www.sciencemag.org/cgi/content/full/341/6143/253/DC1

Supplementary Materials for

Switchable Static and Dynamic Self-Assembly of Magnetic Droplets on

Superhydrophobic Surfaces

Jaakko V. I. Timonen,* Mika Latikka, Ludwik Leibler, Robin H. A. Ras,* Olli Ikkala*

*Corresponding author. E-mail: jaakko.timonen@aalto.fi (J.V.I.T.); robin.ras@aalto.fi (R.H.A.R.);

olli.ikkala@aalto.fi (O.I.)

Published 19 July 2013, Science 341, 253 (2013)

DOI: 10.1126/science.1233775

This PDF file includes:

Materials and Methods

Supplementary Text

Figs. S1 to S10

Captions for Movies S1 to S14

Full Reference List

Other Supplementary Material for this manuscript includes the following:

(available at www.sciencemag.org/cgi/content/full/341/6143/253/DC1)

Movies S1 to S14

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Materials and Methods

Preparation of ferrofluids

Iron oxide nanocrystals were synthesized with the coprecipitation method (21) and stabilized with citric acid near pH 7 in water as described earlier (28, 31). The aqueous ferrofluid was used either as such (in the static self-assembly experiments) or the water was removed partly by evaporation and replaced by ethylene glycol to reduce the evaporation during the experiments (in the case of dynamic self-assembly). The volumetric concentration of iron oxide nanocrystals was in both ferrofluids ~20%. The ferrofluids were characterized in liquid state with a SQUID magnetometer (QuantumDesign MPMS XL7) and the iron oxide nanoparticles with a transmission electron microscope (JEOL JEM-2200FS, 200 keV). Both ferrofluids showed relative susceptibilities of ~3.3 and saturation magnetizations close to 80 emu cm-3 (see Fig. S1A,B for typical magnetic properties of the aqueous ferrofluids). The average diameter of the iron oxide nanocrystals was 4.6 nm with a standard deviation of 0.35 nm (Fig. S1C,D).

Preparation of superhydrophobic substrates

Surface 1 (ultralow friction, used in the experiments unless otherwise stated): Fluorinated Ag-coated Cu substrates were prepared as described in literature (23). AgNO3 (99%, Fluka), 1H,1H,2H,2H-perfluorodecanethiol (HDFT, 97%, Aldrich), and 1,2-dichloromethane (DCM, 99%, Aldrich) were used as obtained. First, a mechanically polished copper substrate (approximately 130 x 28 x 0.8 mm3) was coated with silver by immersing it into aqueous AgNO3 solution (0.01 M). After 60 seconds, the substrate was removed, washed with Milli-Q water and dried under nitrogen flow. Subsequently, the substrate was immersed into a HDFT solution in DCM (0.001 M) for 5 minutes and washed twice with fresh DCM. Finally, the substrate was dried under normal atmosphere, resulting in a high quality superhydrophobic coating. Advancing water contact angle was measured to be approximately (32) 175° and receding contact angle 174°. Advancing and receding contact angles measured for ethylene glycol ferrofluid were 171° and 165° respectively.

Surface 2 (medium friction, used in Movie S12): Silicone nanofilaments coated with fluorosurfactant were prepared as described in literature with small modifications (33). Test grade silicon (100) wafers were cleaned with an alkaline detergent (Deconex 11 Universal), thoroughly rinsed, dried and then activated in H2/O2 plasma (Gatan Solarus Model 950). The nanofilaments were synthesized on the silicon wafer in an in-house built gas-phase reactor operating at ca. 0.1 mbar at room temperature. Deionized water and methyltrichlorosilane (MTCS, 99%, Aldrich) were sequentially evaporated into the reactor from separate lines controlled by Teflon valves. After the reaction, the chamber was flushed with dry nitrogen. The nanofilament-coated substrate was activated in O2/H2 plasma and fluorinated with tridecafluorotetrahydro-octyltrichlorosilane (97%, ABCR). The latter reaction was done in an atmospheric reactor vessel flushed with dry

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nitrogen. The fluorosurfactant was injected into the reactor through a silicone septum, where it evaporated and reacted with the surface of the nanofilaments. Resulting surface had an advancing water contact angle of 164° and receding contact angle of 158°. Advancing and receding contact angles measured for ethylene glycol ferrofluid were 173° and 139° respectively.

