arXiv subjects

Daniel Bonn

Publications and source records attributed to Daniel Bonn.

At least 19 recordsLinked to original sources

arXiv2026

Active Reinforcement of Jammed Emulsions by Living Microswimmers

We show that living microswimmers mechanically reinforce dense emulsions. Castor-oil-in-water emulsions laden with the microalga Chlamydomonas reinhardtii are compared in three states: without algae, with immobilized algae, and with motile algae, over a broad range of oil fractions spanning the jamming transition. Oscillatory rheology reveals that motile algae systematically increase the yield stress, by up to a factor of two, whereas immobilized cells at the same concentration leave it essentially unchanged. The reinforcement thus originates from activity rather than from the mere presence of inclusions. Single-cell tracking shows that the droplet network confines the swimmers in pores that shrink as the oil fraction increases, and confinement is known to amplify the propulsion force of C. reinhardtii. A simple estimate based on this confinement-enhanced force accounts for the measured excess yield stress and indicates an effective, activity-induced depletion-like attraction between the passive droplets. These results identify a feedback loop: the microstructure confines the swimmers, confinement amplifies the forces they exert, and these forces stiffen the microstructure. Active emulsions thus emerge as a model platform for programming the mechanics of disordered soft solids through activity.

cond-mat.soft↗

arXiv2026

Inelastic spreading of viscoelastic drops

When a liquid drop impacts a solid surface, rebound and splashing are known to be suppressed by additives that induce elastic effects; however, the influence of elasticity on the maximum spreading radius remains debated. The difficulty lies in isolating elastic resistance from the enhanced spreading caused by viscous shear thinning. Here, we experimentally decouple these effects by investigating Boger fluid drops (constant-viscosity solutions with high elasticity) of polyethylene oxide (PEO) and polyacrylamide (PAAM). We demonstrate that elastic properties do not alter the maximum spreading ratio, even at high concentrations up to 1000 ppm. We formulate an energy balance that accounts for the inertial, capillary, viscous, and elastic contributions, and yields a dimensionless criterion ($\Gamma$) that estimates the degree of elastic effects. We show that $\Gamma \ll 1$ for all impact conditions tested, demonstrating that elastic effects are energetically negligible during droplet spreading, opposite to what Weissenberg and Deborah numbers would predict.

physics.flu-dyn↗

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

Roughness-controlled Tribocharging Governs Friction in Dry Glass Contacts

Friction is commonly reduced by polishing surfaces, based on the idea that roughness enhances mechanical interlocking and thus friction. Here we show that, for dry glass-glass contacts, increasing nanoscale roughness can instead reduce friction because it suppresses triboelectric adhesion. Using rheometer-based friction measurements in dry nitrogen, super-resolution imaging of the real contact area, soft x-ray discharge, and Faraday-cup electrometry, we demonstrate that sliding generates substantial tribocharges whose electrostatic attraction contributes significantly to friction. As the root-mean-square surface slope h'_rms of the glass ball is increased from 0.01 to 0.09, the real contact area and retained tribocharge both decrease strongly, while the average contact pressure increases by a factor of three; nevertheless, the friction coefficient drops by about 30%. Discharging the interface with soft x-rays largely removes the roughness dependence of friction. Our results show that nanoscale roughness controls tribocharging and electroadhesion in dielectric contacts, inverting the classical relation between roughness and friction and identifying triboelectric effects as a key design parameter for friction control.

cond-mat.soft↗

arXiv2026

Kinematic Closure of Drop Impact

Existing models for droplet impact prescribe the spreading contact time and effective spreading velocity from asymptotic arguments, which prevents a self consistent prediction of the maximum spreading ratio across regimes. Here, the total spreading time and characteristic spreading velocity are formulated from the energy balance, with explicit capillary and viscous contributions. Multiplying this time and velocity to obtain the maximum spreading diameter yields a closed, unified scaling law for the maximum spreading ratio of wetting drops across inertio capillary and inertio viscous regimes. The resulting expression quantitatively collapses the present measurements and literature data over a wide range of Weber and Ohnesorge numbers, droplet sizes, and surface wettabilities without prefactors that need to be adjusted to a certain regime.

