Molecular Simulations of Electrotunable Lubrication: Viscosity and Wall Slip in Aqueous Electrolytes

Abstract

We study the frictional response of water-lubricated gold electrodes subject to an electrostatic potential difference using molecular dynamics simulations. Contrary to previous studies on electrotunable lubrication that were carried out by fixing the charges, our simulations keep electrodes at fixed electrostatic potential using a variable charge method. For pure water and NaCl solutions, viscosity is independent of the polarization of the electrodes, but wall slip depends on the potential difference. Our findings are in agreement with previous analytical theories of how wall slip is affected by interatomic interactions. The simulations shed light on the role of electrode polarization for wall slip and illustrate a mechanism for controlling friction and nanoscale flow in simple aqueous lubricants.

Theory and Simulations of Ionic Liquids in Nanoconfinement

“…Friction forces in ionic liquid films between uncharged surfaces have also been computed using a constant number of particles in the confined region. 213,223,231 The GC-NEMD approach offers the advantage of simulating explicitly "squeeze out" processes upon increasing normal loads, allowing the study of the mechanisms determining film thinning and providing a direct connection with AFM and SFA experiments.…”

“…207,224,229,236,237,367 Some attempts to investigate EL in thick aqueous solutions were reported recently, too. 231 The great majority of EL computations focused on unravelling the microscopic mechanisms of electrotunable friction in RTILs by computing the friction force as a function of surface charge density, which can be controlled by setting up a potential difference between two confining surfaces with respect to a reference electrode (see ref 366).…”

“…Hence, the GC-NEMD implementation mimics the SFA and AFM experiments, allowing the exchange of molecules in the confined regions to respond to shear and changes in the applied load, f L , and surface charge. Friction forces in ionic liquid films between uncharged surfaces have also been computed using a constant number of particles in the confined region. ,, The GC-NEMD approach offers the advantage of simulating explicitly “squeeze out” processes upon increasing normal loads, allowing the study of the mechanisms determining film thinning and providing a direct connection with AFM and SFA experiments.…”

Electrotunable friction with ionic liquid lubricants

“…The surface charge can be modelled using the fixed charge method (FCM), in which the charge on each surface atom is constant throughout the simulation 62, , or the constant potential method (CPM) 72,73 , where the charge distribution on the surfaces is adjusted self-consistently, subject to the electrical capacitance of the frictional contact. Differences between the two methods might be expected for metallic surfaces, as the electrode potential, not the charge, is kept constant.…”

QCM Study of Tribotronic Control in Ionic Liquids and Nanoparticle Suspensions

“…A natural extension of such studies involves charged constituents, particularly in suspensions [5,6], which are ubiquitous in lubricants and are readily manipulated by external electric and magnetic fields [7]. Examples include tailoring of electrolyte lubricants for custom response to electric and magnetic fields, environmentally friendly ionic liquid batteries, automatic friction and braking controls in mechanical systems, tribotronic micro-electro-mechanical systems (MEMS) devices for electrical power generation, thermally controlled friction via surface nanopatterning, and tunable viscosity and wall slip via polarizable electrodes [8][9][10][11][12]. This emerging field is referred to as "tribotronics", and is defined as the "active" or "smart" control of friction by combining machine elements with electronics [13][14][15][16][17].…”