Surface 3 (high friction, used in Movie S4): Silicone nanofilament surface was prepared as described in literature (27, 33). Synthesis of the nanofilaments was done in an in-house built gas-phase reactor operating at atmospheric pressure. Test grade silicon (100) wafers were cleaned with an alkaline detergent (Deconex 11 Universal), thoroughly rinsed, dried and then activated in H2/O2 plasma (Gatan Solarus Model 950). The reactor with the silicon wafer was purged with dry argon followed by pre-humidified argon until the relative humidity reached ca. 30%. The reactor was then sealed and trimethylchlorosilane (TMCS) injected through a silicone septum. MTCS evaporated rapidly and nanofilaments were grown onto the surface. After ca. 1 hour, the reactor was purged with argon to stop the reaction and to remove gaseous reaction byproducts. Resulting surface had an advancing water contact angle of 157° and receding contact angle of 145°. Advancing contact angle measured for ethylene glycol ferrofluid was 169°. Receding angle could not be accurately measured due to high wetting of the surface, but was estimated to be less than 4°.

Contact angle hysteresis force Fµ acting on moving ferrofluid droplets on the superhydrophobic surfaces can be used as a parameter to describe and compare friction between different surfaces (N.B. that also other frictional forces are present, such as viscous dissipation). Its magnitude can be estimated from the measured advancing and receding contact angles: (34)

Fµ = µmg =l2γ cos θR( )− cos θA( )"# $% (Eq. S1)

where l is the length of the three phase contact line, γ is the surface tension and θA

and θR are the advancing and receding contact angles. Surface tension of the ethylene glycol ferrofluid was measured to be 50.28 ± 0.05 mN/m. Friction forces Fµ for 2.8 µl (l = 4.8 mm) ferrofluid droplet used in the dynamic self-assembly experiments were calculated to be 2 ± 2 µN for Surface 1 and 28 ± 5 µN for Surface 2. These forces correspond to coefficients of friction of 0.04 and 0.52, respectively. For Surface 3 such an estimate cannot be calculated due to high wetting of the surface. The contact angles and the surface tensions were measured with an optical contact angle goniometer (Attension Theta).

Experimental procedures: Static self-assembly

A Hall sensor (LakeShore) was attached on the bottom side of the superhydrophobic substrate with double-sided tape. Two aluminum bars were used to support the substrate over a permanent magnet that was placed on a laboratory jack (Swiss Boy 110) directly under the Hall-sensor. Initially, the magnet was kept far away from the superhydrophobic

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substrate so that the magnetic field indicated by the Hall-sensor was below 200 Oe. Then, one droplet of aqueous ferrofluid with a desired volume was deposited on the substrate with a micropipette. The as deposited droplet immediately moved to the position of maximum field strength directly above the magnet and the Hall sensor. The laboratory jack was then used to move the magnet towards the superhydrophobic surface to increase the magnetic field. Still images and regular and high-speed videos were recorded on the deforming, dividing and self-assembling droplets with a digital single-lens reflex camera (Canon EOS 400D or EOS 7D) with extension tubes and a 50 mm objective lens or a high-speed camera (Casio EX-FH25). Experiments with multiple parent droplets were performed similarly, with the exception that multiple droplets were deposited with a micropipette before increasing the magnetic field strength.

Experimental procedures: Dynamic self-assembly

The general approach is illustrated using a concrete example: First, a cylindrical

magnet was attached to a computer-controlled linear actuator (Aerotech PRO165LM) that was placed under the superhydrophobic surface supported by a laboratory stand. A 20 µl droplet of ethylene glycol ferrofluid was then deposited on the substrate, which was subsequently lowered toward the magnet until the desired static pattern of seven droplets was formed by multiple divisions of the pipetted droplet. The linear actuator was used to bring the attached magnet to a sinusoidal oscillatory motion, starting slowly and increasing gradually to the desired amplitude and frequency. High-speed movies of the resulting dynamic self-assemblies were recorded using Canon EOS 7D, EOS 60D and Casio EX-FH25 cameras. At the end of each experiment, the energy feed was stopped in one of two ways: either the substrate was lifted up to decrease the field before the magnet was halted or the magnet motion was halted without changing the field strength. In the former case the droplets that were coalesced during the dynamic self-assembly remained coalesced in the static field. In contrast, in the latter case the coalesced droplets divided back to the original seven-droplet static self-assembly when the magnet motion was halted.