physics.flu-dyn↗

arXiv2025

An Ice Christmas Tree: Fast Three-Dimensional Printing of Ice Structures via Evaporative Cooling in Vacuum

We demonstrate a novel approach to three-dimensional (3D) printing of freeform ice structures by exploiting evaporative cooling. A micrometer-sized water jet is used to 3D print inside a vacuum chamber. The reduced ambient pressure leads to rapid evaporation of the extruded water, extracting latent heat, and quickly cooling the water well below 0 {\deg}C. Once deposited, the water freezes almost instantaneously into stable ice structures. We demonstrate high-fidelity printing of complex geometries (Christmas trees, cones, vertical pillars, and free-standing zigzag structures) without cryogenic infrastructure, supporting materials, or external refrigeration. This approach directly visualizes fundamental thermodynamic principles -- latent heat, evaporative cooling, and pressure-dependent phase transitions -- while offering a relatively simple and scalable platform for ice-templated microfluidics and tissue engineering, or even extraterrestrial 3D printing.

physics.flu-dyn↗

arXiv2025

Collective Asperity Dynamics and the Origin of Static Friction

Solid interfaces resist sliding up to a threshold shear force, called static friction, beyond which they start moving and their resistance drops to the kinetic friction. Static friction at rough interfaces has long been described empirically using system-specific coefficients tabulated in engineering handbooks. Here, through nanometer-resolution sliding experiments, we show that it is set by a friction overshoot during the onset of sliding. We demonstrate that this overshoot originates from the collective configurational evolution of surface asperities under shear, and derive a minimal differential equation governing this evolution. Our theory predicts that such overshoots generically emerge when an athermal frictional system evolves smoothly toward a unique steady-state kinetic friction. These results show that static friction is not an intrinsic material property, but an emergent consequence of collective asperity dynamics.

cond-mat.soft↗

arXiv2025

On the Slipperiness of Surfactants: Charge-Mediated Friction Control at the Molecular Scale

From soap-covered dishes to freshly cleaned floors, surfactants can make surfaces slippery; yet, the underlying mechanism remains poorly understood. Here, we identify the molecular origin behind this ubiquitous phenomenon using macroscopic tribology and surface molecular spectroscopy. We demonstrate that reducing friction through surfactants hinges on charge complementarity: surfactants of opposite charge to the solid surface reduce friction even at extreme contact pressures, whereas like-charged or neutral surfactants are ineffective. Oppositely charged surfactants self-assemble into dense and robust molecular brushes, creating a persistent lubrication beyond the limits of conventional mechanisms. This charge-mediated approach offers a universal and scalable framework for friction control across length scales without significant surface modification.

physics.chem-ph↗

arXiv2025

Polarity-dependent Electroadhesion at Silicon Interfaces with Nanoscale Roughness

We measure and model electroadhesion across multi-asperity silicon interfaces with nanometer scale roughness. When electrically biased, our naturally oxidized silicon interfaces display a leakage current consistent with the Fowler-Nordheim model and electroadhesion that can be successfully captured using a boundary element contact model. We show that polysilicon exhibits electroadhesion only under positive bias applied to the substrate monocrystalline wafer, which we interpret as a result of the reduced mobility of holes, with respect to electrons, within polysilicon. Overall, our findings reveal that electrostatic interactions can significantly enhance adhesion and friction between stiff and smooth surfaces, which can be important for example in precision positioning applications.

cond-mat.soft↗

arXiv2025

Controlled Spherulitic Crystal Growth from Salt Mixtures: A Universal Mechanism for Complex Crystal Self-Assembly

