Abstract
Orientational order in condensed matter plays a key role in determining material properties such as ferromagnetism, viscoelasticity or birefringence. We studied purely orientational ordering in closely-packed one-patch colloidal particles confined between flat substrates, where the particles can only rotate and are ordered via the sticky interaction between the patches. For the first time, we experimentally realized a rich variety of mesoscopic patterns through orientational ordering of colloids by controlling patch size and confinement thickness. The combination of experiment and numerical simulation reveals the decisive role of confinement: An ordered state(s) is selected from the (meta)stable options in bulk when it is commensurate with the system geometry and boundary conditions; otherwise, frustration induces a unique order. Our study offers a new means of systematic control over mesoscopic structures via orientational ordering in patchy particles. The system would also possess unique functionalities through the rotational response of the particles to external stimuli.
Highlights
Material properties of condensed matter owe a great part to the positional and orientational order of unit structures[1]
A colloidal system exhibiting purely orientational ordering recently has been studied in closely-packed spherical patchy particles[12,13,14,15,16], where the particle motion is almost purely rotational (Fig. 1) and the particles are ordered by the inter-patch attraction
The results show a great role of spatial confinement in the ordering: A state which is commensurate with the geometry and boundary conditions of confinement is selected from the degenerate options in a bulk system, or even a unique state appears when none of those options in bulk is commensurate
Summary
Material properties of condensed matter owe a great part to the positional and orientational order of unit structures[1]. Patchy colloidal particles have undergone rigorous study as an ideal model system with a well-defined interaction anisotropy[7] These particles possess sticky patches on their surfaces (Fig. 1), their positions and size dictating the directions and number of inter-patch attractive bonds. They have been called molecular colloids, and interesting results due to their anisotropy have been reported; e.g. new colloidal crystal phases such as a kagome lattice[8,9], equilibrium gel phases[10,11], and phase behaviour closely related to protein solutions[5], making them a powerful mesoscopic model system. Inherently complex: A patch size often gives rise to multiple states with identical internal energy, i.e. degeneracy in ground states, resulting in multiple (meta)stable states with similar free energy[12,13,14,16]
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