Particles with directional interactions are promising building blocks for new functional materials and may serve as models for biological structures1,2,3. Mutually attractive nanoparticles that are deformable owing to flexible surface groups, for example, may spontaneously order themselves into strings, sheets and large vesicles4,5,6. Furthermore, anisotropic colloids with attractive patches can self-assemble into open lattices and the colloidal equivalents of molecules and micelles7,8,9. However, model systems that combine mutual attraction, anisotropy and deformability have not yet been realized. Here we synthesize colloidal particles that combine these three characteristics and obtain self-assembled microcapsules. We propose that mutual attraction and deformability induce directional interactions via colloidal bond hybridization. Our particles contain both mutually attractive and repulsive surface groups that are flexible. Analogously to the simplest chemical bond—in which two isotropic orbitals hybridize into the molecular orbital of H2—these flexible groups redistribute on binding. Via colloidal bond hybridization, isotropic spheres self-assemble into planar monolayers, whereas anisotropic snowman-shaped particles self-assemble into hollow monolayer microcapsules. A modest change in the building blocks thus results in much greater complexity of the self-assembled structures. In other words, these relatively simple building blocks self-assemble into markedly more complex structures than do similar particles that are isotropic or non-deformable.
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We thank B. G. P. van Ravensteijn for providing non-deformable, fluorescein functionalized snowman-shaped particles, S. I. R. Castillo for taking the SEM images, and J. D. Meeldijk and C. T. W. M. Schneijdenberg for help with freeze drying and TEM. This work is part of the research programmes VICI 700.58.442 and TOP-GO 700.10.355, which are financed by the Netherlands Organization for Scientific Research. We thank A. van Blaaderen and M. Dijkstra for discussions, and M. de Jong for reading the manuscript.
Extended data figures
A self-assembled sheet of mutually attractive, isotropic, deformable particles freely moves and rotates in the solution showing its hexagonal ordering and its monolayer thickness.
Microcapsule. Mutually attractive, anisotropic, deformable particles self-assemble into monolayer microcapsules
This microcapsule translates and rotates close to the glass slide, showing particles with both six and five nearest neighbours. The structure moves also with respect to the focal plane. Particles just below the focal plane are dark, and those just above the focal plane are bright.
The position of the focal plane, rz, is increased from the glass slide to 5.3 μm above the glass slide, showing the bottom layer, the hollow interior, and the top layer of a microcapsule.
Mutually attractive, anisotropic, deformable particles flow towards the contact line forming a dense monolayer on the glass slide with a stable cavity. Note that particles at the edge of the dense phase form non-lasting bonds.
Cavity phase. At particle volume fractions of about 0.2, a highly fluctuating cavity phase is observed
Mutually attractive, anisotropic, deformable particles form non-lasting bonds, and we observe coexisting regions on the order of 1-10 μm with either high particle concentrations or virtually no particles, i.e. dense curved structures around cavities.
Mutually attractive, anisotropic, deformable particles are centrifuged in a thin cell and subsequently the cell is turned upside down. In the diluting sediment, cavities are observed.
Simulation snapshot of mutually attractive, anisotropic, deformable particles that form a curved monolayer with in plane protrusions. Each particle has eight satellite spheres of 0.6 times the size of its central sphere.
About this article
Soft Matter (2019)