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Abstract
Positional information is proposed as a key element in morphogenesis, where cells differentiate based on their position in an external morphogen gradient. Here, we demonstrate the position-dependent differentiation of floating surfactant droplets as they self-organize in a pH gradient, using the Marangoni effect to translate gradients of surface-active molecules into motion. We start with a field of surfactant microliter-droplets floating on water, that upon the release of surfactant drive local, outbound Marangoni flows and myelin filament growth. Next, we introduce a competing surfactant based on a hydrolysable amide, which is more surface active than the myelin surfactant and thereby inhibits the local Marangoni flows and myelin growth from the droplets. Upon introducing a pH gradient, the amide surfactant hydrolyses in the acidic region, so that the local Marangoni flows and myelin growth are reestablished. The resulting combination of local and global surface tension gradients produces a region of myelin-growing droplets and a region where myelin-growth is suppressed, separated by a wave front of closely packed droplets – featuring an organization that is reminiscent to the ‘French flag’-patterns emerging from concentration gradients in morphogenesis.
Supplementary materials
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Movie 1
Description
Optical microscopy recording corresponding to Figures 2c-d and S3: C12E3 myelin growth on aqueous solutions with amide surfactant 2 (left panel), with precursor 1-octylamine (middle panel), or with precursor DMMA (right panel). Myelin growth only fully halts if amide surfactant 2 is present. DMMA destabilizes the myelins, but myelins continue to grow. All solutions are set to pH 7 (100 mM PB).
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Movie 2
Description
Optical microscopy recording corresponding to Figure 3a:
Reactivation of C12E3 myelin growth on an aqueous solution with amide surfactant 2 (7.5 mM DMMA, 20 mM 1-octylamine) upon addition of HCl solution at t = 250 s. Initially, the pH of the solution is set to pH 7 (100 mM PB), and after addition of HCl the pH is measured to be 6.5.
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Movie 3
Description
Time lapse recording corresponding to Figure S4:
Control experiment for droplet swarm self-organization. NaCl (2 M, 1 mL) was injected into the solution instead of HCl at t = 0 s. Over the course of approx. 30 minutes myelins do not grow, but the droplets slowly drift towards the spot where NaCl was injected, and some droplets clump together.
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Movie 4
Description
Time lapse recordings corresponding to Figure 5a:
Full color (RGB, left) and contrast enhanced, blue channel (right) recordings of the self-organization of 50 C12E3 droplets in a pH gradient. The aqueous solution contains 7.5 mM DMMA, 20 mM 1-octylamine, 0.0025 wt% bromocresol purple and is set to pH 7 (100 mM PB). At t = 0 s, 1 mL HCl (2 M) is injected into the solution with the droplet dispensing robot. The ‘French flag’-pattern is established approx. 20 minutes after addition of the acid.
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Movie 5
Description
Time lapse recordings corresponding to Figures 6a-b:
Full color (RGB, top row) and contrast enhanced, blue channel (bottom row) recordings of the self-organization of 50 C12E3 droplets in a pH gradient. The aqueous solution contains 7.5 mM DMMA, 20 mM 1-octylamine, 0.0025 wt% bromocresol purple and is set to pH 7 (100 mM PB). At t = 0, the indicated amount of HCl (2 M) is injected into the solution with the droplet dispensing robot.
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Movie 6
Description
Time lapse recordings corresponding to Figure 6e:
Full color (RGB) recordings of the self-organization of 50 C12E3 droplets in pH gradients of different intensities (0.1, 0.5, 1, 2 mL HCl solution added). The aqueous solution contains 7.5 mM DMMA, 20 mM 1-octylamine, 0.0025 wt% bromocresol purple and is set to pH 7 (100 mM PB). At t = 0, the indicated amount of HCl (2 M) is injected into the solution with the droplet dispensing robot.
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