ChE PhD Dissertation Defense: Barrett Smith

Name:
Barrett Smith

Title:
in situ polymer gelation in confined flow

Date:
12/10/2025

Time:
9:00:00 AM

Committee Members:
Prof. Sara M. Hashmi (Advisor)
Prof. Steve Lustig
Prof. Xiaoyu Tang
Prof. Matt Kipper

Location:
425 Shillman Hall

Abstract:
Polymer flows through pores, nozzles and other small channels govern engineered and naturally occurring dynamics in many processes, from 3D printing to oil recovery in the earth’s subsurface to a wide variety of biological flows. Cross-linking within these polymer flows can change their material properties dramatically. Bulk characterization of these changes is insufficient to describe how these materials behave in microfluidic flow. Shear stresses produced by confinement cause changes in gel properties. Additionally, small inhomogeneities which arise during cross-linking become more important at microfluidic length scales. As a result of these complexities, the behavior of polymer solutions which are actively undergoing cross-linking is understudied and few principles have been established which help determine a priori whether emergent behaviors such as clogging will occur.

In this dissertation, we investigate a simple model system of polymer cross-linking in microfluidic flow. Alginate, a common biopolymer, is crosslinked by calcium ions while being driven through a microfluidic channel at constant flow rate. We map the boundaries defining clogging and flow as a function of flow rate, polymer concentration, and crosslinker concentration. Between the dynamic regimes of complete clogging and unrestricted flow, we observe a remarkable phenomenon in which the crosslinked polymer intermittently clogs the channel. This pattern of deposition and removal of the crosslinked gel is simultaneously highly reproducible, long-lasting, and controllable by system parameters. We provide an analytical framework to quantitatively explain and describe the intermittent behavior as resulting from diffusively driven deposition in a high Peclet number flow where convection dominates over diffusion. Fitting the experimental data shows that higher component concentrations lead to more efficient deposition and more swollen gels, while increasing the flow rate increases the deposition rate but produces much less swollen gels. By correlating the analytical analysis with bulk rheology measurements, we find that deposition efficiency increases with the stiffness of the gel formed in flow. Softer gels withstand higher shear stresses before ablation. Upon detaching from the channel, the gel retains its shape, resulting in alginate microrods. To investigate the properties of these alginate rods, we
develop and validate a novel method which uses common laboratory equipment to measure viscoelastic mechanical properties of small, soft rods and fibers. Finally, we discuss future directions for this model system and related experimental setups.

Barrett Smith is a PhD candidate in Chemical Engineering at Northeastern University. Mr. Smith obtained a Bachelor’s of Science in Biological Engineering from Cornell University in 2014. Upon graduating, he worked for five years in the research department of the pharmaceutical company Insmed Inc, where he developed nebulized and intravenous nanoparticle formulations for novel antibiotics. He joined the Hashmi Complex Fluids lab in 2021and is defending his dissertation in December 2025. In his graduate work, Mr. Smith investigates the behavior of polymer hydrogels in microfluidic systems. Outside of school, Mr. Smith enjoys spending time with his wife and two children reading, completing puzzles, and riding bikes.