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Magnetic Liquid Marbles: Manipulation of LiquidDroplets Using Highly Hydrophobic Fe3O4

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By Yan Zhao, Jian Fang, Hongxia Wang, Xungai Wang, and Tong Lin*

[*] Dr. T. Lin, Dr. Y. Zhao, Dr. J. Fang, H. Wang, Prof. X. WangCentre for Material and Fibre InnovationDeakin University, Geelong, VIC 3217 (Australia)E-mail: tong.lin@deakin.edu.au

DOI: 10.1002/adma.200902512

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Controlled manipulation of small volumes of liquid, either flowor droplet, is extremely important in miniature systems forchemical and biological applications, such as many microarraysfor high-throughput analyses and purifications.[1–4] Techniques toachieve this are essentially based onmicrochannel fluidic devices,which have been explored extensively for decades.

In contrast with the microchannel-based fluidics, 2D manip-ulation of discrete droplets without usingmicrofluidic channels isa new field.[5] Without being confined to a closed channelnetwork, the droplets have additional degrees of freedom to travelin different directions, making a single device flexible for diversereaction designs and applications. In addition, the problemsassociated with adsorption on the channel walls are eliminated.So far, there have been several reports on actuating liquid dropletswith an external electric field,[6] magnetic field,[7] or acousticaction[8] and the potential applications of these techniques in‘‘lab-on-a-chip’’ DNA purifications have been demonstrated.[7]

However, an immiscible liquid has to be used in these systems asa host medium for the droplets so that the droplet–substrateadhesion is eliminated and, as a result, the droplets tend to have aspherical shape in the medium.

More recently, the development of ‘‘nonstick’’ liquid/solidinterfaces has led to simpler and more promising strategies tomanipulate liquid droplets with reduced adhesion and otheradvantages.[9–12] Since no immiscible liquid phase is involved, thedroplets move under no extra fluid–fluid resistance and anypossible diffusion between the droplet and the immiscible liquidis also avoided. The formation of nonstick interfaces has beenbased on the principles of using lotus-inspired nonstick surfaces(superhydrophobic surfaces)[13–15] or ‘‘liquid marbles’’.[1,16–21] Ona superhydrophobic surface, water drops can stay in a quasi-spherical shape because of the large water contact angle.However, a dynamically induced contact-angle hysteresis couldhinder droplet motion.[22] As a result, a droplet in theCassie–Baxter state is highly desired because it is more mobilethan that in the Wenzel state.[23]

Liquid marbles are formed due to the self-organization ofhydrophobic particles on the liquid/air interface of liquid drops.They are considered as perfect nonwetting systems because theircontact angles on smooth substrates are close to 1808. The

hydrophobic particles on the liquid surface form nonstickdroplet/substrate interfaces to reduce the motion resistancearising from weak particle–substrate adhesion, making liquidmarbles very attractive for manipulation of aqueous droplets.[1,21]

However, challenges remain in forming mechanically robustliquid marbles and in controlling their movements and the liquidinside.

Magnetic actuation has advantages in large and long-rangeforces and very low interaction with nonmagnetic media.[24] Toachieve magnetic actuation of a liquid marble, a magneticmaterial is required to integrate into the liquid marble as a forcemediator. Liquid marbles that can be magnetically manipulatedhave been prepared by coapplication of hydrophobic lycopodiumparticles and iron microparticles on aqueous drops[16] or bydispersing iron microparticles or Fe2O3 nanoparticles into thedroplet liquid phase.[11,16] However, the problem with thesemarble systems is either the detachment of iron particles fromthe marble surface due to the action of magnetic force, leading tothe loss of magnetic response, or the aggregation of the magneticparticles under the magnetic field, resulting in uneven pullingand deformation of the droplet.

Herein, we report on a mechanically robust magnetic liquidmarble, prepared by coating a water droplet with highlyhydrophobic Fe3O4 nanoparticles, and its magnetic actuation.A notable difference in our overall approach is that onlyhydrophobic magnetite nanoparticles are used as the coatingmaterial to form magnetic liquid marbles. The magnetic Fe3O4

nanoparticles have dual function, as both the force mediator andthe hydrophobic-coating phase. For the first time, we have foundthat the magnetic liquid marbles have a remarkable ability to beopened and closed reversibly under the action of a magnetic field.Based on this, novel functions, such as adjusting the liquid insidethe marble droplet and coalescing of two liquid marbles into alarger one, were also demonstrated.

