Samira Darvishi

Folded micro-threads: Role of viscosity and interfacial tension

The shape and evolution of periodically folded threads are experimentally examined in a microfluidic network. The fluidic system is designed for the production and lubricated transport of very uniform folds. To investigate the influence of viscosity and interfacial tension on buckling deformations, multiphase flows are scrutinized using both miscible and immiscible fluid pairs. The parameters used to analyze folding morphologies include thread diameter, arc-length, fold amplitude, and wavelength. When fluids are immiscible, the onset of viscous folding is characterized as a function of the capillary number and the phenomenon of “capillary unfolding” where a corrugated thread straightens along the flow direction is demonstrated. The spatial transition from folding to coiling-like flow behavior of high-viscosity capillary threads is also shown.

Lubrication of Highly Viscous Core-Annular Flows in Microfluidic Chambers

We investigate the lubrication transition of high-viscosity fluid threads flowing in sheaths of less viscous fluids, i.e., viscous core-annular flows, in microchannels. Focus is given on the flow behavior of threads as they traverse a quasi-two-dimensional diverging-converging slit microfluidic chamber. The role of the viscosity contrast is examined for both miscible and immiscible fluids, and, for the later case, both partially wetting and nonwetting threads are considered. The conditions for lubrication are established in relation to flow rates of injection, interfacial properties, viscosities, and phenomena such as viscous buckling, wetting, breakup, and coalescence.

Formation of capillary structures with highly viscous fluids in plane microchannels

A detailed experimental study of capillary structures made with high-viscosity fluids is conducted in plane microchannels. We examine the possibility to emulsify fluids via viscous folding instabilities using continuous microflows. Non-wetting capillary threads are formed in a sheath of immiscible liquids for a wide range of viscosity and flow rate ratios using symmetric hydrodynamic focusing sections. Downstream, threads are significantly deformed in a diverging channel connected to a long plane microchannel. Adjusting the residence time of thread structures in the plane channel allows us to produce various degrees of fold coalescence. Three typical regimes are identified: thread breakup, partial fold coalescence, and complete fold coalescence. The geometrical features of evolving folds are measured and related to material properties, flow parameters, and microgeometries. This study shows that folded structures offer the opportunity to examine novel interfacial configurations and provides a set of microfluidic techniques for improved manipulation of viscous materials with thin immiscible lubricants at the small scale.

Viscous Core-Annular Flows in Microfluidic Chambers

The manipulation of highly viscous materials at the microscale is a key challenge for implementing lab on chips with the ability to manage a variety of complex and reactive fluids. We describe methods for producing and controlling high-viscosity fluid threads flowing in sheath of less viscous fluids, i.e., viscous core-annular flows, in microchannels. The self-lubrication property of multi-fluid flows having large viscosity contrasts offers a promising means for manipulating interfaces between “thick” and “thin” fluids and for reducing the hydraulic resistance in micro- and nanofluidic devices. In particular, we focus on the flow behavior of threads as they traverse diverging-converging slit microfluidic chambers. The alteration of convective time-scales using extensional microgeometries permits the manipulation of complex phenomena such as viscous buckling, wetting, and coalescence. We examine the interrelation between these phenomena that are useful for passively enhancing mixing between miscible fluids and for initiating continuous emulsification processes between immiscible fluids having widely disparate viscosities.Copyright