Control of the structural evolution in self-organization of soft matter by external stimulations

Abstract

Engineering soft matter has been studied intensively for diverse applications because of its excellent chemical, mechanical, optical, and biological properties. From molecular scale polymer brush to macroscopic gels, soft matter has their own structure, and its structural flexibility makes them valuable. In this thesis, I deal with responsive self-organization of soft matter - colloidal crystals and responsive hydrogels, especially - by external stimulations and discuss for its potential applications. In Chapter 2, I report colloidal photonic crystals whose bandgap can be dynamically tuned by applying an electric field, where the bandgap changes occur at low actuation voltages with fast response times. To achieve this, I synthesized highly charged polystyrene (PS) particles of all equal size that formed spontaneously crystalline colloidal arrays (CCAs) in aqueous medium due to strong electrostatic repulsion among the PS particles. Under alternating current (AC) electric fields, the CCAs exhibited a dynamic modulation of the reflection color over the visible range, synchronized with the imposed AC field. The shift in the bandgap can be attributed to compression or relaxation of the colloidal lattice along the direction of the electric field. Therefore, the magnitude of the bandgap shift was controlled by varying the intensity of the AC field. For higher electric field intensities, the magnitude was also enhanced owing to the stronger electrokinetic force exerted on the charged PS particles. The actuation voltage and response time of the bandgap modulation were 2.2 V and 50 ms, respectively, which are considerably smaller than previously reported values for colloidal crystals due to high colloidal mobility in low viscosity medium. Furthermore, bandgap of CCAs was fixed at desired position when I used direct current (DC) electric field. Bandgap was shifted within seconds after field application, and then find equilibrium position after few seconds. Finally,...