Controlling crystallization of responsive microgel particles via cyclic stimuli

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Self-assembled, crystalline materials offer a wealth of applications in different areas of nanotechnology, including photonics and plasmonics, phononics, and electronics. However, the fundamental optical, acoustic and electronic properties of a structured material are highly sensitive to imperfections and crystallographic defects, which can consequently limit the performance of the material. Here we seek to explore a new and pragmatic non-equilibrium route towards the creation of defect-free crystals, which relies on the periodic application of an external stimulus. We focus on colloidal systems composed of microgel particles that can swell or shrink in response to a light pulse or change in temperature, thus allowing us to dynamically control the effective density and particle interactions. By exposing the material to an external stimulus in a cyclic manner, such that the particles undergo a breathing motion of periodic swelling and collapsing, the particles may overcome local rearrangement barriers and consequently anneal the grain boundaries and other defects in the lattice structure. Under the right conditions, we hypothesize that this cyclic breathing protocol will allow us to drive the material toward its thermodynamic ground state of a perfect crystal. We will explore this new crystallization route using a complementary approach involving experiments of microgel particles on fluid-fluid interfaces and extensive particle-resolved computer simulations, the latter serving both to guide and explain the experiments. Specifically, we aim to establish, by means of combined experimental and numerical studies: i) the proof-of-principle concept for changing the degree of crystallinity in colloidal particles through cyclic external stimuli; ii) the optimum breathing conditions for achieving full control over the crystallization process, ultimately allowing for a defect-free crystal; and iii) the applicability of cyclic breathing as a general and versatile non-equilibrium pathway to provide access to the equilibrium state, especially in cases where the thermodynamic ground state may be difficult to reach, such as in binary mixtures and materials with curved interfaces.