arXiv subjects

Nathan Vani

Publications and source records attributed to Nathan Vani.

8 recordsLinked to original sources

arXiv2026

A single length scale rules ballistic aggregation: travels of a droplet train

Ballistic aggregation is a canonical non-equilibrium process, relevant across scales from granular gases to planetary accretion. Collisions are driven by differences in velocities, building up persistent correlations between neighbors. Here, we provide the first experimental realization of one-dimensional ballistic aggregation in a train of droplets formed by the breakup of a liquid jet. Experiments and simulations confirm the analytically predicted scaling, with the global process shown to be governed by a single length scale. While air drag inverts the sign of neighbor velocity correlations, a 1D ordering constraint protects bulk characteristics of ballistic aggregation such as the scaling exponent and the shape of the large mass tail. More broadly, our results show that Smoluchowski-like mean-field descriptions fail when collisions carry directional memory -- as demonstrated here for jet-generated sprays.

cond-mat.soft↗

arXiv2026

Pressure and asymmetry govern the shape and stiffness of inflatables

Inflatables made of thin sheets constitute a lightweight, scalable alternative to conventional soft robots. Since sheets are essentially inextensible while offering low resistance to bending, the shape of a straight tube should be trivially set by volume maximization. We show that networks of parallel tubes made from two sheets differing in stiffness defy this expectation as their global shape is governed by the binding angle at the junctions of adjacent tubes. Through this angle, the stiffness asymmetry induces a pressure-dependent curling and stiffening of the networks. Modeling a tube cross-section as two coupled rods, we quantitatively describe the geometry and mechanics of this new class of inflatables. Our model captures unexpected mechanical features such as a stiffness scaling as the square root of pressure and a contact-induced stiffening between neighboring tubes -- challenging common assumptions on thin-sheet inflatables. Unlike prior work restricted to the high-pressure regime, the pressure-dependent description further enables multiprogrammable control over a continuous range of curvatures. Discussing a variety of examples, we finally show that networks of asymmetric tubes are a versatile platform for functional shape-morphing objects.

cond-mat.soft↗

arXiv2026

Suspensions of non-Brownian rods: droplet pinch-off, onset of heterogeneity and effective extensional viscosity

The stretching and pinch-off of a liquid bridge is a simple way to probe when a suspension of particles stops behaving as a continuum. In this study, we consider density-matched suspensions of rigid nylon fibers with aspect ratios (length over diameter) ranging from 2 to 84, and volume fractions $\phi$ spanning the dilute to dense regimes. High-speed imaging of pendant-drop breakup reveals three successive regimes, as previously observed for spherical particles: an equivalent-fluid regime at early times, a dislocation regime corresponding to the separation of the rods, and a final regime controlled by the interstitial liquid once the neck is devoid of rods. The thresholds between these regimes follow the previously proposed scaling for spherical particles, in which the rod length, rather than the rod diameter, is used as the relevant discrete scale. In the equivalent-fluid regime, pinch-off also leads to an effective extensional viscosity that increases with both volume fraction and aspect ratio. This viscosity is not equal to the shear viscosity measured in a parallel-plate rheometer, but both sets of data are well described by Mills' law using a critical volume fraction $\phi_c$. Finally, the critical volume fraction $\phi_c$ decreases monotonically with the aspect ratio and is well captured by an empirical law. These results show that pinch-off is a sensitive probe of continuum breakdown in anisotropic suspensions and that, for rigid rods, the rod length controls the onset of heterogeneous thinning.

cond-mat.soft↗

arXiv2024

Asymmetric Bending Boundary Layer: the $\lambda$-test

We investigate the mechanics of two asymmetric ribbons bound at one end and pulled apart at the other ends. We characterize the elastic junction near the bonding and conceptualize it as a bending boundary layer. While the size of this junction decreases with the pulling force, we observe the surprising existence of the binding angle as a macroscopic signature of the bending stiffnesses. Our results thus challenge the standard assumption of neglecting bending stiffness of thin shells at large tensile loading. In addition, we show how the rotational response of the structure exhibits a non-linear and universal behavior regardless of the ratio of asymmetry. Leveraging the independence of the binding angle to the pulling force, we finally introduce the $\lambda$-test -- a visual measurement technique to characterize membranes through simple mechanical coupling.

