Paulo Arratia
Polymer Breakup:
We are currently investigating the dynamics of drop breakup of polymeric and Newtonian fluids in microchannels. Inverse emulsions are obtained in a cross-like geometry by impinging a continuous oil phase (with surfactant) onto either a polymeric or a Newtonian aqueous solution. All fluids (including the oil phase) have similar shear viscosity of approximately 0.2 Pa s. Hence, the viscosity ratio between the two phases is kept close to unity. In these experiments, we keep the aqueous solution flow rate constant (and very slow) and vary the oil flow rate. Solutions containing small amounts (100 ppm) of flexible polymers strongly affect the filament and drop breakup processes when compared to a Newtonian solution of similar viscosity (see PDF file and movies below).
For more info see this PDF and movies below:
1) Newtonian, 10x / 30x
2) Polymer, 10x / 30x
3) Satellite Dynamics
Elastic Instabilities in Cross-Channel Flows (PRL 2006):
Elastic Instabilities in Extensional Flows: When polymer molecules pass near the hyperbolic point of a flow, they are strongly stretched. We are currently investigating 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 (Figure 1), 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. (with Jerry Gollub at Haverford College)
Drop formation in the presence of a chemical reaction: Recently, a chemical reaction that produces surfactant has been shown to produce 'tip streaming' from a pedant drop. The phenomenon occurs due to surface tension gradients produced by the reaction. This process could potentially be very useful in microfluidic devices to produce small (<1 mm) droplets of controllable size and shape by tuning the reaction rate and shear rate. Nanoparticles could also be produced by quenching the solution containing the small droplets. The local chemical production of surfactant seems to enhance the formation of small droplets. However, to date, there is no study that combines both stretching and chemical production of surfactant. The goal of this investigation is to do just that.
