Nina C. Shapley

Biocompatible Nanoparticles Trigger Rapid Bacteria Clustering

This study reveals an exciting phenomenon of stimulated bacteria clustering. Rapid aggregation and microbial arrest are shown to occur in Escherichia coli solutions of neutral pH when chitosan nanoparticles with positive zeta potential are added. Because chitosan nanoparticles can easily be dispersed in aqueous buffers, the rapid clustering phenomenon requires only minuscule nanoparticle concentrations and will be critical in developing new methods for extricating bacterial pathogens. This work establishes the dominant role of electrostatic attraction in bacteria‐nanoparticle interactions by varying the nanoparticle zeta potential from highly positive to strongly negative values, and by exploring concentration effects. For strongly negative nanoparticles, no clusters form, while aggregates are small and loose at intermediate conditions. In addition, optical density measurements indicate that over 90% of the suspended bacteria flocculate within seconds of being mixed with chitosan nanoparticles of a highly positive surface charge. Finally, the nanoparticles are significantly more efficient as a clustering agent compared to an equal mass of molecular chitosan in solution, as the bacteria‐nanoparticle clusters formed are substantially larger. The bacteria‐nanoparticle aggregation effect demonstrated here promises a rapid separation method for aiding pathogen detection and for flocculation of bacteria in fermentation processes.

Flow of a concentrated suspension through an abrupt axisymmetric expansion measured by nuclear magnetic resonance imaging

The objective of this work is to characterize the parameters affecting the particle concentration distribution and flow field of a concentrated suspension undergoing steady flow in an abrupt axisymmetric 1:4 expansion. Of particular interest are the relationships between imposed operating conditions and the resulting spatial particle distribution and the interaction between particles and recirculating flow regions. Experiments were conducted to determine the effect of the bulk particle volume fraction, tube-particle radius ratio, and Reynolds number on observed concentration and flow patterns. Particle concentration and velocity profiles were measured by using nuclear magnetic resonance imaging. Results indicate that inlet concentration profiles formed in the upstream narrow tube greatly influenced behavior downstream, more so than direct interparticle collisions in the abrupt expansion. Also, particle depletion in recirculating flow regions was observed under all conditions studied and is consistent with...

Dialysis without Membranes: How and Why?

Dialysis between two flowing, miscible fluids without an intervening membrane enhances both the transport rate and biocompatibility. Unfortunately, it also presents serious challenges, including the loss of pressure as a driving force for volume transport, the need for sterile dialysate in greater quantity than in conventional dialysis, the possibility of unacceptable protein loss, and even the possibility of blood cell loss. This paper quantifies these advantages and disadvantages, and evaluate the means by which the latter might be surmounted. Preliminary data are provided to show that stable flows of one fluid sheathing another, miscible fluid are achievable and that molecular exchange between the fluids is orderly and in qualitative agreement with the theory. Extension of the concept to other blood purification tasks, especially in the treatment of liver failure and various macromolecular separations, is also discussed. In conclusion, membraneless separations will require a secondary process and a recycle stream. Under these conditions, its advantages can be preserved and its disadvantages controlled.

Flows of concentrated suspensions through an asymmetric bifurcation

Flows of concentrated suspensions at low Reynolds number through an asymmetric T-junction bifurcation composed of rectangular channels are studied experimentally using nuclear magnetic resonance imaging. In contrast to the unequal division of a uniform concentration material, the suspension and the neutrally buoyant, noncolloidal particles are almost equally partitioned between downstream branches, and motion of particles across the dividing stream surface is deduced to occur at the bifurcation. We attribute the rearrangement of particles to enhanced spreading of high concentration (and therefore high local viscosity) regions of the suspension toward the side branch. The partitioning is accompanied by lateral asymmetry in the concentration and velocity profiles of the downstream branches, although the inlet profiles are symmetric. In the spanwise direction, inhomogeneous concentration distributions that develop upstream persist throughout the inlet and downstream channels. Overall, the fractions of the fl...

The effect of inlet conditions on concentrated suspension flows in abrupt expansions

This work explores the influence of inlet conditions on the particle concentration distribution and flow field of a concentrated suspension undergoing steady flow in an abrupt, axisymmetric 1:4 expansion. Specifically, we consider the impact of inlet conditions in the upstream narrow tube on the resulting downstream profiles. Particle concentration and velocity profiles were measured by using nuclear magnetic resonance imaging. Experiments were conducted with two contrasting inlet tube lengths of 10d1 and 128d1, where d1 is the diameter of the narrow inlet tube. In the short inlet case, none of the particle concentration fields were fully developed upon entering the expansion, whereas several of the suspensions had fully developed profiles in the long inlet case. Results indicate that concentration differences between the core and annular regions were greater for the cases with a long inlet tube than in cases with a short inlet tube, likely owing to the higher degree of particle migration occurring in the...

Pressure drop enhancement in a concentrated suspension flowing through an abrupt axisymmetric contraction-expansion

In this work, we investigate the pressure drop of concentrated suspensions flowing through a 4:1:4 axisymmetric contraction-expansion at low Reynolds number. Pressure drop measurements across the narrow tube show an enhanced pressure drop relative to a Newtonian fluid at the same flow rate. A linear increase in pressure drop as a function of flow rate is observed for the suspensions, with a steeper slope at higher particle volume fraction. However, due to shear-induced particle migration, the pressure drop at each concentration is lower than would be expected for a uniform Newtonian fluid of the same viscosity as the bulk suspension viscosity. The Krieger [I. M. Krieger, Adv. Colloid Interface Sci. 3, 111 (1972)] expression for the relative viscosity provides an accurate estimate of the dependence of the normalized suspension pressure drop on particle volume fraction, although the bulk suspension viscosity follows the more rapidly increasing curve of Zarraga [I. E. Zarraga, D. A. Hill, and D. T. Leighton,...