View source for Surface-induced droplet fusion in microfluidic devices

Original entry:  Scott Tsai,  APPHY 226,  Spring 2009

"Surface-induced droplet fusion in microfluidic devices"

Luis M. Fidalgo, Chris Abell, and Wilheml T.S. Huck

''Lab Chip'', 2007, '''7''', 984-986

== Soft Matter Keywords ==
Microfluidics, droplet, surface tension, PDMS

== Overview ==
Refer to paper

== Soft Matter Examples ==
[[Image:Paper9_fig1.jpg|thumb|right|300px|'''Fig.1''']] 
[[Image:Paper9_fig2.jpg|thumb|right|300px|'''Fig.2''']]
[[Image:Paper9_fig3.jpg|thumb|right|300px|'''Fig.3''']] 

Droplets in microfluidic devices have shown to be important for carrying out on-chip reactions. So far, the coalescence of two drops in a microfluidic channel have been done by either applying electric fields to polarize the interfaces of teh droplets, or by forcing physical contact of the two droplets using precise geometries. The authors here demonstrate a method of combining two droplets by altering surface energy in the microfluidic channels. 

The authors grafteed poly(acrylic acid) (PAA) using UV photopolymerization on benzophenone-containing PDMS surfaces. Figure 1(b) shows a picture of a part of a PDMS channel that contained a patterened poly(acrylic acid) part. When water droplets (in a continuous phase of oil) flow past the patterned surface, they are trapped. When the next drop arrives, the two drops combine and mix (Fig. 2). Because the process produces no loss of material, the coalesced droplets are also monodispersed. 

In order for a droplet to be trapped on the surface, the droplet has to have sufficient time on the surface. The authors show that if the droplet moves too fast, it will escape the trap (Fig. 3). The droplet's detachment is governed by viscous drag (<math> w \eta v </math>) and interfacial force (<math> w \gamma </math>), where w is the widtho f the pattern. When the force produced by viscous drag (the oil moving faster than the water droplet) overcomes the interfactial force that holds the droplet to the patterned surface, the droplet breaks off and is no longer trapped.