Details on magnets and magnetic fields Magnetic field strength H, vertical field gradient dH/dz and horizontal field

curvature c of a cylindrical permanent magnet near the symmetry axis depend on the dimensions of the magnet and the distance from the magnet. Four differently sized NdFeB N52 permanent magnets (K&J Magnetics) with an aspect ratio of 2:1 (length:diameter) were used. The lengths of the magnets were 25.6 mm (Magnet 1), 38.9 mm (Magnet 2), 50.7 mm (Magnet 3), and 77.2 mm (Magnet 4). Magnetic field strengths were measured as a function of the distance from the surface of the magnets with a Hall sensor (LakeShore) (Fig. S2A). The vertical field gradient dH/dz (Fig. S2B) and field curvature c (Fig. S2C) were obtained from the measured data by fitting with a theoretical expression (35). Generally, the vertical field gradient dH/dz and the field curvature c are large for small magnets and small for large magnets (Fig. S2B,C). The large curvatures of

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small magnets led to tight confinement of the divided droplets in the static self-assembly experiments, which hindered further droplet splitting as the patterns got crowded (Fig. S2D, black). On the other hand, the low vertical field gradient of larger magnets hindered droplet division (because of limited normal force), even though the patterns were not crowded (Fig. S2D, green). Optimal cleaving was observed for medium-sized Magnets 2 and 3.

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Supplementary Text

The role of substrate wettability and vertical field gradient

Division of the magnetic droplet on a superhydrophobic surface was not observed in a uniform magnetic field created with an electromagnet (Fig. S4A). On a hydrophilic glass substrate, the division was observed in the inhomogeneous fields of the cylindrical magnets, but a liquid film connecting all the droplets remained on the substrate all the time, preventing the formation of distinct separated droplets (Fig. S4B). This was also observed on a moderately hydrophobic substrate (Surface 3, Movie S4). Only when a superhydrophobic substrate was used in combination with inhomogeneous magnetic field, clean divisions into totally isolated daughter droplets and their static self-assembly were observed (Fig. S4C). In addition, dynamic self-assembly required a superhydrophobic surface of a particularly high quality (Surface 1) and was not observed on a less superhydrophobic substrate (Surface 2, Movie S12).

Simulation of dynamic self-assembly The droplet-droplet distances of the seven-droplet pattern were simulated under the

oscillating magnetic field by using Eq. (2) as the potential energy (Fig. S8). Viscous friction proportional to droplet velocity was assumed. Numerical simulations were carried out with Matlab by integrating the coupled differential equations that arise from Eq. (2). The motion of the patterns was simulated over several oscillations of the driving field, and the steady-state droplet-droplet distances were obtained from the last oscillation (to avoid the transient effects during the first two oscillations). The results show that the droplet-droplet distances decrease upon increasing both the driving frequency and amplitude (Fig. S8A), in good agreement with the experiments (Fig. 4C). Notably, the spacing of some of the droplet pairs decreased more than the others, making them more susceptible to coalescence (Fig. S8B). The coalescence order observed in the dynamic self-assembly of a pipetted seven-droplet pattern (Movie S10, Fig. S8C) was in excellent agreement with the simulation (Fig. S8D).

Splitting of equally sized droplets Feedback loop between the magnetic droplets and the magnetic field is

demonstrated with a simple pattern of three separately pipetted, nominally identical droplets (Fig. S9). Under ideal conditions the droplets could be expected to split at the same external field and at the same time. However, in reality the divisions take place in two phases: first, one of the parent droplets divides and alters the local magnetic field affecting the other two droplets, prohibiting their division (Fig. S9, Movie S11). Upon increasing the field further, a second parent droplet splits, again followed by a spontaneous rearrangement of the pattern. This time the rearrangement makes the last parent droplet unstable, leading to its spontaneous division approximately 200 ms

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afterwards without any further increase of the magnetic field strength. Thus, the interplay between the droplets through the local magnetic fields (due to the droplet-droplet dipolar interaction) can make the behavior of even the simplest patterns complex and ultimately unpredictable.

