arXiv subjects

Michelle M. Driscoll

Publications and source records attributed to Michelle M. Driscoll.

At least 19 recordsLinked to original sources

arXiv2026

Influence of Flow on Discharge Behaviors and CO2 Conversion in Gliding Arc Discharges

Plasma reactors present themselves as a unique means of electrified chemical production, particularly in the conversion of greenhouse gases (e.g., CO2) back into chemical feedstocks. Warm plasmas, such as gliding arc discharges, represent a growing class of catalyst-free plasma reactors that demonstrate high energy efficiency and scalability. Here, we introduce mean discharge time as a characteristic descriptor of gliding arc dynamics, extracted directly from voltage and current waveforms. Electrical signatures for distinct plasma behaviors (modes) were established using high-speed photography, and discharge time distributions were evaluated as a function of Reynolds number. Mean discharge time is shown to decrease non-linearly with Reynolds number, providing a link between arc behavior and underlying fluid dynamics, which is otherwise neglected in traditional reactor characterization. Mean discharge time thereby provides a quantitative, operando descriptor of transient gliding arc dynamics and stability, offering insight into discharge behavior that traditional characterization approaches do not capture.

physics.flu-dyn↗

arXiv2026

Persistence of Coffee-Ring Deposits in Concentrated Suspensions of Anisotropic Colloids

Evaporating a droplet containing dispersed colloids leaves behind a dried deposit whose shape is determined by capillary flows and the resulting particle transport. The classical coffee-ring effect occurs when an outward radial flow drives particles toward the droplet's contact line as the droplet evaporates, resulting in uneven deposition. This deposition is often studied in dilute concentration regimes where, hydrodynamically, the effects of particle shape are unimportant. As particle concentration increases, it is expected that particle anisotropy should play a larger role in modifying transport and potentially suppressing coffee-ring formation. We present experiments isolating the effects of particle shape, concentration, and density, as well as solvent temperature, on the geometry of the ring deposit. By analyzing the deposits using surface profilometry to more accurately characterize ring widths, these experiments show that coffee-ring formation is independent of particle anisotropy and is instead controlled by the ratio of the particle sedimentation velocity to the velocity of the droplet's evaporating air-water interface. Hydrodynamic simulations support this finding by providing quantitative estimates for bulk sedimentation velocity. Together, these results offer a unified picture of how multiple physical parameters determine coffee-ring geometry with direct implications for suppressing uneven deposition in practical applications.

cond-mat.soft↗

arXiv2026

Emergent clusters in strongly confined systems

Driven suspensions, where energy is input at a particle scale, are a framework for understanding general principles of out-of-equilibrium organization. A large number of simple interacting units can give rise to non-trivial structure and hierarchy. Rotationally driven colloidal particles are a particularly nice model system for exploring this pattern formation, as the dominant interaction between the particles is hydrodynamic. Here, we use experiments and large-scale simulations to explore how strong confinement alters dynamics and emergent structure at the particle scale in these driven suspensions. Surprisingly, we find that large-scale (many times the particle size) density fluctuations emerge as a result of confinement, and that these density fluctuations sensitively depend on the degree of confinement. We extract a characteristic length scale for these fluctuations, demonstrating that the simulations quantitatively reproduce the experimental pattern. Moreover, we show that these density fluctuations are a result of the large-scale recirculating flow generated by the rotating particles inside a sealed chamber. This surprising result shows that even when system boundaries are far away, they can cause qualitative changes to mesoscale structure and ordering.

cond-mat.soft↗

arXiv2026

Inertia-Dilatancy Interplay Governs Shear-Thickening Drop Impact

Combining high-speed photography with direct force measurements, we investigate the impact dynamics of drops of cornstarch-water mixtures -- a premier example of shear-thickening fluids -- across a wide range of impact conditions. Our study identifies three distinct impact regimes. In addition to the liquid-like and solid-like behaviors generally expected for the impact-induced response of shear-thickening fluids, we uncover a counterintuitive regime in which high-concentration cornstarch-water mixtures display a liquid-like response at the onset of impact when shear rates are high and only transition to a solid-like behavior at later times as shear rates reduce. By integrating the classic drop-impact theory with the Reynolds-Darcy mechanism for dilatancy, we develop a unified model that quantitatively describes the impact dynamics of shear-thickening drops across all regimes. Our work reveals the unexpected response of shear-thickening fluids to ultra-fast deformation and advances fundamental understanding of drop impact for complex fluids.

physics.flu-dyn↗

arXiv2025

Photo-Switchable Cross-Linking in Polymer Gels: Effects on Surface Creasing and Network Relaxation during Swelling

