arXiv subjects

Erin Koos

Publications and source records attributed to Erin Koos.

15 recordsLinked to original sources

arXiv2025

Capillary force-driven particle orientation in rod networks

Hypothesis: Anisotropic rod particles in capillary suspensions form complex network structures with distinctive orientation patterns and rheological properties that differ significantly from spherical particle systems. By identifying the orientation of individual particles, we are able to acquire invaluable experimental insight into the bulk particle orientation measurements. Experiments: Glass microrods were dispersed in capillary suspensions with varied secondary liquid volume fractions. The resulting microstructural characteristics were analyzed using confocal microscopy. Meanwhile, their rheological properties were measured through rheometry and rheoconfocal techniques. Particle networks were quantified in terms of coordination number, clustering coefficient, and orientation distribution. Findings: As the secondary liquid volume fraction increased, rod networks transitioned from point-to-point contact configurations to side-to-side aligned clusters. Unlike spherical systems, the average clustering coefficient decreased with increasing coordination number, indicating the formation of complex particle cluster configurations beyond simple side-to-side alignment. The rod networks demonstrated higher sensitivity to deformation, and samples with higher side-to-side contact probability exhibit higher viscoplastic fragility. These results provide a foundation for designing advanced materials with precisely tunable mechanical properties through controlled anisotropic particle interactions in capillary suspensions.

cond-mat.soft↗

arXiv2024

Dewetting Fingering Instability in Capillary Suspensions: Role of Particles and Liquid Bridges

This study investigates the fingering instability that forms during stretching of capillary suspensions with and without added nanoparticles. The dewetting process is observed using a transparent lifted Hele-Shaw cell. The liquid bridge is stretched under constant acceleration, and the resulting instability patterns are recorded using two high-speed cameras. Finger-like structures, characteristic of the Saffman-Taylor instability are observed. The total length of the dendrites and the intersecting number of branches are quantified. We reveal the roles of microparticles, nanoparticles, and the secondary liquid during the fingering instability. The addition of microparticles to pure liquid enhanced finger length due to increased particle interactions and nucleation sites for bubbles. The addition of secondary fluid reduces fingering length by forming a strong interparticle network. Incorporation of nanoparticles induces an early onset of cavitation and enhances fingering instability. However, nanoparticles make the capillary suspensions' overall microstructure more homogeneous, reduce the sample variation in fingering patterns, and promote the even distribution of gel on both slides during splitting. These findings highlight the complex interactions governing dewetting in capillary (nano)suspensions. This knowledge has potential applications in microfluidics, 3D printing, and thin-film coatings, where controlling dewetting is crucial.

cond-mat.soft↗

arXiv2024

Hierarchical materials with interconnected pores from capillary suspensions for bone tissue engineering

The increasing demand for bone grafts due to the aging population has opened new opportunities for the manufacture of porous ceramics to assist in bone reconstruction. In our study, we investigate a new, promising method to manufacture hierarchically porous structures in a straightforward, and tuneable way. It consists of combining the novel technology of capillary suspensions, formed by mixing solid particles and two immiscible liquids, one less than 5 vol%, with freeze casting. We have successfully achieved alumina and $\beta$-TCP materials with both <2 $\mu$m and 20-50 $\mu$m as the smallest and largest pore size, respectively. The microstructure exhibits fully open pores and high levels of porosity (>60%). The capillary suspensions' rheological behaviour indicates that silica nano-suspensions as a secondary fluid creates a stronger internal particle network than sucrose for the alumina system. Conversely, the opposite was observed with the $\beta$-TCP system. These differences were attributed to the change in affinity between the secondary fluids and the solid loading. Our study, both systems has served to deepen the knowledge about the new area of capillary suspensions and proves their use in hierarchical porous scaffolds for bone tissue engineering.