Recovering one droplet state via droplet recombination by dynamic self-assembly

As described, static droplet self-assemblies can be switched to dynamic self-assemblies by feeding energy. When the energy feed is stopped by halting the oscillation but keeping the magnetic field unchanged, the remaining static magnetic field triggers droplet splitting until the original static self-assembly is achieved. However, it is also possible to adjust the magnetic field strength while the magnet is being oscillated. If the field strength is decreased while keeping the oscillation on, the critical wavelength is increased during the energy feed and as a result the larger droplets remain stable once the oscillation is stopped. This is illustrated in Fig. S10 and Movies S13 an S14. It allows, for example, the initial state of one single static droplet to be recovered.

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Fig. S1. Properties of the two batches of the aqueous iron oxide ferrofluid used in the experiments. A) Low-field part of the magnetization curves and B) the corresponding full magnetization curves. C) A TEM micrograph of the iron oxide nanocrystals and D) the corresponding particle size distribution measured of approximately 200 particles (with a log-normal fit). Ethylene glycol based ferrofluids used in the dynamic self-assembly experiments exhibited similar magnetic behavior.

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Fig. S2 Magnetic fields of the four cylindrical magnets used in experiments. A) Field strength on the magnet axis as a function of distance from the surface of the magnet, showing experimentally measured data (dots) and theoretical fits (lines). B) The corresponding vertical field gradients H’=dH/dz and C) horizontal curvatures c as a function of field strength. D) Number of the daughter droplets formed by the division instability starting from a 10 µl parent droplet as a function of the critical wavelength (calculated from the magnetic properties of the ferrofluid and the magnetic field strength and vertical gradient). Lines are linear fits through origin. Insets show typical patterns obtained with Magnets 1 (M1, black) and 4 (M4, green).

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Fig. S3 High-speed photographs showing the formation of a satellite droplet. Just before the two daughter droplets get completely separated, they are still connected by a thin ferrofluid channel. Due to the rapidity of the process, viscosity and the liquid channel being pulled by both of the daughter droplets, part of the fluid is left between the daughter droplets as a small satellite droplet.

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Fig. S4 Behavior of a 20 µl aqueous ferrofluid droplet on different substrates and magnetic fields. A) In nearly uniform magnetic field (produced by an electromagnet) on a superhydrophobic substrate (Surface 1), B) in non-uniform magnetic field (produced by a cylindrical permanent magnet) on a hydrophilic glass substrate, and C) in non-uniform magnetic field (produced by a cylindrical permanent magnet) on a superhydrophobic substrate (Surface 1).

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Fig. S5 Vertical magnetic force as a function of vertical field gradient (H’=dH/dz). The force experienced by ferrofluid droplets was measured with a balance from the effective weight of the droplets on the superhydrophobic surface when subjected to the non-uniform field of a cylindrical permanent magnet (Magnet 2). The force was found out to be in good agreement with what was expected based on the magnetization of the ferrofluid and the strength and the vertical gradient of the external magnetic field:

€

F =ddz

µ0HMV( ).

Red dots correspond to the measured values and black line to the theoretical force (in the units of gravitational acceleration). The demagnetizing factor of the droplets was assumed to be equal to that of a sphere (i.e. 1/3).

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Fig. S6 Histogram of the droplet sizes formed via droplet splitting. Droplet populations were formed from a single 20 µl ethylene glycol ferrofluid droplet. Size distributions were determined by individually weighing the formed droplets with an analytical balance (Mettler Toledo XA105DU) and converting the obtained masses into volumes. Ethylene glycol ferrofluid was used in order to minimize errors due to the solvent evaporation. At highest number of divided droplets (i.e. 75), the standard deviation of the droplet volume was 27%. In comparison, separately pipetted 2.8 µl droplets showed standard deviation of 3% in volume.

