Microfluidic manometry and rheology of complex fluids

The emergence and proliferation of soft lithography has allowed for the rapid fabrication of microfluidic devices with nearly any geometry imaginable. The growth of microfluidics has allowed lab-on-a-chip devices to infiltrate diverse areas of research due to its ease of fabrication and ability to multiplex and conduct high-throughput assays. In this dissertation, new microfluidic techniques were developed for measuring pressure drop versus flow rate (∆P-Q) relations of fluids ranging from droplets to polymeric solutions. The methods rely on image-based approaches to calculating differential pressures, thereby eliminating the need for on-chip or externally attached pressure sensors.

Measurement of ∆P-Q relations in microscale geometries can be challenging due to the small differential ∆P (O(10 Pa)) caused by moving objects and the limited footprint available. Existing approaches use floor mounted or externally attached pressure sensors to make bulk measurements to make localized ∆P measurements. Importantly, current approaches do not allow measurement of ∆P-Q relations in a large number of geometries simultaneously. To address this gap, a versatile experimental technique was developed referred to as microfluidic bypass manometer for measurement of ∆P-Q relations in a parallelized manner. The method involves introducing co-flowing laminar streams into a microfluidic network that contains a series of loops, where each loop is comprised of a test geometry and a bypass channel as a flow rate sensing element. To demonstrate the capabilities of the technique, single-phase fluids were used to first measure the ∆P-Q relations simultaneously for forty test geometries of varying complexity. Then, the capabilities were expanded to stationary oil droplets trapped in microcavities. Here, the ∆P-Q relations were found to be nonlinear and the flow resistance to be sensitive to droplet confinement.

In addition to trapped droplets, the hydrodynamic resistance due to moving droplets in a square microchannel was also determined. In this case, a complementary technique called a microfluidic comparator was used. Previously, the comparator had been used to determine the excess pressure drop of a single deformable object. However, the method was configured so that it could measure the pressure drop of a train of confined droplets. It was observed that there is a significant increase in the pressure drop when the viscosity ratio between the droplet and continuous phase is increased from 0.01 to 0.1, which was also confirmed with computational fluid dynamic simulations.

Moving beyond systems containing Newtonian fluids, techniques were developed to measure ∆P-Q relations of weakly elastic fluids, there enabling extensional rheological measurements. The extensional rheology of weakly elastic polymeric fluids is difficult to measure with macroscale techniques due to inertial instabilities, and moreover, these approaches do not lend to use-and-throw capability which is useful for biofluids, such as saliva. In contrast, microfluidic devices not only lend to disposability but also offer high elongational rates at low Reynolds number making them well suited for characterizing weakly elastic fluids. Therefore, a disposable microfluidic extensional viscometer based on an optimized hyperbolic contraction-expansion geometry was developed. This “eCapillary” device works on the principle of measuring ∆P-Q while accounting for the viscous contribution to the pressure drop. It was tested with dilute polymer solutions and the measurements showed an onset of extensional-thickening at Deborah number  1 and apparent extensional viscosities 2-4 orders of magnitude higher than the shear viscosities.

Complementing the microfluidic investigations of extensional rheology of elastic fluids, studies were conducted to measure the elastic behavior of saliva using dripping-on-substrate rheology. The relaxation time of citric acid-stimulated saliva was significantly higher than unstimulated or mechanically stimulated saliva by a factor of 2 and 4, respectively. Also, centrifugation or resting saliva at room temperature for a minimum of one hour resulted in a significant decrease in the elastic behavior, by nearly two orders of magnitude. Interestingly, it was found that removing cellular debris from whole saliva with gentle filtration results in a significant decrease in relaxation time, by 2-3 orders of magnitude, indicating that the particulate matter in saliva plays an important role in its extensional rheology.