Langmuir 2008, 24, 12119-12122
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New Investigations on Ferrofluidics: Ferrofluidic Marbles and Magnetic-Field-Driven Drops on Superhydrophobic Surfaces Edward Bormashenko,* Roman Pogreb, Yelena Bormashenko, Albina Musin, and Tamir Stein Ariel UniVersity Center of Samaria, The Research Institute, Ariel 40700, Israel ReceiVed July 22, 2008. ReVised Manuscript ReceiVed September 25, 2008 The motion of ferrofluidic marbles on flat polymer substrates is reported. Nanopowders of polyvinylidene fluoride and γFe2O3 were used for the preparation of ferrofluidic marbles. The marbles are activated easily with an external magnetic field. A microfluidic device based on ferrofluidic marbles (the ferrofluidic bearing) is described. Velocities of marbles as high as 25 ( 3 cm/s were registered. The sliding of ferrofluidic drops on superhydrophobic surfaces was studied. It was demonstrated that the threshold magnetic force necessary for the drop displacement depends linearly on the drop radius, thus the motion of the drop is defined by the processes occurring in the vicinity of the triple line only.
1. Introduction The precise transport of small quantities of liquids has been exposed to intensive research during the past decade because of its importance in biomedical applications. In particular, microfluidic devices were used successfully for DNA amplification and biological cell manipulation.1,2 One of the most promising approaches allowing the precise transport of drops is based on the use of the so-called nonstick-lotus-inspired surfaces.3-13 The alternative method is based on nonstick drops or so-called marbles.3,14-16 The marble is the liquid drop encapsulated with a hydrophobic powder. Such an encapsulated drop behaves like a soft solid and demonstrates a low adhesion to the substrate.14 Our paper deals with both approaches (i.e., with the motion of marbles and the sliding of drops on superhydrophobic surfaces). It is noteworthy that the marble-based approach for the precise transportation of drops is much less developed than the lotuseffect-inspired approach. The question is how the drop motion could be activated. Krupenkin and McHale used a well-known electrowetting effect for the activation of drops.16,17 We demonstrate in this letter that ferrofluid marbles and drops could be activated with a magnetic field. The displacement of ferrofluidic drops has been intensively * Corresponding author. E-mail:
[email protected]. (1) Zhang, Ch.; Xu, J.; Ma, W.; Zheng, W. Biotechnol. AdV. 2006, 24, 243– 284. (2) Yi, Ch.; Li, Ch.-W.; Ji, Sh.; Yang, M. Anal. Chim. Acta 2006, 560, 1–23. (3) de Gennes, P. G.; Brochard-Wyart, F.; Que´re´, D. Capillarity and Wetting Phenomena; Springer: Berlin, 2003. (4) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1–8. (5) Shibuichi, A.; Onda, T.; Satoh, N.; Tsujii, K. J. Phys. Chem. 1996, 100, 19512–19517. (6) Que´re´, D. Rep. Prog. Phys. 2005, 68, 2495–2532. (7) Que´re´, D.; Reyssat, M. Phil. Trans. R. Soc., Sect. A 2008, 366, 1539–1556. (8) Nosonovsky, M.; Bhushan, B. AdV. Funct. Mater. 2008, 18, 843–855. (9) Nosonovsky, M.; Bhushan, B. J. Phys.: Condens. Matter 2008, 20, 225009. (10) Roach, P.; Shirtcliffe, N. J.; Newton, M. I. Soft Matter 2008, 2, 224–240. (11) Bormashenko, Ed.; Stein, T.; Whyman, G.; Bormashenko, Ye.; Pogreb, R. Langmuir 2006, 22, 9982–9985. (12) Bormashenko, Ed.; Bormashenko, Ye.; Stein, T.; Whyman, G.; Bormashenko, E. J. Colloid Interface Sci. 2007, 311, 212–216. (13) Patankar, N. A. Langmuir 2004, 20, 7097–7102. (14) Aussillous, P.; Que´re´, D. Nature 2001, 411, 924–927. (15) Mahadevan, L. Nature 2001, 411, 895–896. (16) McHale, G.; Herbertson, D. L.; Elliott, S. J.; Shirtcliffe, N. J.; Newton, M. I. Langmuir 2007, 23, 918–924. (17) Krupenkin, T. N.; Taylor, J. A.; Schneider, T. M.; Yang, S. Langmuir 2004, 20, 3824–3827.
researched, and it was shown that such drops could be effectively activated and displaced with a magnetic field.18-23 However, very few reports relate to the sliding of ferrofluidic drops on superhydrophobic surfaces.20,22,23 We demonstrate in our paper that ferrofluid marbles could be effectively activated by a magnetic field.
