PDMS Viscometers: Using Microfluidic Technologies for Everyday Applications

The fabrication of the viscometer begins with the creation of a mold that will form the features of the device.  The mold uses a single crystalline silicon wafer as a substrate and is made from a negative photo epoxy called SU-8.  The epoxy is spin coated onto the wafer to a thickness of approximately 60 microns (Figure 1). 

Using photolithography the SU-8 film can then patterned into a negative mold (Figure 2).  A view of the SU-8 channel and length indicators can be seen below. 

The remaining SU-8 that was not removed will form the features that the fluid will flow through.  A liquid polymer called polydimethylsiloxane (PDMS) is mixed with a cross-linker and poured over the SU-8 mold (Figure 3).

This film of PDMS is approximately 3mm thick.  The PDMS is then cured at elevated temperatures and peeled from the surface (Figure 4). 

The non channel side of the PDMS film is non-permanently bonded to a glass slide.  A plasma treatment with Oxygen gas is then carried out for a second glass slide as well as the exposed 250 micron channels, and the two are stuck together (Figure 5).  The plasma treatment bonds the two materials as well as makes the PDMS hydrophilic.  If the PDMS were hydrophobic the fluid motion would be greatly impeded if not halted all together.  This surface treatment with argon plasma bombards the surface with O+ ions.  This treatment forms silanol groups on the surface.  When two treated surfaces (either PDMS-PDMS or PDMS-glass) come in contact with one another the silanol groups condense to form Si-O-Si bonds which are comprised of strong covalent bonds.  This process is non-reversable and forms a tight seal which can withstand pressures up to 50psi.