Experimental Soft Condensed Matter Group Harvard University, Prof. D. A. Weitz Geometrically Controlled Jet-Like Instabilities in Microfluidic Two-Phase Flows

Introduction

We are interested in the effects of confinement in two phase co-flows in microfluidic devices. When the flow rate of the inner fluid is small compared to the flow rate of the outer fluid, and the resulting width of the inner fluid is smaller than the height of the channel, the inner fluid breaks into droplets, as expected for a three-dimensional system. On the other hand, when the width of the second phase becomes comparable to the height of the microfluidic device, Rayleigh capillary instabilities are suppressed, and the inner fluid forms a jet that does not break, as might be expected for a purely two-dimensional system. We show that by changing the dimensions of the microfluidic channel we can transition from a stable co-flow to drop break-up. These results can be explained with a model of this two phase flow.

Microfluidic Device

We have designed and fabricated PDMS microfluidic devices using soft lithographic techniques. The devices are much wider than they are high, often by more than an order of magnitude, resulting in a quasi-two-dimensional device.

Observations

We observe three distinct behaviors in these microfluidic devices: dripping, jetting with break-up, and jetting without break-up.

Dripping and jetting with break-up have been observed in axisymmetric two-phase flow, but the observation of jetting without break up is new. Could this new, stable behavior be due to the confinement in the system?

Jets in Two and Three Dimensions

A cylinder of fluid in three dimensions is unstable because perturbations with a wavelength greater than the circumference of the cylinder cause a decrease in surface area (Lord Rayleigh, Proc. R. Soc. London, 1979). Conversely, a ribbon (the two-dimensional analog of a cylinder) in two dimensions is stable as all perturbations lead to an increased surface area (Miguel, Phys. Rev. A, 1985).

Could the transition from a stable to an unstable flow be due to a transition between these two states?

Analysis

By assuming Poiseuille flow it is possible to calculate the expected width of the inner fluid in the case of the stable jet. We show that we see a transition from unstable to stable behavior when the width of the inner fluid is equal to or greater than the height of the channel.

This transition holds for different outer viscosities, different channel heights, and a range of inner and outer flow rates.

Geometrically Controlled Transistions

It is possible to control the stablity of the flow by changing the confinement of the flow.

This movie shows a PDMS device with flow from left to right. The device width is 700 microns. The device is ~30 microns high on the left-hand side and ~60 microns high on the right hand side. As the flow moves from a more confined to a less confined geometry the stable jet becomes unstable and breaks up.

For More Information

Katie Humphry
office: Engineering Science Laboratory, Room 213
phone: +1 617 496 7451
email: katie@physics.harvard.edu

Other people working on this project

Armand Ajdari, Alberto Fernandez-Nieves, Howard Stone, and David Weitz