arXiv subjects

Charles Maldarelli

Publications and source records attributed to Charles Maldarelli.

9 recordsLinked to original sources

arXiv2025

Axisymmetric Pushing of a Spherical Cargo using an Active Spherical Janus Motor

We analyze the interaction between a self-diffusiophoretic spherical Janus motor and an inert spherical cargo particle in an axisymmetric configuration in the Stokes regime. To study the different configurations of the two spheres and their motions, we develop an analog to the twin multipole approach to numerically determine the axisymmetric stream function for the flow field. We verify the validity and accuracy of this approach using existing literature and COMSOL Multiphysics. We study the effects of the size of the Janus cap, the relative ratio of sizes of the two spheres, and their separation distance on their interactions. For the case of a stationary cargo, we identify the existence of a distinct regime where the Janus motor hovers at a finite separation distance from the cargo and summarize the results using a phase diagram. In the presence of a freely moving cargo, we analyze the steady terminal velocities of the Janus motor and the cargo to identify distinct conditions at which the two spheres can translate with equal velocities while maintaining a finite separation distance.

physics.flu-dyn↗

arXiv2024

Interfacial Rheology of Lanthanide Binding Peptide Surfactants at the Air-Water Interface

Peptide surfactants (PEPS) are studied to capture and retain rare earth elements (REEs) at air-water interfaces to enable REE separations. Peptide sequences, designed to selectively bind REEs, depend crucially on the position of ligands within their binding loop domain. These ligands form a coordination sphere that wraps and retains the cation. We study variants of lanthanide binding tags (LBTs) designed to complex strongly with Tb$^{3+}$. The peptide LBT$^{5-}$ (with net charge -5) is known to bind Tb$^{3+}$ and adsorb with more REE cations than peptide molecules, suggesting that undesired non-specific Coulombic interactions occur. Rheological characterization of interfaces of LBT$^{5-}$ and Tb$^{3+}$ solutions reveal the formation of an interfacial gel. To probe whether this gelation reflects chelation among intact adsorbed LBT$^{5-}$:Tb$^{3+}$ complexes or destruction of the binding loop, we study a variant, LBT$^{3-}$, designed to form net neutral LBT$^{3-}$:Tb$^{3+}$ complexes. Solutions of LBT$^{3-}$ and Tb$^{3+}$ form purely viscous layers in the presence of excess Tb$^{3+}$, indicating that each peptide binds a single REE in an intact coordination sphere. We introduce the variant RR-LBT$^{3-}$ with net charge -3 and anionic ligands outside of the coordination sphere. We find that such exposed ligands promote interfacial gelation. Thus, a nuanced requirement for interfacial selectivity of PEPS is proposed: that anionic ligands outside of the coordination sphere must be avoided to prevent the non-selective recruitment of REE cations. This view is supported by simulation, including interfacial molecular dynamics simulations, and interfacial metadynamics simulations of the free energy landscape of the binding loop conformational space.

cond-mat.soft↗

arXiv2022

Hydrodynamic Interactions Between Charged and Uncharged Brownian Colloids at a Fluid-Fluid Interface

Hypothesis: The collective dynamics and self-assembly of colloids floating at a fluid/fluid interface is a balance between deterministic lateral interaction forces, viscous resistance to colloid motion along the surface and thermal (Brownian) fluctuations. As the colloid size decreases, thermal forces become important and can affect the self assembly into ordered patterns and crystal structures that are the starting point for various materials applications. Numerics: Langevin dynamic simulations involving two particles straddling a liquid/liquid interface with a high viscosity contrast are presented to describe the lateral interfacial assembly of particles in Brownian and non-Brownian dominated regimes. These simulations incorporate capillary attraction, electrostatic repulsion, thermal fluctuations and HI between particles (including the effect of the particle immersion depth). Simulation results are presented for neutrally wetted particles which form a contact angle of 90 degrees at the interface. Findings: Clustering, fractal growth and particle ordering are observed at critically large values of the Pe numbers, while smaller Pe numbers exhibit higher probabilities of yielding states in which particles remain uncorrelated in space and more widely separated.

cond-mat.soft↗

arXiv2020

Sediment creep triggered by porous flow

Quasi-2D experiments of a submerged sediment layer creeping downward were performed, varying the channel tilt and a porous flow under the respective thresholds for yielding. Logarithmic decay rates of the deformation are observed, with the rate increasing with both control parameters. A new dimensionless parameter, $P^*$, accounting for both mean porous flow and gravity force effects on particle motion, allows a collapse of all the deformation results on a single curve. Two distinct creep regimes are identified, and correspond to a systematic change of the void size distribution as $P^*$ increases.

cond-mat.soft↗

arXiv2018

Soil granular dynamics on-a-chip: fluidization inception under scrutiny

Predicting rapid and slower soil evolution remains a scientific challenge. This process involves poorly understood aspects of disordered granular matter and dense suspension dynamics. This study presents a novel two-dimensional experiment on a small-scale chip structure; this allows the observation of the deformation at the particle scale of a large-grained sediment bed, under conditions where friction dominate over cohesive and thermal forces, and with an imposed fluid flow. Experiments are performed at conditions which span the particle resuspension criterion, and particle motion is detected and analyzed. The void size population and statistics of particle trajectories bring insight to the sediment dynamics near fluidization conditions. Specifically, particle rearrangement and net bed compaction are observed at flow rates significantly below the criterion for instability growth. Above a threshold, a large vertical channel through the bed forms. In the range of flow rates where channelization can occur, the coexistence of compacting and dilating bed scenarios is observed. The results of the study enhance our capacity for modeling of both slow dynamics and eventual rapid destabilization of sediment beds. Microfluidics channel soil-on-a-chip studies open avenues to new investigations including dissolution-precipitation, fine particles transport, or micro-organisms swimming and population growth, which may depend on mechanics of the porous media itself.

