Article pubs.acs.org/JPCC
Magnetically Induced Reversible Transition between Cassie and Wenzel States of Superparamagnetic Microdroplets on Highly Hydrophobic Silicon Surface Zhongjun Cheng,‡,§ Hua Lai,‡ Naiqing Zhang,†,‡ Kening Sun,*,†,‡ and Lei Jiang§ †
State Key Laboratory of Urban Water Resource and Environment, School of Municipal and Environmental Engineering and ‡Natural Science Research Center, Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin, Heilongjiang 150090, P. R. China § Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P. R. China S Supporting Information *
ABSTRACT: In this work, we report a magnetic technique for reversible wetting− dewetting transitions of microdroplets on highly hydrophobic surfaces. A superparamagnetic microdroplet can be reversibly switched between the Cassie state and the Wenzel state on a highly hydrophobic microstructured silicon substrate by the application of the magnetic field. The transition can be controlled by both the intensity of the magnetic field and the concentration of the superparamagnetic Fe3O4 nanoparticles in the microdroplet. The magnetic force needed during the transition from the Cassie state to the Wenzel state was found to be apparently smaller than that needed in the reverse process. Such asymmetry is ascribed to the higher energy of the Cassie state compared with the Wenzel state, the change of the gravitational potential energy, and the adhesion hysteresis. This report provides a novel method of dynamically controlling liquid/solid interactions, which can not only help us to understand further the transition between the Cassie state and the Wenzel state but also potentially be used in some important applications, such as lab-on-a-chip devices and chemical microreactors.
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INTRODUCTION Controllable manipulation of the liquid droplets on solid surfaces is very important in both fundamental research and practical applications,1−3 especially on the highly hydrophobic surfaces, which have attracted much attention due to their interesting and unusual surface properties.4−27 Droplets on such surfaces often reside in one of the two states, Cassie state28 or Wenzel state.29 In the Cassie state, there is a layer of air below the liquid, and the three-phase contact line is discrete, whereas in the Wenzel state, the liquid retains intimate contact with the solid surface at all points. In addition, droplets sometimes can reside in another state, the metastable composite state.8,30,31 The presence of the metastable composite state indicates that there is an energy barrier between the Cassie state and the Wenzel state, and external energy is necessary to overcome the energy barrier to realize the transition between the two states.8,30 Experimentally, many methods32−40 have been used to achieve the transition, for example, by applying a force to the droplet,8 dropping the droplet from some height,31 dragging the droplet on the rough surface,34 and so on.37−39 In the meanwhile, many theoretical modes have also been used to investigate these transitions.41−53 In common cases, the irreversible transitions as mentioned above may be just needed; nevertheless, in some special cases, such as switchable devices and bioseparation, the reversible transition54−60 between the two states is expected and has only © 2012 American Chemical Society
recently been reported. Some known examples are: by applying the pulsed electric current through a conductive substrate,56 heating the substrate,57 or electrowetting of liquid droplets in oil environments.58,59 However, all of these methods often need some severe conditions, such as oil environment and pulse electric current, and often the Cassie state is the thermodynamically favorite state while the Wenzel state is the metastable state. On this regard, a new and more convenient method that can induce the reversible transition is necessary and highly desired. Because magnetic nanomaterials have been widely used in biochemical separation,61 immunoassay,62 and targeted drug delivery,63 there is an increasing need for realization of the controlled interactions between their small volumes of liquid and solid surface. Recently, we have succeeded in manipulating superparamagnetic microdroplets on the superhydrophobic polystyrene surfaces without any leaks1 and reported a superhydrophobic iron surface with tunable adhesion for the superparamagnetic microdroplet.64 However, all of this research is focused on the adhesive property of the surface; in fact, the study about the transition of the wetting states (Cassie state and Wenzel state) of the superparamagnetic microdroplet on Received: May 22, 2012 Revised: August 15, 2012 Published: August 15, 2012 18796
