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

Chemical fluxes between immiscible liquids trigger interfacial stirring flows, but their mechanistic origin long remained elusive. In 1959, Sternling and Scriven showed that solute fluxes induce interfacial ‘turbulence’ through local gradients of interfacial tension, paving the way for the modern understanding of a wide range of chemical engineering processes.

This study investigates the radial centering of gas bubbles within a buoyant plume of ethanol injected into a co-flowing water sheath flow in a vertical capillary. Bubbles nucleate in the ethanol stream due to CO_2 supersaturation and rapidly migrate toward the plume axis via solutocapillary (Marangoni) forces driven by interfacial tension gradients in the ethanol-water mixture. Experiments reveal that bubbles of varying sizes reliably align along the plume centerline, facilitated by steep radial concentration gradients near the plume boundary. A reduced-order model supports robust centering across a wide range of bubble radii. For larger bubbles, axial Marangoni effects modulate ascent velocities and can even induce upstream migration under transient conditions, highlighting the complex feedback between bubble dynamics and plume distortion. The results demonstrate that solutocapillary migration provides a reliable mechanism for contact-free bubble focusing, with implications for bubble manipulation in microfluidics, reactors, and phase-separation processes.

The development of large-scale, three-dimensional human tissues is crucial for various applications in therapeutic tissue engineering, disease modeling, and drug testing. However, due to the diffusion limit of oxygen, the lack of functional vascular networks is a significant limitation in maintaining these engineered tissues in the laboratory. To address this challenge, we present a systematic, model-based design process for artificial supply networks that can ensure a sufficient supply of oxygen and nutrients to engineered human tissue. Our approach combines mathematical models of fluid dynamics, cell metabolism, and network properties to identify key parameters influencing the supply performance. We demonstrate the applicability and possibilities of this design process by simulating different network structures, including cuboid and rhombic dodecahedral honeycombs, under various conditions. Our results show that the structure of the artificial supply network, oxygen concentration, and solute flow within the network strongly influence cellular metabolic activity and viability. We also examine the effects of non-uniform cell density, channel blockage, and long channel length on the oxygen distribution inside the cell-containing tissue compartment. Our findings highlight the importance of considering these factors in the design of artificial supply networks for large-scale engineered human tissues. This study provides a promising approach for quickly exploring the vast design space of possible network structures under different conditions for desired cell and tissue states, ultimately contributing to the development of more efficient and effective tissue engineering strategies.

The breakup dynamics of liquid bridges consisting of aqueous solutions of xanthan gum (XG) is investigated experimentally. Two series of experiments with liquid bridges in air (free bridges) and on solid surfaces (wetting bridges) are performed. Phase-space representations are employed to characterize the breakup dynamics over a range of polymer concentrations, solvent viscosities, and substrate wettabilities (for wetting bridges). For free liquid bridges, three distinct dynamic regimes are identified: inertia-capillary, elasto-capillary, and terminal viscous-capillary. Consistent with previous observations for semi-flexible polymers, the elasto-capillary response is found to be short-lived. In the terminal viscous-capillary regime, the effective extensional viscosity scales linearly with polymer concentration and solvent viscosity, indicating an extensional response characteristic for dilute solutions despite the solutions being semi-dilute under equilibrium conditions. On the other hand, wetting liquid bridges exhibit no distinct regime transitions, and their breakup dynamics remains consistent with a single terminal viscous-capillary-like response over the entire breakup process. Remarkably, the same scaling governing the terminal viscous-capillary regime of free bridges is shown to collapse the wetting bridge dynamics for different XG concentrations and solvent viscosities to a master curve. This behavior is hypothesized to result from the combined flow history associated with bridge formation, substrate-induced flow, and capillary thinning, which may modify the polymer conformations and obscure the dynamic transitions observed in free liquid bridges. Additionally, XG wetting liquid bridges exhibit a markedly weaker dependence on substrate wettability than Newtonian liquids. Finally, appropriate rescaling reveals a nearly universal evolution of both the free liquid bridge profile and the wetting contact line profile.

To explore the physicochemical hydrodynamics of phase-separating ternary liquids (Ouzo-type), a binary oil-ethanol mixture is introduced into a co-flowing stream of water. Oil droplets nucleate at the interface between the two liquids, leading to a larger oil droplet interacting with the ethanol-rich jet. Although buoyancy forces and hydrodynamic drag forces push the droplet in downstream direction, we observe an upstream motion. Using computational fluid dynamics simulations of a simplified model system, we identify the nucleation zone for oil droplets and uncover Marangoni forces to be responsible for the upstream motion of the droplet. A semi-analytical model allows us to identify the key parameters governing this effect. A general conclusion is that Marangoni stresses can reverse the motion of droplets through channels, where the surrounding liquid is a multi-component mixture. The insights from this work are not only relevant for channel flow, but more generally, for the physicochemical hydrodynamics of multiphase, multi-component systems.