2. Experimental Section 2.1. Materials. To manufacture marbles, we used polyvinylidene fluoride (PVDF) nanobeads supplied by Aldrich; the molecular weight Mw ) 534 000, Tg ) 38.0 °C, and the density was 1.74 g/cm.3 The average diameter of PVDF particles was established as 130 nm with SEM imaging. Marbles were deposited on extruded polyethylene (PE) and polypropylene (PP) substrates. Ferrofluidic properties were supplied to drops by the introduction of the γ-modification of Fe2O3 nanoparticles, obtained by the hydrolysis of bivalent and trivalent iron followed by precipitation and drying. The average diameter of Fe2O3 particles was established as 12-15 nm (as established with TEM), the density was 5.24 g/cm,3 and the specific surface of nanoparticles was 300 m2/g (as established with the nitrogen absorption (BET) technique). 2.2. Manufacturing of Superhydrophobic Surfaces. Superhydrophobic surfaces were manufactured in a way similar to that previously reported by the authors.11 Polytetrafluoroethylene (PTFE) 100-200 nm powder was spread on a polymethyl methacrylate (PMMA) substrate and pressed with a rifled stamp.11 The PMMA substrate had been softened under the pressing and trapped PTFE particles (which remained solid under the pressing temperature). Hot pressing has been carried out under t ) 95 °C. The SEM image of the superhydrophobic surface is presented in Figure 1A. Drops and marbles were in contact with PTFE beads and air trapped by the surface.11 (18) Tadmor, R.; Rosensweig, R. E.; Frey, J.; Klein, J. Langmuir 2000, 16, 9117–9120. (19) Lehmann, U.; Hadjidj, S.; Parashar, V. K.; Vandevyver, C.; Rida, A.; Gijs, M. A. M. Sens. Actuators, B 2006, 117, 457–463 (magnetic). (20) Garcı´a, A. A.; Egatz-Go´mez, A.; Lindsay, S. A.; Domı´nguez-Garcı´a, P.; Melle, S.; Marquez, M.; Rubio, M. A.; Picraux, S. T.; Yang, D.; Aella, D. P.; Hayes, M. A.; Gust, D.; Loyprasert, S.; Vazquez-Alvarez, T.; Wang, J. J. Magn. Magn. Mater. 2007, 311, 238–243. (21) Egatz-Go´mez, A.; Melle, S.; Garcı´a, A. A.; Lindsay, S. A.; Marquez, M.; Domı´nguez-Garcı´a, P.; Rubio, M. A.; Picraux, S. T.; Taraci, J. L.; Clement, T.; Yang, D.; Hayes, M. A.; Gust, D. Appl. Phys. Lett. 2006, 89, 034106. (22) Hong, X.; Gao, X.; Jiang, L. J. Am. Chem. Soc. 2007, 129, 1478–1479. (23) Guo, Zh.-G.; Zhou, F.; Hao, J.-Ch.; Liang, Y.-M.; Liu, W.-M.; Huck, W. T. S. Appl. Phys. Lett. 2006, 89, 081911.
10.1021/la802355y CCC: $40.75 2008 American Chemical Society Published on Web 10/07/2008
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Figure 2. (A) Ferrofluidic marble (V ) 20 µL) deposited on the flat polyethylene substrate. (B) Scheme of the ferrofluidic marble. Figure 1. (A) Superhydrophobic surface used in the investigation. The scale bar is 4 µm. (B) Scheme of the experiment measuring threshold parameters for ferrofluidic drop motion.
