PDMS Viscometers: Using Microfluidic Technologies for Everyday Applications

A PDMS Micro Viscometer

Using Polydimethylsiloxane (PDMS) viscometers offers an alternative to traditional techniques for measuring the viscosity of Newtonian fluids. PDMS viscometers are attractive because they are easy to fabricate using well known soft lithography techniques.  These devices are also cheap, simple to operate, accurate, require little fluid , and allow for a range of fluids to be measured. The process can also be simplified during the mold fabrication by using solar lithography instead of an expensive aligner to expose the negative photo resist.  Material cost for PDMS is extremely low and the ability of batch processing reduces the cost of the device to about $0.50 per chip. The device utilizes the idea that a fluid will flow through a channel at a certain velocity due to a pressure that is pulling the fluid and a resistance from the fluid-channel interaction.  The viscometer works by comparing the velocity of a reference fluid where the viscosity is known to an unknown fluid. Viscometery results from the PDMS viscometer varied by 3% or less compared to traditional viscometry techniques such as the Ubbelohde device. The PDMS devices also only consume 5 μL or less when testing the fluids. The viscometer was found to work with a range of fluids including aqueous solutions, non-PDMS-swelling organic solvents, fluorinated oil and blood plasma.

Viscosity plays a significant role in many areas of our lives.  Biological events such as protein dynamics, enzymatic kinetics, and cell mitosis viscosity have a key role. Viscosity is a key factor in quality control and in the screening process in the food and chemical manufacturing industry. Viscosity can be used as a screening for medical diagnosis. Anomalies of blood plasma viscosity are associated with diseases such as diabetes, hypertension, infarctions and infections. The viscosities of body fluids, such as synovial fluid and urine, are also factors predictive of certain diseases. A PDMS viscometer could be used in third world countries as a screening test for diseases.

Operation
The viscometer was placed in the vacuum desiccator for 20 min to degas the PDMS. Upon removal from the vacuum air began to diffuse back into the PDMS. PDMS has a high solubility of air, and this can be used to form a pressure to drive fluid flow.  With a 6 mm thick PDMS viscometer with the microchannel and chamber in the center, it took approximately 12 minutes for the PDMS to reach half-saturation with air.  Setup and measurement of a cample's viscosity was usually finished within 4 to 5 min after the viscometer was taken out of the vacuum desiccator. One reservoir was filled with a reference and the other reservoir had the sample fluid. The control channel was blocked to begin fluid flow.  Once all the three inlets of the viscometer were blocked, the air pressure inside the microchannel and the chamber started to drop due to the diffusion of air into PDMS. If the fluid was wetting on PDMS, the flow of the fluid caused by the capillary force would occur; if the fluid was not wetting on PDMS, when the pressure difference over came the resistance caused by the capillary force, the fluid started to flow into the microchannel. The velocity of the reference compared to the sample was used to determine the viscosity of the sample fluid.