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these samples were rinsed thoroughly with ultrapure water and dried with N2 airflow and then immersed in a 1.0 wt % ethanol solution of hydrolyzed heptadecafluorodecyltrimethoxysilane (FAS-17, CF3(CF2)7CH2CH2−Si(OCH3)3, shin-Etsu Chemical, Tokyo, Japan) FAS-17 for 10 h and heated to 100 °C for 1 h. Preparation of the Robust Superhydrophobic Surface. To obtain the robust superhydrophobicity, the hierarchical micro/nanostructure is necessary.5 The silicon wafer with pillar structure was prepared first with the same process as mentioned above. To endow the substrate with nanostructures, the silicon substrate was soaked in H2SO4 (97%)/H2O2 (30%) at a volume ratio of 3:1 for 30 min and then thoroughly rinsed with deionized water. Cleaned silicon substrate was put into a plastic beaker containing etching solution ([HF] 5.0 mol L−1 and [AgNO3] 0.015 mol L−1), sealed at 50 °C for 20 min, immersed in 20 wt % nitric acid for 1 min, and finally rinsed with deionized water two or three times. At last, the substrate with micro/nanostructures was obtained and immersed in a 1.0 wt % ethanol solution of FAS17 for 10 h and heated to 100 °C for 1 h to achieve the superhydrophobic property. Characterizations. A field-emission SEM apparatus (JSM6700F, Japan) was used to obtain SEM images of the silicon surfaces. The water contact angles and advancing/receding contact angles were measured using an OCA20 at ambient temperature. The advancing and receding contact angles were obtained by increasing and decreasing the volume of the microdroplet (about 15 μL) on the surface, and the flow rate of the liquid is ∼1.2 μL s−1. The structure and the crystallite sizes of the Fe3O4 nanoparticles were tested by X-ray diffractometer in the 2θ range 20−80° using Cu Kα radiation (λ = 0.15405 nm). The type of X-ray diffractometer is Shimadzu, Tokyo, Japan. The crystallite sizes are calculated using Scherrer’s relationship D = kλ/B cos θ, where D is the average diameter in nanometers, k is the shape factor, B is the broadening of the diffraction line measured half of its maximum intensity in radians, λ is the wavelength of X-ray, and θ is the Bragg’s diffraction angle. The crystallite sizes of the samples are estimated from the line width of the (311) XRD peaks. Hysteresis loops of the Fe3O4 nanoparticles were obtained by using a vibrating sample magnetometer (VSM) (Digital Measurement System JDM-13) with a maximum magnetic field of 1 T. The values of the magnetic field intensity (H) used to attract the microdroplet were obtained by using a Gauss meter (Digital Measurement System SG-3-A). The optical image of the substrate was investigated on an optical microscope (LW-200-4JS). Control of the Reversible Transition. The highly hydrophobic silicon substrate was fixed on a bracket, and the surface was horizontal (Scheme 1), above which was positioned a special surface with robust superhydrophobicity and low adhesion. (This surface was used to assist the transition; for more detail see Figures S1−S3 of the Supporting Information.) Two bar magnets (with the maximal intensity of about 0.6 T) were fixed on the motors that were placed above and under the substrate, respectively; the motor was controlled by a computer, and thus the magnet could be moved up and down freely under the control of the computer. The intensity of the magnetic field action on the microdroplet (H) was probed by a Gauss meter, and such intensity can be controlled by changing the distance between the magnet and the microdroplet.
the solid surface is also important because it is the wetting states that can determine the static and dynamic properties of the microdroplet simultaneously, and up to now, the correlative reports are extremely rare. Herein we report a novel method to control the transition of a liquid droplet between the Cassie state and the Wenzel state. Noticeably, the Cassie state is the metastable state, whereas the Wenzel state is the thermodynamically favorite state. By switching the direction of the magnetic field intensity, reversible transition of a superparamagnetic microdroplet between the Cassie and Wenzel states on the highly hydrophobic silicon surface can be realized, and such transition can be controlled by both the intensity of the magnetic field and the concentration of the superparamagnetic Fe3O4 nanoparticles in the microdroplet. The magnetic force needed during the transition from the Cassie state to the Wenzel state is found to be apparently smaller than that needed in the reverse process, which is ascribed to the higher energy of the Cassie state compared with the Wenzel state, the change of the gravitational potential energy, and the adhesion hysteresis. This approach provides us a new platform for convenient control of microdroplet on solid surfaces and would be potential used in many fields such as microreactors, switchable lenses, and lab-on-a-chip devices.