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Fig. S7 Switching of hierarchical patterns by size-selective division (Movie S6). Several different hierarchical switching patterns can be designed with different symmetries. Static self-assembly of 13 droplets (1x17 ml (yellow), 6x20 ml (green), and 6x2 ml (red)) was switched to another hierarchical pattern of 20 droplets (2x8.5 ml (yellow), 12x10 ml (green), and 6x2 ml (red)). Magnetic field strength was increased from 930 Oe (dH/dz = 51 Oe/mm) to 1170 Oe (dH/dz = 69 Oe/mm).

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Fig. S8 Simulation results on the droplet-droplet spacing in the seven-droplet pattern under oscillating magnetic field. A) The maximum change in the droplet-droplet spacing as a function of the amplitude and frequency of the magnet. B) Simulated droplet-droplet distances of a stationary pattern and the time-averaged distances for a pattern in motion (amplitude 30 mm, frequency 5 Hz). Distances are expressed in millimeters. C) Snapshots of Movie S10 during the coalescence of the first two droplet pairs marked with red arrows. Time interval between the frames is 17 ms. D) The corresponding simulated droplet arrangement, wherein the two droplet-droplet distances marked in red are exceptionally short, leading to the coalescence seen in subfigure C).

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Fig. S9 Three equally sized droplets dividing in a magnetic field (Movie S11). An aqueous ferrofluid droplet pattern with three-fold symmetry (three 20 µl droplets) in axisymmetric field is switched to a pattern with five-fold symmetry (six 10 µl droplets). Notice that magnetic field is changed only between first two figures (as seen in the change in the critical wavelength). The processes shown in the rest of the figures happen spontaneously without further increasing the magnetic field.

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Fig. S10 A flow chart describing transitions between a single parent droplet, static self-assembly and dynamic self-assembly (Movies S13 and S14). The parent droplet can be divided into static self-assembled pattern of daughter droplets, but direct recovery of the original single droplet from the droplet population is prevented due to kinetic trapping. However, the static self-assembly can be switched to a dynamic self-assembly which allows recovering the initial single droplet state by changing the magnetic field strength while the magnet is being oscillated.

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Movie S1 Magnetically triggered droplet splitting. A 20 µl ferrofluid droplet is placed on a superhydrophobic substrate with a cylindrical magnet on a laboratory jack below it. The magnet is gradually lifted, which creates an increasing magnetic field on the droplet. The droplet gradually deforms and eventually divides into two daughter droplets. The distance of the magnet from the droplet can be monitored from the ruler attached to the laboratory jack shown in the left hand side of the video (tick spacing corresponding to one millimeter).

Movie S2 Magnetically triggered droplet splitting (high-speed video). Droplet division similar as in Movie S1 is shown in slow motion. Movie was recorded with a 1000 fps high-speed video camera.

Movie S3 Droplet pattern in axisymmetric field. As in Movie S1, a ferrofluid droplet on a superhydrophobic surface is subjected to increasing magnetic field of a cylindrical permanent magnet. This results in division into numerous daughter droplets that form different static self-assembled patterns to minimize their total energy. In the end, the magnetic field is decreased, resulting in relaxation of the conical daughter droplets into spherical ones. The mobility of the droplets is demonstrated in the video by moving the magnet horizontally (between 50 seconds and 1 minute).

Movie S4 Static self-assembly on a hydrophobic surface with large contact angle hysteresis. A single 20 µl droplet is split on Surface 3 to several daughter droplets as in Movie S3. The ferrofluid wets the substrate, inhibiting the formation of distinct droplets. In the end, the magnetic field is decreased, showing that some of the droplets coalesce due to the connecting liquid film.

Movie S5 Droplet pattern in non-axisymmetric field. As in Movie S3, but with the exception that the cylindrical permanent magnet is replaced with a rectangular cuboid magnet (length 50.8 mm, width 12.7 mm, height 12.7 mm, magnetized along the short dimension).

