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Room‐temperature liquid metal droplets (LMDs) are a promising material for various applications in soft robotics, active droplets, and biomedical devices. However, controllable and high‐throughput production of LMDs remains challenging due to their high surface tension and density. Here, a novel strategy is presented to produce LMDs by combining electric field‐induced electrocapillary flow with an external flow field. The basic mechanism is that the electrocapillary flow induced at the LMD/electrolyte interface forms a vortex ring in the electrolyte, creating a hurricane‐like effect in the tube, which in turn causes the liquid metal to deform and eventually pinch off into small droplets. It is demonstrated that droplet size and generation frequency can be controlled precisely by adjusting the applied electric current, flow rate, and surfactant concentration, establishing a relationship between radius and experimental parameters through dimensionless analysis. More importantly, this strategy can handle pendant droplets and facilitate programmable droplet patterning. Leveraging established relationships, flexible control over droplet size and spacing during patterning is attained. Furthermore, an iontronic pressure‐sensitive device based on LMDs and hydrogel is developed to showcase the versatility of the approach. This technique opens up new opportunities for fabricating soft circuits, composite materials, and other functional devices with LMDs. [ABSTRACT FROM AUTHOR]

Poor mass transport to or from vertical gas-evolving electrodes can adversely impact energy efficiency and product purity in the production of hydrogen, chlorine, and various metals. A proper description that combines natural convection with micromixing of growing, coalescing, and departing bubbles is presently lacking. This work develops a simple, physically sound analytical model that includes the influence of bubble size, flow regime, and bubble surface coverage. By comprehensively reviewing mass transfer measurements from the water electrolysis literature, we observe that the surface coverage of oxygen bubbles increases much more strongly with increasing current density than an often-used square root scaling predicts. Strong differences are observed in the degree of micromixing of hydrogen and oxygen bubbles in alkaline and acidic electrolytes. These varied results can all be explained by a combination of electrocapillarity, and coalescence induced by either a high surface coverage or Marangoni flows. [Display omitted] • A simple formula for adding micromixing and natural convection is derived. • A single micromixing parameter between 0 and 5 describes all literature data. • Micromixing is strongest for oxygen in alkaline and hydrogen in acidic electrolytes. • Marangoni forces are attractive in this case, causing coalescence and larger bubbles. • Bubble coverage increases with the square/square root of current for oxygen/hydrogen. [ABSTRACT FROM AUTHOR]

Wetting phenomena underpin a plethora of key processes occurring in nature and artificial systems. Subtle control over wetting can boost the performance of devices with applications in diverse technologies, spanning electrochemical energy conversion, storage, and capacitive deionization to variable micro‐/nano‐fluidics platforms and electro(nano)tribology. A common characteristic of these systems is their operation under application of an electric field that induces a series of electrochemical processes that can impact wetting. The dependence of wetting on the applied potential bias is referred to as electrowetting. Carbon‐based materials hold a central role as both active electrodes and additives in such systems due to their distinct physicochemical properties and versatile characteristics. Deciphering the mechanisms of electrowetting on carbon in contact with electrolytes of varied types and compositions requires the fundamental understanding of the underlying processes taking place at the carbon/electrolyte interface. Simultaneously, a systematic investigation of electrowetting can often provide significant insights into the properties of the interface. In this concept article, we discuss the latest advances in the area along with the current challenges and future directions of this recently revived research topic. [ABSTRACT FROM AUTHOR]

Abstract Wetting phenomena underpin a plethora of key processes occurring in nature and artificial systems. Subtle control over wetting can boost the performance of devices with applications in diverse technologies, spanning electrochemical energy conversion, storage, and capacitive deionization to variable micro‐/nano‐fluidics platforms and electro(nano)tribology. A common characteristic of these systems is their operation under application of an electric field that induces a series of electrochemical processes that can impact wetting. The dependence of wetting on the applied potential bias is referred to as electrowetting. Carbon‐based materials hold a central role as both active electrodes and additives in such systems due to their distinct physicochemical properties and versatile characteristics. Deciphering the mechanisms of electrowetting on carbon in contact with electrolytes of varied types and compositions requires the fundamental understanding of the underlying processes taking place at the carbon/electrolyte interface. Simultaneously, a systematic investigation of electrowetting can often provide significant insights into the properties of the interface. In this concept article, we discuss the latest advances in the area along with the current challenges and future directions of this recently revived research topic.