cond-mat.soft↗

arXiv2018

A Molecular Dynamics Study of The Translation and Rotation of Amphiphilic Janus Nanoparticles at a Vapor-Liquid Surface

We study the effects of heterogeneity on interfacial pinning and hydrodynamic drag using molecular dynamics (MD) simulations of Janus nanospheres at a liquid/vapor interface. We construct the free energy landscape for this system, both in the continuum approximation using surfaces tensions and the flat-interface approximation and atomistically using MD and thermodynamic integration. The results of the two methods differ in detail due to interfacial distortion and finite width, as well as thermal fluctuations, and only the MD landscape is consistent with simulations of a nanosphere approaching the interface from the liquid or vapor side. When dragged along an interface, these Janus particles exhibit a velocity-dependent tilt accompanied by a weak variation in drag force, but never an enhancement of the drag force beyond the value when fully immersed. This velocity dependence arises when the interface is pinned at heterogeneities and prevents the particle from rotating, and similar behavior is observed for homogeneous but non-spherical particles. The occurrence of different particle orientations having different drag coefficients may lead to an apparent violation of the Stokes-Einstein relation.

cond-mat.soft↗

arXiv2017

Self-Propelled Colloidal Particle Near a Planar Wall: A Brownian Dynamics Study

Miniaturized, self-propelled locomotors use chemo-mechanical transduction mechanisms to convert fuel in the environment to autonomous motion. Recent experimental and theoretical studies demonstrate that these autonomous engines can passively follow the contours of solid boundaries they encounter. Boundary guidance, however, is not necessarily stable: Mechanical disturbances can cause the motor to hydrodynamically depart from the passively guided pathway. Furthermore, given the scaled-down size of micromotors (typically 100 nm -10 $μ$m), Brownian thermal fluctuation forces are necessarily important and these stochastic forces can randomize passively-steered trajectories. Here we examine theoretically the stability of boundary guided motion of micromotors along infinite planar walls to mechanical disturbances and to Brownian forces. Our aim is to understand under what conditions this passively guided motion is stable. We choose a locomotor design in which spherical colloids are partially coated with a catalytic cap that reacts with solute to produce a product. The product is repelled from the particle surface, causing the particle to move with the inert face at the front (autonomous motion via self-diffusiophoresis). When propelled towards a planar wall, deterministic hydrodynamic studies demonstrate that these locomotors can exhibit, for large enough cap sizes, steady trajectories in which the particle either skims unidirectionally along the surface at a constant distance from the wall, or becomes stationary. We first investigate the linear hydrodynamic stability of these states by expanding the equations of motion about the states, and find that linear perturbations decay exponentially in time. We then study the effects of thermal fluctuations by formulating a Langevin equation for the particle motion which includes the Brownian stochastic force...

cond-mat.soft↗

arXiv2016

Diffusivity and Hydrodynamic Drag of Nanoparticles at a Vapor-liquid Interface

Measurements of the surface diffusivity of colloidal spheres translating along a vapor/liquid inter- face show an unexpected decrease in diffusivity, or increase in surface drag (from the Stokes-Einstein relation) when the particles situate further into the vapor phase. However, direct measurements of the surface drag from the colloid velocity due to an external force find the expected decrease with deeper immersion into the vapor. The paradoxical drag increase observed in diffusion experiments has been attributed to the attachment of the fluid interface to heterogeneities on the colloid surface, which causes the interface, in response to thermal fluctuations, to either jump or remain pinned, creating added drag. We have performed molecular dynamics simulations of the diffusivity and force experiments for a nanoparticle with a rough surface at a vapor/liquid interface to examine the effect of contact line fluctuations. The drag calculated from both experiments agree and decrease as the particle positions further into the vapor. The surface drag is smaller than the bulk liquid drag due to the partial submersion into the liquid, and the finite thickness of the interfacial zone relative to the nanoparticle size. Contact line fluctuations do not give rise to an anomalous increase in drag.

cond-mat.soft↗

arXiv2015

Self-Diffusiophoretic Colloidal Propulsion Near a Solid Boundary

We study the diffusiophoretic self-propulsion of a colloidal catalytic particle due to a surface chemical reaction in a vicinity of a solid wall. Diffusiophoresis is a chemico-mechanical transduction mechanism in which a concentration gradient of an interacting solute produces an unbalanced force on a colloidal particle and drives it along the gradient. We consider a spherical particle with an axisymmetric reacting cap covering the polar angle range $0\le θ\le θ_{cap}$ in the presence of a repulsive solute, near an infinite planar wall, and solve the coupled solute concentration and Stokes equations, using a mixture of numerical and analytic arguments. The resulting particle trajectory is determined by $θ_{cap}$ and the initial orientation of the symmetry axis with respect to the plane. At normal incidence the particle initially moves away from or towards the wall, depending on whether the cap faces towards or away, respectively, but even in the latter case the particle never reaches the wall due to hydrodynamic lubrication resistance. For other initial orientations, when $θ_{cap}\le 115^{\circ}$ the particle either moves away immediately or else rotates along its trajectory so as to cause the active side to face the wall and the particle to rebound. For higher coverage we find trajectories where the particle skims along the wall at constant separation or else comes to rest. We provide a phase diagram giving the nature of the trajectory (repulsion, rebound, skimming or stationary) as a function of $θ_{cap}$ and the initial orientation.

cond-mat.soft↗