Spherulites are complex polycrystalline structures that form through the self-assembly of small aggregated nanocrystals starting from a central point and growing radially outward. Despite their wide prevalence and relevance to fields ranging from geology to medicine, the dynamics of spherulitic crystallization and the conditions required for such growth remain ill-understood. Here, we report on the conditions to induce controlled spherulitic growth of sodium sulfate from evaporating aqueous solutions of sulfate salt mixtures at room temperature. We reveal that introducing divalent metal ions in the solution cause spherulitic growth of sodium sulfate. For the first time, we quantify the supersaturation at the onset of spherulitic growth from salt mixtures and determine the growth kinetics. Our results show that the nonclassical nucleation process induces the growth of sodium sulfate spherulites at high supersaturation in highly viscous solutions. The latter reaches approximately 111 Pa$\cdot$s, triggered by the divalent ions, at the onset of spherulite precipitation leading to a diffusion limited growth. We also show that spherulites, which are metastable structures formed under out-of-equilibrium conditions, can evolve into other shapes when supersaturation decreases as growth continues at different evaporation rates. These findings shed light on the conditions under which spherulites form and offer practical strategies for tuning their morphology.

cond-mat.soft↗

arXiv2025

Water-rich amorphous state from drying mixed-metal sulfate solutions

Amorphous and glassy materials are important for many advanced applications, from flexible solar cells to drug delivery systems. To this end, new glasses are in high demand, but precise chemical design of amorphous materials remains challenging. By studying the crystallization of mixed salt solutions, we have discovered an entirely new type of amorphous material: water-rich amorphous mixed sulfates. Specifically, we show that drying of sulfate salt mixtures of both mono- and higher valency cations almost exclusively yields a glassy or amorphous state, where the stability of the amorphous state depends on the cations present and ranges from seconds to months. Furthermore, we show that the glassy state is viscoelastic, behaves like a soft solid (G' 10^5 - 10^6 Pa), retains a large amount of water (30 to 40 weight percent), and is X-ray amorphous. Additionally, confocal Raman microspectroscopy reveals disordered sulfate orientations and Fourier-transform infrared spectroscopy highlights increased hydrogen bonding during drying, which together with strong cation hydration is hypothesized to prevent crystallization. These results provide insights for the production of a new class of amorphous materials, and help to elucidate the mystery of the high abundance of such amorphous salts found on Mars.

cond-mat.soft↗

arXiv2025

Anomalous Creep as a Precursor to Failure in Granular Materials

Granular materials, composed of discrete solid grains, can be modeled as simple mechanical systems. However, these materials can undergo spontaneous slow deformation, or creep, even under small forces and while in apparent mechanical equilibrium; a phenomenon central to understanding soil mechanics and the behavior of earthquake faults. We show that creep in granular materials originates from frictional dynamics at the contact points between grains. We reveal that the stability of these materials is governed by the interplay between creep and aging at these frictional contacts. Near the yield threshold, the frictional interactions result in anomalously accelerating creep, eventually leading to the delayed failure of the fragile packing. This behavior may serve as an early warning signal for catastrophic events like earthquakes and landslides.

cond-mat.soft↗

arXiv2024

What determines the breakup length of a jet?

The breakup of a capillary jet into drops is believed to be governed by initial disturbances on the surface of the jet that grow exponentially. The disturbances are often assumed to be due to external sources of noise, to turbulence, or to imperfections of the nozzle. However, even in conditions where external perturbations are minimal, the jet's length cannot grow indefinitely, suggesting that its fragmentation cannot be entirely attributed to such factors. Here we show that the initial disturbances are thermal capillary waves. By extrapolating the observed growth of the instability back in time, we demonstrate that the initiating disturbances must be of the order of an {\aa}ngstr\"om, consistent with fluctuations induced by thermal noise. Further, by performing many experiments with different nozzles, we find no significant variation in breakup length linked to nozzle type, shape or inner roughness. By systematically varying the jet diameter and velocity, and the surface tension, viscosity, and density of the fluid, we validate our thermal disturbance model over four orders of magnitude in jet length; seven orders of magnitude if simulations of nanojets are included.

physics.flu-dyn↗

arXiv2024

Drop-drop coalescence: a simple crossover function between inertial and viscous dynamics