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EXPERIMENTAL SECTION Preparation of the Superparamagnetic Microdroplet. The superparamagnetic microdroplet was composed of the aqueous solution of magnetite nanoparticles, which were prepared by a similar method to the report.65 In brief, purified, deoxygenated water (25 mL, by nitrogen gas bubbling for 30 min) was prepared first, and FeCl3 (5.2 g) and FeCl2 (2.0 g) were successively dissolved in the solution with stirring. The resulting solution was added dropwise into NaOH solution (250 mL, 1.5 M) under vigorous stirring. The last step generated an instant black precipitate. The precipitate was isolated in the magnetic field, and the supernatant was removed from the precipitate by decantation. Purified deoxygenated water was added to the precipitate, and the solution was decanted after centrifugation at 4000 rpm. After repeating the last procedure three times, HCl solution (500 mL, 0.01 M) was added to the precipitate (with stirring) to neutralize the anionic charges on the nanoparticles. The cationic colloidal nanoparticles were again separated by centrifugation (4000 rpm) and peptized by adding water. Preparation of the Highly Hydrophobic Silicon Substrate. Contact lithographic masks were constructed by Microelectronics R&D Center, Chinese Academy of Sciences. The instrument KARL SUSS MA6 (Germany) was used to transfer the patterns of masks onto silicon wafers by photolithography method. The deep etching process was completed by a STS ICP ASE (U.K.) instrument. Thus rough surfaces of a flat silicon wafer were prepared on which geometrical structures of a patterned square pillars (30 μm high, 20 μm width, and with a spacing of 20 μm between pillars) were introduced. The as-prepared structured silicon wafer was first rinsed though the ultrasonication in acetone, ethanol, and ultrapure water for 20 min, respectively. Then, the samples were dried with N2 airflow. To obtain the highly hydrophobic surface, the low free surface energy is necessary. These samples were first submerged in a freshly prepared mixture of H2SO4 (98%) and H2O2 (30%) at a volume ratio of 3:1, and the solution was heated to 80 °C for ∼10 min to endowed the surface with fresh hydroxyl groups. Afterward, 18797
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Scheme 1. Schematic Illustration of the Experimental Device
Figure 1. (a) SEM images of the regular arrays of silicon (left) compared with that of flat substrate (right). (b) Schematic illustration of the pillar structures. a (20 μm) represents the width of a pillar, b (20 μm) is the spacing between the pillars, and h (30 μm) means the height of a pillar. The insert is the shape of a superparamagnetic microdroplet (4 μL) on the flat silicon substrate coated with FAS with a contact angle of ∼107°.
When a superparamagnetic microdroplet (4 μL, the concentration of the Fe3O4 nanoparticles is ∼1.98 × 1020 nanoparticles mL−1) is placed on the highly hydrophobic silicon surface, the contact angle is ∼145°, which agrees well with the value derived from the Cassie model (Figure 2a).
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RESULTS AND DISCUSSION As reported, liquid droplets can have multiple wetting states on the hydrophobic surfaces.8 For a moderate hydrophobic surface, the apparent contact angle should be given by the Wenzel model, and is expressed by the following equation cos θ W = r cos θi (1) where r is defined as the ratio between the actual area of the rough surface and the geometric projected area and the θi is the intrinsic contact angle of liquid on the flat surface. As for the substrate either very hydrophobic or very rough, the apparent contact angle should be given by the Cassie model and can be expressed by the eq 2 cos θC = fs cos θi + fs − 1
(2)
where fs is the Cassie roughness factor, defined as the fraction of the solid/liquid interface below the liquid droplet (here fs is dimensionless and smaller than the unity) and (1 − fs) is the fraction of the liquid/gas interface at the same position. However, for some special surfaces, liquid drops are observed to be in the Cassie state, although they should be in the Wenzel state in theory. This suggests that the two states (Cassie state and Wenzel state) might coexist, and the Cassie state on such surface should be the metastable Cassie state.8,30,31 In this experiment, regular pillar structured silicon coated with hydrophobic fluoroalkylsilane (FAS) was used as the substrate. Figure 1a shows the scanning electron microscopy (SEM) image of the microstructures on the substrate, on which well-ordered arrays of pillars are observed with the width a, spacing b, and the height h of 20 μm, 20 μm, and 30 μm, respectively (Figure 1b). As for this surface, the structural parameters66 fs = a2/(a + b)2 = 0.25, r = 1 + 4ah/(a + b)2= 2.5, and the contact angle of a superparamagnetic microdroplet (4 μL, the aqueous solution composed of Fe3O4 nanoparticles, more detailed see Figures S4 to S5 of the Supporting Information) on the flat surface of silicon coated with FAS is θi = 107° (Figure 1a, insert). Therefore, according to eqs 1 and 2, the contact angles derived from the Wenzel model and the Cassie model are 137 and 145°, respectively.