Porous gas‐diffusion electrodes (GDEs) are widely used in electrochemical applications where a gaseous reactant is converted to a target product. Important applications for silver‐based GDEs are the chlor‐alkali and the CO2 electrolysis processes in which silver catalyzes the oxygen‐ or carbon dioxide reduction reaction. The wetting of the porous GDEs is of utmost importance for the achieved performance of the electrode: a completely dry electrode will result in low current densities due to the reduced active surface area while on the other hand, a completely flooded electrode will deteriorate the access of the gaseous reactant. Therefore, we investigated silver‐based GDEs for the oxygen reduction reaction with different amounts of the hydrophobic agent polytetrafluoroethylene (PTFE) and analyzed the potential‐induced wetting behavior (electrowetting). The electrolyte breakthrough was recorded by a digital microscope and subsequently evaluated via imaging analysis of the observed breached electrolyte droplets. In order to characterize the wetting state during transition to the steady‐state, we applied electrochemical impedance spectroscopy measurements and retrieved the double‐layer capacitance. Our results indicate that a higher overvoltage facilitates the breakthrough of electrolytes through the gas‐diffusion electrode. Surprisingly, a faster breakthrough of electrolyte was observed for electrodes with higher PTFE content. Porometry measurements revealed that the GDE with low PTFE content has a monomodal pore size distribution, whereas electrodes with higher PTFE amount exhibit a bimodal pore size distribution. In GDEs with monomodal pore size distribution the time in which the double layer capacitance is leveling off correlates with the breakthrough time of the electrolyte. In summary, we emphasize that the wetting of GDEs is a complex interplay of the applied potential, electrode composition, and resulting porous structure which requires further advanced measurements and analysis considering the parameters affecting the wetting behavior as a whole. [ABSTRACT FROM AUTHOR]

Abstract Porous gas‐diffusion electrodes (GDEs) are widely used in electrochemical applications where a gaseous reactant is converted to a target product. Important applications for silver‐based GDEs are the chlor‐alkali and the CO2 electrolysis processes in which silver catalyzes the oxygen‐ or carbon dioxide reduction reaction. The wetting of the porous GDEs is of utmost importance for the achieved performance of the electrode: a completely dry electrode will result in low current densities due to the reduced active surface area while on the other hand, a completely flooded electrode will deteriorate the access of the gaseous reactant. Therefore, we investigated silver‐based GDEs for the oxygen reduction reaction with different amounts of the hydrophobic agent polytetrafluoroethylene (PTFE) and analyzed the potential‐induced wetting behavior (electrowetting). The electrolyte breakthrough was recorded by a digital microscope and subsequently evaluated via imaging analysis of the observed breached electrolyte droplets. In order to characterize the wetting state during transition to the steady‐state, we applied electrochemical impedance spectroscopy measurements and retrieved the double‐layer capacitance. Our results indicate that a higher overvoltage facilitates the breakthrough of electrolytes through the gas‐diffusion electrode. Surprisingly, a faster breakthrough of electrolyte was observed for electrodes with higher PTFE content. Porometry measurements revealed that the GDE with low PTFE content has a monomodal pore size distribution, whereas electrodes with higher PTFE amount exhibit a bimodal pore size distribution. In GDEs with monomodal pore size distribution the time in which the double layer capacitance is leveling off correlates with the breakthrough time of the electrolyte. In summary, we emphasize that the wetting of GDEs is a complex interplay of the applied potential, electrode composition, and resulting porous structure which requires further advanced measurements and analysis considering the parameters affecting the wetting behavior as a whole.

