Capillary threads and droplet-decorated streams in microchannels

We investigate the evolution of high-viscosity fluid threads flowing in a sheath of immiscible liquid in a diverging/plane microchannel. A steady viscous-core annular flow is produced upstream in a square microchannel. Downstream, the fluids enter a diverging channel that causes the thread to bend and deform into a complex microstructure having a large interfacial area. Using a variety of fluids, we study the effect of interfacial tension, viscosities, and flow rates on the flow morphology and we characterize the evolution of the thread thickness, arc length, fold wavelength, and envelope amplitude in the plane and straight microchannel downstream from the divergence. In particular, we focus on the coalescence mechanism between adjacent folds that can produce small droplets embedded into a highly viscous matrix (i.e., ``droplet-decorated'' streams). Other original phenomena, such as capillary breakup by folding at high capillary numbers and secondary folding, are also examined.