Controlling crystallization of responsive microgel particles via cyclic stimuli

自组装的晶体材料在纳米技术的不同领域提供了丰富的应用，包括光子学和等离子、声子学和电子学。然而，结构材料的基本光学、声学和电学性质对缺陷和晶体缺陷高度敏感，这可能会限制材料的性能。在这里，我们试图探索一条新的、实用的非平衡路线，以创造无缺陷的晶体，这依赖于外部刺激的周期性应用。我们专注于由微凝胶颗粒组成的胶体系统，这些微凝胶颗粒可以随着光脉冲或温度变化而膨胀或收缩，从而使我们能够动态控制有效密度和颗粒相互作用。通过以循环方式将材料暴露在外部刺激下，使得颗粒经历周期性膨胀和坍塌的呼吸运动，颗粒可以克服局部重排障碍，从而使晶界和晶格结构中的其他缺陷退火化。在适当的条件下，我们假设这种循环呼吸方案将允许我们将材料推向其热力学基态--完美晶体。我们将使用一种互补的方法来探索这一新的结晶路线，包括流体-流体界面上的微凝胶颗粒实验和广泛的颗粒分辨计算机模拟，后者既用于指导和解释实验。具体地说，我们的目标是通过实验和数值研究相结合的方法，建立：i)通过循环外部刺激改变胶体粒子结晶度的证明原理概念；ii)实现对结晶过程的完全控制的最佳呼吸条件，最终允许无缺陷的晶体；以及iii)循环呼吸作为一种通用和多功能的非平衡途径的适用性，以提供进入平衡态的途径，特别是在热力学基态可能难以达到的情况下，例如在二元混合物和具有弯曲界面的材料中。

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.