Modulating Interfacial Energy Dissipation via Potential-Controlled Ion Trapping

As a metal (gold) surface at a given, but variable potential slides past a dielectric (mica) surface at a fixed charge, across aqueous salt solutions, two distinct dissipation regimes may be identified. In regime I, when the gold potential is such that counterions are expelled from between the surfaces, which then come to adhesive contact, the frictional dissipation is high, with coefficient of friction μ ≈ 0.8–0.9. In regime II, when hydrated counterions are trapped between the compressed surfaces, hydration lubrication is active and friction is much lower, μ = 0.05 ± 0.03. Moreover, the dissipation regime as the surfaces contact is largely retained even when the metal potential changes to the other regime, attributed to the slow kinetics of counterion expulsion from or penetration into the subnanometer intersurface gap. Our results indicate how frictional dissipation between such a conducting/nonconducting couple may be modulated by the potential applied to the metal.


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. A. Schematic of the Surface Force Balance (SFB), with the principle components labeled, and a picture of the interference fringes of equal chromatic order from whose wavelengths and shape the surface separation D and curvature radius R are determined. Gold and mica are mounted on fused silica lenses in a crossed-cylinder configuration, equivalent to the geometry of sphere on a flat, with the lower lens mounted on a leaf spring of spring costant K n = 81.5±2.7 N/m. The top lens is moved to a distance D between the surfaces by a 3-stage mechanism with the most sensitive stage being a sectored piezoelectric tube (PZT), and the normal force Fn between the surfaces is monitored through the bending of the spring. Shear forces Fs transmitted to the bottom surface when the top surface is moved laterally by the sectored PZT are monitored via the bending of the shear spring (of spring constant K s =300N/m) measured with a sensitive capacitance probe (see also in figure S4). The inset, left, shows the 3-electrode configuration where the gold surface is mounted on the upper cylindrical lens facing the mica surface on the lower lens. B. Schematic of the custom-designed three electrode cell used in the SFB. In this setup, gold acts as a working

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(W) electrode and two platinum wires as the (quasi) reference (R) and counter (C) electrodes. The boat is made from quartz to prevent leakage of current to the ground and the lower lens is placed on a leaf spring. During an experiment, all three electrodes are immersed in the quartz boat potential is applied to the gold surface using a potentiostat (CHI600C, CH Instruments, Inc.) which serves as the control unit. From the distance change Δd=|d1-d2|, the maximum static friction force Fs=KsΔd can then be measured.

Variation of frictional forces following a potential step
The ability to trap hydrated counterions between two surfaces (gold and mica) while regulating surface potentials is crucial for controlling frictional forces in our system. Hence, we have explored whether counterions diffuse out of the intersurface gap over a period of ca. 100sec, as expected theoretically and experimentally 1-3 once the gold surface potential is toggled from regime II to regime I.
To this end, we applied a step potential between -0.3V (regime II) and -0.15V (regime I) (corresponding to gold surface potentials of -0.18V and 0.004V, respectively) while measuring the variation of the frictional force over a period of 130sec; then, the potential was toggled back to -0.3V ( figure S5). As depicted, following a potential change from -0.3V to -0.15V, the frictional force gradually (over a time scale of 20 sec or more) increased up to a point where it exceeded the maximal applied shear force Fs, max so that the surfaces did not slide (i.e., their lateral motion was coupled). Such an increase upon transition from regime II to regime I suggests that counterions may indeed slowly diffuse out of the gap. Once the potential was changed back to -0.3V (regime II) the frictional force decreased close to its initial value (i.e. its value prior to toggling the potential to -0.15V). It is of interest that in both cases an initial rapid change was followed by a much slower change over a time scale of 20 sec or longer. This suggests that a slow ion diffusion both in and out of the gap contributes to the dissipation on sliding, but that other dissipative mechanisms such as plastic deformation of gold asperities and gradual increase of nominal contact area may also play a role. Figure S5. Variation of in situ frictional force Fs between mica and gold across 2mM LiClO4 (trace iii) as the upper gold surface is moved laterally (trace i) and a potential step -0.3V to -0.15V is applied to the gold (trace ii).

Estimation of the total load and friction coefficient
We define the total load (Fn) applied to the surfaces as the sum of the external mechanical load Fext applied by the surface force balance motor and the pull-off force Fpull-off required to separate the surfaces while under adhesive interaction (in regime II). Following our estimation of the total load (Fn = Fpulloff + Fext) after altering the gold surface potential, the friction coefficient is then  = (Fs/Fn). is the electrostatic energy and S9 W vdW is the vdW interaction energy between the surfaces 4 . In principle, while W vdW is hardly affected by small changes in electrolyte concentration the electrostatic force per unit area F e (D)/A and thus W e (D)will be lower at any given D for higher salt concentration because of the shorter Debye screening length (see figure 1 and S2). This, in turn, reduces the magnitude of the adhesion energy W A at higher electrolyte concentration in regime I, as observed in figure 1B. This effect was not observed in our friction measurements where similar friction coefficient values were obtained in regime II for 1mM and 2mM, as can be seen in figure 2B. This is because at both salt concentrations Li + counterions are trapped between the surfaces and frictional dissipation is modulated by hydration lubrication.