Study the Rheology Property of Soft Particle Suspension under Electric Fiel

In the past decades, colloidal suspensions with special exchangeable rheological properties were studied as "smart" material, e.g. electrorheological (ER) fluids. In precisely controlled electric field, the state of ER fluids can go back and forth from liquid to solid. A main theory explains this phenomenon as the colloidal particles in ER fluid are strongly polarized by the applied electric field. And every particle acts as a huge "dipole" that interacts and aligns along the electric field, forming a partly ordered structure, which helps the system to resist the shear force perpendicular to the direction of the electric field. These ER fluids can turn from liquid to solid. ER fluids have been widely used in transmission technology, control tools, dampers, brakes and tactile technology.;Typical ER fluid contains two parts: colloidal particles with big dielectric constant as dispersate; insulating dispersion liquid, such as silicone oil. Colloids in traditional ER fluids are hard particles, for example, metal doped TiO2, SiO2 particles and conductive polyaniline (PANI) particles. It should be noticed that the polarity of particles can be affected by their shape deformation. In general, there's linear relationship between the degree of polarity and the applied electric field strength for hard particles; as for soft particles, such relationship can be nonlinear, because of deformation. It can be inferred that the ER effect of soft particle ER fluids should be quite different. Studying on the ER effect of soft particles can also be helpful to understand the relationships between deformation and polarity.;In this thesis, poly(N-isopropylacrylamide) (PNIPAM)-based microgels was firstly introduced to be the model soft colloids. It is easy to be synthesized by surfactant-free precipitation polymerization, and it can be functionalized by copolymering with other comonomers. Comonomers with different capabilities of polarization were chosen: methylacrylate (MAA), 3-[2-(methacryloyloxy)- ethyl](dimethyl)-ammonio]-1- propanesulfonate (DMAPS), 4-vinylpyrridine (4-VP) and [2-(methacryloyloxy) ethyl trimethylammonium] [bis(trifluoromethanesulfonyl).;Functionalized microgels were dispersed and swelled in hydroxyl silicone oil to form suspensions. From the ER experiments, the phenomena of microgel suspensions differed greatly from hard colloidal ER fluids. The yielding plateau appeared in hard colloidal ER fluids did not show in the ER experiments of microgel suspensions. Instead, the shear stress is enhanced by power law, a typical phenomenon of pseudoplastic fluids. In addition, the increased shear stress cannot return to its original state, even after the field is removed. The reason is suggested to be that microgels deformed and its polymer chains entangled under strong electric field; which leaded to the maintenance of fibrous structure formed by microgels after the field was gone. Those highly entangled fibrous structure was further identified in the super resolution confocal microscopy.;Besides microgels, hybrid elastic particles based on silicone rubber material (polydimethylsiloxane (PDMS)) were also used as a soft particle. Through emulsion template, ionic liquid filled PDMS particles were synthesized. Thanks to their elastic property, their ER effect was believed to be quite special. In ER experiments, these hybrid elastic particle systems represented two typical phenomena in different intervals: pseudoplastic fluid similar to microgel system and yielding like traditional hard colloidal system. Such exchangeable state, soft to hard states, can be controlled by tuning the shear rate, which may get promising application by this changeable property.