FEDSM-ICNMM 2010-30164 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 microand 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. INTRODUCTION Microfluidic platforms offer great opportunities for controlling multi-fluid flows and interfaces at the small scale. The unique behavior of microflows finds applications in many fields of interests and facilitates studies of fast reactions with reduced reagent cost and chemical waste, complex cellular processes, biomedical detection and diagnostic, environmental monitoring, as well as micro-power generation. The manipulation of fluid interfaces is important for developing optofluidic elements, including lenses and waveguides as well as for sequestrating reactive components into individual droplets and bubbles. Motivated by this broad range of applications, the continuous formation of bubbles and droplets has been studied using a variety of microgeometries. A broad range of industrial and biological fluids are highly viscous and their manipulation at the small-scale would provide new facilities for lab-on-chip devices. Most microfluidic studies, however, have been conducted using fluids that exhibit a relatively low viscosity coefficient η. Indeed, as the dimension shrinks, the hydraulic resistance significantly increases and many of the early microflow investigations have focused on low-viscosity fluids to mitigate the large pressure drop required to displace fluids in confined microsystems. To precisely handle such fluids, a particularly promising strategy consists in the formation of a highly viscous fluid core lubricated by a less viscous fluid, i.e., a viscous core-annular flow or a viscous thread. The self-lubrication property of fluids having a large difference in viscosities 25 has been relatively unexplored in microfluidic systems. Yet, in addition to its application for fluid transport in narrow geometries, the situation of a high-viscosity fluid flowing with a low-viscosity fluid is also important for designing continuous flow-reactors between soft materials and solvents. Therefore, understanding the stability of viscous core-annular flow is essential for the development of novel microfluidic devices. Here, we examine the behavior of viscous threads flowing in a quasi two-dimensional pore. A diverging-converging microfluidic chamber is connected with two microchannels that are square in cross-section and are used as fluid inlet and outlet. In a diverging microchannel, a viscous thread experiences a flow rate-controlled folding instability. Slender viscous structures are known to buckle in a fashion similar to elastic solids during short intervals of time. In this paper, we show that the folding instability can be exploited for the interfacial control of high-viscosity fluids. Our microfluidic cell is also particularly advantageous for investigating the role of fluid properties and flow parameters in the stability of viscous coreannular flows.