Soft Materials: Microfluidics

The Soft Materials Discussion Group regularly convenes investigators in the New York region with an interest in soft materials research and development, and provides a forum for scientists; engineers; and other key stakeholders working in academia, industry, and not-for-profit entities to exchange ideas and discuss advances. To ensure impact globally, the meeting proceedings will be disseminated electronically through the Academy's eBriefings program. The interdisciplinary topics include a range of technologically important materials in colloids, polymers, emulsions, liquid and organic crystals, membranes, proteins, cells, and tissue.

The PhaseChip: Manipulating Phase Diagrams with Microfluidics
Seth Fraden
Brandeis University

X-ray diffraction of protein crystals reveals protein structure, which is needed to advance fundamental understanding of protein function and for drug development. Currently the physical process of crystallization is the bottleneck in protein structure determination.
The PhaseChip, is a microfluidic device that can precisely meter, mix, and store nanoliter volumes of sample, solvent, and other reagents. Thousands of nanoliter drops of different protein solutions can be stored in individual wells. Through the controlled kinetic manipulation of the solution chemical potential the process of nucleation and growth can be decoupled, which is crucial for optimizing protein crystallization.
Movies illustrating the PhaseChip in action: http://www.elsie.brandeis.edu

Folding, Swirling, and Mixing Instabilities of Viscous Liquid Threads in Microfluidic Channels
Thomas Mason
UCLA
Collaborator: Thomas Cubaud, Stony Brook University

Viscous fluid threads, which have been formed by hydrodynamic focusing, can exhibit interesting instabilities as they are swept along by the flow of a different outer fluid in microfluidic channels. By examining pairs of miscible liquids for which interfacial tension is essentially absent, such as silicone oils having different molecular weights, we reveal a rich variety of fluid instabilities that occur in the laminar flow regime. When a single thread that propagates stably in the center of a straight channel encounters a divergence in the channel's width, the thread simply dilates if its viscosity is comparable to the viscosity of the outer fluid. However, due to the extensional flow and deceleration in the diverging channel, a thread that is sufficiently viscous reduces energy dissipation by folding in sinuous bending oscillations rather than dilating. By tuning the flow rates, we reveal a novel period-doubling route to chaotic folding. The folding and stretching of a thread in a diverging channel provides a simple means of mixing viscous liquids and creating controlled viscosity gradients. Moreover, using a sequence of two cross-channels, we make a pair of viscous threads that become unstable when swept along near the walls of a straight channel as a result of the viscous torque induced by the velocity gradient. The amplification of lateral undulations ultimately causes the threads to break up and form an array of discrete viscous swirls, the miscible counterparts of droplets. By injecting three different miscible liquids into a dual cross-channel geometry, we examine the complex patterns that form when several fluid instabilities interact and compete. Overall, we anticipate that these measurements will provide a basis for investigating the behavior of flowing threads for which interfacial tension plays a more substantial role.