Although ‘active’ surfactants, which are responsive to individual external stimuli such as temperature1, electric2,3 or magnetic4 fields, light5,6, redox processes6,7 or chemical agents8, are well known, it would be interesting to combine several of these properties within one surfactant species. Such multi-responsive surfactants could provide ways of manipulating individual droplets and possibly assembling them into larger systems of dynamic reactors9,10. Here we describe surfactants based on functionalized nanoparticle dimers that combine all of these and several other characteristics. These surfactants and therefore the droplets that they cover are simultaneously addressable by magnetic, optical and electric fields. As a result, the surfactant-covered droplets can be assembled into various hierarchical structures, including dynamic ones, in which light powers the rapid rotation of the droplets. Such rotating droplets can transfer mechanical torques to their non-nearest neighbours, thus acting like systems of mechanical gears. Furthermore, droplets of different types can be merged by applying electric fields and, owing to interfacial jamming11,12, can form complex, non-spherical, ‘patchy’ structures with different surface regions covered with different surfactants. In systems of droplets that carry different chemicals, combinations of multiple stimuli can be used to control the orientations of the droplets, inter-droplet transport, mixing of contents and, ultimately, sequences of chemical reactions. Overall, the multi-responsive active surfactants that we describe provide an unprecedented level of flexibility with which liquid droplets can be manipulated, assembled and reacted.
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We acknowledge support from the Institute for Basic Science Korea, project code IBS-R020-D1.
Magnetic droplets attracted by magnetic field (00:00 – 00:39s). Video demonstrates attraction of droplets covered with magnetic nanosurfactants towards an externally placed permanent magnet (NdFeB block, K&J Magnetics D4Y0, magnetized along the long dimension, µ0M = 1.32T). Initially, the magnet was placed at the right side of the PTFE Petri dish and removed at 00:00s. At 00:04s, the magnet was moved to the left side of the Petri dish, and droplets were rapidly attracted to this side. Magnetic-field-directed assembly of magnetic droplets coated with Au-3O4 nanosurfactants (00:40 - 00:56s). The video demonstrates that magnetic droplets can be assembled by magnetic fields into closely-packed clusters. At 00:41s, magnet is placed below the PTFE Petri dish. Blue-colored droplets are moving and pack closely in the region of high magnetic field. Magnetic-field-directed assembly of magnetic droplets into a “hierarchical” structure (00:57 - 01:32s). Three different dyes are used to color the droplets, as explained in Fig. 1 in the main text. Initially, droplets are randomly distributed. When a magnet is placed below the PTFE Petri dish (00:58s), blue droplets with the highest content of magnetic nanosurfactants move the fastest towards the high-field region. The red and yellow droplets then follow.
The 900 µm droplets are coated with Au-Fe3O4 nanosurfactants and, initially (00:00 – 00:06s) are randomly distributed and spaced by, on average, several millimeters. When light (660 nm, 70 mW) irradiates one of the droplets (00:07s), the other droplets move rapidly toward the irradiated locus, forming a two-dimensional, closely-packed structure in less than 10s. Next, sequential light-driven assembly / spontaneous disassembly cycles are demonstrated (00:17 - 02:16s). Initially, the laser light is off and the droplets are randomly distributed (00:17s). When the laser is turned on (00:18s), droplets approach the irradiated spot, forming a bilayer aggregate (00:19 - 00:37s). The droplets in the bottom layer locate into the triangular gaps of the upper layer. When the light is off (00:38s), the ordered structure disassembles by thermal diffusion (00:37 - 00:57s). When the light is back on (00:58s), a compact structure forms again (00:58 - 01:12s). When the light is off (01:13s), the structure disassembles (01:13 - 01:21s). When the light is again on (01:21s), droplets assemble again (01:21 - 01:46s). When the light is off (01:47s), the ordered structure disassembles (01:47 - 02:03s). Finally, when the light is on, an ordered structure is yet again assembled (02:03 -02:16s). Light-induced assembly of droplets into a three-dimensional closely-packed structure (02:16 - 02:59s). The droplets are coated with Au-Fe3O4 nanosurfactants. The diameter of the droplets is ~ 500 µm. The laser light (660 nm, 70 mW) irradiation starts at (02:17s). The size of the assembled structure first increases to ~5 mm (02:17-02:47s) and then remains roughly constant (02:48 - 02:59s). Note that some droplets at the border of the aggregate are in equilibrium with the aggregate and detach/reattach to it.