When dissolved, weak electrolytes only partially dissociate into ions in a temperature-dependent process. We show herein that such incomplete dissociation yields an enormous thermoelectric response in an electrolyte-filled nanochannel along which a temperature gradient is applied. For this purpose, an extended version of the Nernst-Planck equations is developed that takes into account the temperature-dependent dissociation-association equilibrium. The results indicate that in this way, Seebeck coefficients can be achieved that outperform all previously reported values.

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.

This review is about drops of a liquid with high dielectric permittivity that slide over a solid surface with high electrical resistivity. A typical situation is a water drop sliding down a tilted hydrophobic plate. It has been realized recently that such drops spontaneously acquire a charge. The opposite charge is deposited behind the drop as a surface charge. Generated electric potentials in the drops can easily reach 1 kV and more. This phenomenon has been termed slide or contact electrification. It is the soft matter analog to triboelectrification, which occurs in solid friction. Slide electrification turned out to be ubiquitous in everyday life and technical applications. It will change our common knowledge of dynamic wetting. Studying slide electrification is complex because the outcome of a wetting experiment depends on the history. For this reason, a series of drops, rather than single drops, are analyzed to gain quantitative understanding. Here, we review the fundamental understanding of slide electrification and its limits. We describe consequences, e.g. on drop motion and advancing and receding contact angles and we address open questions.

Thermoelectricity in electrolyte-filled slit channels is studied using analytical methods for solving the combined Stokes, Nernst-Planck, Poisson, and heat transport equations. Next to a temperature gradient, the scenario considered accounts for a pressure gradient and an electric field applied along the channel. Closed-form analytical expressions are derived for the thermoelectric current and the thermovoltage expressed by the Seebeck coefficient, which are compared to numerical results obtained using the finite-element method. A special focus is put on convective charge transport resulting from thermoosmotic flow, which is massively augmented when considering nanochannels with a Navier-slip condition at the channel walls. The corresponding convective transport has a negligible effect on thermoelectricity in the regime of strong electric double layer overlap. By contrast, when the Debye length is much smaller than the channel width, convection can yield a massive increase of the thermoelectric current and power. This occurs in the case of a nonvanishing slip length, where thermoelectricity in the absence of convection is taken as a reference. Generally, it can be concluded that boundaries with a nonvanishing slip length can induce a significant thermoelectric response via convective transport even in cases where the Debye length is much smaller than any length scale of the compartments or channels in which the liquid is contained.

The breakup dynamics of viscous liquid bridges on solid surfaces is studied experimentally. It is found that the dynamics bears similarities to the breakup of free liquid bridges in the viscous regime. Nevertheless, the dynamics is significantly influenced by the wettability of the solid substrate. Therefore, it is essential to take into account the interaction between the solid and the liquid, especially at the three-phase contact line. It is shown that when the breakup velocity is low and the solid surface is hydrophobic, the dominant channel of energy dissipation is likely due to thermally activated jumping of molecules, as described by the molecular kinetic theory. Nevertheless, the viscous dissipation in the bulk due to axial flow along the bridge can be of importance for long bridges. In view of this, a scaling relation for the time dependence of the minimum width of the liquid bridge is derived. For high viscosities, the scaling relation captures the time evolution of the minimum width very well. Furthermore, it is found that external geometrical constraints alter the dynamic behaviour of low and high viscosity liquid bridges in a different fashion. This discrepancy is explained by considering the dominant forces in each regime. Lastly, the morphology of the satellite droplets deposited on the surface is qualitatively compared with that of free liquid bridges.

The Leidenfrost effect allows a gas film to form between a high-temperature substrate and a volatile liquid, resulting in a highly mobile drop. Previous research has shown that the surface texture needs to break the spatial symmetry to enable self-propulsion. Here, we demonstrate that spontaneous symmetry breaking in the gas-liquid flow above the texture generates substantial self-propulsion forces, driving drops on symmetric textured surfaces. Using a model of hydrodynamically coupled driven oscillators and computational fluid dynamic simulations, we show that drops are propelled by the pressure field of the vapor. We analyze the influence of drop size, temperature, and structural parameters of the substrate on the propulsion velocity. Furthermore, we observe similar self-propulsion in liquid nitrogen, ethanol, and liquid rings, indicating broad applicability. These findings highlight the importance of dynamic symmetry breaking of the gas-liquid interface in driving large-scale liquid motion and provide a potential path for efficient thermal-mechanical energy conversion.

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.

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 fluid 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 0.7 kBT 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 toward real-time controllable nanoparticle traps.

