Direct measurement of the viscoelectric effect in water

Macroscopic movement of electrolyte-droplet reveals the characteristics of microscopic ion dynamics

This paper aims to elucidate the microscopic ion dynamics at the solid-liquid interface of a moving droplet, which is difficult to characterize experimentally. We investigated the ion dynamics at the solid-liquid interface by measuring the induced current at the semiconductor surface due to ion-electron interaction at the semiconductor-electrolyte interface. The electric signal exhibited two non-monotonic behaviors corresponding to the ionic concentration and velocity of the electrolyte droplet, neither of which could be explained by the previous model. Revising the characteristic equation of the device to include microscopic ion dynamics led us to the following conclusions: in a sliding electrolyte-droplet, (i) the proportion of ions in the bulk solution that are adsorbed is almost the same regardless of the velocity when ion adsorption is not saturated, (ii) ion-electron interactions decrease rapidly beyond a certain distance, and (iii) the time scale on which the ion-electron interaction becomes a steady state is on the order of seconds. We further extended the charge neutrality condition incorporating ion-electron interaction at the semiconductor-electrolyte interface, combined with the modified Poisson-Boltzmann model. Through the macroscopic motion of the droplet, we are able to estimate the microscopic dynamics inside the droplet.

Cell-inspired, massive electromodulation of friction via transmembrane fields across lipid bilayers

Transient electric fields across cell bilayer membranes can lead to electroporation and cell fusion, effects crucial to cell viability whose biological implications have been extensively studied. However, little is known about these behaviours in a materials context. Here we find that transmembrane electric fields can lead to a massive, reversible modulation of the sliding friction between surfaces coated with lipid-bilayer membranes-a 200-fold variation, up to two orders of magnitude greater than that achieved to date. Atomistic simulations reveal that the transverse fields, resembling those at cell membranes, lead to fully reversible electroporation of the confined bilayers and the formation of inter-bilayer bridges analogous to the stalks preceding intermembrane fusion. These increase the interfacial dissipation through reduced hydration at the slip plane, forcing it to revert in part from the low-dissipation, hydrated lipid-headgroup plane to the intra-bilayer, high-dissipation acyl tail interface. Our results demonstrate that lipid bilayers under transmembrane electric fields can have striking materials modification properties.

Nonlinear conductivity of aqueous electrolytes: Beyond the first Wien effect

The conductivity of strong electrolytes increases under high electric fields, a nonlinear response known as the first Wien effect. Here, using molecular dynamics simulations, we show that this increase is almost suppressed in moderately concentrated aqueous electrolytes due to the alignment of the water molecules by the electric field. As a consequence of this alignment, the permittivity of water decreases and becomes anisotropic, an effect that can be measured in simulations and reproduced by a model of water molecules as dipoles. We incorporate the resulting anisotropic interactions between the ions into a stochastic density field theory and calculate ionic correlations as well as corrections to the Nernst-Einstein conductivity, which are in qualitative agreement with the numerical simulations.

The surface force balance: direct measurement of interactions in fluids and soft matter

Over the last half-century, direct measurements of surface forces have been instrumental in the exploration of a multitude of phenomena in liquid, soft, and biological matter. Measurements of van der Waals interactions, electrostatic interactions, hydrophobic interactions, structural forces, depletion forces, and many other effects have checked and challenged theoretical predictions and motivated new models and understanding. The gold-standard instrument for these measurements is thesurface force balance(SFB), orsurface forces apparatus, where interferometry is used to detect the interaction force and distance between two atomically smooth planes, with 0.1 nm resolution, over separations from about 1 µm down to contact. The measured interaction forcevs.distance gives access to the free energy of interaction across the fluid film; a fundamental quantity whose general form and subtle features reveal the underlying molecular and surface interactions and their variation. Motivated by new challenges in emerging fields of research, such as energy storage, biomaterials, non-equilibrium and driven systems, innovations to the apparatus are now clearing the way for new discoveries. It is now possible to measure interaction forces (and free energies) with control of electric field, surface potential, surface chemistry; to measure time-dependent effects; and to determine structurein situ. Here, we provide an overview the operating principles and capabilities of the SFB with particular focus on the recent developments and future possibilities of this remarkable technique.