). Interestingly, the marble could even suspend in the water by elaborately controlling the laser irradiation. To perform such a manipulation, we stopped moving the laser spot once the marble transported to a given position. As shown in Figure 4b and Movie 2 of the Supporting Information, the marble almost immediately stopped ascending but suspended in the water for about 2 s. Then it slowly moved to a higher level after
(1)
where ρ, t, cp, T, u⃗, and Q are the density, time, specific heat capacity, local temperature, velocity vector, and heat source term, respectively. ki is the thermal conductivity of liquid or gas. The simulation results show that the encapsulated layer quickly increases its local temperature to 351 K (∼78 °C) after irradiation for only one second (Figure 3a−c). The temperature almost remains unchanged during the whole irradiation process, indicating effective heat transfer from the irradiated site to surrounding water and CHCl3. Actually, a heat flow with a rate of 4.24 × 10−6 m s−1 is simulated inside the marble at the initial 1 s (Figure 3d). A fast heat flow (9.43 × 10−4 m s−1 rate) also appears at the interface of the irradiated site and water (Figure 3g). The rate of the former is slowly increased to 4.65 × 10−6 m s−1 after irradiation for 15 s, while the later quickly accelerates its rate to 2.73 × 10−3 m s−1 (Figure 3e and h). Then both flows keep their rates in the subsequent irradiation (Figure 3f and i). These heat flows effectively impede fast temperature increase of the irradiated site during the irradiation. It is reasonable that a temperature of ∼80 °C can vaporize the CHCl3 droplet. Therefore, our simulation predicts that continuous NIR laser irradiation will result in an “airbag” on the marble by vaporizing the encapsulated CHCl3 droplet, which is well consistent with the experimental results. More importantly, effective heat transfer allows us to elaborately 1276
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Figure 4. Laser controlled (a) shuttling, (b) suspending, and (c) horizontal movement of the marble in water at room temperature; (d) merging of two marbles. (1) The marble remained still on the bottom, (2) the marble tranported to the water surface after the 1st irradiation, (3) the marble fell to the bottom without irradition, (4) the marble transported to the water surface after the second irradiaiton, (5) the marble fell to the bottom without irradiation, and (6) the marble transported to the water surface after the third irradiation. Scale bar: 4 mm.
syringe (Figure 5a). Then the remaining CHCl3 formed a smaller marble after its encapsulating layer closed. Similarly, a
continuous irradiation. By focusing the laser spot on its surface, the marble was also able to suspend at the new position for 1−2 s. Notably, the marble could float on the water surface for more than 60 s before its “airbag” burst. To our knowledge, this is the first report on the suspending of a microdroplet in an aqueous environment, which has potential applications in chemical reactions and biological analysis. The suspending of the marble originated from an effective heat transfer from the irradiated nanoparticles to surrounding water and CHCl3. Such a transfer to some extent hindered fast accumulation of heat in the marble and thereafter “airbag” growth. Consequently, the “airbag” was able to keep a nearly constant volume necessary for the suspending. After continuous irradiation, heat slowly accumulated in the marble and thus the “airbag” slightly enlarged its volume. When the marble moved to a higher level, it transferred a part of heat to relatively cold surrounding water. This means that the marble needed extra irradiation to counteract the heat loss. Therefore, the marble could stay at the new position for 1−2 s under the irradiation. The explanation is well supported by the temperature and flow distribution of the marble in the irradiation process (Figure 3). It should be noted that the marble was able to horizontally move on the bottom by simply changing the irradiation angle (Figure 4c and Movie 3 of the Supporting Information). Taking advantage of the unique capability, we could merge two marbles into a bigger one under magnetic field (Figure 4d). The manipulation allows us to remotely conduct chemical reactions in aqueous conditions. Since the marble was able to float on the water surface over 60 s, it could be further manipulated through a capillary device. Here we performed the draining and filling of the marble on the water surface. After the marble was opened by a magnet bar at the water surface, its inner CHCl3 could be easily drained by a
Figure 5. (a) Draining and (b) filling of the marble near the water surface with the assistance of a magnet bar. Scale bar: 2 mm.
small volume of CCl4 (dyed red) could also be directly injected to the opened marble through the syringe (Figure 5b). After removing the magnet bar, the encapsulating layer spontaneously closed and a larger marble fell on the bottom. This feature allows us to adjust the volume and composition of a liquid marble, as well as to perform further analyses on instruments like ultraviolet (UV) or fluorescent spectrometers. Although filling or draining of a liquid marble was achieved on solid surfaces,21,23,33 to our knowledge such manipulations have rarely been conducted on the water surface. The above results 1277
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offer a facile strategy for remotely manipulating a small volume of liquids in aqueous environments, while avoiding the use of valves, pumps, or a microfluidic device.
4. CONCLUSIONS In summary, we demonstrated that a tiny CHCl3 droplet could be remotely manipulated in water at ambient temperature by encapsulating with Fe3O4@PDA nanoparticles and subsequent exposure to laser irradiation. The resulting marble ascended, shuttled, horizontally moved, and even suspended in water by simply tuning the laser irradiation. The unique movements of the marble were attributed to effective heat exchange between the irradiated nanoparticles and surrounding liquids. As a result, the volume of the “airbag”, which determined the motion of the marble in water, could be elaborately controlled. Interestingly, draining or filling of the marble was also conducted on the water surface with the assistance of a magnetic field. Because of its simplicity and effectiveness, the strategy is attractive for manipulating and monitoring a single liquid droplet, as well as for constructing smart microreactors, microengines, or microrobots in aqueous environments.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b09952. TEM images and XRD pattern of Fe3O4 nanoparticles; XPS spectra of the Fe3O4@PDA nanoparticles; infrared thermographs of the Fe3O4@PDA nanoparticles immersed in water; optical images of a bare CHCl3 droplet irradiated by the laser; effect of irradiation time, distance, and water temperature on bubble formation; GC-MS analysis results; calculation of photothermal conversion efficiency; theoretical simulations (PDF) Marble with a diameter of 2 mm continuously shuttled for 6 cycles until it failed to form an “airbag” (AVI) Suspension of marble in water by elaborately controlling the laser irradiation (AVI) Horizontal movement of marble with changing irradiation angle (AVI)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by a self-planned task of the State Key Laboratory of Robotics and System of Harbin Institute of Technology (SKLRS200901C) and National Natural Science Foundation of China (51473041). The authors thank Prof. Xiangjun Zhang and Prof. Hua Yang for the assistance of infrared thermal imager measurements.
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