Controlling crystallization of responsive microgel particles via cyclic stimuli

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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.