2.3. Preparing Ferrofluidic Marbles. Ferrofluidic marbles were prepared in a two-stage procedure. In the first stage, a dispersion of solid Fe2O3 nanoparticles in water was prepared. The mixture of Fe2O3 with distilled water was placed in a sonicator. As a result of ultrasonic treatment for 30 min, the dispersion of Fe2O3 nanoparticles was formed. The concentration of magnetic powder in marbles was 2.5-25 g/L. Then 5-70 µL drops were deposited with a precise microdosing syringe on the superhydrophobic surface described in paragraph 2.2 and covered with a layer of PVDF beads. Slight tilting of the superhydrophobic surface caused rolling of the drop and coating with PVDF beads. Afterwards, ferrofluidic marbles were rolled to polyethylene substrates. 2.4. Magnetic Activation of Ferrofluidic Droplets. Magnetic activation of droplets was carried out with a 3/4-in.-diameter permanent neodymium magnet on an iron base. Measurements of magnetic field were carried out with magnetic field sensor Pasco Cl-6520A. The establishment of threshold parameters necessary for drop motion on a superhydrophobic surface was performed as follows (Figure 1B). The dispersion of solid Fe2O3 nanoparticles in water was prepared and placed in a sonicator bath. The dispersion was not stable, so the samples were kept in the sonicator bath until the experiment. Droplets of various volumes in the range of 20 to 200 µL and with a known mass of Fe2O3 were placed on the superhydrophobic surface. The permanent magnet moved constantly with a stable low velocity (about 1 to 2 cm/s) toward the droplet until it began to slide toward the magnet. Threshold magnetic fields sufficient to move the droplets were measured.The velocity of marbles and drops was established with a CCD camera in the first centimeter of their total 2-2.5 cm displacement. 2.5. Measurements of Contact Angles. The contact angles of marbles and drops were measured with a homemade goniometer and an image-processing technique. A horizontal laser beam illuminated the drop profile and produced its enlarged image on the screen using a system of lenses. Measurements were made on both sides of the drop and averaged. The contact angles established for drops and marbles are so-called apparent contact angles.3
3. Results and Discussion 3.1. Motion of Ferrofluidic Marbles. Spontaneous coating of ferrofluidic drops with PVDF nanobeads gives rise to liquid
marbles such as those depicted in Figure 2A. (The scheme of the marble is presented in Figure 2B.) Marbles deposited on polyethylene and polypropylene substrates have apparent contact angles as high as 145° as depicted in Figure 2A. Marbles with volumes of Fth, the drop begins to slide on the superhydrophobic surface depicted in Figure 1A. It is reasonable to suggest that the magnetic force F acting on the drop depends on two factors (i.e., the external magnetic field B exerted on the drop and the
mass of magnetic powder in the drop m20,21). We assumed that these dependencies are linear: F ≈ mB. It is also clear that the ferrofluidic drop starts to slide when the condition Fth ) Fpin is fulfilled, where Fpin is the frictional force pinning the droplet to the superhydrophobic surface. The dependence of the threshold values of mB (which is proportional to the threshold value of magnetic force) on the radius of the drop is presented in Figure 5. The radius of the drop R could be calculated according to the well-known equation
R )
(
3V π(1 - cos θ)2(2 + cos θ)
)
1⁄3
(1)
where V is the volume of the drop and θ is the apparent contact angle. It is readily seen from Figure 5 that starting from a certain radius the dependence is strictly linear. This fact allows an
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Figure 6. Scheme illustrating the relationship between the radius of the drop R and the radius of the contact area r.
important scaling interpretation. Indeed, it means that Fpin is also proportional to R, Fpin ∝ R. Now let us take into account that the radius of the drop R and the radius of the contact area r are related to one another according to r ) R sin θ (see Figure 6). Thus, it is clear that when θ ) const, r ∝ R and, consequently, Fth ∝ r. This means that the pinning force is proportional to the perimeter of the triple line and not to the surface of the contact area. Thus, we conclude that the sliding of the ferrofluidic drop occurs when the critical value of the force pinning the triple line is exceeded, as already suggested by the authors,24,25 hence the sliding of the drop looks like a 1D and not a 2D affair. It is also seen from Figure 5 that the sliding of small drops with radius R < 2.25 mm is governed by other laws when compared to large drops. We relate this experimental finding to the dependence of the contact angle of small drops on their radius, which has already been discussed in our recent paper.26 (24) Bormashenko, E.; Pogreb, R.; Whyman, G.; Erlich, M. Langmuir 2007, 23, 6501–6503. (25) Bormashenko, E.; Pogreb, R.; Whyman, G.; Erlich, M. Langmuir 2007, 23, 12217–12221. (26) Bormashenko, E.; Bormashenko, Y.; Stein, T.; Whyman, G.; Pogreb, R.; Barkay, Z. Langmuir 2007, 23, 4378–4382.
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Indeed, when a drop radius becomes small, the Laplace pressure under the droplet surface grows according to P ≈ 2γ/R and promotes the penetration of water into depressions of the relief and consequently leads to the so-called Cassie-Wenzel wetting transition.24-26 Actually, we observed a decrease in the apparent contact angle for small drops on the presented superhydrophobic surfaces. Thus, it could be concluded that the Cassie-Wenzel transition has to be taken into account when the sliding of drops on rough surfaces is under consideration.
4. Conclusions Ferrofluidic marbles reported in this letter demonstrate promising technological applications. The marbles are obtained with the use of polyvinylidene fluoride and γFe2O3 nanopowder. Ferrofluidic marbles activated by a magnetic field develop velocities of up to 25 cm/s. The operation of microfluidic device based on the marbles is presented, which is a low-friction bearing activated with an external magnetic field. The sliding of ferrofluidic droplets on the superhydrophobic surfaces is investigated. The mechanism of ferrofluidic droplet motion is discussed. Acknowledgment. We are grateful to M. Zinigrad for his continuous support of our research and Professor M.V. Panchagnula and Dr. G. Whyman for fruitful discussions. We also thank Dr. S. Sutovsky and Dr. V. Boiko for preparing the Fe2O3 nanoparticles. This work was supported by the Israel Ministry of Absorption. LA802355Y
