arXiv subjects

Aaron D. Ratschow

Publications and source records attributed to Aaron D. Ratschow.

12 recordsLinked to original sources

arXiv2026

Coupled self-charging and self-propulsion of evaporating sessile droplets

Evaporating sessile water droplets universally deposit surface charges while simultaneously becoming self-charged---a phenomenon that remains under-appreciated despite its potentially important consequences. Here, we investigate one such consequence: droplet self-propulsion driven by self-charging. This self-propulsion arises spontaneously in the absence of external stimuli and produces self-avoiding trajectories that can displace droplets by many times their own diameter. Our results have important practical implications for processes requiring spatial precision, such as inkjet printing. The coupling between self-charging and self-propulsion produces mosaic surface charge patterns characterized by alternating polarities, which we explain with a physical model based on properties of the electric double layer.

cond-mat.soft↗

arXiv2026

Mechanisms in Slide Electrification of Liquid and Frozen Drops on Hydrophobic Surfaces

The microscopic and fundamental origin of slide electrification, where droplets of water move across insulating surfaces accumulating and depositing electrical charges, is still debated. Charge transfer is often attributed to ion transfer at the receding contact line. However, it is still unclear whether ion transfer alone can fully account for the observed charge separation. We examined slide electrification of two polar, self-ionizing liquids (water, formamide) and two non-polar liquids (diiodomethane, bromonaphthalene). By cooling below the melting temperature, we were able to compare this process to tribocharging of the respective frozen components. Despite reduced ion mobility at sub-freezing temperatures, the frozen polar compounds continue to accumulate significant charge. Non-polar liquids exhibit lower charging (<25% of polar liquids) and nearly identical charging behaviour in both their liquid and frozen phases on five different substrates. Since non-polar liquids contain few free ions, these observations indicate an alternative charging mechanism, which could be electron transfer. Our findings suggest that slide electrification operates through at least two mechanisms, with the dominant charge transfer pathway shifting between ions and electron transfer depending on the electronegativity, phase, and temperature.

cond-mat.soft↗

arXiv2026

How Spontaneous Electrowetting and Surface Charge affect Drop Motion

Water drops sliding on hydrophobic surfaces spontaneously separate charges at their rear. It is unclear how this charge separation affects the contact angles of a sliding drop. We slide grounded and insulated drops on hydrophobic surfaces at low capillary numbers (\leq 10^{-4}). We find that drop charge leads to spontaneous electrowetting, which decreases the contact angles. Additionally, the deposited charges lead to a surface charge effect and decrease the contact angle. Both phenomena compensate each other at the receding contact line, resulting in an insignificant change in the receding contact angle of a sliding drop.

physics.flu-dyn↗

arXiv2025

Convection can enhance the capacitive charging of porous electrodes

Charge transport in porous electrodes is foundational for modern energy storage technologies like supercapacitors, fuel cells, and batteries. Supercapacitors in particular rely solely on storing energy in charged pores. Here, we simulate the charging of a single electrolyte-filled pore using the modified Poisson-Nernst-Planck and Navier-Stokes equations. We find that electroconvection can substantially speed up the charging dynamics. We uncover the fundamental mechanism of electroconvection during pore charging through an analytical model that predicts the induced flow field and the electric current arising due to convection. Our findings suggest that convection is especially important in the limit of slender pores with thin electric double layers, and becomes significant beyond a certain threshold voltage that is an inherent electrolyte property.

cond-mat.soft↗

arXiv2025

Spontaneous Coulomb fissions of drops on lubricated surfaces

Charged water drops are more widespread than commonly acknowledged. For example, raindrops typically carry charges of order Q ~ 1 pC, while routine pipetting in the laboratory produces drops with Q ~ 50 pC. Here, we show that such modest charging can spontaneously generate periodic Coulomb fissions for evaporating water drops on lubricated surfaces, with more than 60 successive cycles observed over 30 min. Interestingly, the underlying instability can be quantitatively predicted by two fissility thresholds: one marking the onset of drop elongation and another triggering fission. Each fission culminates with a fine liquid jet that disintegrates into 40-50 microdroplets, expelled within microseconds. The phenomenon spans an extraordinary range of length scales (from millimetres to microns) and time scales (hour to microseconds), with broad potential applications ranging from nanoscale fabrication to electrospray ionization.

