A small-volume microcapillary rheometer with sample recovery

Rheology of small volumes is necessary for high value fluids such as biological samples, but many rheometers require milliliter volumes. A capillary device is constructed by connecting a single silica microcapillary to a round glass millimeter diameter capillary tube. Sample liquid is driven pneumatically to fill the microcapillary and partially fill the larger glass capillary. The glass capillary is mounted on an optical linear sensor array and the meniscus is tracked in real time to measure flow rate and enable flow reversal by switching the pressure differential with a pneumatic valve. The flow rate and the pressure drop are used to determine viscosity as a function of shear rate using capillary rheology equations. A given sample of at least 50 \textmu L can be measured over 2 to 3 decades within the shear rate range of 10 s$^{\mathrm{-1}}$ to 10$^{\mathrm{5}}$ s$^{\mathrm{-1}}$, and be essentially fully recovered. Validation is performed by comparing measurements of Newtonian and non-Newtonian fluids with reference measurements. The operational limits and sources of uncertainty are analyzed, including instrumentation error, meniscus effects, and inertial effects. Future work towards automation in well plates and temperature control are discussed.