arXiv subjects

Andreas Carlson

Publications and source records attributed to Andreas Carlson.

At least 19 recordsLinked to original sources

arXiv2026

Tuning nonlinear waves in nonreciprocal active filaments

The instabilities of slender structures power biological locomotion across scales, and offer a compelling method to actuate soft robots. Nonreciprocal elastic solids have been found to amplify flexural waves in one direction only, but design principles to tune and stabilize these waves are missing. Here we develop a geometrically exact theory of nonreciprocal filaments and provide simulations that capture their post-instability nonlinear dynamics. We find that nonreciprocity, when coupled to inertia or pre-stress, amplifies and advects curvature variations. The resulting one-way patterns of shape morphing can then be selected via dissipative interactions with the environment. Our work offers a continuum-based strategy for how internal stresses can drive active unidirectional waves without need for additional degrees of freedom.

cond-mat.soft↗

arXiv2026

Capillary Slinky: Equilibrium and Dynamics of Droplets in a Soft Spring

Springs can be found in many applications and biological systems, and when these are soft, they easily deform. At small scales, capillarity can induce a force leading to spring deformations when the elastocapillary number is small. We demonstrate through experiments the non-trivial equilibrium shape liquid droplets adopt in these soft springs, which form an annulus, Eruciform, and spherical shapes. When these droplets are set in motion, they display different flow regimes with significant dissipation generated by the internal rotational flow. The static and dynamics of droplets in such a capillary slinky is also used to demonstrate how surface tension can actuate springs by stretching/compression, while providing a way for active flow control in soft springs.

cond-mat.soft↗

arXiv2025

Nonreciprocal buckling makes active filaments polyfunctional

Active filaments are a workhorse for propulsion and actuation across biology, soft robotics and mechanical metamaterials. However, artificial active rods suffer from limited robustness and adaptivity because they rely on external control, or are tethered to a substrate. Here we bypass these constraints by demonstrating that non-reciprocal interactions lead to large-scale unidirectional dynamics in free-standing slender structures. By coupling the bending modes of a buckled beam anti-symmetrically, we transform the multistable dynamics of elastic snap-through into persistent cycles of shape change. In contrast to the critical point underpinning beam buckling, this transition to self-snapping is mediated by a critical exceptional point, at which bending modes simultaneously become unstable and degenerate. Upon environmental perturbation, our active filaments exploit self-snapping for a range of functionality including crawling, digging and walking. Our work advances critical exceptional physics as a guiding principle for programming instabilities into functional active materials.

cond-mat.soft↗

arXiv2025

Viscous adhesion in vibrated sheets: elastohydrodynamics with inertia and compressibility effects

Inspired by recent experiments demonstrating that vibrating elastic sheets can function as seemingly contactless suction cups, we investigate the elastohydrodynamic hovering of a thin elastic sheet vibrating near a rigid substrate. Previous theoretical work suggests that the hovering height results from a balance between the active forcing that triggers the vibrations, the bending forces associated with the sheet's deformation, the viscous lubrication flow between the sheet and the substrate, and the sheet's weight. Here, we extend this analysis beyond the asymptotic regime of weak forcing and explore the regime of strong forcing through numerical simulations. We further quantify the influence of fluid inertia and compressibility on the equilibrium hovering height and the maximum load that can be supported. Both effects are found to introduce repulsive contributions to the net force on the sheet, which can significantly reduce its adhesive strength. Beyond providing insights into soft contactless grippers and swimming near surfaces, our analysis is relevant to the elastohydrodynamics of squeeze films and near-field acoustic levitation.