Diameters of the large and small droplets are ~1500 µm and ~500 µm, respectively. Densities of the large and small droplets are the same (ρ = 1.05 g mL-1). Light irradiates the center of the large droplet (00:01s), which causes attraction of the small droplets to its periphery. (00:17 - 00:32s) Diameters of the large and small droplets are ~2000 µm and ~400 µm, respectively. The density of the small droplets (ρ = 1.02 g mL-1) is lower than that of the large droplet (ρ = 1.05 g mL-1). Laser beam is at the center of the large droplet (00:17 - 00:20s). Smaller, lighter droplets are not only attracted towards the larger drop, but also cover its top surface (00:20 - 00:32s).
Droplets are ~900 µm in diameter and are covered with Au-Fe3O4 nanosurfactants. Initially (00:00s), light irradiates the edge of the central droplet (#0 in the Fig. 3b) and the droplets rotate. The irradiation spot then moves to this droplet’s center (00:00 - 00:08s) causing the droplets to stop rotating. The laser beam then moves to the edge (00:08 - 00:14s), center (00:15 - 00:17s), edge (00:18 - 00:24s), center (00:25 - 00:31s), edge (00:32 - 00:34s), and back to center (00:35 - 00:47s).
Part 1. Mechanical transmission of torque to non-nearest-neighbor “droplet gears”. This is a top view of droplets below the surface of DCB/toluene in a Petri dish (see scheme in Supplementary Fig. 15). Rotation of the droplet near the bottom of the image is powered by laser-induced convection. This torque is “transmitted” to the neighboring droplet which, in turn, powers its opposite-side neighbor. Part 2. Direction of rotation changes by changing the position of the focused beam around the droplet’s perimeter. The history of movement of the laser beam is summarized in Supplementary Fig. 16. The diameter of the droplets is ~900 µm. Initially, light irradiates the left-side edge of the central droplet (00:26s) (marked as A in Supplementary Fig. 16). The irradiation site then moves to the down-side edge (00:29s) (B in Supplementary Fig. 16). Then, the irradiation site moves to the neighboring droplet at the bottom of the image (00:31s) (from C to D in Supplementary Fig. 16). Part 3. Rotation of colored droplets without exchange of contents. The large droplet is ~ 1200 µm in diameter and contains uncolored solution; the small droplets are ~500 µm in diameter and their contents are colored with a blue dye (colours are visible towards the end of the video, when the laser is turned off). Although laser-irradiated the assembled droplets are rotating and transmitting torques, they do not exchange their contents.
Focusing laser light onto and heating the region between two proximal droplets creates short-lived (can disappear in less than 50 ms) bubbles – probably filled with water vapor and emerging from gaps in the disrupted nanosurfactant layer.
Experiment demonstrating the direction of droplet’s rotation is independent of the position of the laser spot.
For the schematic side views corresponding to this video, see Supplementary Fig. 5. The video itself shows the top view – i.e., the direction of gravity is into the video frame. When laser is focused on the lower part of the droplet (with respect to gravity), rotation starts (00:04s - 00:10s). Notice the refraction in the rising hot liquid. When the laser spot is shifted to the upper portion of the droplet (00:11s - 00:17s), rotation accelerates, but direction remains the same.
