Controlling crystallization of responsive microgel particles via cyclic stimuli

微凝胶颗粒可以通过外部刺激（如温度或光）改变其体积和物理化学特性，为胶体结晶提供了极好的可控模型系统。当被限制在液体界面时，这些粒子代表了二维强相互作用的经典多体系统，这使得自组装现象的基础研究成为可能。该项目的目标是通过以周期性方式（即时间周期性方式）施加刺激来控制结晶过程和缺陷动态。基本假设是，通过使用优化的循环刺激，缺陷可以更快地退火，以产生具有长程有序的晶体阵列。在第一个资助期内，这一假设得到了概念验证研究的支持，该研究使用了在空气/水界面形成的单组分二维胶体晶体，这些晶体暴露于声学驻波。这种循环呼吸运动，其中可用的界面面积周期性地增加，被证明是有效的退火缺陷和晶界，并显着增加了系统中的整体秩序。此外，对刺激响应性微凝胶和二元混合物在液体界面处的行为的基础研究为限制下的体积相变和相变的确定性预测提供了新的见解。在第二个资助期内，我们将利用这些洞察力到三个不同的方向：i）在干燥过程中发生的毛细作用力及其在结构形成中的作用，ii）弯曲流形上的颗粒，如皮克林乳液中的球形表面，以及iii）从颗粒的二元和多元混合物中获得的新颖，奇异的结构。为了实现这些目标，我们将以互补的方式利用实验和理论进行成功的合作。

Microgel particles, which can change their volume and physicochemical characteristics by external stimuli, such as temperature or light, provide excellent controllable model systems for colloidal crystallization. When confined to a liquid interface, such particles represent two-dimensional strongly interacting classical many-body systems, which allow fundamental studies of self-assembly phenomena. The goal of the project is to control the crystallization process and the defect dynamics by applying stimuli in a cyclic, i.e. time-periodic, way. The basic hypothesis is that, by using optimized cyclic stimuli, defects can be annealed more quickly to produce crystalline arrays with long range order. In the first funding period, this hypothesis was supported by a proof-of-concept study using one-component, two dimensional colloidal crystals formed at the air/water interface, which were exposed to acoustic standing surface waves. This cycling breathing motion, in which the available interfacial area was periodically increased, proved effective in annealing defects and grain boundaries and significantly increased the overall order in the system. In addition, fundamental investigations on the behaviour of stimuli-responsive microgels and binary mixtures at liquid interfaces provides new insights into the volume phase transition under confinement and the deterministic predictions of phase transitions. In the second funding period, we will capitalize on these insights into three different directions: i) capillary forces occurring during drying and their role in structure formation, ii) particles on curved manifolds such as spherical surfaces in Pickering emulsions, and iii) novel, exotic structures obtainable from bi- and multinary mixtures of particles. To achieve these goals, we shall pursue our successful collaboration using experiments and theory in a complementary way.