Figure 2. Reversible transition between the Cassie state and the Wenzel state: the superparamagnetic microdroplet resides in the Cassie state (a) and the Wenzel state (b) with the contact angles of about 145 and 135°, respectively. (c,d) The amplificatory photos of the interface between the liquid and the substrate, showing the presence and absence of air, respectively. (e,f) Schematic illustration of the two wetting states: Cassie state and Wenzel state. A microdroplet can roll on the surface (h) while be pinned (i) when it resides in the Cassie state and Wenzel state, respectively. Reversible transition between the Cassie state and the Wenzel state can be achieved by application of the magnetic fields.
Meanwhile, it can roll easily on the surface with a sliding angle of ∼18° (Figure 2h), indicating that the microdroplet resides in the low adhesive Cassie state. In addition, such low adhesive Cassie state can be further proved more directly from the amplificatory photo of the interface between the liquid and the substrate (Figure 2c), and it can be seen that the light can pass through the space between the droplet and the substrate, indicating that there is a layer of air present in the grooves between the pillars (Figure 2e). When a magnet (the maximal intensity H is ∼0.6 T) under the substrate is used to attract the microdroplet, the microdroplet can transit to another state with a contact angle of ∼135° (Figure 2b). This value is approximate 18798
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with the theoretical value derived from the Wenzel model (137°). Meanwhile, the microdroplet cannot roll and be pinned on the surface even when the surface is turned upside down (Figure 2i), which means that the microdroplet resides in the high adhesive Wenzel state (Figure 2f), which can be further confirmed by the disappearance of the air between the liquid and the substrate (Figure 2d). When another magnet with the same intensity upon the microdroplet is used to attract the microdroplet, the microdroplet can fly upward and at last return to its initial Cassie state with a contact angle of ∼145°. In a word, by the application of magnetic field, the superparamagnetic microdroplet can be reversibly switched between the Cassie state and the Wenzel state on the highly hydrophobic silicon surface. In this work, we also investigated the contact angle hysteresis for microdroplet on the surface, and we believe that the different dynamic properties (Figure 2h,i) of the microdroplet in different states are mainly ascribed to the different contact angle hysteresis. It can be seen from Table 1 (for more detail, see Figure S6 of the Supporting Table 1. Advancing/Receding Contact Angles (θa/θr) and the Contact Angle Hysteresis (Δθ) for the SuperParamagnetic Microdroplet on the Flat and Micropillar Substrates Modified with FAS, Respectively
Figure 3. Process of the reversible transition of a superparamagnetic microdroplet between the Cassie state and the Wenzel state. (a) A superparamagnetic microdroplet was placed on the silicon surface and resided in the Cassie state, above which was positioned a special surface with robust superhydrophobic and ultralow adhesive properties (more detailed see Figures S1−S3 of the Supporting Information). (b) When a magnet under the surface was used (Magnet B On), the microdroplet was attracted downward and transited to the Wenzel state. (c) The microdroplet still stated in the Wenzel state even though the magnet was removed (Magnet B Off). (d) When another magnet upon the upper surface was used (Magnet A On), the microdroplet was attracted to fly upward. (e) The microdroplet left the silicon surface and adhered to upper surface under the attraction of the magnetic field (Magnet A On). (f) The microdroplet returned to its initial state as a result of the gravity of the microdroplet when the magnetic field is removed (Magnet A Off). Inserts are the corresponding contact angles of the microdroplet on the superhydrophobic silicon surface (the under surface).
