View source for Dynamic Pattern Formation in a Vesicle-Generating Microfluidic Device.

Entry by [[Yuhang Jin]], AP225 Fall 2011

== Reference ==

Todd Thorsen, Richard W. Roberts, Frances H. Arnold, and Stephen R. Quake, ''Phys. Rev. Lett.'', 2001, '''86''', 4163.

== Keywords ==

pattern formation, emulsions, self-organization, microfluidics

== Introduction ==

This Letter presents a simple microfluidic device designed to generate ordered patterns. Viscous forces, especially those on the interface between two phases of flow, lead to the production of motifs from droplets to helices etc. The physics far from equilibrium is responsible for the mechanism of this pattern formation.

== Experiments and results ==

[[Image:Fig2-1.jpg|300px|thumb|left|'''Fig.1 ''' The microfluidic structure for the production of emulsions.]]

[[Image:Fig2-2.jpg|300px|thumb|right|'''Fig.2 ''' Droplet formation in rectangular channels. The pressures for water and oil are noted (in psi).]]

[[Image:Fig2-3.jpg|300px|thumb|right|'''Fig.3 ''' Illustrations of complex pattern formation seen in rounded channels and a phase diagram of experimental observations. Pressure of water and oil are noted. Solid circle: solid water stream; open circle: elongated droplets; solid triangle: triple droplet layer; open triangle: double droplet layer; solid square: jointed droplets; open square: separated droplet.]]

Most microfluidic devices work at low Reynolds number, resulting in a stable laminar flow in the channels according to the Navier-Stokes equation, if only one fluid phase is involved. However, interaction between two immiscible fluids can give rise to instabilities and complex pattern formation, which is an interesting example of self-organization out of equilibrium.

In this Letter, the authors design a device in which emulsions are formed by shearing one liquid into a second immiscible one. In this process, a proper concentration of surfactant is critical to reduce the surface energy and therefore facilitate the generation of droplets. The structure is shown in Fig. 1. High shear forces at the leading edge of the water perpendicular to the oil flow generate picoliter-scale droplets. This droplet formation is a result of competition between surface tension and shear forces.

In rectangular channels fabricated following the standard protocols of photolithography and soft lithography, only monodisperse reverse micelles with regular periodicity will form, as is illustrated in Fig. 2. The relative water/oil pressures determine the size and spacing between the droplets.

Heating of the SU-8 mold for soft lithography produces rounded channels, where interesting and more complex phenomena may occur. Fig. 3 displays some of the experimentally observed droplet morphology with varied water/oil pressures. A phase diagram summarizing all those patterns is also presented.