Quantifying and controlling microflow is often necessary for the performance of on-chip processes and for accurate quantitative measurements of fluid properties. Beyond flow, microscale devices lend themselves to rapid, accurate measurements of viscosity and rheological properties of complex fluids.
Many drugs must be delivered at specific doses, with very narrow margins of error, and many manufacturing and R&D processes demand that fluids be mixed in extremely precise ratios under carefully controlled conditions. To support those and other applications, NIST researchers are developing standards that can ultimately be used to measure flows as small as 10 nanoliters per minute with uncertainties of 1%. (For comparison, 10 nanoliters is about five thousand times smaller than the volume of a single drop of water.)
There are multiple ways to gauge flow. One — the gravimetric approach – involves measuring the mass of fluid delivered through a microchannel over time; it is more accurate at higher flow rates. Another method tracks the changing position of particles suspended in liquid to determine velocity, and is more accurate at lower flows. NIST is at work on advanced, compact candidate standard systems for both methods. Measurements with each currently differ by less than 3%, and further improvement is expected soon.
Elsewhere, NIST researchers are working to test and improve the performance of thermal mass flow meters, which use the amount of heat transferred to a moving fluid from a thermal source to quantify flow. The amount of temperature change indicates the velocity of the fluid, if the channel’s properties and the fluid’s characteristics are well known and accounted for. These devices can be fabricated at sub-millimeter dimensions.
At the same time, NIST researchers are finding more accurate, high-resolution ways to characterize the behavior of viscous and semi-solid fluids using light beams, advanced microscopy, and holographic imaging of suspended particles, to cite a few. This capability is important for understanding the behavior of fluids under stress, including non-Newtonian fluids. Most biological materials and fluids (such as blood) are non-Newtonian, and novel chip-scale sensors are being developed to measure blood coagulation, aggregation of antibodies, and injectability of thick fluids. (High viscosity leads to poor syringeability due to the high injection force required.)
Other NIST labs have devised chip-scale methods of measuring how properties such as pH and temperature affect the behavior of protein solutions or clusters of antibodies. They use pressure sources to drive fluids through tiny channels called microcapillaries and record the flow under different conditions. This research has already led to development of a protocol to optimize the performance and statistics of commercial microfluidic flow meters.