micropillar substrate flat substrate
Cassie state
Wenzel state
θa = 111° θr = 91° Δθ = 20°
θa = 153° θr = 142° Δθ = 11°
θa = 144° θr = 83° Δθ = 61°
Information) that the contact angle hysteresis Δθ = θa − θr for the microdroplet in the Cassie state (∼11°) is much smaller than the Δθ in the Wenzel state (∼61°). Furthermore, one can find that compared with the flat substrate the microstructure on the substrate can lead an opposite behavior of the Δθ in the two states, decreases in the Cassie state, and increases in the Wenzel state, which is in good agreement with the data reported by Demirel et al.67 To elucidate further the dynamic process of the transition, video recording of the transition process was performed (Figure 3). First, a superparamagnetic microdroplet (4 μL) was placed on the silicon surface and resided in the Cassie state (Figure 3a), above which was positioned a special surface with robust superhydrophobicity (on which the liquid droplet can always reside in the low adhesive state, for more detail, see Figures S1−S3 of the Supporting Information). When a magnet under the surface was used to attract the microdroplet (Figure 3b, Magnet B On), the microdroplet distorted from a ball to an ellipse as a function of the magnetic force. Meanwhile, the length of three-phase contact line increased apparently with the liquid penetrating in the interspaces between the pillars on the silicon substrate. As the microdroplet in the interspaces contacted the bottom and air was excluded, the transition from the Cassie state to the Wenzel state was realized. Then, the microdroplet still stayed in this state even though the magnet was removed (Figure 3c, Magnet B Off). Subsequently, another magnet upon the upper surface was used, and the microdroplet was attracted and flew upward (Figure 3d, Magnet A On). In this case, the length of three-phase contact line greatly decreased compared with the Wenzel state. With increasing the intensity of the magnetic field (this can be achieved by decreasing the distance between the magnet and
the substrate), the microdroplet would leave the silicon surface and be adhered to the upper surface as a result of the magnetic force (Figure 3e, Magnet A On); in the meanwhile, the microdroplet would distort and present as an ellipse. At last, when the magnet was removed (Magnet A Off), because of the robust superhydrophobicity and ultralow adhesion of the upper surface, the microdroplet would leave the upper surface and return to the initial Cassie state on the silicon surface as a result of the gravity. It is worthy of noting that no apparent loss of liquid was observed during the transition, and the substrate remains clean after a whole transition. (See Figure S7 of the Supporting Information.) In the meanwhile, we also recorded the contact angles of the microdroplet on the silicon surface during the transition process (inserts in Figure 3), from which we can find that different from the dewetting process (Figure 3c−e) the contact angle decreased monotonically until the microdroplet left the surface. During the wetting process (Figure 3a−c), the contact angle increased first from 145 to 158°, and then decreased to 135°, which maybe ascribe to the presence of the energy barrier and will be discussed in the latter sections. We emphasize that in this work, the upper surface must be robust superhydrophobic and ultralow adhesive, on which the superparamagnetic microdroplet cannot transit to the Wenzel state after the attraction by the magnetic field and can restore to its initial superhydrophobic low adhesive state when 18799
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the magnetic field is removed. If the upper surface without robust superhydrophobicity or is high adhesive, after attraction by the magnet upon the upper surface, the microdroplet may transit to the high adhesive Wenzel state and be pinned on the upper surface, thus resulting in the unsuccessful transition of the microdroplet. To obtain a better understanding of the transition process, we investigated separately the transition from the Cassie state to the Wenzel state and the reverse process. When the intensity of external magnetic field is constant (H = 0.54 T), different extent of the transition can be observed on microdroplets with different concentration (N) of magnetic Fe3O4 nanoparticles. (Figure 4) When the concentration is too low (N < 7.81 × 1019
