3D and 2D Microrheology of soft matter systems consisting of particles

Soft matter systems consist of a large range of materials, including polymers, emulsions, foams, granular materials, etc. Cells, tissues and organisms also belong to the category of soft matter systems. All these systems play an important role in our daily life and in industrial applications. Because soft matter systems are sensitive to external applied forces and stimuli, they can be used to fabricate stimulus-responsive materials, such as actuators and sensors. The rheological properties of soft matter systems determine how they deform under forces. Understanding the rheological properties of soft matter systems on different length scales can help to design the stimulus-responsive materials. Depending on the probed length scale, rheology can be divided into bulk rheology, microrheology and nanorheology. Among these methods, microrheology probes the rheological properties of soft matter systems on the micrometer scale. It gives essential information on the interactions between the building blocks of the systems. In this thesis, the microrheological properties of two soft matter systems are studied. The first studied systems are the magneto-responsive hybrid gels (MRGs). A MRG consists of a polymer network swollen in a good solvent, and embedded micro-sized magnetic particles. MRGs have potential applications as soft actuators, artificial muscles, and sensors, etc. It is well known that the magnetic responsibility of the MRGs arises from the magnetic interactions between the embedded magnetic particles. However, the details about how the magnetic particles couple to the surrounding polymer network on a microscopic scale are still not clear. In this thesis, an experimental model system for MRGs is developed. It consists of paramagnetic particle chains inside the soft gels. Using laser scanning confocal microscopy, the 3D rearrangement of the magnetic particles in the soft gels under a homogeneous magnetic field is studied. Under an applied magnetic field, the magnetic interactions between the magnetic particles tend to align the magnetic particle chains along the field direction. However, this is impeded by the cross-linked polymer network. In the experiments, I find that the interplay between these two effects leads to rich morphological changes (e.g., buckling) of the paramagnetic particle chains under the magnetic field. Together with theorists from Heinrich-Heine-Universität Düsseldorf and Universität Stuttgart, we develop a simplified theoretical model and perform simulations to understand this magnetic-field-induced buckling behavior. In addition, in order to understand how the magnetic particles interact with each other in the gel matrix, theorists from Heinrich-Heine-Universität Düsseldorf propose a theory which can calculate the effective interactions between particles in the elastic matrix analytically. To confirm the theory, I perform experiments on small groups of paramagnetic particles embedded in a soft gel matrix. Applying an external magnetic field induces magnetic forces between the particles. Rotating the magnetic field tunes these forces. In this thesis, it will be shown that the theory can correctly predict the change of positions of the magnetic particles embedded in the gel under the magnetic field. The second studied systems consist of microgels at the water/oil interface. Microgels have been used to prepared emulsions which are responsive to temperature or pH value. However, it is still not clear how the microgels interact at the interface, and how these interactions influence the rheological properties of the microgel-laden interfaces. In this thesis, the rheological properties of the microgel monolayers at the water/oil interface are studied on the single-microgel level. It is observed that driven by the capillary interactions, the microgels at the water/oil interface tend to aggregate. At low surface coverage, the aggregated microgels form open networks which show dominating elasticity. Passive microrheology is used to determine the elasticity of these microgel networks at the interface. At high surface coverage, when the microgels become closely packed at the water/oil interface, the elasticity of the microgel monolayers is dominated by the overlapping of the microgels. Their elasticity can be measured using the active microrheology with the magnetic dimers as probes. The advantage of using microrheology is that the structure of the microgel monolayers can be well observed during the rheological measurements