ChE PhD Dissertation Defense: Sabrina Marnoto

Name: Sabrina Marnoto

Title: Dynamics of Droplet and Soft Particle Systems in Confined Microflows

Date: 08/06/2025

Time: 12:00:00 PM

Committee Members:
Prof. Sara M. Hashmi (Advisor)
Dr. Petia Vlahovska
Prof. Xiaoyu Tang
Prof. Ambika Bajpayee
Prof. Mansoor Amiji

Location: EXP-610A

Abstract:

Droplets and soft particles are ubiquitous, exhibiting behavior in nature through rainfall and in biological materials, and serving industrial needs such as hopper de-cloggers and cell encapsulators. While droplets and particles appear in natural and industrial settings, their behavior is especially rich when flowing in confined geometries. In confined flow systems, droplets and soft particles exhibit unique phenomena that cause passive sorting and shape deformation.  Many researchers utilize this distinctive behavior for applications in understanding the micro-vascular system, creating lab-on-a-chip platforms, and manipulating droplets and soft particles. In this thesis, we exploit droplet and soft particle dynamics in confined flow for two primary objectives: to develop a technique for measuring mechanical properties of individual droplets or particles and to gain a deeper understanding of the passive softness-driven separation of droplet and particle suspensions.

To begin, we focus on developing a continuous fluidic tool that combines droplet and particle formation with mechanical measurement in a single device. Current measurement tools are known to be challenging to use and time-consuming. Specifically, methods measure a single droplet or particle at a time. Furthermore, the measurement tools are discontinuous, requiring the creation of droplets and particles, followed by a separate measurement of mechanical properties. Discontinuous methods can introduce experimental errors in measurements. Confined geometries, such as microfluidic devices, offer the opportunity to measure the properties of multiple droplets and soft particles continuously. High-throughput continuous methods have implications in automation, material design optimization, and provide precise quality control. In this thesis, we explore the mechanical properties of two systems: droplets and soft particles. For droplets, we develop a robust yet simple fluidic tensiometer for surface tension measurements. The tensiometer accurately measures surface tension for a wide range of emulsion systems and is validated by established techniques.

Our tensiometer not only measures surface tension, but also can measure the restoring stress of soft particles. Restoring stress is the stress for a particle to reform back to its initial shape in response to an applied viscous shear. A higher restoring stress is indicative of a stiffer particle. We use polyethylene glycol diacrylate (PEGDA) as our model for soft particles. PEGDA is biocompatible, tunable, and photocurable, making it a widely used soft particle in fluidic measurements. In fluidic devices, PEGDA particles are formed by first pinching off PEGDA-filled droplets using oil, followed by exposure to UV light downstream, either on or off-chip. Off-chip curing presents complications, as droplets coalesce as they exit fluidic channels, causing polydispersion and negating the monodisperse advantage of fluidic techniques.  On-chip exposure is possible with the rise of transparent fluidic device materials. In devices, many researchers expose particles to a single UV intensity and assume that particles are fully crosslinked as they flow. However, intensity and particle curing have a direct relationship. The UV curing mechanisms in flow are also not well understood. There are numerous methods for monitoring particle gelation, but few researchers combine techniques to comprehensively understand UV curing under precise flow conditions and UV control. We utilize our fluidic measurement tool, combined with a multitude of other analysis techniques, to fully grasp crosslinking kinetics both in and out of flow. We find that crosslinking particles in flow introduces additional complexities because UV intensity and exposure time depend on velocity and trajectory and can result in nonuniform curing.

In confined shear flows, wall effects cause individual droplets and particles to migrate towards the center of the channel and even deform. Shifting from individual droplet and particle measurements, we explore both emulsion and particle suspensions in flow. Suspensions of droplets and particles experience additional hydrodynamic forces due to pair-wise interactions with each other. In many computational and theoretical analyses, researchers assume monodisperse or bidisperse suspensions and simple shear flow for simplicity. However, real-world systems often deviate from these assumptions, particularly in polydispersed systems and Poiseuille flow. We employ a scaling behavior analysis on a polydispersed emulsion, considering both simple shear and Poiseuille phenomena. We investigate the validity of the scaling theory applied to varied shear rates, volume fractions, and viscosity ratios.

Multicomponent suspension flows through confined spaces are ubiquitous in nature. One of the most commonly investigated confined particle suspension systems is blood. In blood vessels in the microcirculation, red blood cells migrate towards the center of the channel, creating a cell-free layer, and other particulates, such as white blood cells, platelets, and leukocytes, partition towards the walls of the channels. Among the particulates circulating with red blood cells are drug delivery vehicles. Understanding carrier migration in blood vessels is crucial for improving drug efficacy and reducing adverse side effects. Both particle and flow properties control carrier migration. We provide a comprehensive review explaining how various properties affect particle migration in flow. Further, we suggest a method for quantifying said migration.

The thesis explores the individual and collective behavior of both particles and droplets in confined flow. By developing novel measurement tools and gaining a better fundamental understanding of droplet and particle behavior in confined flows, we present opportunities for broader industrial and academic applications, including automation, cell mechanics, and more.