cond-mat.soft↗

arXiv2024

Enhanced contact flexibility from nanoparticles in capillary suspensions

Hypothesis: Sample-spanning particle networks are used to induce structure and a yield stress, necessary for 3D printing of porous ceramics and paints. In capillary suspensions, a small quantity of immiscible secondary fluid is incorporated into a suspension. By further adding nanoparticles with a range of hydrophobicities, the structure of the bridges and microparticle-microparticle contacts should be modified, resulting in a tunable yield stress and shear moduli. Moreover, the compressibility of these samples, important in many processing and application steps, should be sensitive to these changes. Experiment: The nanoparticle hydrophobicity was altered and their position relative to the microparticles and the bridges was examined using confocal microscopy where the correlation between bridge size and network structure was observed. A step-wise uniaxial compression test on the confocal was conducted to monitor the microparticle movement and structural changes between capillary suspension networks with and without nanoparticles. Findings: Our observation suggests that nanoparticles induce the formation of thin liquid films on the surface of the microparticles, mitigating contact line pinning and promoting internal liquid exchange. Additionally, nanoparticles at microparticle contact regions further diminish Hertzian contact, enhancing the capacity for rearrangement. These effects enhance microparticle movement, narrowing the bridge size distribution.

cond-mat.soft↗

arXiv2022

Effects of particle roughness on the rheology and structure of capillary suspensions

We show that particle roughness leads to changes in the number, shape and resulting capillary force of liquid bridges in capillary suspensions. We created fluorescently labeled, raspberry-like particles with varying roughness by electrostatically adsorbing silica nanoparticles with sizes between 40 nm and 250 nm on silica microparticles. Rougher particles require more liquid to fill the surface asperities before they form pendular bridges, resulting in smaller and weaker bridges. In a system where the effective bridge volume is adjusted, higher particle roughness leads to less clustered networks, which show a higher yield strain for a matching storage modulus compared to the smooth particle networks. This finding suggests that the particle-particle frictional contacts also affects the strength of capillary suspensions. Using asymptotically nonlinear oscillatory rheology, we corroborate the non-cubical power law scaling of the third harmonic in the shear stress response that results from both Hertzian contacts and friction between particles connected by capillary bridges. We demonstrate that the repulsive Hertzian contact parameter $A$ is sensitive to the liquid bridge strength and that roughness appears to shift the relative scaling of the power law exponents from adhesive-controlled friction to load-controlled friction.

cond-mat.soft↗

arXiv2021

Spreading of a droplet impacting on a smooth flat surface: how liquid viscosity influences the maximum spreading time and spreading ratio

Existing energy balance models, which estimate maximum droplet spreading, insufficiently capture the droplet spreading from low to high Weber and Reynolds numbers and contact angles. This is mainly due to the simplified definition of the viscous dissipation term and incomplete modeling of the maximum spreading time. In this particular research, droplet impact on a smooth sapphire surface is studied for seven glycerol concentrations between 0% - 100%, and 294 data points are acquired using high-speed photography. Fluid properties such as density, surface tension, and viscosity are also measured. For the first time according to the authors' knowledge, we incorporate the fluid viscosity in the modeling of the maximum spreading time based on the recorded data. We also estimate the characteristic velocity of the viscous dissipation term in the energy balance equation. These viscosity-based characteristic scales help to formulate a more comprehensive maximum droplet spreading model. Thanks to this improvement, our model successfully fits the data available in the literature for various fluids and surfaces compared to the existing models.

physics.flu-dyn↗

arXiv2021

The behavior of capillary suspensions at diverse length scales: from single capillary bridges to bulk

Liquid-liquid-solid systems are becoming increasingly common in everyday life with many possible applications. Here, we focus on a special case of such liquid-liquid-solid systems, namely, capillary suspensions. These capillary suspensions originate from particles that form a network based on capillary forces and are typically composed of solids in a bulk liquid with an added secondary liquid. The structure of particle networks based on capillary bridges possesses unique properties compared with networks formed via other attractive interactions where these differences are inherently related to the properties of the capillary bridges, such as bridge breaking and coalescence between adjacent bridges. Thus, to tailor the mechanical properties of capillary suspensions to specific requirements, it is important to understand the influences on different length scales ranging from the dynamics of the bridges with varying external stimuli to the often heterogeneous network structure.