The electroosmotic flow (EOF) fields in the vicinity of solids with high dielectric permittivity are studied for the case of charge-asymmetric electrolyte solutions. Corresponding solutions of the coupled Poisson-Nernst-Planck and Navier-Stokes equations are obtained analytically and numerically. When a direct-current (DC) electric field is applied to a high-permittivity uncharged sphere, a net EOF develops that translates into a non-zero electrophoretic mobility of the sphere, although it does not carry any charge. Similarly, a DC field acting on a channel in a high-permittivity material results in a net flow through the channel, although the solid is uncharged. Such phenomena are expected to occur frequently whenever high-permittivity solids are immersed in charge-asymmetric electrolyte solutions and do not rely on special scenarios such as ion crowding. Also, the net flow velocities are very significant for realistic values of the electric field strength. The derived scaling relationships even predict giant net flow velocities through nanochannels of the order of metres per second for practically relevant scenarios.

The influence of chaotic capillary waves on the time-averaged shape of a liquid volume is studied experimentally and theoretically. In that context, a liquid film containing a stable hole is subjected to Faraday waves. The waves induce a shrinkage of the hole compared to the static film, which can be described using the Young-Laplace equation by incorporating an effective capillary length. In the regime of chaotic Faraday waves, the presented theoretical model explains the hole shrinkage quantitatively, linking the effective capillary length to the wave energy. The effect of chaotic Faraday waves can be interpreted as a dynamic surface force that acts against surface tension.

Conical nanochannels employed to create ionic current rectification (ICR) in nanofluidic devices are prone to clogging due to the contraction at one end. As an alternative approach for creating ICR, a cylindrical nanochannel covered with a polyelectrolyte layer (PEL) of variable thickness is proposed in the present study. The efficacy of the proposed design is studied by numerically solving the governing equations including the Poisson, Nernst-Planck, and Stokes-Brinkman equations. Furthermore, the fundamental mechanism behind ICR is explained using a simplified one-dimensional model. The effects of the nanochannel radius, concentration of PEL fixed charges, and bulk ionic concentration on the rectification factor are then investigated in detail. It is shown that the proposed nanochannel provides larger rectification factors as compared to conical nanochannels over wide ranges of the fixed charge concentration and bulk ionic concentration. Such a performance can be achieved even at channel radii much larger than the tip radius of conical nanochannels, indicating not only the better performance of the proposed nanochannel but also its likely longer service life, because of reducing the probability of total ionic current blockage. This means that the proposed nanochannel could find widespread use in fluidic devices, as a replacement for conical nanofluidic diodes.

The thermoelectric response of thermally activated electrolytes (TAEs) in a slit channel is studied theoretically and by numerical simulations. The term TAE refers to electrolytes whose charge carrier concentration is a function of temperature, as recently suggested for ionic liquids and highly concentrated aqueous electrolyte solutions. Two competing mechanisms driving charge transport by temperature gradients are identified. For suitable values of the activation energy that governs the generation of charge carriers, a giant thermoelectric response is found, which could help explain recent experimental results for nanoporous media infiltrated with TAEs.

Microfluidic isotachophoresis (ITP) is a powerful technique that can significantly increase the reaction rate of homogeneous chemical reactions by cofocusing reactants in a narrow sample zone. Correspondingly, ITP has been utilized to reduce the reaction time in various bioanalytical assays. However, in conventional ITP, it is hardly possible to control the reaction rate in real time, i.e., speeding up or slowing down a reaction on demand. Here, we experimentally demonstrate a new mode of ITP that allows the spatial overlap of two ITP zones to be precisely controlled over time, which is a crucial first step toward controlling reaction rates. Two nonreactive samples are initially focused and separated by a spacer using a DC electric field. By superimposing an oscillating field component with sufficiently high amplitude on the DC field, the spatial overlap of their concentration profiles is temporarily increased due to electromigration dispersion. The time-average of this overlap can be precisely controlled by varying the frequency and amplitude of the oscillation. We suggest that this scheme can be transferred to chemical reactions between ionic species with sufficiently different electrophoretic mobilities. Tuning the parameters of the oscillatory electric field should allow direct control of the corresponding reaction rate.

A liquid drop containing a long fiber is a complex system whose configuration is determined by an interplay of elastic stresses in the fiber and capillary forces due to the liquid. We study the morphological evolution of fibers that are much longer than the drop diameter in evaporating sessile drops. After insertion, the fibers are either found in an ordered or disordered state, with increasing disorder for increasing fiber length. Upon evaporation, the order increases, in such a way that the final configuration deposited on the solid surface is either a circle, an ellipse, or 8-shaped. The morphology of the deposit depends on the fiber length and the elastocapillary length, both non-dimensionalized with the characteristic drop size, which we classify in a morphology regime map. The disorder-to-order transition allows depositing ordered fiber structures on solid surfaces even in cases of a strongly disordered state after fiber insertion. Combined with technologies such as inkjet printing, this process could open new avenues to decorate surfaces with filamental structures whose morphology can be controlled by varying the fiber length.