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Arshad Kudrolli

Publications and source records attributed to Arshad Kudrolli.

At least 19 recordsLinked to original sources
arXiv2026

Elasto-Hydrodynamic Propulsion of a Magnetically Actuated Filament

We investigate the low-Reynolds-number propulsion of a slender elastic filament with a dipolar magnetic head actuated by an oscillating field in a viscous fluid by studying its strokes and net forward motion. To capture these dynamics, we employ an elasto-hydrodynamic (EH) framework that couples Euler-Bernoulli beam mechanics with resistive force theory. Unlike prescribed-kinematics models, filament shapes here emerge self-consistently from the actuation and the force and torque boundary conditions (BCs). We demonstrate that viscous boundary contributions are crucial for quantitative agreement and show that the swimming dynamics are governed by the EH length and a magneto-viscous-elastic stroke amplitude introduced here. The swimming speed is non-monotonic with increasing ratio of the swimmer length to the EH length, and is shown to reach a maximum when the swimmer length is on the order of the EH length. We further discuss the analytical limit in which the tail BCs can be described as free, and the limitations that arise when viscous contributions to the BCs are ignored.

cond-mat.soft↗
arXiv2026

Dynamical Boundary Following and Corner Trapping of Undulating Worms

We investigate the behavior of {\it Lumbriculus variegatus} in circular and polygonal chambers and show that the worms align with the boundaries as they move forward and then become dynamically trapped at the concave corners over prolonged periods. We model the worm as a self-propelled rod and derive analytical expressions for the evolution of its orientation when it encounters the flat and the circular boundaries of the chamber. By further incorporating translational and rotational diffusion, arising due to the undulatory and peristaltic body strokes, we demonstrate through numerical simulations that the self-propelled rod model can capture both the boundary aligning and the corner trapping behavior of the worm. The Péclet number $Pe$, representing the ratio of forward propulsion to rotational diffusion, is found to characterize the boundary alignment dynamics and trapping time distribution of the worm. Simulations show that the angle of the worm's body with the boundary while entering a concave corner plays a key role in determining the trapping time, with shallow angles leading to faster escapes. Our study demonstrates that directed motion combined with limited angular diffusion can lead to spatial localization that mimics shelter seeking behavior in slender undulating limbless worms, even in the absence of thigmotaxis or contact seeking behavior.

cond-mat.soft↗
arXiv2025

Self-Propulsion of floating ice blocks caused by melting in water

We show that floating ice blocks with asymmetric shapes can self-propel with significant speeds due to buoyancy driven currents caused by melting. In water baths with temperatures between $10\,^\circ$C and $30\,^\circ$C, model right-angle ice wedges are found to move in the direction opposite to the gravity current which descends along the longest inclined side. We describe the measured speed as a function of the length and angle of the inclined side, and the temperature of the bath in terms of a propulsion model which incorporates the cooling of the surrounding fluid by the melting of ice. The heat pulled from the surrounding liquid by the melting ice block generates a thermal convection flow, leading to momentum exchange and to a net propulsion force. The translation velocity is explained by balancing the propulsion force by drag. We further show that the ice block moves robustly in a saltwater bath with ocean-like salinity and maintains the same direction of motion as in freshwater. A simplified model is further developed to describe the propulsion of asymmetric ice blocks in saltwater, incorporating the effects of rising meltwater and the sinking of the surrounding bath water due to cooling. For sufficiently large temperature, we find that the cooling-induced sinking flow generates a stronger force than the upward flow from the meltwater. Consequently, the net propulsion force is in the same direction and nearly the same magnitude as that observed in freshwater. These findings suggest that melting-driven propulsion may be relevant to the motion of icebergs in sufficiently warm oceanic environments.

physics.flu-dyn↗
arXiv2025

Memory in cyclically crumpled sheets

We investigate the crumpling of a sheet as it is repeatedly crushed onto itself by rolling it into a cylinder and twisting it axially while allowing the end-to-end length to evolve freely. As deduced from its plastic deformations, the sheet creases and collapses into structures which repeat and sharpen over hundreds of cycles to a remarkable degree before forming new configurations. The observed metastablilty increases with applied cycles leading to recurrent structures over a significant range of loading, but reconfigurations can continue to occur for large enough loading as the creases develop tears. The evolution of the sheet structure as measured by the mean curvature and the total crease length is found to increase logarithmically with cycle number with a rate which increases with degree of compression. We explain the overall extent of creasing using flat folding models, and show the logarithmic growth as being a consequence of individual creases becoming sharper with number of folding cycles, and due to the bifurcation in the curvature field leading to the formation of new creases and folding pathways. Thus, we show that elastoplastic sheets can follow complex folding pathways to form convergent structures after a sufficiently large number of training cycles provided material fatigue remains unimportant.