cond-mat.soft↗

arXiv2021

Spreading-splashing transition of nanofluid droplets on a smooth flat surface

Even a small fraction of nanoparticles in fluids affects the splashing behavior of a droplet upon impact on a smooth surface. Nanofluid drop impact onto a smooth sapphire substrate is experimentally investigated over wide ranges of Reynolds ($10^2<\mathrm{Re}<10^4$) and Weber ($50<\mathrm{We}<500$) numbers for three nanofluid mass concentrations (0.01%, 0.1%, 1%) using high-speed photography. Nanofluids are prepared by diluting a commercial Al$_2$O$_3$-water nanofluid in aqueous glycerol solutions without dispersants. In total, 30 samples are prepared and 1799 data points are acquired. Every sample is experimentally characterized prior to droplet impact measurements in terms of stability, density, viscosity, and surface tension to demonstrate the observed outcomes on the We-Re maps. Each droplet impact condition is repeated at least 3 times to ensure good repeatability. The non-monotonic behavior of the spreading-to-splashing transition remains the same for nanofluids. However, nanofluids influence this boundary by promoting splashing at low Reynolds numbers. We explain this behavior by increased lamella spreading speed and lift during the lamella spreading stage. Finally, we develop an empirical correlation which describes the splashing threshold dependency on nanoparticle concentration for the first time.

physics.flu-dyn↗

arXiv2021

Lees-Edwards boundary conditions for translation invariant shear flow: implementation and transport properties

Molecular dynamics (MD) simulations represent a powerful investigation tool in the field of soft matter. By using shear flows, one can probe the bulk rheology of complex fluids, also beyond the linear response regime, in a way that imitates laboratory experiments. One solution to impose a shear flow in particle-based simulations is the Lees-Edwards technique which ensures that particles experience shear by imposing rules for motion and interactions across the boundary in the direction of the shear plane. Despite their presentation in 1972, a readily available public implementation of Lees-Edwards boundary conditions has been missing from MD simulation codes. In this article, we present our implementation of the Lees-Edwards technique and discuss the relevant technical choices. We used ESPResSo, the extensible simulation package for research on soft matter, for molecular dynamics simulations which can be used as a reference for other implementers. We illustrate our implementation using bulk dissipative particle dynamics fluids, compare different viscosity measurement techniques, and observe the anomalous diffusion in our samples during continuous and oscillatory shear, in good comparison to theoretical estimates.

cond-mat.soft↗

arXiv2021

Particle contact dynamics as the origin for non-integer power expansion rheology in attractive suspension networks

We show that Hertzian particle contacts are the underlying cause of the as-yet-unexplained noninteger power laws in weakly nonlinear rheology. In the medium amplitude oscillatory shear (MAOS) region, the cubic scaling of the leading order nonlinear shear stress ($\sigma_\mathrm{3} \sim \gamma_\mathrm{0}^{m_\mathrm{3}}$, $m_\mathrm{3}=3$) is the standard expectation. Expanding on the work by Natalia et al. [J. Rheol. 64 625-635 (2020)], we report an extensive data set of noncubical, noninteger power law scalings $m_\mathrm{3}$ for particle suspensions in two immiscible fluids with a capillary attractive interaction, known as capillary suspensions. Here, we show that distinct power law exponents are found for the storage and loss moduli and these noninteger scalings occur at every secondary fluid concentration for two different contact angles. These compelling results indicate that the noninteger scalings are related to the underlying microstructure of capillary suspensions. We show that the magnitude of the third harmonic elastic stress scaling $m_\mathrm{3,elastic}$ originates from Hertzian-like contacts in combination with the attractive capillary force. The related third harmonic viscous stress scaling $m_\mathrm{3,viscous}$ is, found to be associated with adhesive-controlled friction. These observations, conducted for a wide range of compositions, can help explain previous reports of noninteger scaling for materials involving particle contacts and offers a new opportunity using the variable power law exponent of MAOS rheology to reveal the physics of particle bonds and friction in the rheological response under low deformation instead of at very high shear rates.

cond-mat.soft↗

arXiv2020

Influence of drying conditions on the stress and weight development of capillary suspensions

Cracking of suspensions during drying is a common problem. While additives, e.g. binders and surfactants, can mitigate this problem, some applications, such as printing conductive pastes or sintering green bodies, do not lend themselves to the use of additives. Capillary suspensions provide an alternative formulation without additives. In this work, we use simultaneous stress and weight measurements to investigate the influence of formulation and drying conditions. Capillary suspensions dry more homogeneously and with lower peak stresses, leading to an increased robustness against cracking compared. An increase in dry film porosity is not the key driver for the stress reduction. Instead, the capillary bridges, which create strong particle networks, resist the stress. Increasing the relative humidity enhances this effect, even for pure suspensions. While lower boiling point secondary liquids, e.g. water, persist for very long times during drying, higher boiling point liquids offer further potential to tune the the drying process.