deduce from the above formula that when the intensity of the magnetic field (H) is fixed, the magnetic force (F) is increased as the concentration (N) of magnetic Fe3O4 nanoparticles is increased. As mentioned above, when the intensity of the external magnetic field is constant, compared with the transition from the Cassie state to the Wenzel state, to realize the reverse process, the transition from the Wenzel state to the Cassie state, the microdroplet with higher concentration of Fe3O4 nanoparticles should be used. Therefore, it can be deduced that the magnetic force needed in the transition from the Cassie state to the Wenzel state would be smaller than that needed in the reverse process. The magnetic force is the only external energy provided to the system; consequently, it can be concluded that smaller energy would be needed for the transition from the Cassie state to the Wenzel state than that needed in the reverse process, and such difference may be ascribed to the following reasons; the first is different energy states for the Cassie state and the Wenzel state, respectively, the second is the variation of the gravitational potential energy, and the last is the adhesion hysteresis. In this experiment, the volume of the microdroplet (4 μL) is relatively small, the radius of the droplet is smaller than the capillary length, the effect of the gravity that influences the shape of the microdroplet can be negligible, and the microdroplet can be seen as a spherical cap.8 Therefore, the energy of the droplet (G) on the surface in equilibrium can be expressed by the following equation70,71 G=
9π V 2/3γLV(1 − cos θr)2/3 (2 + cos θr)1/3
(4)
where γLV and V are the surface tension coefficient and the volume respectively for the droplet and θr represents the apparent contact angle of the droplet on the surface. When the microdroplet is in the Cassie state, θr = θc, similarly, when the microdroplet is in the Wenzel state, θr = θw. From eq 4, it can be easily verified that the energy G is increased monotonically as θr (0° ⩽ θr ⩽ 180°) is increased. Herein, the contact angles of the microdroplet in the Cassie state and the Wenzel state are 145 and 135°, that is to say θc > θw; therefore, the energy of the microdroplet in the Cassie state (Gc) is larger than that in the Wenzel state (Gw), which means that the Wenzel state is the thermodynamically favorite state, whereas the Cassie state is the metastable state. As mentioned in Figure 3, the contact angle increased first and then decreased when the microdroplet transited from the Cassie state to the Wenzel state, and this is due to the presence of the energy barrier. To realize the transition from the Cassie state to the Wenzel state, the energy barrier must be conquered, and the microdroplet has to go through a higher intermediate energy state (defined as Gint, and Gint > Gc > Gw); according to the above equation, the contact angle for the intermediate energy state (defined as θint) should be θint > θc > θw. Therefore, it would be intelligible why the contact angle first increased from 145 to 158° and then decreased to 135°.70 As mentioned above, the energy needed in the transition from the Cassie state to the Wenzel state is smaller than that needed in the reverse process, and this maybe ascribe to the following reasons. First, because the energy for the droplet in the Cassie state is larger than that in the Wenzel state (Gc > Gw), during the wetting process, the variation of the microdroplet is from the high energy state to the low energy state, and the transition from the Cassie state to the Wenzel state would be realized just when the external energy is larger
Figure 4. Schematic illustration of different extent of the transition can be realized for the microdroplet with different concentration of magnetic nanoparticles under the constant external magnetic field intensity (H = 0.54 T).