Movie S6 Hierarchical pattern switching. First part: A hierarchical initial pattern of 9 droplets with four-fold symmetry (1x20 µl, 4x13 µl, and 4x6 µl) is switched to another hierarchical pattern of 14 droplets with two-fold symmetry (2x10 µl, 8x6.5 µl, and 4x6 µl) by increasing the magnetic field by bringing a cylindrical magnet closer to the

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superhydrophobic substrate. Second part: A hierarchical pattern of 13 droplets with six-fold symmetry (1x17 µl, 6x20 µl and 6x2 µl) is switched to another hierarchical pattern of 20 droplets with reflection symmetry (2x8.5 µl, 12x10 µl, and 6x2 µl).

Movie S7 Reversible switching between static and non-alternating dynamic self-assembly. The static self-assembly of 7 droplets is reversibly switched to different dynamic self-assemblies by feeding energy. The cylindrical magnet below the superhydrophobic substrate is attached to a computer-controlled linear actuator that puts the magnet into a sinusoidal motion with different frequencies and amplitudes. At low energy feeds (small amplitudes and frequencies), the initial seven-droplet pattern follows the magnet without changing the appearance (Mode I). At sufficiently high energy feed, the static self-assembly switches to different dynamic self-assemblies (Modes II-V) that transform back to the initial static self-assembly when the energy feed is stopped. The dynamic self-assemblies in this movie consist of droplets with similar mobility (i.e. of only spherical or only elongated droplets), thus leading to non-alternating patterns.

Movie S8 Reversible switching between static and alternating dynamic self-assembly. As in Movie S7, but with the exception that the dynamic self-assembly modes consist of droplets with different mobility (combinations of spherical and elongated droplets), thus leading to alternating patterns (Modes VI-X).

Movie S9 Complexity in pattern switching. As in Movies S7 and S8, the static self-assembly of 7 droplets (Mode I) is oscillated four times under identical energy feed (f=3 Hz, A=12.5 mm), yielding three different patterns (Modes I, VII and VIII). Evolutions of the Modes VIII and VII are shown in slow motion (video speed 25% of the original).

Movie S10 Static and dynamic self-assembly of separately pipetted droplets. Static self-assembly of 7 droplets (total volume of 20 µl) is prepared by separately pipetting the droplets on the substrate to obtain a well-defined pattern. The pattern is switched to a dynamic self-assembly (Mode V) as in Movie 7, showing no qualitative difference between droplet populations that are directly pipetted or formed through droplet divisions.

Movie S11 Splitting of three equal sized droplets. An initial pattern of three separately pipetted 20 µl droplets is switched to a pattern of six droplets. When the field is increased, one of the parent droplets splits, adjusting the positions of the other droplets and stabilizing the pattern. Upon increasing the field further, a second parent droplet splits, making the

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pattern unstable and triggering the autonomous splitting of the last parent droplet, highlighting the complex nature of the pattern evolution. Movie was recorded with a frame rate of 120 fps (video speed 25% compared to real-time).

Movie S12 Static and dynamic self-assembly on a superhydrophobic surface with large contact angle hysteresis. A single 20 µl parent droplet is split on Surface 2 into a static self-assembly of 7 droplets with the magnetic field. This pattern is switched to a dynamic self-assembly (Mode V) as in Movie S7. Due to the high contact angle hysteresis of the surface, the moving droplets pin to the surface, breaking up the dynamic self-assembly pattern. When the energy feed is stopped, the pattern doesn’t return to initial seven-droplet pattern, but decays to several smaller droplets.

Movie S13 Droplet recombination using dynamic self-assembly. A single 20 µl parent droplet is split to a static self-assembly of 7 droplets with the magnetic field. This pattern is switched to a dynamic self-assembly (Mode V) as in Movie S7. When the Mode V is achieved, the magnetic field is decreased by increasing the distance between the magnet and the substrate. The starting configuration (i.e. a single droplet) is obtained upon switching off the oscillations.

Movie S14 Pattern recovery from dynamic self-assembly. As in Movie S13, a dynamic self-assembly (Mode X) is switched to a static pattern by decreasing the magnetic field strength before stopping the energy feed. A static self-assembly with two large and one small droplet is obtained.