cond-mat.soft↗

arXiv2020

Using an added liquid to suppress drying defects in hard particle coatings

Hypothesis: Lateral accumulation and film defects during drying of hard particle coatings is a common problem, typically solved using polymeric additives and surface active ingredients, which require further processing of the dried film. Capillary suspensions with their tunable physical properties, devoid of polymers, offer new pathways in producing uniform and defect free particulate coatings. Experiments: We investigated the effect of small amounts of secondary liquid on the coating's drying behavior. Stress build-up and weight loss in a temperature and humidity controlled drying chamber were simultaneously measured. Changes in the coating's reflectance and height profile over time were related with the weight loss and stress curve. Findings: Capillary suspensions dry uniformly without defects. Lateral drying is inhibited by the high yield stress, causing the coating to shrink to an even height. The bridges between particles prevent air invasion and extend the constant drying period. The liquid in the lower layers is transported to the interface via corner flow within surface pores, leading to a partially dry layer near the substrate while the pores above are still saturated. Using capillary suspensions for hard particle coatings results in more uniform, defect free films with better printing characteristics, rendering high additive content obsolete.

cond-mat.soft↗

arXiv2020

Connecting particle clustering and rheology in attractive particle networks

The structural properties of suspensions and other multiphase systems are vital to overall processability, functionality and acceptance among consumers. Therefore, it is crucial to understand the intrinsic connection between the microstructure of a material and the resulting rheological properties. Here, we demonstrate how the transitions in the microstructural conformations can be quantified and correlated to rheological measurements. We find semi-local parameters from graph theory, the mathematical study of networks, to be useful in linking structure and rheology. Our results, using capillary suspensions as a model system, show that the use of the clustering coefficient, in combination with the coordination number, is able to capture not only the agglomeration of particles, but also measures the formation of groups. These phenomena are tightly connected to the rheological properties. The present sparse networks cannot be described by established techniques such as betweenness centrality.

cond-mat.soft↗

arXiv2014

Capillary suspensions: Particle networks formed through the capillary force

The addition of small amounts of a secondary fluid to a suspension can, through the attractive capillary force, lead to particle bridging and network formation. The capillary bridging phenomenon can be used to stabilize particle suspensions and precisely tune their rheological properties. This effect can even occur when the secondary fluid wets the particles less well than the bulk fluid. These materials, so-called capillary suspensions, have been the subject of recent research studying the mechanism for network formation, the properties of these suspensions, and how the material properties can be modified. Recent work in colloidal clusters is summarized and the relationship to capillary suspensions is discussed. Capillary suspensions can also be used as a pathway for new material design and some of these applications are highlighted. Results obtained to date are summarized and central questions that remain to be answered are proposed in this review.

cond-mat.soft↗

arXiv2014

Restructuring and aging in a capillary suspension

The rheological properties of capillary suspensions, suspensions with small amounts of an added immiscible fluid, are dramatically altered with the addition of the secondary fluid. We investigate a capillary suspension to determine how the network ages and restructures at rest and under applied external shear deformation. The present work uses calcium carbonate suspended in silicone oil (11 % solids) with added water as a model system. Aging of capillary suspensions and their response to applied oscillatory shear is distinctly different from particulate gels dominated by the van der Waals forces. The suspensions dominated by the capillary force are very sensitive to oscillatory flow, with the linear viscoelastic regime ending at a deformation of only 0.1 % and demonstrating power-law aging behavior. This aging persists for long times at low deformations or for shorter times with a sudden decrease in the strength at higher deformations. This aging behavior suggests that the network is able to rearrange and even rupture. This same sensitivity is not demonstrated in shear flow where very high shear rates are required to rupture the agglomerates returning the apparent viscosity of capillary suspensions to the same viscosity as for the pure vdW suspension. A transitional region is also present at intermediate water contents wherein the material response depends very strongly on the type, strength, and duration of the external forcing.

cond-mat.soft↗