Suspension Dynamics and Hydrodynamic Interaction in Viscoelastic Fluids

One of the unusual phenomena observed of suspensions involving viscoelastic liquids, like polymer solutions and melts, is the tendency of solid particles to form chains and microstructures. Till now, there is very little physical understanding of chaining dynamics. Limited experimental results arise from the absence of appropriate quantification methods in chain dynamics. The first part of this dissertation introduces a new approach for quantifying the chain dynamics based on the evolution of singlets, doublets, triplets, and multiplets as an explicit way of defining chain formation. Capturing the microstructural formation in this way, reveals the average chaining length, and the distribution of structural length scales in the medium, and the time scales associated with their formation. Then a global String Factor (SF) is suggested based on considering the long range orientational correlations within the cluster to help better understanding the effect of different parameters on chaining. By using a highly shear thinning polymer solution the effect of particle size, shear rate, viscoelasticity of the fluid, and shear thinning properties of the liquid phase on microstructure formations are studied. Based on these calculation chaining time scales are estimated and chaining velocities were on the order of 0.1-10 μm/s. Singlet depletion is modeled with a first order reaction mechanism, and reaction rate constants are derived. By using single-mode Giesekus model to fit the viscosity data, extensional properties of the fluid is predicted. Using the extensional properties and tying that to the negative wake as a possible mechanism for chain formation, a new criteria for chain formation is proposed and verified. The second part descrides an investigation into the effect of particle interaction in viscoelastic fluids on different non-intrusive velocity measurement techniques like Particle Image Velocimetry (PIV) and Micro-Particle Image Velocimetry (μ PIV), which are both based on cross-correlation. Similar effects are examined for Particle Tracking Velocimetry (PTV) in these fluids, which is based on tracking individual particles. The main assumption of all these velocity measurement techniques is that the tracer particles follow the flow faithfully and not interact with each other. Then the fluid velocity field can be extracted directly from the tracer particle velocity. PIV measurements over a wide range of particle volumetric concentrations (0.005 < &phis; < 0.2% v/v) did not expose any particle interaction effects in a square cross-section channel flow. Measurement errors are quantified in detail by finding the RMS fluctuations in velocity measurements. Micro-PIV measurements were performed in a rectangular micro-channel, for a wide range of flow rates, with a fixed tracer concentration (0.2%). Velocity fluctuations did not show any dependence on the flow rate and average shear rate. Also we showed that the uncertainty in velocity measurement is one order of magnitude larger than chaining velocity. PTV is also used for comparing the velocity of singlets versus clusters over time in a cone and plate geometry. Over the limited time in which a particle is traveling through the field of view, no meaningful differences between the velocities could be observed. A theoretical approach for finding the limits of cross-correlation techniques (PIV, μPIV) is introduced and the minimum detectible velocity is estimated to be on the order of 1-10 μm/s. Chaining velocity is shown to be less than the minimum detectable velocity. In the last chapter, by using viscoelastic properties of polymer solutions, a novel technique (Low-Voltage Near-Field Electrospinning) for depositing and writing nano-fibers on a 3D substrate has been introduced and demonstrated.