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Self-shaping of oil droplets via the formation of intermediate rotator phases upon cooling

Nature volume 528, pages 392–395 (2015)Cite this article

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Abstract

Revealing the chemical and physical mechanisms underlying symmetry breaking and shape transformations is key to understanding morphogenesis1. If we are to synthesize artificial structures with similar control and complexity to biological systems, we need energy- and material-efficient bottom-up processes to create building blocks of various shapes that can further assemble into hierarchical structures. Lithographic top-down processing2 allows a high level of structural control in microparticle production but at the expense of limited productivity. Conversely, bottom-up particle syntheses3,4,5,6,7,8 have higher material and energy efficiency, but are more limited in the shapes achievable. Linear hydrocarbons are known to pass through a series of metastable plastic rotator phases before freezing9,10. Here we show that by using appropriate cooling protocols, we can harness these phase transitions to control the deformation of liquid hydrocarbon droplets and then freeze them into solid particles, permanently preserving their shape. Upon cooling, the droplets spontaneously break their shape symmetry several times, morphing through a series of complex regular shapes owing to the internal phase-transition processes. In this way we produce particles including micrometre-sized octahedra, various polygonal platelets, O-shapes, and fibres of submicrometre diameter, which can be selectively frozen into the corresponding solid particles. This mechanism offers insights into achieving complex morphogenesis from a system with a minimal number of molecular components.

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Figure 1: Schematic of the shape transformations observed during cooling of emulsion droplets of pure hydrocarbons in water, in the presence of 1.5 wt% surfactant.
Figure 2: Choice of surfactant and cooling rate can determine particle shape and aspect ratio.
Figure 3: Illustrative examples of liquid drop shapes that would be unstable in the absence of elastic properties of the fluid-drop material.
Figure 4: Schematic presentation of the drop-shape deformation mechanism, driven by the formation of interfacial plastic phases.

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Acknowledgements

This work was funded by the European Research Council (ERC) grant to S.K.S., EMATTER (#280078). The study falls under the umbrella of European networks COST MP 1106 and 1305 and the capacity building project BeyondEverest of the European Commission (grant no. 286205).

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Authors and Affiliations

  1. Department of Chemical and Pharmaceutical Engineering, Faculty of Chemistry and Pharmacy, Sofia University, Sofia, 1164, Bulgaria

    Nikolai Denkov, Slavka Tcholakova, Ivan Lesov & Diana Cholakova

  2. Department of Materials Science & Metallurgy, Active and Intelligent Materials Laboratory, University of Cambridge, Cambridge, CB3 0FS, UK

    Stoyan K. Smoukov

Authors
  1. Nikolai Denkov
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  2. Slavka Tcholakova
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  3. Ivan Lesov
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  4. Diana Cholakova
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Contributions

N.D. and S.T. conceived the main idea for the study, designed the experiments and performed most of the result interpretation, N.D. wrote the first draft, I.L. and D.C. performed the experiments and prepared the figures, S.K.S. contributed important ideas for the experiments and data and mechanism interpretation, and edited the text.

Corresponding author

Correspondence to Stoyan K. Smoukov.

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Extended data figures and tables

Extended Data Figure 1 Solid particles with various shapes.

a–h, The shapes were obtained by freezing of deformed hexadecane (a–d), heptadecane (e), tetradecane (f), or eicosane (g, h) drops in emulsions, stabilized by 1.5 wt% of different surfactants: a, non-ionic Tween 60; b, c, non-ionic Brij 78; d, cationic CTAB; e, f, non-ionic Tween 40; g, h, Brij 78. Scale bars, 20 μm.

Extended Data Figure 2 Snapshot images of multiple hexadecane particles, obtained in 1.5 wt% solutions of various surfactants.

a–h, Tween 60 (a–e), Brij 58 (f) and Tween 40 (g, h). a–d, Consecutive images from the evolution of emulsion droplets stabilized by Tween 60. e, Rod-like particles before freezing. f, Frozen triangular particles. g, Frozen tetragonal platelets. h, Frozen toroidal particles. The initial drop sizes of the particles are indicated on the pictures. Cooling rates are 0.5 K min−1, except for h, 2 K min−1.

Extended Data Figure 3 Images proving that the deformed drops are still fluid.

Extending drops collide with each other and bend, as shown with the black arrow. Images of hexadecane drops in 1.5 wt% aqueous solution of Brij 58, cooled with rate of 1 K min−1.

Extended Data Figure 4 Experimental setup.

a, Schematic presentation of the cooling chamber with optical windows, used for microscope observation of the emulsion samples. b, The studied emulsions are contained in glass capillaries, placed in the thermostatic chamber and observed through the optical windows.

Extended Data Table 1 Hydrophilic–lipophilic balance values of the used non-ionic surfactants
Full size table

Supplementary information

Supplementary Information

This file contains Supplementary Text and Data and an additional reference. (PDF 150 kb)

Video with the shape transformations of hexadecane-in-water drops

Video with the shape transformations of hexadecane-in-water drops, stabilized by the nonionic surfactant Brij 58 and cooled with rate of 0.4 K/min. This video illustrates the drop-shape evolution shown in Figures 1 and 2. (AVI 4952 kb)

Video with the shape transformation of hexadecane-in-water drops

Video with the shape transformation of hexadecane-in-water drops, stabilized by the nonionic surfactant Tween 60 and cooled with rate of 0.4 K/min. This video demonstrates the formation of doughnut-shaped drops which could exist only if the drops contain liquid-crystalline material with certain elastic properties. (AVI 3637 kb)

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Denkov, N., Tcholakova, S., Lesov, I. et al. Self-shaping of oil droplets via the formation of intermediate rotator phases upon cooling. Nature 528, 392–395 (2015). https://doi.org/10.1038/nature16189

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