Fluid dynamics at a pinch: droplet and bubble formation in microfluidic devices

The formation of microscopically small droplets and bubbles with an accurately controlled and narrow size distribution is crucial in a wide variety of products and applications. For example, in medical applications such as diagnostic ultrasound imaging, targeted drug delivery, and drug inhalation therapy, but also in inkjet printing, cosmetics and in the modern food in- dustry. The typical size of the droplets and bubbles investigated in this thesis are much smaller than the thickness of a human hair (100 �m) and the time scale of the formation process is of the order of several microsec- onds or less. Approaching the singularity that arises when the droplet or bubble detaches from its source, the time and length scales become ever smaller. Hence, to capture the final moment of pinch-off experimentally, ultra high-speed microscopic imaging is required. The research presented in this thesis discusses various aspects of droplet and bubble formation in dif- ferent microfluidic systems both experimentally and numerically. Chapter 1 starts with a brief introduction to microscale fluid dynamics. The role of the flow-focusing channel geometry on the microbubble formation process and the dynamics of the bubble pinch-off are discussed in Chapter 2 and 3, re- spectively. In Chapter 4 a new micro uidic operating regime is described to efficiently generate bubbles with a size that is typically much smaller than the channel dimension. A prediction for the size of droplets and bubbles formed from the breakup of the inner jet in a co- owing stream is presented in Chapter 5. In Chapter 6 the extremely fast droplet formation process from the natural breakup of a microscopically thin liquid jet into droplets is investigated both experimentally using ultra high-speed imaging and nu- merically by means of a 1D model based on the lubrication approximation. Chapter 7 concludes the thesis by reviewing the results of the preceding chapters.