physics.flu-dyn↗

arXiv2025

The seed-carrying stalk of the linden diaspore ensures autorotating flight

The dispersal of seeds by wind is one of the most evolved mechanisms plants use to invade new territories. Linden trees grow diaspores with a curved bract acting as a wing, where the seed pods are connected underneath by a stalk. Besides the seed-carrying capacity, the other functions of the stalk remain unknown. We demonstrate that the stalk of linden (genus Tilia L.) diaspores plays an essential role in their flight. The stalk length to wing span ratio is found to be the key parameter for facilitating steady autorotating flight in Tilia x europaea diaspores, effectively doubling their flight time compared to an unsteady tumbling motion. Flight experiments with biological and synthetic diaspores reveal a critical stalk length needed to induce autorotation, which correlates with analysis of collected biological samples and points to a possible evolutionarily selected trait. Flow measurements show that the vortical structures around the curved wing cause the elevated lift force during the autorotating flight.

physics.bio-ph↗

arXiv2025

Patterning surface textured plates with a viscoplastic fluid

The deposition of a viscoplastic fluid onto a substrate can be achieved by simply moving apart two plates initially separated by a fluid filled gap, where the footprint shape depends on the initiation of a fingering instability. Here, we present another approach for the controlled deposition of a viscoplastic fluid by designing the macroscopic structures of the solid substrate. Through experiments in a lifted Hele-Shaw cell, we explore how slot, square, pyramid, and triangle-shaped patterns affect the dynamics of liquid deposition. These substrate structures directly control the final shape of the viscoplastic fluid footprint. It turns out that these substrate patterns have little influence on the normal adhesive force acting on the plates.

physics.flu-dyn↗

arXiv2025

Stochastic elastohydrodynamics of contact and coarsening during membrane adhesion

Contact between fluctuating, fluid-lubricated soft surfaces is prevalent in engineering and biological systems, a process starting with adhesive contact, which can give rise to complex coarsening dynamics. One representation of such a system, which is relevant to biological membrane adhesion, is a fluctuating elastic interface covered by adhesive molecules that bind and unbind to a solid substrate across a narrow gap filled with a viscous fluid. This flow is described by the stochastic elastohydrodynamics thin-film equation, which combines the effects of viscous nanometric thin film flow, elastic membrane properties, adhesive springs, and thermal fluctuations. The average time it takes the fluctuating elastic membrane to adhere is predicted by the rare event theory, increasing exponentially with the square of the initial gap height. Numerical simulations reveal a phase separation of membrane domains driven by the binding and unbinding of adhesive molecules. The coarsening process displays close similarities to classical Ostwald ripening; however, the inclusion of hydrodynamics affects power-law growth. In particular, we identify a new bending-dominated coarsening regime, which is slower than the well-known tension-dominated case.

cond-mat.soft↗

arXiv2025

Surface mobility of a glass-forming polymer in an ionic liquid

The free surface of glassy polymers exhibits enhanced segmental dynamics compared to the bulk, forming a liquid-like layer that lowers the glass transition temperature (Tg) in nanometersized polymer samples. Recent studies have shown that immersing polymers in ionic liquids can suppress this enhanced surface dynamics. To investigate how ionic liquids influence polymer dynamics near the ionic-liquid-polymer interface, we measure the surface leveling of nanometersized stepped polystyrene films immersed in ionic liquids, and compared the results to the case of films in vacuum. Our results reveal that ionic liquids significantly slow the leveling process both above and below Tg. However, our results indicate that the liquid-like surface layer below Tg does exist in ionic liquids. Numerical solutions of the thin-film equation, incorporating appropriate boundary conditions, show that the surface mobility of PS films in ionic liquids can match that of PS films in vacuum. Thus, while ionic liquids alter the polymer flow process, they do not eliminate the dynamical heterogeneity inherent to glassy polymers.

cond-mat.soft↗

arXiv2025

Hovering of an actively driven fluid-lubricated foil

Inspired by recent experimental observations of a harmonically excited elastic foil hovering near a wall while supporting substantial weight, we develop a theoretical framework that describes the underlying physical effects. Using elastohydrodynamic lubrication theory, we quantify how the dynamic deformation of the soft foil couples to the viscous fluid flow in the intervening gap. Our analysis shows that the soft foil rectifies the reversible forcing, breaking time-reversal symmetry; the relative spatial support of the forcing determines whether the sheet is attracted to or repelled from the wall. A simple scaling law predicts the time-averaged equilibrium hovering height and the maximum weight the sheet can sustain before detaching from the surface. Numerical simulations of the governing equation corroborate our theoretical predictions, are in qualitative agreement with experiments, and might explain the behavior of organisms while providing design principles for soft robotics.

