(5co) Complex Fluids in Microfluidic Devices

Arratia, P. E. - Presenter, University of Pennsylvania

My research interests focus in the field of transport phenomena (fluid mechanics), in particular the flow and dynamics of complex fluids. Complex fluids are a broad class of materials that are usually homogeneous at the macroscopic scale and disordered at the microscopic scale, but possess structure at an intermediate scale (e.g. colloids, polymer solutions, blood). Such fluids exhibit many useful macroscopic properties stemming from the variety of structures at the intermediate scale. They are also 'soft', in the sense that they flow or distort easily in response to shear or other external forces; their macroscopic flow behavior is a strong function of the fluid microstructure. Hence, understanding and control over the physical forces at the structural scale and its interaction with the imposed flow (or forces) may lead to the development of materials with unique macroscopic properties and new technologies.

The recent focus in microfluidics research is both technological and scientific. The technological appeal of microfluidics devices is their potential to produce and process small amounts of complex fluids with efficiency and speed ('lab-on-a-chip'). There is also plenty of science in small scale fluid systems. For example, the geometric length scale of microdevices can easily approach the 'structural' scale of a complex fluid. This confinement may result in flow behavior that is markedly different than the equivalent macroscopic experiment.

My future research will focus on complex fluids in microdevices and is comprised by two main thrusts. The first is devoted to fluid manipulation and dynamics, its interaction with the fluid microstructure, and the effects on macroscopic fluid behavior. The second is to understand the underlying physical and molecular mechanisms that govern fluid microstructure. Below are some current examples.

1) Drop Formation & Breakup in Microchannels: Polymeric Emulsions

Polymeric emulsions are common in many applications including pharmaceuticals, drug delivery, and cosmetics. A common rheological property of many polymeric fluids is viscoelasticity, particularly when flexible polymers are present. We are currently investigating the effects of elasticity and surface tension on drop formation and breakup process in microchannels. The drop breakup process produces a wealth of interesting phenomena, including tip-streaming, ?beads-on-string', and iterated stretching. The overall process can produce emulsions in the nanometer scale, which can be further processed to obtain nanoparticles of controllable size and shape. Of particular interest is the fabrication of materials with novel optical and mechanical properties using ?emulsion templating' or polymer encapsulation. We are also investigating the effects of fast chemical reactions on drop formation process. (with D.J. Durian & J.P. Gollub)

2) Soft Colloidal Hydrodynamics: NIPA Hydrogels

NIPA particles (dp ~400 nm) are soft materials that can be used for drug delivery. One of the main featured of these soft hydrogels is that they are very sensitive to temperature and pH, particularly particle size. For example, they show first order transition in size with respect to temperature. We are currently investigating the flow of these soft colloids in channel flows as a function of density (temperature). The microscopic organization (glass vs. crystal, etc.) seems to have a significant impact on the flow profile and velocity fluctuations. This is of importance when designing these particles as carries of medicine, for example. The goal is to study the interaction between microscopic organization and "macroscopic" flow. (with Arjun Yodh & J.P. Gollub).

3) Elastic Instabilities & Mixing in Polymeric Flows in Elongational Flow:

Rheologically complex materials are common in many applications and can produce unexpected hydrodynamics instabilities. When polymer molecules pass near the hyperbolic point of a flow, they are strongly stretched. Here, we investigate the flow of Newtonian and polymeric fluids (flexible and stiff polymeric solutions) in a well-defined and controlled elongational flow in microchannels. As the strain rate is varied at low Reynolds number, the stretching produces two flow instabilities, one in which the velocity field becomes strongly asymmetric, and a second in which it fluctuates non-periodically in time. Velocity fields are obtained using time-resolved particle tracking methods. These instabilities do not occur for stiff polymer solutions. The flow is strongly perturbed even far from the region of instability and this phenomenon can be used to produce mixing in microchannels. (with J.P. Golllub)

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