| Droplet based lab on a chip systems have already been developed for bio analysis, particle fabrication using microfluidics, and the creation of complex emulsions using microfluidic systems. Initial droplet based lab on a chip devices have used a single droplet with several components as a reactor vessel, but in the future multiple step complex reactions will require the contents and volumes of drops to be adjusted during a single reaction. In order to develop future droplet based lab on a chip devices that can replicate complex laboratory processes, controlling droplets' sizes and contents will be a fundamental necessity. In order to manipulate droplets and their contents, we will need to be able to control a number of step functions like droplet generation, coalescence and splitting of preformed droplets. Some of these step functions have been accomplished using electrostatic or magnetic external forces to manipulate droplets in microchannels, but these methods require the use of dielectric or magnetic fluids. We propose that using simple channel geometries and the flow fields they create we can generate, coalesce or split droplets of any type of Newtonian fluid. Furthermore, because many applications will introduce long chain molecules (like DNA and biopolymers) to our droplet phase, it will also be necessary to understand how viscoelastic fluids affect change the behavior for generation, coalescence and splitting of droplets. Using a single geometry, a microfluidic T-junction, in a number of orientations, we will characterize the flow induced generation, coalescence and splitting of both Newtonian and viscoelastic droplets. Furthermore, we will compare observed phenomenon to results for similar experiments done at the macroscale, and compare the similarity between the microfluidic and microfluidic results, allowing use to extend our results to a broader field.;The generation of droplets at a T-junction is achieved by crossflowing an immiscible continuous phase around an emerging perpendicular dispersed phase at the T-junction. Due to the channel geometry, a combination of viscous shear forces and pressure forces act to create drops of the dispersed phase in the continuous phase with neither force being dominate during breakup. The effects of viscosity ratio, channel geometry parameters, and flow parameters on drop size, frequency and monodispersity are characterized for both Newtonian and viscoelastic fluids is characterized. Using conservation of mass we find that power law dependences of drop volume and frequency on capillary number are related by their exponents which is corroborated by our data. We finally develop a 2 stage scaling model which we find qualitatively matches results from the highest viscosity ratio tested.;Reorienting the T-junction such that 2 drops approach each other from either branch and are forced to flow down a single outlet, we characterize the behavior of drops meeting at the T-junction in head on collisions. Drops exhibit three behaviors: splitting, non splitting, and coalescing, and a phase diagram based on capillary number and droplet size is developed. The coalescence behavior of the drops is compared to macroscale coalescence experiments, and the critical capillary number for coalescence dependence on both drop size and viscosity ratio is found to be well described by macroscale experiments.;Finally, reorienting the T-junction a final time, drops flow towards the branching channels, either splitting and traveling down both branches are staying whole and traveling down a single branch. The behavior of viscoelastic drops at the T-junction is found to be qualitatively similar, but viscoelastic drops are found to be less stable and breakup at lower relative viscous stresses. Furthermore, we observe the formation of filaments forming between splitting drop halves that form the iconic beads on a string structure. The kinetics of the thinning filaments are compared favorably to elastocapillary thinning of a filament, allowing us to extract relevant extensional rheology parameters from this experiment. |
