Fabrication of Monodisperse Toroidal Particles by Polymer Solidification in Microfluidics
Original entry: Tamas Szalay (APPHY225 2012)
"Fabrication of Monodisperse Toroidal Particles by Polymer Solidification in Microfluidics"
Baoguo Wang, Ho Cheung Shum, and David A. Weitz
ChemPhysChem 2009, 10, 641–645
In this article, the authors describe a means of fabricating toroidal particles by solidifying droplets in a particular fluid flow. They use a variety of polymers to form the toroids, and examine the resulting distributions of particle sizes as a function of the experimental parameters.
Soft matter keywords
emulsion, micelle, polymer, microfluidics
The general experimental setup can be seen in part (a) of the image above, with a nozzle supplying droplets of polymer solution which are then carried down a microfluidic channel. The solidification of the polymer solution occurs by solvent diffusion into the bulk. In all experiments, the continuous phase is PDMS oil and the solvent is DMF (N,N-dimethylformamide).
The diffusion rate of the solvent is measured and analyzed by looking at the particle size ratios as a function of the lengthwise travel down the channel away from the nozzle. It can be seen in the chart that there is a sudden discontinuous jump in (a) where the particles solidify (and the solvent phase separates), about 2.6 cm away from the capillary nozzle. In (b) there are just droplets of DMF, showing the bulk diffusion rate, and in (c) there are droplets with DMF-saturated bulk PDMS, such that no shrinking occurs.
The stipulated mechanism of toroid formation is as follows: the laminar flow regime in the tube sets up a nonuniform (parabolic) flow rate profile. The presence of the drop modifies this profile, such that the liquid in the center of the drop is more stagnant and the liquid on the outside edges is moving faster. As a result, the solvent is carried away more rapidly at the outside edges, causing the polymer to concentrate there. As a result, the center of the droplet gradually gets thinner and eventually breaks and forms a hole; eventually, the entire droplet solidifies. This is demonstrated in the chart below, which shows the effects of different flow rates on the droplet morphology. Most notably, you can see that the slowest rate leads to near-spherical particles. The inset demonstrates how narrow the distribution is for a flow rate of <math>10000 \mu L/hr</math>.
As a final demonstration of the utility of this method of synthesis, the authors mixed the polymer solution with both fluorescent solution and magnetic nanoparticles, to make glowing and magnetic toroids. In the image below, (a) is PSF with a fluorescein salt, (b) is PMMA with rhodamine B, (c) is PMMA with 10 nm magnetic nanoparticles, and (d) is a demonstration of the magnetic nanoparticles. In the left hand vial, they sit at the interface of oil and water; on the right side, they are attracted to a magnet behind the vial.