physics.flu-dyn↗

arXiv2025

Droplet spreading in a wedge: A route to fluid rheology for power-law liquids

Measuring the rheology of liquids typically requires precise control over shear rates and stresses. Here, we describe an alternative route for predicting the characteristic features of a power-law fluid by simply observing the capillary spreading dynamics of viscous droplets in a wedge-shaped geometry. In this confined setting, capillary and viscous forces interact to produce a spreading dynamics described by anomalous diffusion, a process where the front position grows as a power-law in time with an exponent that differs from the value $1/2$ found in classical diffusion. We derive a nonlinear diffusion equation that captures this behavior, and we show that the diffusion exponent is directly related to the rheological exponent of the fluid. We verify this relationship by using both experiments and simulations for different power-law fluids. As the predictions are independent from flow-specific details, this approach provides a robust tool for inferring rheological properties from the spreading dynamics.

physics.flu-dyn↗

arXiv2024

A trade-off between hydrodynamic performance and morphological bias limits the evolution of symmetric lattice animal wings

Bristled and membranous insect wings have co-evolved despite apparently serving the same functionality. We emulate flight physics using an automated free-fall experiment to better understand how and why several distinct wing forms may have developed. Biomimetic two-dimensional lattice animals were laser cut from a continuous sheet of paper, and their descent in a settling tank was tracked using a camera. Data from 31 generic symmetric polyominos (1,692 experiments) reveal that morphology impacts the drag coefficient $C_D$ and hence flight efficiency. Some polyominos rapidly sediment while others remain suspended for longer. Positioning the search for an optimal shape within an evolutionary context, we relinquished control of the automated setup to a genetic algorithm. Hereditary information passes between generations in proportion to fitness and is augmented by rare mutations. High-performing morphologies are observed, but {experimental repeats do} not re-evolve the same shapes. Adaptation rates also differ if the selective pressure is reversed from suspension to sedimentation. Our data (50,086 experiments) reveal several physical sources of indeterminism in the simulated natural selection process, including fluid flow variability and morphological entropy. This provides experimental support for the idea that the lack of convergence in insect wing shape may partly reflect the competition between fitness and entropy, making it nearly impossible to achieve unique optimal forms.

physics.bio-ph↗

arXiv2024

Singular viscoelastic perturbation to soft lubrication

Soft lubrication has been shown to drastically affect the mobility of an object immersed in a viscous fluid in the vicinity of a purely elastic wall. In this theoretical study, we develop a minimal model incorporating viscoelasticity, carrying out a perturbation analysis in both the elastic deformation of the wall and its viscous damping. Our approach reveals the singular-perturbation nature of viscoelasticity to soft lubrication. Numerical resolution of the resulting non-linear, singular and coupled equations of motion reveals peculiar effects of viscoelasticity on confined colloidal mobility, opening the way towards the description of complex migration scenarios near realistic polymeric substrates and biological membranes.

cond-mat.soft↗

arXiv2024

Plateau-Rayleigh instability of a soft layer coated on a rigid cylinder

We study the Plateau-Rayleigh instability of a viscoelastic soft solid layer coated on a rigid cylinder i.e., a soft fibre with a rigid core. The onset of instability is examined using a linear stability analysis. We find that increasing the rigid cylinder radius reduce the growth rate of the fastest growing mode. For each rigid cylinder radius, a critical elastocapillary number is found below which all wavelengths of disturbances are stable. The critical value for a soft fibre with a thick rigid cylindrical core can be several orders of magnitudes larger than that for a totally soft fibre (no rigid core), which highlights the strong stabilizing effect of the rigid core on the system. Increasing the relaxation timescale of the viscoelastic material also slows down the growth of disturbance, but has no effect on the critical elastocapillary number. Interestingly, the wavelength of the fastest growing mode is independent of the rigid cylinder radius for the purely elastic case.