Highly hydrophobic Fe3O4 nanoparticles were synthesized bycoprecipitation of Fe(II) and Fe(III) salts in an ethanol–watersolution with ammonia in the presence of a fluorinated alkylsilane, which hydrolyzed in solution to form a low-free-energycoating on the Fe3O4 nanoparticles. The obtained sample wascharacterized with transmission electron microscopy (TEM) andX-ray powder diffraction (XRD) (see Supporting Information, S1).The particle size, calculated from the reflection peak of (311) inthe XRD pattern according to the Scherrer equation, was 8.6 nm,which is close to the value of 10.2� 2.7 nm determined by theTEM image. Magnetic measurements showed that the Fe3O4

nanoparticles exhibited a typical superparamagnetic behavior (seeSupporting Information, S2). The apparent contact angle of a

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water drop placed on a bed of the as-synthesized Fe3O4

nanoparticles was 156.5� 3.28 (see Supporting Information, S3).Liquid marbles were obtained by rolling a small volume of

water in the highly hydrophobic Fe3O4 nanoparticles. Thespontaneous attachment of the nanoparticles at the liquid/airinterface can be understood by the minimization of the freeenergy of the surface.[20] The liquid marbles have a quasi-spherical shape when the droplet is less than 2mm in radius.With an increase in droplet size (radius >2mm), the liquidmarbles deformed under increased gravity and finally becamepuddle-shaped (see Supporting Information, S4).

Sufficient mechanical strength is an essential requirement forliquid marbles in practical microfluidic devices. The mechanicalrobustness of the liquid marbles was demonstrated by impactdeformation and rebound of the marbles. Figure 1 presentssequences of still frames from speedy videos when a 4mL liquidmarble impacted a glass surface driven by gravity or an externalmagnetic field. A typical rebound is shown in Figure 1A, where aliquid marble impacted the surface at a velocity V¼ 0.73m s�1.When landing on the glass surface, the initial spherical liquidmarble was first deformed to a pancake-like shape and thenretracted and bounced off the surface. The bouncing continueduntil the potential energy was consumed by the inertial flow andthe surface friction. After the impact deformation, we noticed atrace of Fe3O4 nanoparticles on the glass plate. Despite this, theliquid marble was still able to recover to its original shape within16ms. It is important to note here that the loosely packedFe3O4-nanoparticle coating was flexible enough to deform itself tofollow the contours of the droplet, making the liquid marbleelastic and bouncy without leaking of the fluid inside.

Figure 1B shows the critical situation that a liquid marbleimpacts a glass plate. From a height of 32mm, the liquid marblefell and impacted the glass surface at a velocity of 0.85m s�1.Based on this critical velocity, the average impact force, F, thatbroke the liquid marble can be calculated to be 1.47 g m s�2

according to the conservation of momentum equation,F¼mV/Dt, where m is the mass of the liquid marble and Dt isthe impact duration, which was taken to be 2R/V for calculation(R¼ droplet radius).

Figure 1C shows the magnetically actuated liquid marblemoving on a glass surface. The magnet bar was placed on the leftside and moved slowly toward the liquid marble until the marble

Figure 1. Still frames of speedy videos depicting a liquid marble (4mL) impacA,B) Two distinct types of impact behavior of a liquid marble falling from a32mm, respectively. The time interval between two frames is 4 ms. C) Maga liquid marble moving horizontally from right to left to impact a gldistance¼ 14mm). Scale bar: 5mm.

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started to move. The critical magnetic field actuating thedroplet motion was 0.02 T. The distance the liquid marblemoved was 14mm. It is noted that the minimal force neededto actuate the liquid marble should be determined by themaximum static friction between the marble and the substrate.Herein, the magnetic force acting on the liquid marble isproportional to the intensity of external magnetic field(0.02 T) and the mass of magnetic powders on the droplet(0.18mg). Under the action of the static magnetic field, themovement of the droplet sped up to reach a velocity of 0.32m s�1

before it impacted the glass wall. In this case, no detachment ofFe3O4 particles from the liquid marble was observed. Theseresults indicate that the as-prepared liquid marbles are robustenough for manipulation of liquid transport in microfluidicdevices.