Polymer gels with photo-responsive cross-links enable tunable mechanics and surface morphologies, making them promising for adaptive materials. While prior work on coumarin cross-linked gels has focused on photo-mediated events in dilute solution, their network-level mechanical responses remain unclear. Here, we design PEG hydrogels with both permanent covalent and dynamic coumarin cross-links, allowing in situ modulation of cross-linking under wavelength specific UV light. Real-time FTIR and dynamic mechanical analysis show that post-cure 365 nm irradiation drives rapid dimerization, increasing storage modulus by up to 69\%, whereas cleavage of coumarin cross-links via 254 nm post-cure irradiation has a more limited effect due to attenuation in bulk samples. Surface imaging reveals that dynamic cross-linking governs swelling-induced crease formation and evolution. Together, these results establish design principles for hydrogels with programmable mechanics and adaptive surface topographies, advancing application in smart coatings, actuators, and responsive biomaterials.

cond-mat.soft↗

arXiv2024

Rocking, Rolling, and Hopping: Exploring the Multi-motion Capabilities of Rigid and Soft Ellipsoidal Actuators

The problem of a rigid disk rolling down a ramp is a classic problem given to students in introductory mechanics courses. In contrast, systematic studies on the rolling behavior of an ellipse have only recently emerged. Unlike a rolling disk, here the geometric center remains at a constant height from the floor, the center of a rotating ellipse changes nonlinearly due to its eccentric shape. This eccentricity introduces new modes of motion beyond rolling, including rocking and hopping. Leveraging this multi-motion behavior, we design an ellipsoidal actuator which exhibits both rolling and hopping behaviors in response to changes in the applied angular velocity. Using a simple geometric framework, we successfully capture the motion of the actuator as a force-driven rigid ellipsoid on a non-slip flat surface, and identify the critical angular velocity for the rolling-to-hopping transition. Furthermore, by adding deformability to the actuator, we unlock new functionalities, enabling soft actuators that can climb slopes and work together to collectively ascend stairs.

cond-mat.soft↗

arXiv2024

Unconstrained dynamic gel swelling generates transient surface deformations

Polymer gels are comprised of a three-dimensional, cross-linked network that can typically withstand the mechanical deformation associated with both swelling and de-swelling. Thus, gels can be designed with smart behaviors that require both stress generation and dissipation, making them well-suited to many applications including membrane technology, water capture devices, and drug delivery systems. In contrast to the fully swelled equilibrium state, limited research characterizes the unsteady-state swelling regime prior to equilibrium. It is in this regime where unique surface deformations can occur. Here we show how internal network constraints and external diffusive pressure can be leveraged to manipulate swelling kinetics and surface deformations in poly(ethylene glycol) gels during unconstrained, three-dimensional swelling. We find that increasing cross-linker molecular weight and swelling in ethanol, as opposed to water, are both effective routes to increase the time it takes to reach equilibrium but do so through different mechanisms. Networks with fewer internal constraints, manipulated via cross-linker chain-length, imbibe more solvent over a longer time. In contrast, swelling in ethanol reduces the amount of solvent imbibed by the network while increasing the time to reach equilibrium. Measurements of surface patterns during swelling establishes that an immediate, fast relaxation at the surface occurs during the first five minutes of swelling. However, the density and persistence of these features varies with solvent quality. These results serve establish a framework for how soft materials undergo dynamic deformation. Engineering transient surface properties while mitigating unwanted instabilities opens the door for emerging technologies such as smart anti-fouling and sensors.

cond-mat.soft↗

arXiv2024

Drop Impact Dynamics of Complex Fluids: A Review

The impact of fluid drops on solid substrates has widespread interest in many industrial coating and spraying applications, such as ink-jet printing and agricultural pesticide sprays. Many of the fluids used in these applications are non-Newtonian, that is they contain particulate or polymeric additives that strongly modify their flow behaviour. While a large body of experimental and theoretical work has been done to understand the impact dynamics of Newtonian fluids, we as a community have much progress to make to understand how these dynamics are modified when the impact fluid has non-Newtonian rheology. In this review, we outline recent experimental, theoretical, and computational advances in the study of impact dynamics of complex fluids on solid surfaces. Here, we provide an overview of this field that is geared towards a multidisciplinary audience. Our discussion is segmented by two principal material constitutions: polymeric fluids and particulate suspensions. Throughout, we highlight promising future directions, as well as ongoing experimental and theoretical challenges in the field.

cond-mat.soft↗

arXiv2024

Restructuring a passive colloidal suspension using a rotationally driven particle

