Hydrophilic PDMS microchannels for high-throughput formation of oil-in-water microdroplets and water-in-oil-in-water double emulsions.
Entry by Yuhang Jin, AP225 Fall 2011
Wolfgang-Andreas C. Bauer, Martin Fischlechner, Chris Abell and Wilhelm T. S. Huck, Lab Chip, 2010, 10, 1814.
The paper presents a method of layer-by-layer (LbL) deposition of polyelectrolytes to modify the surface wettability of PDMS-based microfluidic devices. The resulting coatings show stable hydrophylicity and hydrophobicity, and allow for high throughput generation of microdroplets or double emulsions.
This approach has several advantages over previously available ones: long-term stability, simple protocols and negligible distortion of channel dimension.
Results and discussion
The LbL method is largely automated by sequentially pumping segments of poly(allylamine hydrochloride) (PAH) solution, poly-(sodium 4-styrenesulfonate) (PSS) and aqueous NaCl washing solution into the plasma-oxidized channel with a syringe, as is shown in Fig. 1. The polyelectrolytes attach to the surface of PDMS via electrostatic interactions. A PAH layer is highly hydrophobic, whereas a PSS layer is hydrophilic. Therefore the wettability of the channel walls rely on the innermost layer of deposition. This modification of surface properties is essentially a permanent effect.
Hydrophilic channels produced in this way enable generation of oil microdroplets in water by a flow focusing geometry, as is seen in Fig. 2. The shear forces arising from the encounter of oil with water around the nozzle break the oil phase into discrete droplets, whose size is rather monodisperse according to subsequent measurements.
This LbL deposition approach can also be extended to selectively coat certain parts of the channel with polyelectrolytes. Fig. 3 illustrates how this selective coating works. When polyelectrolyte solutions are injected from inlet D and water from inlet B, flow rates can be adjusted such that only the bottom part of the lower nozzle is coated, since the laminar flows do not mix in the channel. Consequently, the lower wall of the channel becomes hydrophilic, while the upper part remains hydrophobic. After this treatment, water is pumped into the device from inlets A and C, and oil enters from inlet B. Water droplets in oil forms at the upper nozzle, which is entirely uncoated and hydrophobic. These droplets are further enclosed by the second water phase coming from inlet A around the partially coated lower nozzle, giving birth to double emulsions. Proper flow rates guarantee that only one smaller water droplet is present in each double emulsion. An example of stable production of such double emulsions is shown in Fig. 4