Polymerization of liquid oil polyhedra:
from physics of droplet faceting transitions to colloidal synthesis

O. Marin1,2,*, D. Zitoun2,3, E. Sloutskin1,2
1Bar-Ilan University, Physics Department, Ramat-Gan 5290002 Israel
2Bar-Ilan Institute of Nanotechnology & Advanced Materials, Ramat-Gan 5290002 Israel
3Bar-Ilan University, Department of Chemistry, Ramat-Gan 5290002 Israel
*[email protected] URL: https://elisloutskin.wixsite.com/softlab

KEY WORDS: colloids, icosahedra, polyhedra, interfacial freezing, surfactant-stabilized emulsions, scanning electron microscopy, focused ion beam.

The recently-discovered temperature-controlled self-faceting of liquid emulsion droplets provides a new route towards the production of polyhedral solid particles by UV-polymerizing the faceted emulsion droplets to fix their shapes. The self-faceted liquid droplets could be hitherto studied only by optical microscopy, with the low resolution limiting their accurate three-dimensional shape observation. Furthermore, their internal structure could not be resolved, leading recently to a significant controversy1,2 with respect to the physical mechanism of the self-faceting transition.

Guttman et al.1 attributed the transition to the elasticity of a two-dimensional (2D) hexagonally-packed monolayer molecular crystal forming at the surface of the droplets. In particular, the incompatibility of the hexagonal molecular packing with the closed-surface topology of the droplets’ surface induces 12 topological defects: lattice sites with a coordination number of 5 rather than 6, known as `disclinations’. The out-of-plane buckling of the disclinations, partly relieving the stresses within the 2D crystal, makes the liquid droplet adopt a shape of an icosahedron. A few other polyhedral shapes form at a lower temperature, where partial merging of the disclinations takes place.

The team of Denkov et al.2 attributed the self-faceting of liquid droplets to the formation within the droplet of a surface-adjacent, cylindrically curved, layer of a crystalline rotator-alkane phase. The layer was postulated to be ∼0.3µm thick, with a µm-size radius. Thus, this layer, if exists, is invisible for the optical microscopy. Furthermore, the polyhedral droplet shapes have been identified by Denkov et al.2 as octahedra, rather than icosahedra.

Here we employ UV-initiated polymerization, to fix the shapes of these droplets. Depending on the temperature chosen for the droplets' polymerization, we obtain solid icosahedra, polyhedral platelets, or rods. We determine their three-dimensional shape and internal structure with nanoscale resolution by combined focused ion beam slicing and scanning electron microscopy. Our observations strongly support the surface-freezing-driven mechanism proposed by Guttman et al.1 for the faceting phenomenon. In particular, no internal structure is observed inside the droplets. We also demonstrate the formation of such particles at hitherto-unreported sizes, well below the three-dimensional printing resolution (~20 m). Further development of this method may eventually enable mass-production of shaped micron- to nano- sized colloidal building blocks for three-dimensional metamaterials and other applications.

[1] S. Guttman et al., PNAS 113, 493 (2016); ibid., Langmuir 33, 1305 (2017).
[2] N. Denkov et al., Nature 528, 392 (2015); P. A. Haas et al., Phys. Rev. Lett. 118, 088001 (2017).