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In the previous paper (Gutman, JOSSEC 18:3217-3237, 2014), we have shown that the main problem in capillarity and electrocapillarity of solid surfaces is the lack of clarity in determining the surface stress and basic equations. Now, we continue the survey of efforts to solve this problem and show origins of erroneous results, accenting some important items: comparative analysis of Gibbs and Guggenheim approaches in surface thermodynamics (a geometrical dividing surface and finite-thickness surface layer, respectively), transformation of fundamental equations on per-unit-area basis to obtain Gibbs adsorption equation for finite-thickness surface layer, different attempts to derive the thermodynamic definition of 'surface stress' in frames of Gibbs' theory (including Shuttleworth's approach), atomistic calculations of surface stress, surface stress in rational continuum mechanics, 'modifications' of Gibbs-Duhem relations made for solid interface, and Maxwell relations in capillarity and electrocapillarity of solid interface. It is shown that the erroneous Shuttleworth's approach is present in an explicit or implicit form in all efforts to introduce the surface stress in frames of Gibbsian theory (although Gibbs did not introduce surface stress). Therefore, 'modernizations' or 'generalizations' of the Gibbs-Duhem relation, the Gibbs adsorption equation, and the Lippmann equation to adopt them for a solid surface are unnatural and not necessary. Therefore, we recommend withdrawing the Shuttleworth equation and its consequences from circulation, including the IUPAC Recommendations. [ABSTRACT FROM AUTHOR]

Impregnating the pores of nanoporous gold with aqueous electrolyte yields a hybrid nanomaterial with two separate and interpenetrating charge transport paths, electronic conduction in the metal and ionic conduction in the electrolyte. As the two paths are capacitively connected, space-charge layers along the internal interfaces are coupled to electric potential differences between the paths and can be controlled or detected thereby. The present experiments show that the space charge couples to mechanical deformation of the hybrid material, so that external loading generates an electric current. The electric signal originates from charge displacement along the entire internal interface; the signal is particularly robust since the interface area is large. The charge transfer in response to load constitutes a piezoelectric response, yet the mechanism is quite different to classic piezoelectricity. The analysis in this work predicts links between electromechanical coupling parameters for strain sensing and actuation, which are in excellent agreement with the experiment. [ABSTRACT FROM AUTHOR]

Non-Newtonian fluids can present a complex rheological behaviour involving shear-thinning, viscoelastic or thixotropic effects. We focus here on the characterization of generalized Newtonian fluids as shear-thinning fluids. Rotational rheometers are torque-sensitive, which makes the measurement of viscosity at low shear-rates very difficult, in particular for low viscosity fluids. An accurate knowledge of this value is, however, particularly important for the characterization of several types of flows, in particular those featuring a free surface, a symmetry axis or a symmetry plane. The experimental determination of viscosity and surface tension is enabled from the propagation of attenuated capillary waves. An optical technique has been implemented in order to determine the shear-thinning viscosity at values of the shear-rate as small as 10 −3 s −1 from measurements of the spatial attenuation and wavelength. We have performed measurements on two different polymer solutions; we show that this technique is complementary to the classical rotational rheometer techniques since it gives access to the zero shear viscosity, which could not be measured by using rotational rheometers. The present technique is thus used to characterize the Newtonian plateau, while a rotational rheometer is used to characterize the fluid behaviour at moderate to high shear rates. [ABSTRACT FROM AUTHOR]

The main problem in electrocapillarity of solid electrodes is the lack of clarity in determining the surface stress and basic equations. Within the framework of the Gibbs concept of geometrical dividing surface, the 'surface stress' cannot be defined because methods of continuum mechanics can be applied to a physical surface layer (of finite thickness), but not to a mathematical surface. Gibbs never used the concept of surface stress, introducing only 'surface tension' for a liquid electrode and 'closely related quantity' for a solid electrode. Revisiting the derivation of the Gibbs adsorption equation, we prove its applicability to solid surfaces without the limiting requirement of constant state of strain, which was undeservedly interpreted by Eriksson as a shortcoming of the Gibbs theory caused to look for other approaches to surface stress problem. A critical analysis shows that the attempts (Shuttleworth, Eriksson, Couchman, Gokhstein, Weissmüller, etc.) to create a thermodynamic definition of the surface stress (as well as the formulation of fundamental thermodynamic equations and Maxwell relations operating with surface stresses) contain mathematical defects. It is shown that confusing interpretations of some Gibbs' concepts encountered in the literature have led to 'modifications' of the Lippmann equation based on the critical error in the Gibbs-Duhem relation due to the occurrence of an extensive variable, which is inadmissible. The famous Lippmann equation should not be modified, and it remains a unique electrocapillary relation applicable to liquid and solid electrodes. [ABSTRACT FROM AUTHOR]