cond-mat.soft↗
arXiv2025

Bundling architecture in elastic filaments with applied twist

We investigate the formation of helical multifilament bundles and the torque required to achieve them as a function of applied twist. Hyperelastic filaments with circular cross sections are mounted parallel in a uniform circle onto end-clamps that can move along the twist axis depending on the applied axial load. With increasing twist, the filaments describe a hyperbolic hyperboloid surface before coming into contact in a circle, and then packing in a tight helical bundle in the center with increasing twist. While the bundle appears ordered for sufficiently small number of filaments, they are disordered for large enough number of filaments and applied twist. We reveal with x-ray tomography, that the packing of the filaments becomes disordered following a radial-instability which leads to a decrease in bundle radius, and migration of filaments relative to each other in the bundle. Nonetheless, the helical angle of the filaments in the bundle are found to be essentially constant, resulting in inclination angles which increase with distance from axis of rotation. We develop energy minimization analysis to capture the observed variations in bundle length and torque as a function of number of filaments considering the neo-Hookean nature of the filaments. We show that the bundle geometry and the applied load can be used to describe the non-linear torque profile measured as a function of twist angle.

cond-mat.soft↗
arXiv2024

Bendability parameter for twisted ribbons to describe longitudinal wrinkling and delineate the near-threshold regime

We propose a dimensionless bendability parameter, $ε^{-1} = [\left(h/W\right)^2 T^{-1}]^{-1}$ for wrinkling of thin, twisted ribbons with thickness $h$, width $W$, and tensional strain $T$. Bendability permits efficient collapse of data for wrinkle onset, wavelength, critical stress, and residual stress, demonstrating longitudinal wrinkling's primary dependence on this parameter. This new parameter also allows us to distinguish the highly bendable range ($ε^{-1} > 20$) from moderately bendable samples ($ε^{-1} \in (0,20]$). We identify scaling relations to describe longitudinal wrinkles that are valid across our entire set of simulated ribbons. When restricted to the highly bendable regime, simulations confirm theoretical near-threshold (NT) predictions for wrinkle onset and wavelength.

cond-mat.soft↗
arXiv2024

Energetics of twisted elastic filament pairs

We investigate the elastic energy stored in a filament pair as a function of applied twist by measuring torque under prescribed end-to-end separation conditions. We show that the torque increases rapidly to a peak with applied twist when the filaments are initially separate, then decreases to a minimum as the filaments cross and come into contact. The torque then increases again while the filaments form a double helix with increasing twist. A nonlinear elasto-geometric model that combines the effect of geometrical nonlinearities with large stretching and self-twist is shown to capture the evolution of the helical geometry, the torque profile, and the stored energy with twist. We find that a large fraction of the total energy is stored in stretching the filaments, which increases with separation distance and applied tension. We find that only a small fraction of energy is stored in the form of bending energy, and that the contribution due to contact energy is negligible. Our study highlights the consequences of stretchablility on filament twisting which is a fundamental topological transformation relevant to making ropes, tying shoelaces, actuating robots, and the physical properties of entangled polymers.

cond-mat.soft↗
arXiv2023

Dynamics of magnetoelastic robots in water-saturated granular beds

We investigate the dynamics of a magnetoelastic robot with a dipolar magnetic head and a slender elastic body as it performs undulatory strokes and burrows through water-saturated granular beds. The robot is actuated by an oscillating magnetic field and moves forward when the stroke amplitude increases above a critical threshold. By visualizing the medium, we show that the undulating body fluidizes the bed, resulting in the appearance of a dynamic burrow, which rapidly closes in behind the moving robot as the medium loses energy. We investigate the applicability of Lighthill's elongated body theory of fish locomotion, and estimate the contribution of thrust generated by the undulating body and the drag incorporating the granular volume fraction-dependent effective viscosity of the medium. The projected speeds are found to be consistent with the measured speeds over a range of frequencies and amplitudes above the onset of forward motion. However, systematic deviations are found to grow with increasing driving, pointing to a need for further sophisticated modelling of the medium-structure interactions.

cond-mat.soft↗
arXiv2023

Dissolution-driven propulsion of floating solids

We show that unconstrained asymmetric dissolving solids floating in a fluid can move rectilinearly as a result of attached density currents which occur along their inclined surfaces. Solids in the form of boats composed of centimeter-scale sugar and salt slabs attached to a buoy are observed to move rapidly in water with speeds up to 5 mm/s determined by the inclination angle and orientation of the dissolving surfaces. While symmetric boats drift slowly, asymmetric boats are observed to accelerate rapidly along a line before reaching a terminal velocity when their drag matches the thrust generated by dissolution. By visualizing the flow around the body, we show that the boat velocity is always directed opposite to the horizontal component of the density current. We derive the thrust acting on the body from its measured kinematics, and show that the propulsion mechanism is consistent with the unbalanced momentum generated by the attached density current. We obtain an analytical formula for the body speed depending on geometry and material properties, and show that it captures the observed trends reasonably. Our analysis shows that the gravity current sets the scale of the body speed consistent with our observations, and we estimate that speeds can grow slowly as the cube-root of the length of the inclined dissolving surface. The dynamics of dissolving solids demonstrated here applies equally well to solids undergoing phase change, and may enhance the drift of melting icebergs, besides unraveling a primal strategy by which to achieve locomotion in active matter.