Part 1. Attraction and rotation of two droplets. Two droplets are initially close to each other. When the laser beam is centered onto the side of the “right” droplet closest to the “left” droplet, the former droplet attracts to the latter. While the laser remains focused at the region of droplets’ contact, the droplets keep rotating in opposite directions, but do not “repel” each other. Part 2. Droplet dynamics when laser heats its lower part. Laser beam is focused onto the lower part of the droplet (relative to gravity; gravity direction in the video is into the image). Droplet exhibits a “bouncing” motion: cycles of brief repulsion from the laser followed by attraction to the laser.
Part 1. Magnetic “flipping” of Janus droplets whose surface is in half covered with magnetic nanosurfactants. Magnetic/nonmagnetic droplets with diameters ~1000 µm were produced with the droplet soldering technique as described in the main-text Supplementary Fig. 19. Initially, a permanent magnet was placed ~ 5 mm below a PTFE dish (00:08s). The magnet was then moved to ~8 mm above the dish (00:09s). Part 2. Magnetically active pear-shape Janus droplets. At the beginning, magnetic/diamagnetic pear-shape droplets were aligned with their magnetic parts pointing “to the left” (00:21s). The droplets then responded to the changes in the position of the external magnet. Part 3. A magnetic “rocket” droplet. Initially, the magnetic/diamagnetic rocket-shaped droplet was placed at the upper left of the area shown. The droplet then followed an external permanent magnet.
The video accompanies main-text Fig. 5a. A dumbbell-shaped droplet is created by merging a droplet containing Congo red dye and a droplet containing acidic (HCl) solution, solvent in both parts is a 1:1 mixture of ethylene glycol and water. Laser is modulated (frequency 20 Hz, duty cycle 50%, average power density 200 W/cm2) and video frames are taken when laser is off. The speed of video is 10X the real time. Droplet is held stationary by a permanent magnet placed beneath the dish housing the droplet.
The video accompanies main-text Fig. 5b. A ~1.5 mm droplet containing 0.085 M CoCl2 in water and a ~ 1.2 mm droplet containing 3.3 M 2-methylimidazole in water are suspended in DCB and brought together using a laser. Upon contact and electrostatic welding, the droplets merge. Laser light is used to accelerate mixing. Eventually, the reaction produces crystals of a ZIF-67 metal organic framework (see main text).
The video accompanies main-text Fig. 5c. Sequence of reactions 2Cu2++ 4I- → 2CuI + I2; CuI + EDTA4-→ Cu-EDTA3- + I-. Reactor R1 at the top carries 1 µmol CuSO4 in 1:1 v/v mixture of EG/H2O; R2 in the middle carries 2 µmol KI in 1:1 v/v mixture of EG/H2O; R3 at the bottom carries 1 µmol Na4EDTA in 1:1 v/v mixture of EG/H2O. Reactors are oriented by the external magnetic field and electrostatically welded to connect via ~ 200 µm wide channels (00:01s). The channels are widened and the contents mixed by laser light (R1/R2 at 00:02s – 00:11s; R2/R3 at 00:17s – 00:20s). Yellow-brownish color is due to I2 produced in the first reaction; green is due to the mixing of I2 with the Cu-EDTA3- complex produced in the second reaction.
Calculated flows inside and outside of a single droplet when the region heated by the laser is located below, at or above the droplet’s “equator”.
Area heated by the laser is marked in yellow in the top-left scheme of Supplementary Fig. 7, 8, 9. Flows are shown in the yz-sections of the droplet. Grey streamlines trace the flow field. Black arrows indicate direction of fluid velocity, arrow length is logarithmic in velocity. Red arrows show viscous stress force on the droplet’s surface. Blue arrow shows horizontal y-component of the net force acting on the droplet. Droplet diameter is 0.5 mm. Laser spot diameter is 0.2 mm. Laser power is 70 mW, 30% of it is absorbed by the nanosurfactants at the droplet’s surface.
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Nature Reviews Materials (2018)