cond-mat.soft↗

arXiv2024

Role of the constriction angle on the clogging by bridging of suspensions of particles

Confined flows of particles can lead to clogging, and therefore failure, of various fluidic systems across many applications. As a result, design guidelines need to be developed to ensure that clogging is prevented or at least delayed. In this Letter, we investigate the influence of the angle of reduction in the cross-section of the channel on the bridging of semi-dilute and dense non-Brownian suspensions of spherical particles. We observe a decrease of the clogging probability with the reduction of the constriction angle. This effect is more pronounced for dense suspensions close to the maximum packing fraction where particles are in contact in contrast to semi-dilute suspensions. We rationalize this difference in terms of arch selection. We describe the role of the constriction angle and the flow profile, providing insights into the distinct behavior of semi-dilute and dense suspensions.

cond-mat.soft↗

arXiv2023

Caging and fluid deformations in dense bidisperse suspensions

We investigate the link between the geometric environment of particles, the local deformations of the solvent, and the bulk effective viscosity in non-Brownian suspensions. First, we discuss the caging of particles by their neighbors,and especially the caging of small particles by large ones in bidisperse suspensions.We develop a model that attributes an effective volume to particles depending on their environment, and yields the local deformations and effective viscosity. We compare this model to data from the literature, as well as to our own experiments with suspensions of non-Brownian polystyrene beads. Using dissolved polymers and their coil-stretch transition as strain probes, we measure the local deformation of the liquid and the effect of caging thereon. We obtain a linear relationship between the amplified local strain rate and the particle volume fraction, in which the critical volume fraction $\phi_\mathrm{c}$ appears as an effective volume of the particles; this relationship is found valid up into the dense regime.

cond-mat.soft↗

arXiv2023

Deposition and alignment of fiber suspensions by dip coating

The dip coating of suspensions made of monodisperse non-Brownian spherical particles dispersed in a Newtonian fluid leads to different coating regimes depending on the ratio of the particle diameter to the thickness of the film entrained on the substrate. In particular, dilute particles dispersed in the liquid are entrained only above a threshold value of film thickness. In the case of anisotropic particles, in particular fibers, the smallest characteristic dimension will control the entrainment of the particle. Furthermore, it is possible to control the orientation of the anisotropic particles depending on the substrate geometry. To test the hypotheses, we performed dip-coating experiments with dilute suspensions of non-Brownian fibers with different length-to-diameter aspect ratios. We characterize the number of fibers entrained on the surface of the substrate as a function of the withdrawal velocity, allowing us to estimate a threshold capillary number below which all the particles remain in the liquid bath. Besides, we measure the angular distribution of the entrained fibers for two different substrate geometries: flat plates and cylindrical rods. We then measure the film thickness for more concentrated fiber suspensions. The entrainment of the fibers on a flat plate and a cylindrical rod is primarily controlled by the smaller characteristic length of the fibers: their diameter. At first order, the entrainment threshold scales similarly to that of spherical particles. The length of the fibers only appears to have a minor influence on the entrainment threshold. No preferential alignment is observed for non-Brownian fibers on a flat plate, except for very thin films, whereas the fibers tend to align themselves along the axis of a cylindrical rod for a large enough ratio of the fiber length to the radius of the cylindrical rod.

cond-mat.soft↗

arXiv2022

Influence of the solid fraction on the clogging by bridging of suspensions in constricted channels

Clogging can occur whenever a suspension of particles flows through a confined system. The formation of clogs is often correlated to a reduction in the cross-section of the channel. In this study, we consider the clogging by bridging, i.e., through the formation of a stable arch of particles at a constriction that hinders the transport of particles downstream of the clog. To characterize the role of the volume fraction of the suspension on the clogging dynamics, we study the flow of particulate suspensions through 3D-printed millifluidic devices. We systematically characterize the bridging of non-Brownian particles in a quasi-bidimensional system in which we directly visualize and track the particles as they flow and form arches at a constriction. We report the conditions for clogging by bridging when varying the constriction width to particle diameter ratio for different concentrations of the particles in suspension. We then discuss our results using a stochastic model to rationalize the influence of solid fraction on the probability of clogging. Understanding the mechanisms and conditions of clog formation is an important step for optimizing engineering design and developing more reliable dispensing systems.

cond-mat.soft↗