References and Notes

1. P. Ball, Nature's Patterns: A Tapestry In Three Parts (Oxford Univ. Press, Oxford, 2011).

2. G. M. Whitesides, B. Grzybowski, Self-assembly at all scales. Science 295, 2418–2421

(2002). doi:10.1126/science.1070821

3. F. S. Bates, M. A. Hillmyer, T. P. Lodge, C. M. Bates, K. T. Delaney, G. H. Fredrickson,

Multiblock polymers: Panacea or Pandora’s box? Science 336, 434–440 (2012).

doi:10.1126/science.1215368

4. A.-V. Ruzette, L. Leibler, Block copolymers in tomorrow’s plastics. Nat. Mater. 4, 19–31

(2005). doi:10.1038/nmat1295 Medline

5. Y. Xia, T. D. Nguyen, M. Yang, B. Lee, A. Santos, P. Podsiadlo, Z. Tang, S. C. Glotzer, N. A.

Kotov, Self-assembly of self-limiting monodisperse supraparticles from polydisperse

nanoparticles. Nat. Nanotechnol. 6, 580–587 (2011). doi:10.1038/nnano.2011.121

Medline

6. E. V. Shevchenko, D. V. Talapin, N. A. Kotov, S. O’Brien, C. B. Murray, Structural diversity

in binary nanoparticle superlattices. Nature 439, 55–59 (2006). doi:10.1038/nature04414

Medline

7. K. Liu, N. Zhao, E. Kumacheva, Self-assembly of inorganic nanorods. Chem. Soc. Rev. 40,

656–671 (2011). doi:10.1039/c0cs00133c Medline

8. T. Kato, N. Mizoshita, K. Kishimoto, Functional liquid-crystalline assemblies: Self-organized

soft materials. Angew. Chem. Int. Ed. 45, 38–68 (2006). doi:10.1002/anie.200501384

Medline

9. O. Ikkala, G. ten Brinke, Functional materials based on self-assembly of polymeric

supramolecules. Science 295, 2407–2409 (2002). doi:10.1126/science.1067794

10. S. Park, D. H. Lee, J. Xu, B. Kim, S. W. Hong, U. Jeong, T. Xu, T. P. Russell, Macroscopic

10-terabit-per-square-inch arrays from block copolymers with lateral order. Science 323,

1030–1033 (2009). doi:10.1126/science.1168108

11. J. Yoon, W. Lee, E. L. Thomas, Self-assembly of block copolymers for photonic-bandgap

materials. MRS Bull. 30, 721–726 (2005). doi:10.1557/mrs2005.270

12. B. A. Grzybowski, C. J. Campbell, Complexity and dynamic self-assembly. Chem. Eng. Sci.

59, 1667–1676 (2004). doi:10.1016/j.ces.2004.01.023

13. B. A. Grzybowski, C. E. Wilmer, J. Kim, K. P. Browne, K. J. M. Bishop, Self-assembly:

From crystals to cells. Soft Matter 5, 1110–1128 (2009). doi:10.1039/b819321p

14. S. Kauffman, At Home in the Universe: The Search for Laws of Self-Organization and

Complexity (Oxford Univ. Press, Oxford, 1995).

15. E. Karsenti, Self-organization in cell biology: A brief history. Nat. Rev. Mol. Cell Biol. 9,

255–262 (2008). doi:10.1038/nrm2357 Medline

16. A. M. Mateus, N. Gorfinkiel, A. M. Arias, Origin and function of fluctuations in cell

behaviour and the emergence of patterns. Semin. Cell Dev. Biol. 20, 877–884 (2009).

doi:10.1016/j.semcdb.2009.07.009 Medline

17. B. A. Grzybowski, H. A. Stone, G. M. Whitesides, Dynamic self-assembly of magnetized,

millimetre-sized objects rotating at a liquid-air interface. Nature 405, 1033–1036 (2000).

doi:10.1038/35016528 Medline

18. K. V. Tretiakov, K. J. M. Bishop, B. A. Grzybowski, The dependence between forces and

dissipation rates mediating dynamic self-assembly. Soft Matter 5, 1279–1284 (2009).

doi:10.1039/b811254a

19. R. E. Rosensweig, Ferrohydrodynamics (Dover Publications, New York, 1997).

20. J.-C. Bacri, F. Elias, in Morphogenesis: Origins of Patterns and Shapes, P. Bourgine, A.

Lesne, Eds. (Springer, Heidelberg, 2011), pp. 15–19.