The interaction between passive and active/driven particles has introduced a new way to control colloidal suspension properties from particle aggregation to crystallization. Here, we focus on the hydrodynamic interaction between a single rotational driven particle and a suspension of passive particles near the floor. Using experiments and Stokesian dynamics simulations that account for near-field lubrication, we demonstrate that the flow induced by the driven particle can induce long-ranged rearrangement in a passive suspension. We observe an accumulation of passive particles in front of the driven particle and a depletion of passive particles behind the driven particle. This restructuring generates a pattern that can span a range more than $10$ times of the driven particle's radius. We further show that size scale of the pattern is only a function of the particle's height above the floor.

cond-mat.soft↗

arXiv2023

A simple catch: thermal fluctuations enable hydrodynamic trapping of microrollers by obstacles

It is known that obstacles can hydrodynamically trap bacteria and synthetic microswimmers in orbits, where the trapping time heavily depends on the swimmer flow field and noise is needed to escape the trap. Here, we use experiments and simulations to investigate the trapping of microrollers by obstacles. Microrollers are rotating particles close to a bottom surface, which have a prescribed propulsion direction imposed by an external rotating magnetic field. The flow field that drives their motion is quite different from previously studied swimmers. We found that the trapping time can be controlled by modifying the obstacle size or the colloid-obstacle repulsive potential. We detail the mechanisms of the trapping and find two remarkable features: The microroller is confined in the wake of the obstacle, and it can only enter the trap with Brownian motion. While noise is usually needed to escape traps in dynamical systems, here, we show that it is the only means to reach the hydrodynamic attractor.

cond-mat.soft↗

arXiv2022

Rupture dynamics of flat colloidal films

Here, we report experimental results on the rupture of flat colloidal films over a large range of volume fractions, 0.00 $\le ϕ\le$ 0.47. The films are formed using a constant fluid volume, ruptured with a needle, and recorded using a high-speed camera. We show that colloidal films rupture in a manner quantitatively similar to Newtonian fluids, even well into the shear thinning regime. These results are consistent with the well-known mechanism of the rupture of Newtonian films, where the rupture rim rolls outward collecting more fluid and thus film rupture is a shear-free process. However, in the case of spontaneous rupture under controlled humidity conditions, the same dense colloidal films show exotic instabilities reminiscent of a wrinkling fabric on the film surface. These instabilities were absent in manually ruptured films. We hypothesize that these instabilities occur when the film thickness becomes thin enough to compete with the colloidal particle size, due to film drainage before spontaneous rupture. Thus, although non-Newtonian flow properties do not influence film rupture dynamics for thick enough films, the effect of microstructure has dramatic consequences in thinner films.

cond-mat.soft↗

arXiv2022

FSVPy: A Python-based Package for Fluorescent Streak Velocimetry (FSV)

Predictive constitutive equations that connect easy-to-measure transport properties (e.g., viscosity and conductivity) with system performance variables (e.g., power consumption and efficiency) are needed to design advanced thermal and electrical systems. In this work, we explore the use of fluorescent particle-streak analysis to directly measure the local velocity field of a pressure-driven flow, introducing a new Python package (FSVPy) to perform the analysis. Fluorescent streak velocimetry (FSV) combines high-speed imaging with highly fluorescent particles to produce images that contain fluorescent streaks, whose length and intensity can be related to the local flow velocity. By capturing images throughout the sample volume, the three-dimensional velocity field can be quantified and reconstructed. We demonstrate this technique by characterizing the channel flow profiles of several non-Newtonian fluids: micellar Cetylpyridinium Chloride solution, Carbopol 940, and Polyethylene Glycol. We then explore more complex flows, where significant acceleration is created due to micro-scale features encountered within the flow. We demonstrate the ability of FSVPy to process streaks of various shapes, and use the variable intensity along the streak to extract position-specific velocity measurements from individual images. Thus, we demonstrate that FSVPy is a flexible tool that can be used to extract local velocimetry measurements from a wide variety of fluids and flow conditions.

physics.flu-dyn↗

arXiv2022

Coexistence of solid and liquid phases in shear jammed colloidal drops

Complex fluids exhibit a variety of exotic flow behaviours under high stresses, such as shear thickening and shear jamming. Rheology is a powerful tool to characterise these flow behaviours over the bulk of the fluid. However, this technique is limited in its ability to probe fluid behaviour in a spatially resolved way. Here, we utilise high-speed imaging and the free-surface geometry in drop impact to study the flow of colloidal suspensions. We report, for the first time, observations of coexisting solid and liquid phases due to shear jamming caused by impact. In addition to observing Newtonian-like spreading and bulk shear jamming, we observe the transition between these regimes in the form of localised patches of jammed suspension in the spreading drop. We capture shear jamming as it occurs via a solidification front traveling from the impact point, and show that the speed of this front is set by how far the impact conditions are beyond the shear thickening transition.

