Prediction of bubble formation during filling of microchambers is often critical for determining the efficacy of microfluidic devices in various applications. In this study experimental validation is performed to verify the predictions from a previously developed numerical model using lumped analyses for simulating bubble formation during the filling of microchambers. The lumped model is used to predict bubble formation in a micro-chamber as a function of the chamber geometry, fluid properties (i.e. viscosity and surface tension), surface condition (contact angle, surface roughness) and operational parameters (e.g., flow rate) as user defined inputs. Several microchambers with different geometries and surface properties were microfabricated. Experiments were performed to fill the microchambers with different liquids (e.g., water and alcohol) at various flow rates to study the conditions for bubble formation inside the microchambers. The experimental data are compared with numerical predictions to identify the limitations of the numerical model. Also, the comparison of the experimental data with the numerical results provides additional insight into the physics of the micro/nano-scale flow phenomena. The results indicate that contact angle plays a significant role on properties of fluids confined within small geometries, such as in microfluidic devices.

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Electrokinetically Driven Mixing in a Microchamber for Lab-on-a-Chip Applications Narayan Sundaram Abstract Micro-Total-Analysis-Systems (μTAS) have been the focus of recent world wide research due to their varied applications. Much of the motivation for the development of μTAS stems from applications in biotechnology and biomedicine. A typical μTAS device includes a number of functional units ranging from sample injection or ingestion, pre-concentration, mixing with reagents, chemical reactions, separation, detection, and possibly a chemical response. Mixing of constituents is one of the key functions desired of these systems for conducting analyses in a short span of time. The flow regime in these small devices (typical sizes 100μm) being predominantly laminar (Reynolds number, Re < 1), it becomes difficult to rapidly mix the constituent species. Hence for effective mixing, it is necessary to increase the Reynolds number and/or induce bulk motion such that the material interface between the components to be mixed is continously augmented. The method developed to induce such motion is by the application of an AC fluctuating potential field across a microchamber in which mixing is to be performed. The externally applied electric field applies a force on free ions in the charged Debye layer very close to the surface (1-10 nanometers) and induces a flow velocity which is proportional to the electric field. This applied fluctuating electric field gives rise to hydrodynamic instabilities which are responsible for increasing the material contact surface and hence augmenting the rate of mixing by an order of magnitude or more over pure diffusion. To further enhance mixing, microbaffles are strategically placed inside the microchamber and the mixing time was further decreased by a factor of two. Mixing was also studied in a neutral (no charge on the walls) microchamber. It was found that the mixing achieved in the absence of surface charge was comparable to the mixing achieved in the case with microbaffles. This work establishes that CFD is a useful tool that is capable of providing insight into the flow physics in devices with very small length scales. / Master of Science