Low Reynolds number flows for microfluidic technologies: Instabilities, drops, and inertially ordered particles

The presence or absence of drop formation is studied for two immiscible co-flowing liquids in a microfluidic channel, where the channel width is considerably larger than the channel height. We show that stability of the inner fluid thread depends on the channel geometry. We present a model that accounts for the data and experimentally exploit this effect of geometric confinement to induce the break up of a jet at a spatially defined location. In channels with rectangular cross-sections, and for a range of channel aspect ratios and particle confinement, we experimentally find that both the location and the number of inertial focusing positions depend on the number of particles per unit length along the channel, which is a function of both the channel cross-section and the particle volume fraction. We rationalize these results using simulations to help understand hydrodynamic interactions in these confined flows. We present a criterion based on volume fraction and channel geometry for the occurrence of a step-wise transition from one to two or more trains of particles. We characterize inertial particle migration in channels with rectangular cross-sections. We use numerical simulations and experimental findings to show that results for point particles in plane channel flows remain valid for finite-sized particles in rectangular channels, thus providing guidelines for the incorporation of inertial focusing into the design of microfluidic devices. We trap monodisperse drops in planar arrays in microfluidic channels and characterize the solidification of supercooled aqueous glycerol solutions. We measure rates of nucleation and the ice-water interfacial energy, and enable the dynamics of solidification to be observed for over a hundred drops in parallel without any loss of specificity. We present a technique to locally and rapidly heat water drops in microfluidic devices with microwave dielectric heating. Our microfluidic device integrates a flow-focusing drop maker, drop splitters, and metal electrodes to locally deliver microwave power from a source and amplifier. We improve the encapsulation efficiency of cells and beads in drops by evenly spacing cells as they travel within a high aspect-ratio microchannel; cells enter the drop generator with the frequency of drop formation.