physics.flu-dyn↗

arXiv2025

Contact line friction of bubbles

Contact line friction (CLF) of bubbles is ubiquitous, from bubbles on a beer glass to H2 bubbles sliding over electrodes in electrolysis. However, a fundamental understanding of CLF of bubbles is still missing, mainly due to the challenge of precisely controlling bubble sliding. For example, it is not clear how bubbles start sliding and how CLF of bubbles depends on velocity. We therefore developed a bubble friction force instrument to directly measure bubble CLF. This force develops from a static regime, through a transition, to a kinetic regime. This entire process is quantitatively described by a modified Kawasaki-Furmidge equation. Bubble CLF was measured for velocities from 0.2 micron/s to 2 mm/s, revealing a transition from a constant CLF regime below about 60 micron/s to a velocity-dependent CLF regime on surfaces with various wettability. The velocity dependence stems from interfacial adaptation governed by the liquid ionic environment with a relaxation time of around 10 microsecond. Moreover, CLF of bubbles can be measured on hydrophilic surfaces and under a challenging H2 atmosphere, overcoming the limitations of current droplet-based methods. Our results provide a quantitative basis for understanding CLF of bubbles with relevance to many applications, including bubble manipulation and electrochemistry.

physics.flu-dyn↗

arXiv2025

Bipolar surface charging by evaporating water droplets

Surface charging is a ubiquitous phenomenon with important consequences. On one hand, surface charging underpins emerging technologies such as triboelectric nanogenerators; on the other, uncontrolled charging can damage delicate nanostructures and devices. Despite its significance, surface charging by evaporating water droplets remains poorly understood. Here, using Kelvin Probe Force Microscopy, we spatially resolve the surface-charge patterns from evaporating droplets and propose a physical model that quantitatively explains the origin of bipolar charging.

cond-mat.soft↗

arXiv2024

Electrostatics slows down the breakup of liquid bridges on solid surfaces

We experimentally study the breakup of water-glycerol liquid bridges on non-conductive surfaces and find that spontaneous charge deposition at the receding contact line, slide electrification, can have a substantial influence. Electrostatic forces slow down the dynamics during, and cause spontaneous motion of satellite drops after the bridge breakup. We show that our experimental observations align with slide electrification theory. Our findings demonstrate that slide electrification plays an important role in dewetting beyond drop-related scenarios.

physics.flu-dyn↗

arXiv2023

Gate Electrodes Enable Tunable Nanofluidic Particle Traps

The ability to control the location of nanoscale objects in liquids is essential for fundamental and applied research from nanofluidics to molecular biology. To overcome their random Brownian motion, the electrostatic fluidic trap creates local minima in potential energy by shaping electrostatic interactions with a tailored wall topography. However, this strategy is inherently static -- once fabricated the potential wells cannot be modulated. Here, we propose and experimentally demonstrate that such a trap can be controlled through a buried gate electrode.We measure changes in the average escape times of nanoparticles from the traps to quantify the induced modulations of $0.7k_\rm{B}T$ in potential energy and 50 mV in surface potential. Finally, we summarize the mechanism in a parameter-free predictive model, including surface chemistry and electrostatic fringing, that reproduces the experimental results. Our findings open a route towards real-time controllable nanoparticle traps.

cond-mat.soft↗

arXiv2023

How charges separate when surfaces are dewetted

Charge separation at moving three-phase contact lines is observed in nature as well as technological processes. Despite the growing number of experimental investigations in recent years, the physical mechanism behind the charging remains obscure. Here we identify the origin of charge separation as the dewetting of the bound surface charge within the electric double layer by the receding contact line. This charge depends strongly on the local electric double layer structure close to the contact line, which is affected by the gas-liquid interface and the internal flow of the liquid. We summarize the charge separation mechanism in an analytical model that captures parametric dependencies in agreement with our experiments and numerical simulations. Charge separation increases with increasing contact angle and decreases with increasing dewetting velocity. Our findings reveal the universal mechanism of charge separation at receding contact lines, relevant to many dynamic wetting scenarios, and provide a theoretical foundation for both fundamental questions, like contact angle hysteresis, and practical applications.

cond-mat.soft↗

arXiv2023

Surface charge deposition by moving drops reduces contact angles

Slide electrification - the spontaneous charge separation by sliding water drops - can lead to an electrostatic potential of 1 kV and change drop motion substantially. To find out, how slide electrification influences the contact angles of moving drops, we analyzed the dynamic contact angles of aqueous drops sliding down tilted plates with insulated surfaces, grounded surfaces, and while grounding the drop. The observed decrease in dynamic contact angles at different salt concentrations is attributed to two effects: An electrocapillary reduction of contact angles caused by drop charging and a change in the free surface energy of the solid due to surface charging.

cond-mat.soft↗

arXiv2022

Resonantly-driven nanopores can serve as nanopumps

Inducing transport in electrolyte-filled nanopores with dc fields has led to influential applications ranging from nanosensors to DNA sequencing. Here we use the Poisson-Nernst-Planck and Navier-Stokes equations to show that unbiased ac fields can induce comparable directional flows in gated conical nanopores. This flow exclusively occurs at intermediate driving frequencies and hinges on the resonance of two competing timescales, representing space charge development at the ends and in the interior of the pore. We summarize the physics of resonant nanopumping in an analytical model that reproduces the results of numerical simulations. Our findings provide a generic route towards real-time controllable flow patterns, which might find applications in controlling the translocation of particles such as small molecules or nanocolloids.

cond-mat.soft↗