A recent study has shown that the mechanical robustness ofliquid marbles created with nanoparticles is greater than thosemade from microparticle materials due to more uniformcoverage of the liquid/air interface by nanoparticles.[21] In ourcase, the small particle size of the highly hydrophobic Fe3O4

nanoparticles (about 10 nm) could be a reason that leads to thedurable liquid marbles.

Driven by a magnet bar, the liquid marble can be transportednot only on flat surfaces, as illustrated in Figure 1C, but also oncurved surfaces (see Supporting Information, S5), which willfacilitate the generation of topologically complex microfluidicsystems. Moreover, the liquid marbles can also be manipulatedspatially. As shown in Figure 2A, a liquid marble was transferredvertically between two parallel glass plates with a distance of4.5mm. When a magnet bar was lowered slowly toward the glassplate, a liquid marble resting on the lower glass plate was draggedupward and, then, lifted rapidly to the upper glass surface andheld against the surface by the magnetic force. When the magnetwas removed, the droplet fell back onto the lower substrate ongravity. This feature enables the liquid marble to be transportedfrom one spot to another without a guiding surface. Uponreaching the destination, the liquid marbles can be destroyed bybringing them into contact with a trace of organic solvent thatwets the hydrophobic Fe3O4 nanoparticles, similar to other liquidmarbles reported.[16]

More interestingly, we have found that the magnetic liquidmarble placed on a thin glass slide can be opened by a strong

ting a glass plate.height of 26 andnetic actuation ofass wall (moved

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magnet positioned below the glass slide and theopened liquid surface can also recover to itsoriginal state when the magnet is moved away.As shown in Figure 2B, when the magnet wasmoved slowly upward, the Fe3O4 nanoparticleswere pulled down toward the glass surface anda liquid marble, containing a blue-coloredwater droplet, was opened up to show anexposed blue-liquid surface on the top. Themaximummagnetic field exerting on the liquidmarbles was about 0.1 T. As long as themagnetic field existed, the liquid marbleremained in the opened state. When themagnet was removed, however, the Fe3O4

nanoparticles moved rapidly back to theexposed liquid surface to minimize the freeenergy of the surface. Such a magnet-

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Figure 2. A) Transfer of a liquidmarble between two parallel glass plates 4.5mm apart by placingand displacing a magnet bar above the top glass plate. B) Still frames of a video depicting theopening and closing of a liquid marble containing a blue-colored water droplet under magneticforce (see also movie S1 of the Supporting Information). C) Still frames captured from a speedyvideo showing different stages of coalescing two liquid marbles under magnetic force (see alsomovie S2 of the Supporting Information).

manipulated opening–closing cycle can be repeated many times.To the best of our knowledge, this is the first time that a liquidmarble shows such a remarkable capability to be opened andclosed reversibly in a controlled manner.

Because of the nonstick feature and small droplet size,normally, it is difficult to extract fluid from a liquid marble.

Figure 3. A water drop on a glass surface was recovered by forcing highly hydrophobic Fe3O4

nanoparticles to separate water from the glass surface, followed by either forming a liquid marble(see also movie S3 of the Supporting Information) or being picked up.

However, themagnet-induced opening featureenabled the opened liquid surface to wet acontacting glass capillary, thus transferringliquid out of the liquid marble (see SupportingInformation, S6). Also, the remaining liquidcould form a smaller liquid marble when theexposed surface was closed. Similarly, a smallvolume of liquid could also be added into theliquid marble. This would open new ways toadjust the volume and to modify the composi-tion of the liquid marble.

The mixing of droplets is an essentialprocess for chemical reactions in microfluidicdevices. For liquid marbles, however, thepresence of hydrophobic powders impedesthe combination of droplets,[16] althoughSailor and co-workers[25] have reported thatthe coalescence of two liquid marbles can beaided by increasing the rate at which theycollide. For the magnetic liquid marblesdeveloped in our study, the coalescence canbe easily accomplished under the force of a

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magnetic field, as demonstrated in Figure 2C.Two adjacent liquid marbles were opened fromthe top by putting a strong magnet bar belowthe glass slide (Fig. 2C, image 2). Under theinfluence of the magnet, the exposed dropletscoalesced immediately (images 3–5), resultingin a larger marble (image 6). We also found thata liquid marble can be split into two smallerones simply by cutting with a spatula. Dropletsplitting is a typical process in microfluidicapplications.