cond-mat.soft↗

arXiv2024

Spreading and engulfment of a viscoelastic film onto a Newtonian droplet

We use the conservative phase-field lattice Boltzmann method to investigate the dynamics when a Newtonian droplet comes in contact with an immiscible viscoelastic liquid film. The dynamics of the three liquid phases are explored through numerical simulations, with a focus on illustrating the contact line dynamics and the viscoelastic effects described by the Oldroyd-B model. The droplet dynamics are contrasted with the case of a Newtonian fluid film. The simulations demonstrate that when the film is viscoelastic, the droplet dynamics become insensitive to the film thickness when the polymer viscosity and relaxation time are large. A viscoelastic ridge forms at the moving contact line, which evolves with a power-law dependence on time. By rescaling the interface profile of the ridge using its height and width, it appears to collapse onto a similar shape. Our findings reveal a strong correlation between the viscoelastic stress and the interface shape near the contact line.

physics.flu-dyn↗

arXiv2024

Crowding-Regulated Binding of Divalent Biomolecules

Macromolecular crowding affects biophysical processes as diverse as diffusion, gene expression, cell growth, and senescence. Yet, there is no comprehensive understanding of how crowding affects reactions, particularly multivalent binding. Herein, we use scaled particle theory and develop a molecular simulation method to investigate the binding of monovalent to divalent biomolecules. We find that crowding can increase or reduce cooperativity--the extent to which the binding of a second molecule is enhanced after binding a first molecule--by orders of magnitude, depending on the sizes of the involved molecular complexes. Cooperativity generally increases when a divalent molecule swells and then shrinks upon binding two ligands. Our calculations also reveal that, in some cases, crowding enables binding that does not occur otherwise. As an immunological example, we consider Immunoglobulin G-antigen binding and show that crowding enhances its cooperativity in bulk but reduces it when an Immunoglobulin G binds antigens on a surface.

cond-mat.soft↗

arXiv2023

Plateau-Rayleigh instability of a viscous film on a soft fiber

We theoretically study the Plateau-Rayleigh instability of a thin viscous film covering a fiber consisting of a rigid cylindrical core coated with a thin compressible elastic layer. We develop a soft-lubrication model, combining the capillary-driven flow in the viscous film to the elastic deformation of the soft coating, within the Winkler-foundation framework. We perform a linear-stability analysis and derive the dispersion relation. We find that the growth rate is larger when the soft coating is more compliant. As such, softness acts as a destabilising factor. In contrast, increasing the thickness of the soft coating reduces the growth rate, due to the dominating geometrical effect.

cond-mat.soft↗

arXiv2023

Coalescence of elastic blisters filled with a viscous fluid

Pockets of viscous fluid coalescing beneath an elastic plate are encountered in a wide range of natural phenomena and engineering processes, spanning across scales. As the pockets merge, a bridge is formed with a height increasing as the plate relaxes. We study the spatiotemporal dynamics of such an elasto-hydrodynamic coalescence process by combining experiments, lubrication theory and numerical simulations. The bridge height exhibits an exponential growth with time, which corresponds to a self-similar solution of the bending-driven thin-film equation. We address this unique self-similarity and the self-similar shape of the bridge, both of which are corroborated in numerical simulations and experiments.

physics.flu-dyn↗

arXiv2023

Static wetting of a barrel-shaped droplet on a soft-layer-coated fiber

A droplet can deform a soft substrate due to capillary forces when they are in contact. We study the static deformation of a soft solid layer coated on a rigid cylindrical fiber when an axisymmetric barrel-shaped droplet is embracing it. We find that the elastic deformation increases with decreasing rigid fiber radius. Significant disparities of deformation between the solid-liquid side and the solid-gas side are found when their solid surface tensions are different. When the coated layer is soft enough and the rigid fiber radius is less than the thickness of the coated layer, pronounced displacement oscillations are observed. Such slow decay of deformation with distances from the contact line position suggests a possible long-range interaction between droplets on a soft-layer-coated fiber.

cond-mat.soft↗