Integrating novel rheological tools with microscopy to characterize viscoelastic properties of complex materials

Abstract

Rheological property of any material is the mechanical response when it undergoes stress or deformation. Characterizing these properties is essential because it can help in optimizing the formulation and processing of the materials. Over the years, different rheological techniques have evolved, however, with the growth of science and technology, the realm of material characterization faces new challenges, and this demands for developing innovative rheological tools. In this thesis, we identify certain limitations of the available techniques in the field of microrheology and interfacial rheology and overcome them using the novel rheological techniques that we have developed. Passive particle tracking microrheology (PTM) is a technique that uses Brownian motion of colloidal probe particles to characterize the mechanical properties of materials at micron length scales. However, this method cannot be used in higher modulus materials (G*>10^1 Pa) because the particles experience restricted Brownian motion. To overcome this, we have developed a form of active microrheology that uses electromagnetic tweezers to induce an artificial thermal noise on a superparamagnetic particle in the form of a random white noise signal. The main advantage of this technique is that the induced random motion of the particle allows one to use conventional hydrodynamic models to obtain material functions without needing to measure a defined strain field. Fluid-fluid interfaces undergo variety of deformations during the processing steps. Often the contribution of individual deformation type needs to be separated but it’s difficult to achieve with most of the traditional methods. Further, interfacial rheological properties of complex interfaces are strongly influenced by the film microstructure. Experimental investigations for isolating the deformation type, correlating interfacial morphology and rheology are challenging. We developed a miniaturized radial Langmuir trough to study complex fluid-fluid interfaces under purely dilatation deformations that operates in tandem with a conventional inverted microscope for simultaneous interfacial visualization. In our findings, we were able to evaluate rheological properties like compressional and viscoelastic dilatational modulus of complex interfaces and were able to correlate them with the underlying interfacial microstructure.