Another remarkable feature of the highlyhydrophobic Fe3O4 nanoparticles is their abilityto recover a water drop from a hydrophilicsurface, as demonstrated in Figure 3. To dothis, we first put some Fe3O4 nanoparticlesaround a water drop on a normal glass slide(Fig. 3, image 2) and, then, moved a magnetbar, placed underneath the slide, slowly upwardtoward the nanoparticles. Under the influenceof the magnetic field the nanoparticlesbecame magnetized and they aggregated intofibril-like clusters due to the induced magneticdipolar interaction (image 3). Moving themagnetic bar close to the particles increasedthe magnetic field. As a result, the particleswere gradually drawn toward the central area ofthe water drop and they also attached firmly tothe glass surface due to the horizontal andvertical components of the magnetic field force,respectively (image 4). Thanks to the magnetic

force (vertical component) the Fe3O4 nanoparticles moved whileclinging to the glass slide, where they compressed the water drop.Because of the negative capillary force formed among the Fe3O4

nanoparticles, the water was forced to separate from the glasssurface and finally formed a droplet on the newly formed highlyhydrophobic substrate (image 5), which can either form a liquid

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marble for further manipulations (images 6 and 7), or be pickedup easily with a glass pipette (images 60 and 70).

In summary, we have demonstrated the easy preparation ofmagnetic liquid marbles and their novel properties that can beused for magnetic manipulations of water droplets in a controlledmanner. Under the action of a magnetic field the magnetic liquidmarbles can not only be manipulated in both 2D and 3D but alsobe opened and closed reversibly, depending on the intensity of theapplied magnetic field. Liquid can be easily removed or evenrefilled into the opened liquid marbles and two opened liquidmarbles can be merged into one larger one. Moreover, as apotential solution to the problem associated with the spilling of asmall volume of aqueous liquid, a water drop was recovered froma hydrophilic surface via magnetic manipulation. All of theseunique features are very promising in the development ofmagnetically actuated channel-free microfluidic systems andsmart microreactors.

Experimental

Synthesis of Hydrophobic Fe3O4 Nanoparticles: Aqueous NH4OHsolution (1.5 M) was added dropwise to 200mL water/ethanol solution(4:1 v/v) containing FeCl3 � 6H2O (0.85 g, 3.14mmol), FeCl2 � 4H2O (0.30 g,1.51 mmol), and tridecafluorooctyltriethoxysilane (0.20mL, 5.23 mmol)under nitrogen protection and vigorous stirring until pH 8. After stirring for24h, the resulting precipitate was isolated from the solution with a bar magnet,washed with a water/ethanol mixture for three times, and dried at 60 8C.

Measurements: The contact angle was measured using a contact-anglemeasurement system (CAM101, KSV Instruments Ltd.). TEM images wereobtained using a FEI Tecnai F30 Cryo-TEM. XRD patterns were recordedon a Philips 1140/90 diffractometer using Cu Ka (l¼ 1.54 A) radiation.Magnetic properties were measured with a Quantum Design MPMS-5direct-current superconducting quantum interference device (DC-SQUID)magnetometer. The magnetic field intensity was tested by a PASCOmagnetic field sensor PS-2162. Speedy videos were taken by a high-speedcamera (TroubleShooter LE250ME, Fastec Imaging). Magnetic manipula-tion of liquid marbles was carried out with permanent neodymium cylindermagnets of 10-mm diameter and 12-mm length.

Acknowledgements

We acknowledge the funding support from Deakin University under theAlfred Deakin Postdoctoral Fellowship scheme and the support from theAustralian Microscopy and Microanalysis Research Facility (ARMMF) forthe TEM measurements. We also thank Dr. Matthias Floetenmeyer from

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the University of Queensland for assisting us with the TEM tests.Supporting Information is available online fromWiley InterScience or fromthe author.

Received: July 27, 2009

Published online: November 24, 2009

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