The Nernst-Planck-Poisson-Boltzmann system describes the evolution of ionic concentrations and electrocapillarity effects in porous media. The aim of this paper is a theoretical study of various drift-diffusion modellings. The well-posedness of the systems is proved and some qualitative properties of the global solution are shown to be satisfied (energy law, entropy law in the weak sense of Lyapunov functions, stationary states, Maxwellian distribution, influence of an external electric field). Moreover, some explicit solutions are established in the one-dimensional case. [ABSTRACT FROM AUTHOR]

We present a method to control the interfacial tension of a liquid alloy of gallium via electrochemical deposition (or removal) of the oxide layer on its surface. In sharp contrast with conventional surfactants, this method provides unprecedented lowering of surface tension (∼500 mJ/m2 to near zero) using very low voltage, and the change is completely reversible. This dramatic change in the interfacial tension enables a variety of electrohydrodynamic phenomena. The ability to manipulate the interfacial properties of the metal promises rich opportunities in shape-reconfigurable metallic components in electronic, electromagnetic, and microfluidic devices without the use of toxic mercury. This work suggests that the wetting properties of surface oxides-which are ubiquitous on most metals and semiconductors-are intrinsic "surfactants." The inherent asymmetric nature of the surface coupled with the ability to actively manipulate its energetics is expected to have important applications in electrohydrodynamics, composites, and melt processing of oxide-forming materials. [ABSTRACT FROM AUTHOR]

More than one century ago, Lippmann found that capillary forces can be effectively controlled by external electrostatic forces. As a simple example, by applying a voltage between a conducting liquid droplet and the surface it is sitting on we are able to adjust the wetting angle of the drop. Since Lippmann's findings, electrocapillary phenomena - or electrowetting - have developed into a series of tools for manipulating microdroplets on solid surfaces, or small amounts of liquids in capillaries for microfluidic applications. In this article, we briefly review some recent progress of fundamental understanding of electrowetting and address some still unsolved issues. Specifically, we focus on static and dynamic electrowetting. In static electrowetting, we discuss some basic phenomena found in DC and AC electrowetting, and some theories about the origin of contact angle saturation. In dynamic electrowetting, we introduce some studies about this rather recent area. At last, we address some other capillary phenomena governed by electrostatics and we give an outlook that might stimulate further investigations on electrowetting. [ABSTRACT FROM AUTHOR]

It is shown that the so-called 'equation of solid-state electrocapillarity' derived by Gokhshtein from the simplest thermodynamic model, earlier used by Lippmann, is an incorrect modification that is returned to the classic Lippmann equation if to take into account the usual definition of differential capacity of the electrical double layer. Consequently, Gokhshtein's experiments can actually confirm only the validity of the Lippmann equation (with deviations caused by physicochemical processes in the double layer, which could not be taken into account in the thermodynamic equations). The thermodynamic model is insufficient for the interpretation s of these experimental results, whose understanding requires physical models of the different phenomena occurring at the electrode surface. [ABSTRACT FROM AUTHOR]

Highlights: [•] Strain-dependent kinetic rate equation for hydrogen evolution reaction. [•] Experimental in situ monitoring of strain-induced reactivity modulation. [•] Tensile strain enhances reactivity at low overpotential. [•] Tensile strain diminishes reactivity at high overpotential. [•] Adsorption and activation enthalpies depend on strain. [ABSTRACT FROM AUTHOR]

This work focuses on performing the homogenization of the Nernst-Planck-Poisson system, in order to obtain an efficient modelling of the ionic transfer and electrocapillary effects at the macroscopic scale for saturated porous media. The homogenization is performed with the powerful two-scale convergence method, which enables to obtain both the homogenized model and the associated convergence result. [ABSTRACT FROM AUTHOR]

