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Abstract
Motile droplets provide an attractive platform for liquid matter-based applications and protocell analogues displaying life-like features. The functionality of collectively operating droplets increases by the advance of well-designed (physico)chemical systems directing droplet-droplet interactions. Here, we report a strategy based on crystalline surfactant layers at air/water interfaces, which sustain the propulsion of floating droplets and at the same time shape the paths for other droplets attracted by them. First, we show how decylamine forms a closed, crystalline layer that remains at the air/water interface. Second, we demonstrate how aldehyde-based oil droplets react to decylamine in the crystalline layer by forming an imine, causing the droplets to move through the layer while leaving behind an open channel (comparable to “Pacman”). Third, we introduce tri(ethylene glycol) monododecylether (C12E3) droplets in the crystalline layer. Whereas the crystalline layer suppresses the motion of the C12E3 droplets, the aldehyde droplets create surface tension gradients upon depletion of surfactants from the air/water interface, thereby driving Marangoni flows that attract the C12E3 droplets as well as the myelin filaments they grow: Causing the C12E3 droplets to chase, and ultimately catch, the aldehyde droplets along the channels they have created, featuring a predator-prey analogy established at an air/water interface.
Supplementary materials
Title
Video S1 – Optical microscopy recording corresponding to Figure 2b.
Description
Decylamine droplet (0.5 µL) deposited on a 1 mM DA aqueous solution at the air/water interface. The DA droplet forms short transient filaments, which are simultaneously released to form a crystalline layer at the a/w interface.
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Video S2 – Optical microscopy recording corresponding to Figure 3b-c.
Description
The self-propelled motion of a OE-CHO (0.5 µL) oil droplet on a DA crystalline layer. The OE-CHO consumes the DA crystalline layer, creating a millimeter-wide open channel and the shape of OE-CHO changes during the motion. The oil droplet avoids open channels by changing its direction of motion and showing a self-evading behavior.
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Video S3 – Camera recordings corresponding to Figure 3d-e.
Description
Three replicate experiments on the self-propelled motion of an OE-CHO oil droplet (1.0 µL) through a DA crystalline layer formed on a large size Petri dish (100 mm x 15 mm). The self-propelled motion was sustained for 60 min.
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Video S4 – Camera recordings corresponding to Figure 4b.
Description
Four replicate experiments on the self-propelled motion of 4 OE-CHO oil droplets (1.0 µL) through a DA crystalline layer formed on a large size Petri dish (100 mm x 15 mm). The OE-CHO droplets merged, bounced to the wall of the Petri dish or stopped moving after a certain time; showing the variability in the behavior of OE-CHO droplets.
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Video S5 – Optical microscopy recording corresponding to Figure 6e.
Description
A C12E3 droplet (1.0 µL) is positioned at one end (top) of a 1 cm long open channel formed in a DA crystalline layer. The myelins started to grow from the C12E3 droplet after introducing the OE-CHO oil droplet (0.5 µL) at the other end of the channel (bottom).
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Video S6 – Optical microscopy recording corresponds to Figure 7b.
Description
Three replicate experiments on the predator-prey interaction of 4 C12E3 droplets (0.5 µL) with a OE-CHO oil droplet (0.5 µL) deposited on the DA crystalline layer at the air-water interface.
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Supplementary Information
Description
Materials, Methods, SI Figures.
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