PDMS Viscometers: Using Microfluidic Technologies for Everyday Applications

Conclusion

Various techniques of measuring fluid viscosity exist today. The microfluidic viscometer presented here offers several advantages over traditional techniques namely;  the device can be built extremely cheap, no advanced measurement instruments are required, and minimal technical skills are required for operation.

The fundamental equation governing the viscometer’s operation is the Hagen-Poiseuille equation which relates fluid velocity to fluid viscosity. This equation is an exact solution of the Navier-Stokes theorem. The two forces responsible for driving the fluids through the serpentine microchannels are capillary force and vacuum force. Capillary force is caused by the fluids’ surface tension and the small hydraulic diameter of the channel. A vacuum pressure differential is generated by air reabsorbing into the PDMS chamber once the device is taken out of the vacuum chamber. A lower pressure develops inside the air reservoir thereby drawing the fluids into the channels to reduce the air reservoir volume.

Calculating the viscosity is done by comparing a reference fluid to a sample fluid. Both fluids are driven through the channels by the same vacuum force, but with two different capillary forces. This results in two different fluid velocities which can be compared graphically to determine the difference in viscosities. For each trial the  reference velocity times length traveled versus sample velocity times length traveled is plotted and the slope equals the difference in viscosities. Since the reference viscosity is known the sample viscosity is easily found.

Fabrication of the device is performed using standard soft lithography techniques. An SU-8 mold is patterned onto a silicon wafer to form a mold for the PDMS. After pouring the PDMS slab glass slides are adhered to top and bottom so that air re-absorption preferentially occurs inside the air reservoir.

Our Device

The PDMS viscometer was successfully fabricated at the Cal Poly microfabrication facilities. Preliminary testing suggests the fluids can be successfully driven through serpentine microchannels using capillary force and PDMS re-absorption. During secondary testing reference and sample fluids did not run through the device on account of the sample fluid being too high of a viscosity to move through the channel. More testing will be conducted in the future and the results compared to traditional measurement techniques.

 

Future Devices

Future microfluidic devices can be built based on the same operating principles as the micro viscometer. Most importantly this device has demonstrated the possibility of driving fluids without complicated pumping and valve mechanisms. Consumer devices could eventually be made so the operator simply unwraps an airtight package, places a few drops of fluid on the chip, and a number of tests are automatically performed. Devices such as this have particular relevance to developing nations where microfluidic chips can monitor such things as; water quality, soil fertility, or health. Microfluidic devices are not simply confined to the research laboratory; their real future is in scalable and economical analytic tools for the masses.