cond-mat.soft↗
arXiv2023

Soft matter physics of the ground beneath our feet

Inspired by presentations by the authors during a workshop organized at the Princeton Center for Theoretical Science (PCTS) in January 2022, we present a perspective on some of the outstanding questions related to the "physics of the ground beneath our feet." These identified challenges are intrinsically shared with the field of Soft Matter but also have unique aspects when the natural environment is studied.

cond-mat.soft↗
arXiv2023

Escape dynamics of confined undulating worms

We investigate the escape dynamics of oligochaeta {\it Lumbriculus variegatus} by confining them to a quasi-2D circular chamber with a narrow exit passage. The worms move by performing undulatory and peristaltic strokes and use their head to actively probe their surroundings. We show that the worms follow the chamber boundary with occasional reversals in direction and with velocities determined by the orientation angle of the body with respect to the boundary. The average time needed to reach the passage decreases with its width before approaching a constant, consistent with a boundary-following search strategy. We model the search dynamics as a persistent random walk along the boundary and demonstrate that the head increasingly skips over the passage entrance for smaller passage widths due to body undulations. The simulations capture the observed exponential time-distributions taken to reach the exit and their mean as a function of width when starting from random locations. Even after the head penetrates the passage entrance, we find that the worm does not always escape because the head withdraws rhythmically back into the chamber over distances set by the dual stroke amplitudes. Our study highlights the importance of boundary following and body strokes in determining how active matter escapes from enclosed spaces.

cond-mat.soft↗
arXiv2023

A computational model of twisted elastic ribbons

We develop an irregular lattice mass-spring-model (MSM) to simulate and study the deformation modes of a thin elastic ribbon as a function of applied end-to-end twist and tension. Our simulations reproduce all reported experimentally observed modes, including transitions from helicoids to longitudinal wrinkles, creased helicoids and loops with self-contact, and transverse wrinkles to accordion self-folds. Our simulations also show that the twist angles at which the primary longitudinal and transverse wrinkles appear are well described by various analyses of the Föppl-von Kármán (FvK) equations, but the characteristic wavelength of the longitudinal wrinkles has a more complex relationship to applied tension than previously estimated. The clamped edges are shown to suppress longitudinal wrinkling over a distance set by the applied tension and the ribbon width, but otherwise have no apparent effect on measured wavelength. Further, by analyzing the stress profile, we find that longitudinal wrinkling does not completely alleviate compression, but caps the magnitude of the compression. Nonetheless, the width over which wrinkles form is observed to be wider than the near-threshold analysis predictions -- the width is more consistent with the predictions of far-from-threshold analysis. However, the end-to-end contraction of the ribbon as a function of twist is found to more closely follow the corresponding near-threshold prediction as tension in the ribbon is increased, in contrast to the expectations of far-from-threshold analysis. These results point to the need for further theoretical analysis of this rich thin elastic system, guided by our physically robust and intuitive simulation model.

cond-mat.soft↗
arXiv2023

Tensional twist-folding of sheets into multilayered scrolled yarns

Twisting sheets as a strategy to form functional yarns relies on millennia of human practice in making catguts and fabric wearables, but still lacks overarching principles to guide their intricate architectures. We show that twisted hyperelastic sheets form multilayered self-scrolled yarns, through recursive folding and twist localization, that can be reconfigured and redeployed. We combine weakly nonlinear elasticity and origami to explain the observed ordered progression beyond the realm of perturbative models. Incorporating dominant stretching modes with folding kinematics, we explain the measured torque and energetics originating from geometric nonlinearities due to large displacements. Complementarily, we show that the resulting structures can be algorithmically generated using Schläfli symbols for star-shaped polygons. A geometric model is then introduced to explain the formation and structure of self-scrolled yarns. Our tensional twist-folding framework shows that origami can be harnessed to understand the transformation of stretchable sheets into self-assembled architectures with a simple twist.

cond-mat.soft↗
arXiv2023

Density Mediated Spin Correlations Drive Edge to Bulk Flow Transition in Active Chiral Matter

