Microfluidic devices based on the Coulter principle require a small aperture for cell counting. For applications using such cell counting devices, the volume of the sample also needs to be metered to determine the absolute cell count in a specific volume. Hence, integrated methods to characterize and meter the volume of a fluid are required in these microfluidic devices. Here, we present fluid flow characterization and electrically-based sample metering results of blood through a measurement channel with a cross-section of 15 μm × 15 μm (i.e. the Coulter aperture). Red blood cells in whole blood are lysed and the remaining fluid, consisting of leukocytes, erythrocyte cell lysate and various reagents, is flown at different flow rates through the measurement aperture. The change in impedance across the electrodes embedded in the counting channel shows a linear relationship with the increase in the fluid flow rate. We also show that the fluid volume can be determined by measuring the decrease in pulse width, and increase in number of cells as they pass through the counting channel per unit time.

Dynamic networks formed by physically crosslinked, entangled polymers have emerged as self-healing, stretchable, and functional materials. Entangled associative gels with remarkable toughness and extensibility have been produced by several distinct chemical approaches, suggesting that these enhanced mechanical properties result from molecular-scale topology. Previously, artificially engineered associative proteins were designed to provide an ideal model system to investigate the role of entanglement on gel mechanics via welldefined entangled or unentangled physical gels. Herein, uniaxial strain-induced structural changes in these model gels were observed using in situ small-angle x-ray scattering (SAXS) and in situ polarized optical microscopy (POM) up to 2000% engineering strain. Anisotropic optical responses to uniaxial strain at the nano-, micro-, and macroscales suggest that stress dissipation mechanisms enable high extensibility and toughness. Nano-and microscopic anisotropy observed by SAXS indicate stretching and alignment of flexible polymer strands along the straining axis, and nonmonotonic macroscopic anisotropy observed by POM suggests relaxation within the hydrogel due to rearrangement of associative network junctions. Unentangled hydrogels exhibit low toughness and a strain-rate-dependent transition from ductile to brittle tensile behavior, which is typical for associative polymer solutions. These findings indicate that topological entanglements and the freedom of individual chains to align at the nanoscale due to junction relaxation are both critical to achieving high toughness and elongation in entangled physical gels.

Evolution of composition, rheology, and morphology during phase separation in complex fluids is highly coupled to rheological and mass transport processes within the emerging phases, and understanding this coupling is...

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Understanding the phase behavior of polyelectrolyte coacervation is crucial for many applications, including consumer formulations, wet adhesives, processed food, and drug delivery. However, in most cases, modeling coacervation is not easily accessed by molecular simulation methods due to the long-range nature of electrostatic forces and the typically high molecular weights of the species involved. We present a modeling strategy to study complex coacervation leveraging the strengths of both particle simulations and polymer field theory. Field theory is uniquely suited to capture larger-length scales that are inaccessible to particle simulations, but its predictive capability is limited by the need to specify emergent parameters. Using model coacervate-forming systems consisting of poly(acrylic acid) and poly(allylamine hydrochloride), we show an original way to use small-scale, all-atom simulations to parameterize field-theoretic models via the relative entropy coarse-graining approach. The dependence of coacervation on the salt concentration, molecular weight, and charge stoichiometry is predicted without fitting to experimental data and is consistent with experimental trends including asymmetric phase behavior from non-stoichiometric mixtures of polyelectrolytes. This demonstrates a unique simulation approach to study phase behavior in coacervate-forming systems, which is particularly useful when chemical specificity is of interest.

3D printing holds potential as a faster, cheaper alternative compared with traditional photolithography for the fabrication of microfluidic devices by replica molding. However, the influence of printing resolution and quality on device design and performance has yet to receive detailed study. Here, we investigate the use of 3D-printed molds to create staggered herringbone mixers (SHMs) with feature sizes ranging from ∼100 to 500 μm. We provide guidelines for printer calibration to ensure accurate printing at these length scales and quantify the impacts of print variability on SHM performance. We show that SHMs produced by 3D printing generate well-mixed output streams across devices with variable heights and defects, demonstrating that 3D printing is suitable and advantageous for low-cost, high-throughput SHM manufacturing.