particles mL−1), there no apparent transition can be observed. With increasing the concentration (7.81 × 1019 particles mL−1 < N < 9.94 × 1019 particles mL−1), the microdroplet can transit from the Cassie state to the Wenzel state but is irreversible. Only when the concentration is large enough (N > 9.94 × 1019 particles mL−1) can the transition between the Cassie state and the Wenzel state be reversible. From the above, it can be observed that when the intensity of the external magnetic field is constant, compared with the transition from the Cassie state to the Wenzel state, the transition from the Wenzel state to the Cassie state can be realized only when the microdroplet with higher concentration of Fe3O4 nanoparticles is used, indicating that the transition between wetting and dewetting is an asymmetry process, and different magnetic forces are needed for the wetting and dewetting processes, respectively. Magnetic force is a pivotal factor during the transition for it provides the energy to realize the transition, which results from magnetic field gradients (gradB) acting on magnetic dipole moments, and can be described as the following formula68 F = NSμB gradB
3
(3)
where S and N are the number of the Bohr magnetons (μB) and superparamagnetic nanoparticles per microdroplet, respectively, and gradB is the magnetic field gradients. The magnetic force can be described as F ∝ NH (where H ∝ gradB).69 It is easy to 18800
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CONCLUSIONS In summary, a new method of dynamically magnetic control of the liquid/solid interactions on hydrophobic surface is proposed and experimentally demonstrated. This approach allows the superparamagnetic microdroplets reversible transition between the Cassie state and the Wenzel state without observable loss of the liquid by switching the direction of the magnetic field intensity. The magnetic force needed in the transition from the Cassie state to the Wenzel state is obviously smaller than that needed in the reverse process, and this is ascribed to the higher energy of the Cassie state compared with the Wenzel state, the variation of the gravitational potential energy, and the adhesion hysteresis. This novel method for reversible controlling the microdroplet between wetting and nonwetting states on a highly hydrophobic surface could potentially be used in a wide range of applications, such as droplet-based microfluidics, chemical microreactors, and switchable devices.
than the energy barrier. While in the process of dewetting, besides the energy barrier, the transition is from the low energy state to the high energy state, and the whole energy of the system is increased; thus, compared with the wetting process, more energy would be consumed. Second, even though the gravity dose not change the spherical shape of the droplet, the scale of the change of the gravitational potential energy during the transition is at least mg V1/3 (m is the mass of the droplet, g is the gravitational acceleration) and should not be neglected.69 In this work, θc > θw, the center of the mass of the droplet in the Wenzel state would be lower than that in the Cassie state; thus, during the process from the Cassie state to the Wenzel state, all or part of the gravitational potential energy may be available to overcome the energy barrier, whereas in the reverse process, external energy would be needed to overcome the increase in the gravitational potential energy. Third, different from the wetting process, during the dewetting process, some solid/liquid interface needed to be destroyed as a result of the adhesion hysteresis, which also would consume some energy. Therefore, it can be concluded that more energy needed in the dewetting process than that needed in the wetting process is mainly due to the higher energy of the Cassie state compared with the Wenzel state, the variation of the gravitational potential energy, and the adhesion hysteresis. For the substrate with the fixed microstructures, the minimal energy in the reversible transition would be constant, and accordingly, the minimal magnetic force would be constant. Therefore, the intensity of the minimum external magnetic field would be dependent on the concentration of Fe3O4 nanoparticles N, and as shown in Figure 5, when N increased, the minimum H gradually decreased.
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ASSOCIATED CONTENT
S Supporting Information *
SEM images, the photos of water contact angles, and the sliding angle on the surface with robust superhydrophobicity. XRD and magnetization curve of the magnetic nanoparticles. Advancing and receding contact angles for the microdroplet on the flat and micropillar silicon substrate. The optical photo of the substrate after the transition. This material is available free of charge via the Internet at http://pubs.acs.org.
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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 is supported by the Open Project of State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (no. ES201007); Project (HIT. NSRIF. 2009079) Supported by Natural Scientific Research Innovation Foundation in Harbin Institute of Technology; China Postdoctoral Science Foundation (20100471045) and (201104428); The Research Fund for the Doctoral Program of Higher Education (20112302120062).
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Figure 5. Dependence of the minimal intensity of external magnetic fields on the number of the superparamagnetic nanoparticles per microdroplet.
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In the system, the microdroplet must be superparamagnetic, meaning that it is only magnetic under a very strong applied magnetic field but retains no permanent magnetism once the magnetic field is removed. (See Figure S5 in the Supporting Information.) It is crucial for the controlled manipulation. If the nanoparticles are not superparamagnetic, such as ferromagnetic, then after action in the magnetic field, the particles may congregate for the presence of remanence and separate with the droplet, resulting in the unsuccessful control of the transitions. 18801
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The Journal of Physical Chemistry C
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dx.doi.org/10.1021/jp304965j | J. Phys. Chem. C 2012, 116, 18796−18802
