The goal of this project is to gain fundamental insight into micro and nanoscale particles and flows. The first part of the experimental project quantifies the spreading of parallel streams with viscosity contrast in multilayer microfluidic flows. Three streams converge into one channel where a test fluid is sheathed between two layers of a Newtonian reference fluid. The test fluids are Newtonian fluids with viscosities ranging from 1.1 to 48.2 cP and suspensions of 10 μm diameter PMMA particles with particle volume fractions (&phis;) ranging from 0.16 to 0.30. The steady-state width of the center stream is strongly dependent on the viscosity ratio between the adjacent fluids and exhibits a near power-law relationship. Such dependence occurs for both the Newtonian fluids and the suspensions, although the slopes differ. The high-concentration suspension (&phis; = 0.30) diverges from Newtonian behavior, while the low-concentration suspensions (&phis; = 0.16, 0.22) closely approximate that of the Newtonian fluids. The observed suspension behavior can be attributed to shear-induced particle migration.

The second part of the project is a systematic study of the synthesis and characterization of novel biopolymer gel nanoparticles, followed by implementation in a bacteria aggregation study. Chitosan nanoparticles are synthesized through ionic gelation between chitosan and tripolyphosphate (TPP) at a fixed 5:1 mass ratio, while varying the synthesis solution concentration. The chitosan nanoparticle diameter and zeta potential increase significantly with increasing synthesis solution concentration. Also, the total surface charge is a linear function of the solution concentration. At low synthesis solution concentrations, tightly cross-linked chitosan nanoparticles form and the ratio of TPP to chitosan molecules is high compared to that of the chitosan nanoparticles formed at the high synthesis solution concentrations, which yield loosely cross-linked chitosan nanoparticles.

Suspensions containing nanoparticles of distinct surface charges are mixed with dilute E. coli suspensions and the bacteria-nanoparticle interactions are observed by phase contrast and fluorescence microscopy. Rapid aggregation and microbial arrest are observed when bacteria interact with nanoparticles of a highly positive zeta potential, establishing the primary role of electrostatic interactions.