The filtration behavior of suspensions in various physical systems presents both chal-lenges and opportunities. In confined flows, filtration is often an unintended and unde- sired effect, such as in irrigation systems, where flowing suspended particles can become trapped internally, leading to a gradual clogging effect. Conversely, when it is necessary to remove certain particle types from a suspension, the filtration effect can be desirable and useful. This work aims to use experimental methods to further the understanding of suspension filtering/clogging and empower others with methods to either prevent or leverage the filtration of suspensions, depending on their needs.

The first part of this thesis examines the filtration and sorting of fiber-type suspen-sions based on fiber size; this presents a challenging problem because of the fibers’ two characteristic dimensions, L and D. However, the process of dip coating by immersing a substrate in a suspension bath presents some interesting sorting capabilities; as the substrate is withdrawn, the meniscus acts as a filter which repels certain particles from the liquid bath and accepts others, becoming entrained in the coating film. With flat substrates, particles are categorically sorted by diameter; however, the curved meniscus created by withdrawing a cylindrical substrate forces a preferential fiber alignment which can sort a well-mixed fiber suspension by particle length. This length sorting process is investigated and recommendations are made on best practices.

The focus then turns to the clogging behavior of rigid fiber suspensions flowing arounda 90-degree bend. The fiber suspensions travel at constant flow rate through a channel with a uniform cross-section, and their movement is analyzed as they pass through the bend, where both the channel width and radii of curvature are precisely controlled. Fibers are characterized as “clog”, “slip”, or “no-slip” cases, and numerical methods are used to produce predictive models of clogging. Additionally, guidelines are presented for designing clog-resistant bends within millifluidic systems dispensing fiber suspensions. Finally, a comprehensive review of clogging in drip irrigation systems is presented from a fluid mechanics point-of-view, studying specifically the phenomena of physical clogging by sand, silt, and clay type particles. The current research in the area of improving anti-clogging performance of drip emitters is investigated, highlighting the role of both active and passive methods in decreasing the replacement rate of drip emitters due to clogs. This review gives a direction to possible future studies on geometry optimization of drip emitters, and presents the current best practices in extending the lifetime of drip irrigation systems using fluid mechanics principles.

Capillary flows of suspensions are critical in applications ranging from inkjet printing and 3D printing to the manufacturing of paints and coatings. These processes involve the interplay of capillary, viscous, inertial, and gravitational forces that govern the behavior of suspensions at microscopic scales. This thesis investigates the pinch-off dynamics of bidisperse spherical suspensions and the impact and spreading of fiber suspensions, which are relevant for optimizing droplet formation, extrusion, and deposition processes. The study on bidisperse suspensions reveal a modified pinch-off dynamics. Through our experiments, we reveal that this modified pinch-off dynamics is a result of the new length-scale introduced by the size of the particles close to break-up. In our study on drop impact of fiber suspensions, a notable result is the decrease in the size of the droplet as it spreads, due to increase in the suspension viscosity. A consequence of this is a decrease in the droplet size and modified coating properties as a function of the fiber volume fraction in the suspension.

The thesis also provides a broader framework for understanding the rheology of particulate suspensions, with a focus on both spherical and anisotropic particles. By leveraging classical models for viscosity, such as the Maron-Pierce correlation, it explores how particle shape, size distribution, and volume fraction influence suspension viscosity across different regimes. In dense systems, the maximum packing fraction becomes a limiting factor, particularly for fiber suspensions, where the geometry of the fibers plays a significant role. This work advances the understanding of suspension behavior in unbounded flow systems, such as during the extrusion of a droplet from the nozzle, offering insights that are critical for applications requiring precise control of suspension flow and deposition.

The flow of suspensions of particles in confined systems is ubiquitous in both natural and industrial contexts, ranging from extrusion-based additive manufacturing to water infiltration in soil. However, characterizing and modeling the underlying physics, especially when the size of a constriction is similar to that of the suspended particles, remains a challenge. This PhD work has considered the interplay of particles with two distinct types of confinements featuring: soft and hard.

Soft confinements refer to deformable liquid-air interfaces, such as the ones involved in coating processes. Two processes sharing common features are considered: withdrawing a suspension from a capillary tube, leaving a thin film on the inner wall, and dip-coating, which consists of pulling a solid substrate out of a suspension bath to deposit a thin coating film on the surface. It is demonstrated that the stagnation point at the liquid-air meniscus acts as a tunable "filter" that governs the thickness of the coating film. The local thickness controls whether particles of a certain size will be deposited in the film or repelled back to the suspension bath. The deposition threshold of single spherical and anisotropic particles is investigated, and based on these findings, a filtration method for sorting particles of various sizes and guidelines for controlling the composition of the suspension film are proposed.

The discussion then shifts to the flow of suspension through hard confinements in millifluidic channels, where particles are confined within solid walls. The research focuses on the clogging mechanism known as bridging, where particles of a size comparable to the constriction form a stable arch, blocking the channel and prevent further particle transport. Through experimental investigations, the influence of the geometry of the constriction, including its size and angle, on the particle throughput before the formation of a clog is characterized. Additionally, unique clogging phenomena observed in three-dimensional systems are explored.

Suspensions, whether natural or engineered, are encountered in nearly all physical systems involving flow. For various reasons, they are crucially important in manufacturing, water resource management, and medicine. As a result, many scientists are actively working to understand suspension flows. How do particles modify the flow? How can we separate the discrete and continuous phases? How do suspensions behave in complex or confined flows? In this thesis work, I address these questions using two different experimental platforms.

In the first half, I focus on capillary suspension flows, utilizing a dip coating platform. Dip-coating is a manufacturing technique used to apply thin films and specialized coatings to a variety of wares. When suspensions are involved, the thickness of the stagnation point below the dip-coating meniscus determines whether or not suspended particles will be entrained during the dip-coating process. For a planar substrate, we control this thickness by varying the dip-coating withdraw speed and viscosity of the suspension. Here, we extend this understanding to cylindrical substrates, and show that their curvature can be used as an additional parameter to control particle entertainment. We also demonstrate how to use these controls to separate large and small particles from a bidisperse suspension.

While dip-coating provides a low-maintenance clogging-free environment, due to it's lack of solid confinement, most other suspension systems do not posses this luxury. Indeed, clogging is a major challenge in many applications, ranging from pharmaceuticals to irrigation, resulting in a growing desire for versatile and robust clog prevention techniques. Ideally these techniques should be preventative, rather than restorative, and should avoid changing the suspension itself. Thus, in the second half of this work, I probe the relationship between clogging and hydrodynamics, specifically how a pulsatile flow environment yields different clogging dynamics than a steady flow environment. I provide a primer on pulsatile flows in microfluidic systems, which highlights various applications of pulsatile flows as well as techniques for generating them. Then I present a study which systematically characterizes the mechanisms for clog mitigation in a pulsatile environment, which depend on both the pulsatile amplitude and frequency.