The coalescence of liquid drops is a fundamental process that remains incompletely understood, particularly in the intermediate regimes where capillary, viscous, and inertial forces are comparable. Here, we experimentally investigate the dynamics of drop-to-drop coalescence during the transition between viscous and inertial regimes using high-speed imaging. Our results reveal that the liquid bridge between droplets shows power-law growth with exponents between 1/2 and 1 during drop coalescence. We propose a novel scaling approach using a dimensionless crossover function that smoothly transitions between viscous and inertial limits. This simple approach, inspired by previous work on drop impact, successfully collapses the experimental data for a wide range of liquid viscosities and coalescence times onto a single master curve. We further compare our results with recent theoretical models and demonstrate how our approach complements and extends current understanding in the crossover of drop coalescence. This study contributes to both the fundamental physics of drop coalescence and its practical applications in various industrial processes.

physics.flu-dyn↗

arXiv2024

Damage due to Ice Crystallization

The freezing of water is one of the major causes of mechanical damage in materials during wintertime; surprisingly this happens even in situations where water only partially saturates the material so that the ice has room to grow. Here we perform freezing experiments in cylindrical glass vials of various sizes and wettability properties, using a dye that exclusively colors the liquid phase; this allows to precisely observe the freezing front. The visualization reveals that damage occurs in partially water-saturated media when a closed liquid inclusion forms within the ice due to the freezing of air/water meniscus. When this water inclusion subsequently freezes, the volume expansion leads to very high pressures leading to the fracture of both the surrounding ice and the glass vial. The pressure can be understood quantitatively based on thermodynamics which correctly predicts that the crystallization pressure is independent of the volume of the liquid pocket. Finally, our results also reveal that by changing the wetting properties of the confining walls, the formation of the liquid pockets that cause the mechanical damage can be avoided.

cond-mat.soft↗

arXiv2024

The Decrease of Static Friction Coefficient with Interface Growth from Single to Multiasperity Contact

The key parameter for describing frictional strength at the onset of sliding is the static friction coefficient. Yet, how the static friction coefficient emerges at the macroscale from contacting asperities at the microscale is still an open problem. Here, we present friction experiments in which the normal load was varied over more than 3 orders of magnitude, so that a transition from a single asperity contact at low loads to multiasperity contacts at high loads was achieved. We find a remarkable reduction in the friction drop (the ratio of the static friction force to the dynamic friction force) with increasing normal load. Using a simple stick-slip transition model we identify the presence of presliding and subcritical contact points as the cause of smaller static friction coefficient at increased normal loads. Our measurements and model bridge the gap between friction behavior commonly observed in atomic force microscopy experiments at microscopic forces, and industrially relevant multiasperity contact interfaces loaded with macroscopic forces.

cond-mat.soft↗

arXiv2024

Structural Relaxation in Simple Yield Stress Materials Influences Their Rheology

Simple yield stress materials are composed of soft particles, bubbles, or droplets with purely repulsive forces. The constituent elements are typically too large to undergo thermal fluctuations, suggesting that the internal structure of the material, and therefore the rheology, should not change over time. We explore the rheology of Carbopol, a prototypical simple yield stress material, and show that gradual structural relaxation of the material results in a small yet significant reduction in the dynamic yield stress. This relaxation process can lead to a non-monotonic creep deformation rate under constant stress, culminating in delayed fluidization of the material. These findings show that the yield stress is not merely a static material property but may be a function of the internal structure of the material.

cond-mat.soft↗

arXiv2024

Early Stages of Drop Coalescence

Despite the large body of research on coalescence, firm agreement between experiment, theory, and computation has not been established for the very first moments following the initial contact of two liquid volumes. Combining a range of experimental and computational modeling approaches in two different geometries, namely drop-drop and drop-bath configurations, we have been able to elucidate the influence of the intervening gas and van der Waals forces on coalescence. For simple liquids considered here, the gas influences both pre- and post-contact regimes, with jump-to-contact being the primary mode of merging. Subsequently, wave-like air pockets are observed and ultimately influence the initial opening dynamics of the neck.

physics.flu-dyn↗