21. R. Massart, Preparation of aqueous magnetic liquids in alkaline and acidic media. IEEE

Trans. Magn. 17, 1247–1248 (1981). doi:10.1109/TMAG.1981.1061188

22. W. Barthlott, C. Neinhuis, Purity of the sacred lotus, or escape from contamination in

biological surfaces. Planta 202, 1–8 (1997). doi:10.1007/s004250050096

23. I. A. Larmour, S. E. J. Bell, G. C. Saunders, Remarkably simple fabrication of

superhydrophobic surfaces using electroless galvanic deposition. Angew. Chem. Int. Ed.

46, 1710–1712 (2007). doi:10.1002/anie.200604596 Medline

24. X. F. Gao, X. Yan, X. Yao, L. Xu, K. Zhang, J. Zhang, B. Yang, L. Jiang, The dry-style

antifogging properties of mosquito compound eyes and artificial analogues prepared by

soft lithography. Adv. Mater. 19, 2213–2217 (2007). doi:10.1002/adma.200601946

25. H. Mertaniemi, V. Jokinen, L. Sainiemi, S. Franssila, A. Marmur, O. Ikkala, R. H. Ras,

Superhydrophobic tracks for low-friction, guided transport of water droplets. Adv. Mater.

23, 2911–2914 (2011). doi:10.1002/adma.201100461 Medline

26. H. Mertaniemi, R. Forchheimer, O. Ikkala, R. H. A. Ras, Rebounding droplet-droplet

collisions on superhydrophobic surfaces: From the phenomenon to droplet logic. Adv.

Mater. 24, 5738–5743 (2012). doi:10.1002/adma.201202980 Medline

27. T. Verho, J. T. Korhonen, L. Sainiemi, V. Jokinen, C. Bower, K. Franze, S. Franssila, P.

Andrew, O. Ikkala, R. H. Ras, Reversible switching between superhydrophobic states on

a hierarchically structured surface. Proc. Natl. Acad. Sci. U.S.A. 109, 10210–10213

(2012). doi:10.1073/pnas.1204328109 Medline

28. R. Massart, E. Dubois, V. Cabuil, E. Hasmonay, Preparation and properties of monodisperse

magnetic fluids. J. Magn. Magn. Mater. 149, 1–5 (1995). doi:10.1016/0304-

8853(95)00316-9

29. D. Castelvecchi, New instrument for solo performance. Phys. Rev. Focus 15, 18 (2005).

30. B. Berkovsky, V. Bashtovoi, Instabilities of magnetic fluids leading to a rupture of

continuity. IEEE Trans. Magn. 16, 288–297 (1980). doi:10.1109/TMAG.1980.1060613

31. Y. Sahoo, A. Goodarzi, M. T. Swihart, T. Y. Ohulchanskyy, N. Kaur, E. P. Furlani, P. N.

Prasad, Aqueous ferrofluid of magnetite nanoparticles: Fluorescence labeling and

magnetophoretic control. J. Phys. Chem. B 109, 3879–3885 (2005).

doi:10.1021/jp045402y Medline

32. S. Srinivasan, G. H. McKinley, R. E. Cohen, Assessing the accuracy of contact angle

measurements for sessile drops on liquid-repellent surfaces. Langmuir 27, 13582–13589

(2011). doi:10.1021/la2031208 Medline

33. G. R. J. Artus, S. Jung, J. Zimmermann, H.-P. Gautschi, K. Marquardt, S. Seeger, Silicone

nanofilaments and their application as superhydrophobic coating. Adv. Mater. 18, 2758–

2762 (2006). doi:10.1002/adma.200502030

34. D. Quéré, Non-sticking drops. Rep. Prog. Phys. 68, 2495–2532 (2005). doi:10.1088/0034-

4885/68/11/R01

35. J. M. D. Coey, Magnetism and Magnetic Materials (Cambridge Univ. Press, Cambridge,

2010).