Switching Colloidal Superstructures by Critical Casimir Forces

Recent breakthroughs in colloidal synthesis promise the bottom-up assembly of superstructures on nano- and micrometer length scales, offering molecular analogues on the colloidal scale. However, a structural control similar to that in supramolecular chemistry remains very challenging. Here, colloidal superstructures are built and controlled using critical Casimir forces on patchy colloidal particles. These solvent-mediated forces offer direct analogues of molecular bonds, allowing patch-to-patch binding with exquisite temperature control of bond strength and stiffness. Particles with two patches are shown to form linear chains undergoing morphological changes with temperature, resembling a polymer collapse under poor-solvent conditions. This reversible temperature switching carries over to particles with higher valency, exhibiting a variety of patch-to-patch bonded structures. Using Monte Carlo simulations, it is shown that the collapse results from the growing interaction range favoring close-packed configurations. These results offer new opportunities for the active control of complex structures at the nano and micrometer scale, paving the way to novel temperature-switchable materials.

A novel strategy to assemble colloidal superstructures by critical Casimir forces is presented. These forces provide thermodynamic analogues of quantum mechanical Casimir forces, acting between surfaces immersed in critical solvents. Using patchy particles, specific patch-to-patch binding into colloidal superstructures as analogues of molecular structures is achieved. This opens new routes toward complex material assembly at micrometer and nanometer length scales.

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