Surface Forces and Structure in a Water-in-Salt Electrolyte.

Water-in-salt electrolytes are a fascinating new class of highly concentrated aqueous solutions with wide electrochemical stability windows that make them viable as aqueous battery electrolytes. However, the high ion concentration of water-in-salt electrolytes means that these systems are poorly understood when compared to more dilute electrolyte solutions. Here, we present direct surface force measurements across thin films of a water-in-salt electrolyte at several concentrations.

A new methodology for a detailed investigation of quantized friction in ionic liquids.

When confined at the nanoscale between smooth surfaces, an ionic liquid forms a structured film responding to shear in a quantized way, i.e., with a friction coefficient indexed by the number of layers in the gap.

Are Buckminsterfullerenes Molecular Ball Bearings?

Buckminsterfullerenes (C) are near-spherical molecules, which freely rotate at room temperature in the solid state and when dissolved in solution. An intriguing question arises as to whether C molecules can act as "molecular ball bearings," that is, preventing direct contact between two solid surfaces while simultaneously dissipating shear stress through fast rotation. To explore this, we performed measurements of friction across a solution of C in the boundary lubrication regime.

Controlled defects to link wetting properties to surface heterogeneity.

Small-scale heterogeneities have long been understood to give rise to contact angle hysteresis. More recently, the question of how they influence contact line dynamics has generated interest. Models that express the hysteresis or dynamics in terms of defect properties exist but have yet to be conclusively tested by experiment.

Thermally activated motion of a contact line over defects.

At the nanometer scale, the motion of a contact line separating a dry from a wet region is limited by the presence of surface heterogeneities that pin it. Here we revisit the seminal model proposed by Joanny and de Gennes to include the influence of thermal noise and viscosity using a Langevin model with two degrees of freedom: the average position of the contact line and its distortion. We identify the conditions under which the dynamics in a velocity-driven experiment can in fact be described by a constant forcing at small scale.

A moving contact line as a rheometer for nanometric interfacial layers.

How a liquid drop sits or moves depends on the physical and mechanical properties of the underlying substrate. This can be seen in the hysteresis of the contact angle made by a drop on a solid, which is known to originate from surface heterogeneities, and in the slowing of droplet motion on deformable solids. Here, we show how a moving contact line can be used to characterize a molecularly thin polymer layer on a solid.

Defects at the Nanoscale Impact Contact Line Motion at all Scales.

The contact angle of a liquid drop moving on a real solid surface depends on the speed and direction of motion of the three-phase contact line. Many experiments have demonstrated that pinning on surface defects, thermal activation and viscous dissipation impact contact line dynamics, but so far, efforts have failed to disentangle the role of each of these dissipation channels. Here, we propose a unifying multiscale approach that provides a single quantitative framework.