Electrocapillarity is the basis of modern electrowetting. And room temperature ionic liquids, an increasingly important set of electrolytes and organic salts that are liquid at room temperature, are considered a novel class of electrowetting agents, because of non-significant vapor pressure, nonflammability, good thermal sta-bility, and a wide useable temperature range. In this paper, a simple device has been fabricated to investigate the electrocapillarity of ionic liquids. It shows attractive features involving wide operating temperature and in particular high stability, fast response and good reversibility at high temperatures. Besides, electrocapil-larity of ionic liquids is strongly dependent on power supply frequency. In particular, high frequency, stable, reversible and wide height of rise modulations were obtained. The phenomenon of ionic liquids-based elec-trocapillarity was found to be affected by the structure and physicochemical properties of ionic liquids such as density, surface tension, alkyl carbon chain length of cation, type of the anion, etc., and the efficiency could be further improved through optimal choice of ambient phase or rational design and synthesis of ionic liquids. [ABSTRACT FROM AUTHOR]

Abstract: The ultraslow relaxation, on the order of a few seconds or longer, of the structure of the electrical double layer in response to the change in the phase-boundary potential across the ionic liquid (IL)|water(W) interface, which was recently reported for trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide, has been confirmed in two new hydrophobic ionic liquids, trihexyltetradecylphosphonium bis(nonafluorobutanesulfonyl)amide and trihexyltetradecylphosphonium tetrakis(pentafluorophenyl)borate. A comparison of the degree of the hysteresis in electrocapillary curves for these ILs with those for trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide demonstrates that the degree of the hysteresis is not correlated with the viscosity of these ILs. The ultraslow relaxation of the electrical double layer seems to be a general feature of ILs at electrified interfaces. [ABSTRACT FROM AUTHOR]

Abstract: A new model has been formulated for the electrochemical instability at the liquid∣liquid interface, for which only a rudimentary model [T. Kakiuchi, J. Electroanal. Chem. 536 (2002) 63–69] had been available. The new model considers the diffuse double layers in the presence of the specifically adsorbed surface-active ions, whose adsorption Gibbs energy linearly varies with the phase-boundary potential. The electrocapillary equation in the presence of the common ions partitioning in both phases is employed under the assumption that the partition of the surface-active ions is described by the Nernst equation. The excess surface charge due to the specific adsorption of surface-active ions is allowed for in calculating the structure of the diffuse part of the electrical double layers on both sides of the interface. The calculated electrocapillary curves agree with the experimental results. It is confirmed that electrocapillary curves can have positive curvature in a certain potential region, which signifies the presence of an instability window. [Copyright &y& Elsevier]

Interfacial interactions between a hydroalcoholic solution of potassium hydrogen tartrate (KHT) and a stainless steel surface are studied, when an electrical potential is applied to the metal substrate. The capacitive domain of the metal–solution interface is determined by cyclic voltammetry. In order to study the influence of the potential on KHT nucleation and crystal adhesion, the solid–liquid interfacial energy is assessed from contact angle and capillary rise measurements. Experimentally, the contact angle between a NaF solution and a stainless steel vs. the potential has a parabolic behaviour. The metal topography has no apparent influence on physicochemical properties of the interface when ethanol is present in a KHT solution. The metal substrate promotes the formation of KHT crystals, which is improved by the application of an anodic potential. The adhesion of crystals becomes more effective when wires of 25 μm diameter are used in comparison with those of 250 μm. [ABSTRACT FROM AUTHOR]

The electrocapillarity and the barrier properties of camphor-10-sulphonic acid (CS) in various aqueous electrolytes (1 M NaSO, 3 M LiClO, 2 M KNO, 3 M KCl, 3 M KBr) were studied. From the electrocapillary curves the charge-density-potential and the surface-pressure-potential-curves were calculated and analyzed using the Lippmann-Helmholtz and Gibbs-equations. The cross-sectional areas a of CS, resulting from the maximum excess surface concentration, suggest a perpendicular orientation of CS-molecules in the condensed film at the potential of maximum adsorption. This orientation causes a dense packing and a strong interaction between the CS-molecules in the condensed film. The barrier properties that represent the reduction of Cu-ions in the presence of the adsorption layer were studied dc polarographically at high surface coverage. A change in the inhibition mechanism was observed due to the change of the structure in the adsorptions layer. The electrode reaction is strongly inhibited within the potential and concentration range of film condensation. [ABSTRACT FROM AUTHOR]