We demonstrate that edge currents develop in active chiral matter -- composed of spinning disk-shaped grains with chirally arranged tilted legs confined in a circular vibrating chamber -- due to boundary shielding over a wide range of densities corresponding to a gas, fluid, and crystal. The edge currents are then shown to increasingly drive circulating bulk flows with area fraction $ϕ$ due to increasing spin-coupling between neighbors mediated by frictional contacts, as percolating clusters develop. Edge currents are observed even in the dilute limit. While, at low $ϕ$, the average flux vanishes except within a distance of a single particle diameter of the boundary, the penetration depth grows with increasing $ϕ$ till a solid body rotation is achieved corresponding to the highest packing, where the particles are fully caged with hexagonal order and spin in phase with the entire packing. A coarse-grained model, based on the increased collisional interlocking of the particles with $ϕ$ and the emergence of order, captures the observed flow fields.

cond-mat.soft↗
arXiv2022

Nonadditive drag of tandem rods drafting in granular sediments

We examine the drag experienced by a pair of vertical rods moving in tandem through a granular bed immersed in a fluid as a function of their separation distance and speed. As in Newtonian fluids, the net drag experienced by the rods initially increases with distance from the value for a single rod before plateauing to twice the value. However, the drag acting on the two rods is remarkably different, with the leading rod experiencing roughly similar drag compared to a solitary rod, while the following rod experiences far less drag. The anomalous relationship of drag and the distance between the leading and following body is observed in both dry granular beds and while immersed in viscous Newtonian fluids across the quasi-static and the rate-dependent regimes. Through refractive index matching, we visualize the sediment flow past the two rods and show that a stagnant region develops in their reference frame between the rods for small separations. Thus, the following rod is increasingly shielded from the granular flow with decreasing separation distance, leading to a lower net drag. Care should be exercised in applying resistive force theory to multi-component objects moving in granular sediments based on our result that drag is not additive at short separation distances.

cond-mat.soft↗
arXiv2021

Alcove formation in dissolving cliffs driven by density inversion instability

We demonstrate conditions that give rise to cave-like features commonly found in dissolving cliffsides with a minimal two-phase physical model. Alcoves that are wider at the top and tapered at the bottom, with sharp-edged ceilings and sloping floors, are shown to develop on vertical solid surfaces dissolving in aqueous solutions. As evident from descending plumes, sufficiently large indentations evolve into alcoves as a result of the faster dissolution of the ceiling due to a solutal Rayleigh-Bénard density inversion instability. By contrast, defects of size below the boundary layer thickness set by the critical Rayleigh number smooth out, leading to stable planar interfaces. The ceiling recession rate and the alcove opening area evolution are shown to be given to first order by the critical Rayleigh number. By tracking passive tracers in the fluid phase, we show that the alcoves are shaped by the detachment of the boundary layer flow and the appearance of a pinned vortex at the leading edge of the indentations. The attached boundary layer past the developing alcove is then found to lead to rounding of the other sides and the gradual sloping of the floor.

physics.flu-dyn↗
arXiv2021

Drag anisotropy of cylindrical solids in fluid-saturated granular beds

We study the direction-dependent drag acting on a cylindrical solid intruder with length $L$ and diameter $D$ as it moves in water-saturated granular beds at constant depth. Polysterene and hydrogel grains with diameter $d$ are used to investigate materials which have high contact friction and those which are nearly frictionless, respectively. The drag on the intruder is measured while oriented perpendicular $F_\perp$ and parallel $F_\parallel$ to its axis as a function of speed $U$ from the quasi-static to the rate-dependent regime. We find that the drag anisotropy $ξ= F_\perp/F_\parallel$ is not constant, and increases significantly with driving rate and $L/D$ in both mediums. In particular for $L/D = 40$, $ξ$ increases from 2.6 to 4.5, and from 7.0 to 8.2 in the high and low friction beds, respectively, as the nondimensional Froude number $Fr =U/\sqrt{g(D+d)}$ is varied between $10^{-4}$ and $2 \times 10^{-2}$. On average, $ξ$ is observed to increase logarithmically with $L/D$ for $L/D \gg 1$. Exploiting the index-matched nature of hydrogel grains in water, we further show that the sediment flow around the cylinder in the two orientations is consistent with skin friction dominated drag. The relative contributions of the cylindrical side and the circular flat-ends on $ξ$ are estimated with thin disks to understand the observed variation of drag with aspect ratio and surface friction.

cond-mat.soft↗
arXiv2020

Unstable invasion of sedimenting granular suspensions

We investigate the development of mobility inversion and fingering when a granular suspension is injected radially between horizontal parallel plates of a cell filled with a miscible fluid. While the suspension spreads uniformly when the suspension and the displaced fluid densities are exactly matched, even a small density difference is found to result in a dense granular front which develops fingers with angular spacing that increase with granular volume fraction and decrease with injection rate. We show that the time scale over which the instability develops is given by the volume fraction dependent settling time scale of the grains in the cell. We then show that the mobility inversion and the non-equilibrium Korteweg surface tension due to granular volume fraction gradients determine the number of fingers at the onset of the instability in these miscible suspensions.

physics.flu-dyn↗