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Abstract
Marangoni and elastocapillary effects are well-known as driving forces in the self-organization of floating objects at air-water interfaces. The release of surface active compounds generates Marangoni flows that cause repulsion, whereas capillary forces drive attraction. Typically, these interactions are non-directional and mechanisms to establish directional connections between the self-organizing elements are lacking. In this work, we unravel the mechanisms involved in the self-organization of a linear amphiphile into millimeter-long filaments that form connections between floating droplets. First, we show how the release of the amphiphile tetra(ethylene glycol) monododecyl ether from a floating source droplet onto the air-water interface generates a Marangoni flow. This flow extrudes self-assembled amphiphile filaments which grow from the source droplet, and concomitantly repels floating droplets in the surroundings. A hydrophobic drain droplet that depletes the amphiphiles from the air-water interface directs the Marangoni flow and thereby the growing filaments. We show how these filaments, upon tethering to the drain, potentially facilitate internal Marangoni convection and elastocapillary effects, which attract the drain back towards the source droplet. Furthermore, this concept establishes connections that are selective to the composition of the drain droplets – which influences the rate at which they deplete the amphiphile – such that repulsive and attractive forces can be balanced. Thereby, we provide a novel method through which directional attraction can be established in synthetic self-organizing systems, and advance our understanding of how complexity arises from simple building blocks.
Supplementary materials
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Supplementary Information
Description
Additional experimental details, Supplementary Figures and Supplementary movies corresponding to
the microscopy images.
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Title
SI Movie 1
Description
Fluorescence microscopy movie corresponding to Figure 2b-2f. First, a 1.0 µL C12E4 source droplet is deposited, followed by deposition of 1.0 µL of 10% NaOleate/OA drain solution at t = 0s. The drain moves towards the source while fluorescent particles suspended in solution visualize the flow of the medium.
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SI Movie 2
Description
Fluorescence microscopy movie corresponding to Figure 2h. 1.0 µL of 10% NaOleate/OA drain solution has been deposited onto an air/water interface on which a 1.0 µL C12E4 source droplet was already present. The drain moves towards the source while fluorescent particles suspended in solution visualize the flow of the medium.
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SI Movie 3
Description
Optical microscopy movie corresponding to Figure 3. 1.0 µL of OA (left); 10% NaOleate/OA (middle); and 10% C12E4/OA (right) was deposited, respectively, onto an air/water interface on which a 1.0 µL C12E4 source droplet containing 5.5 mg/mL Oil Red O was already present. Left, right: the drain droplets move rapidly towards the source, absorbing the red dye along with the filaments in which it is contained. Middle: the dye is not absorbed, even upon collision with the source, as filaments cluster around the drain.
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SI Movie 4a
Description
Fluorescence and optical microscopy movie corresponding to Figure 4a-c. 1.0 µL of OA (a); 10% NaOleate/OA (b); and 10% C12E4/OA (c) was deposited, respectively, onto an air/water interface on which a 1.0 µL C12E4 source droplet was already present. Fluorescent particles (a,b) and microdroplets (c) visualize the movement of the liquid inside the drain droplets. Notably, no significant movement is observed inside the 10% NaOleate/OA drain droplet, while convection patters appear irregularly in the OA and 10% C12E4/OA drains.
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SI Movie 4b
Description
(see legend SI Movie 4a)
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SI Movie 4c
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(see legend SI Movie 4a)
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SI Movie 5
Description
Optical microscopy movie corresponding to Figure 5a. First, a 1.0 µL C12E4 source droplet is deposited, followed by deposition of a MolSieve drain at t = 0s. The drain initially moves away from the source, but eventually filaments attach to keep it in place and even attract the MolSieve slightly towards the source.
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SI Movie 6
Description
Optical microscopy movie corresponding to Figure 6. First, a 1.0 µL 20% C12E4/OA drain droplet is deposited, followed by deposition of a 1.0 µL C12E4 source. Zooming in on the drain, we observe how filaments cluster and wrap around the drain droplet as it approaches the source.
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SI Movie 7
Description
Fluorescence microscopy movie corresponding to Supplementary Figure 1. First, a 1.0 µL C12E4 source droplet is deposited, followed by deposition of 1.0 µL of OA drain solution at t = 0s. The drain moves towards the source while fluorescent particles suspended in solution visualize the flow of the medium.
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SI Movie 8
Description
Confocal microscopy movie corresponding to Supplementary Figure 3. A MolSieve is deposited onto NaAlg/NaCl solution seeded with fluorescent particles. The particles indicate a constant flow towards the floating MolSieve.
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SI Movie 9
Description
Optical microscopy movie corresponding to Supplementary Figure 4. A 1.0 µL C12E4 source and 1.0 µL 20% C12E4/OA drain droplet were deposited. Upon suddenly changing the humidity by lifting the covering petri dish, the source droplet bursts into multiple fragments and the filament cluster partially detaches from the drain.
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