Microfluidic devices are being actively investigated in many scientific and industrial arenas. Due to the large surface to volume ratio in microchannels, surface tension and viscous forces play considerable roles in the flow characteristics.

We have investigated a variety of miscible flows of liquids having large viscosity contrasts in the absence of surface tension. Although miscible liquids ultimately evolve into a homogeneous mixture due to intermolecular diffusion, microfluidic manipulations enable us to investigate their viscid interactions before they mix. We have revealed the possibility of forming lubricated threads of more viscous liquid flowing co-currently with a miscible less viscous liquid. We have investigated the behavior of these threads as a function of microgeometrical and flow parameters. This broad body of microfluidic work has been selected for the Gallery of Fluid Motion by the American Institute of Physics (AIP) and the Division of Fluid Dynamics of the American Physical Society (APS) in 2006 and 2007.

Non-diffusive folding between silicone oils Thread viscosity = 500 cP Surroundings viscosity = 6 cP Acquisition rate: 1,600 fps

The flow morphologies exhibit periodic oscillations depending upon the flow rates. In the diverging channel, the folds increase in amplitude, the wavelength decreases, and the thread thins. The peak folding amplitude defines an envelope between the less viscous liquid and the more viscous folded central region. [T. Cubaud and T.G. Mason, Phys. Rev. Lett., 96, 114501 (2006)]

The interplay between folding and the high viscosity gradient leads to the formation of heterogeneous viscous mixtures downstream. In this case, the heterogeneous deposition of the thread creates a pile having nonperiodic viscous ripples. As opposed to purely diffusive flows, viscous folding offers the practical ability to rapidly enhance mixing between liquids having different viscosities in microfluidic devices. [T. Cubaud and T.G. Mason, Phys. Rev. Lett., 96, 114501 (2006)]

Heterogeneous folding Thread viscosity = 500 cP Surroundings viscosity = 0.5 cP Acquisition rate: 1,600 fps