cond-mat.soft↗

arXiv2021

Heterogeneity-stabilized homogeneous states in driven media

Understanding the relationship between symmetry breaking, system properties, and instabilities has been a problem of longstanding scientific interest. Symmetry-breaking instabilities underlie the formation of important patterns in driven systems, but there are many instances in which such instabilities are undesirable. Using parametric resonance as a model process, here we show that a range of states that would be destabilized by symmetry-breaking instabilities can be preserved and stabilized by the introduction of suitable system asymmetry. Because symmetric states are spatially homogeneous and asymmetric systems are spatially heterogeneous, we refer to this effect as heterogeneity-stabilized homogeneity. We illustrate this effect theoretically using driven pendulum array models and demonstrate it experimentally using Faraday wave instabilities. Our results have potential implications for the mitigation of instabilities in engineered systems and the emergence of homogeneous states in natural systems with inherent heterogeneities.

cond-mat.dis-nn↗

arXiv2020

Sedimentation of a Colloidal Monolayer Down an Inclined Plane

We study the driven collective dynamics of a colloidal monolayer sedimentating down an inclined plane. The action of the gravity force parallel to the bottom wall creates a flow around each colloid, and the hydrodynamic interactions among the colloids accelerate the sedimentation as the local density increases. This leads to the creation of a universal "triangular" inhomogeneous density profile, with a traveling density shock at the leading front moving in the downhill direction. Unlike density shocks in a colloidal monolayer driven by applied torques rather than forces [Phys. Rev. Fluids, 2(9):092301, 2017], the density front during sedimentation remains stable over long periods of time even though it develops a roughness on the order of tens of particle diameters. Through experimental measurements and particle-based computer simulations, we find that the Burgers equation can model the density profile along the sedimentation direction as a function of time remarkably well, with a modest improvement if the nonlinear conservation law accounts for the sub-linear dependence of the collective sedimentation velocity on density.

cond-mat.soft↗

arXiv2020

Gel rupture in a dynamic environment

Hydrogels have had a profound impact in the fields of tissue engineering, drug delivery, and materials science as a whole. Due to the network architecture of these materials, imbibement with water often results in uniform swelling and isotropic expansion which scales with the degree of cross-linking. However, the development of internal stresses during swelling can have dramatic consequences, leading to surface instabilities as well as rupture or bursting events. To better understand hydrogel behavior, macroscopic mechanical characterization techniques (e.g.\ tensile testing, rheometry) are often used, however most commonly these techniques are employed on samples that are in two distinct states: (1) unswollen and without any solvent, or (2) in an equilibrium swelling state where the maximum amount of water has been imbibed. Rarely is the dynamic process of swelling studied, especially in samples where rupture or failure events are observed. To address this gap, here we focus on rupture events in poly(ethylene glycol)-based networks that occur in response to swelling with water. Rupture events were visualized using high-speed imaging, and the influence of swelling on material properties was characterized using dynamic mechanical analysis. We find that rupture events follow a three-stage process that includes a waiting period, a slow fracture period, and a final stage in which a rapid increase in the velocity of crack propagation is observed. We describe this fracture behavior based on changes in material properties that occur during swelling, and highlight how this rupture behavior can be controlled by straight-forward modifications to the hydrogel network structure.

cond-mat.soft↗

arXiv2016

Tunable failure: control of rupture through rigidity

We investigate how material rigidity acts as a key control parameter for the failure of solids under stress. In both experiments and simulations, we demonstrate that material failure can be continuously tuned by varying the underlying rigidity of the material while holding the amount of disorder constant. As the rigidity transition is approached, failure due to the application of uniaxial stress evolves from brittle cracking to system-spanning diffuse breaking. This evolution in failure behavior can be parameterized by the width of the crack. As a system becomes more and more floppy, this crack width increases until it saturates at the system size. Thus, the spatial extent of the failure zone can be used as a direct probe for material rigidity.

cond-mat.soft↗

arXiv2014

Geometric control of failure behavior in perforated sheets

Adding perforations to a continuum sheet allows new modes of deformation, and thus modifies its elastic behavior. The failure behavior of such a perforated sheet is explored, using a model experimental system: a material containing a one-dimensional array of rectangular holes. In this model system, a transition in failure mode occurs as the spacing and aspect ratio of the holes are varied: rapid failure via a running crack is completely replaced by quasi-static failure which proceeds via the breaking of struts at random positions in the array of holes. I demonstrate that this transition can be connected to the loss of stress enhancement which occurs as the material geometry is modified.

cond-mat.mtrl-sci↗