Study On The Characteristics Of Microscale Flow Of Viscoelastic Fluids

Micro- and nano-scaled flow phenomena and devices with great potential in applications have attracted much attention from researchers, since the fast development of modern fabrication technology, especially MEMS, have pioneered a new area and new stage for microscopic flow and heat transfer. For 20 years, tremendous experimental investigation and simulations have been done by lots of domestic and foreign researchers. Not only did they find many unique flow behaviors, but also developed new fabrication techniques, such as soft lithography for microscale applications. However, for the microscale flow like droplet stream and continuous flow, there still exist many problems on the mechanism to be solved, especially when the viscoelasticity is introduced in the flow systems.Currently, the flow media for most of microfluidics are Newtonian fluids, and much achievement has been obtained. Whereas viscoelastic fluids commonly exist in the daily life, such as, the biological fluids in human body resemble viscoelastic fluids flow in microscopic environment. The investigation of viscoelastic fluids flow in microchannels can make one get further understanding on the behaviors of biofluids, and lead us manipulate properly the physical/chemical processes involving mixing, molecular detection, reagents reactions, bubble/droplet coalescence, etc.. Moreover, the study on the elasticity-induced chaotic flow provides reference for the flow informations in the Batchelor regime of hydrodynamic turbulence. Therein, the passive scalar evolvement in the flow mixing is important for the cognition of mixing essence.This thesis focuses on the two basic microfluidic problems, i.e., the droplet stream and continuous flow. By altering the fluids properties, experimental study, numerical simulations and theoretical analysis are performed. In the experiments, firstly, the fluids properties, surface characteristics and diffusion capability are measured. The experimens on Newtonian and viscoelastic droplets are conducted for appropriate fluids matches, and viscoelastic fluids with different concentrations are used for mixing in the four different microchannels. In order to complement experiments and obtain the flow details which present experiments cannot provide, numerical simulations are carried out for Newtonian droplets formation, mainly concentrating on local pressure and velocity variations in the vicinity of T-junction. The lubrication theory is also used to analyze the pressure drop along the droplet prior to its detachment, thus the forces competition are obtained during the droplet formation. Besides, direct numerical simulation (DNS) is performed for the study on scalar structure in the fluids mixing in the curvilinear microchannels.For the micro-droplet formation, the Newtonian-Newtonian and Newtonian-viscoelastic fluids system are adopted respectively. On the phenomenology of Newtonian droplets, four different regimes of droplet stream are found by both experiments and simulations. The former three regimes are corresponding to the well-defined Squeezing, Dripping and Jetting regimes, and the fourth regime, i.e., the parallel flow is the development of Jetting regime. The relationship between droplet volume growth and flow field, pressure variations are analyzied in the two stable squeezing and dripping regimes, by measuring the geometrical parameters of droplet tip. The local pressure and velocity changings at the T-junction are displayed in the simulation results, and it is found that fluid flow containing droplets train augments the pressure drop greatly, accompanied with the results of droplet inner circulation obtained by the lubrication analysis. On the formation of viscoelastic droplets, the droplet coalescence conditions, filament stretching and Hencky strain are investigated at the normal interfacial tension condition. Filament stretching shows two parts of linear stretching and quasi-linear fast stretching. Especially at super-low interfacial tension condition, a new regime of droplet formation is found, i.e., the droplet fragmentation, where three different processes including kinetics characteristic process (cone angle emerging), elasticity performance process (filament extracted and stretched) and interfacial instability process (capillary instability) are analyzied.For the study on viscoelastic fluids continuous flow and the corresponding scalar mixing, the mixing length and mixing time at viscoelastic Batchelor regime are measured by calculating the statistical moments, and it is found they both have exponential decays. The large-scale burst events are found via the analysis of scalar power decay on the wavenumber space and scalar structure functions. It derives from the stretched stripes near the wall flinging into the bulk, and the intrinsic intermittency of scalar motion comes from scalar itself. From DNS, one can learn that the flow field deformation rate tensor and flexible molecules deformation rate tensor are weakly connected, and it is determined by the curved geometries. The stretching of flexible molecules induces strong secondary flow in the cross section to enhance the mixing effects, and hereby constantly changes the structures of scalar gradients.