Nanotribology of Ionic Liquids: Transition to Yielding Response in Nanometric Confinement with Metallic Surfaces

Room-temperature ionic liquids (RTILs) are molten salts which exhibit unique physical and chemical properties, commonly harnessed for lubrication and energy applications. The pure ionic nature of RTIL leads to strong electrostatic interactions among the liquid, furthermore exalted in the presence of interfaces and confinement. In this work, we use a tuning-fork-based dynamic surface force tribometer, which allows probing both the rheological and the tribological properties of RTIL films confined between a millimetric sphere and a surface, over a wide range of confinements. When the RTIL is confined between metallic surfaces, we see evidence of an abrupt change of its rheological properties below a threshold confinement. This is reminiscent of a recently reported confinement-induced capillary freezing, here observed with a wide contact area. In parallel, we probe the tribological response of the film under imposed nanometric shear deformation and unveil a yielding behavior of the interfacial solid phase below this threshold confinement. This is characterized by a transition from an elastic to a plastic regime, exhibiting striking similarities with the response of glassy materials. This transition to yielding of the RTIL in metallic confinement leads overall to a reduction in friction and offers a self-healing protection of the surfaces avoiding direct contact, with obvious applications in tribology.

Popular Summary

Room-temperature ionic liquids (RTILs) are molten salts with unique properties commonly used for lubrication and energy applications. Under confinement, their distinctive behavior arises from strong electrostatic interactions between their charged constituents as well as with the charges within the confinement material. Here, we unveil new thermodynamic and mechanical behaviors of RTILs when confined between metallic surfaces. Such behaviors originate in a subtle electronic screening effect in the metals. The unique friction behavior of RTILs in metallic confinement points to further applications in lubrication.

We make use of a recently introduced force instrument that allows one to measure the mechanical and frictional response of RTILs confined between metallic surfaces. A mechanical resonator is used to measure the mechanical impedance of the RTILs confined between a millimeter-sized gold sphere and a flat gold substrate. We notice a freezing transition when the fluid is confined between metallic surfaces at a distance below about 50 nm. Then we observe a shear-induced fluidization when increasing the applied strain, reminiscent of the response of a yielding material.

Overall, the confinement-induced freezing of the RTIL in metallic confinement offers new routes in the use of RTILs as boundary lubricants, with an emphasis on antiwear applications. The strong electrostatic nature of the RTILs’ interactions also makes them great candidates for voltage-controlled lubrication, which would enable full on-demand mechanical control at the nanoscale.

Figure 1

Experimental setup. (a) Schematic representation of the force measurement apparatus. The prongs of the tuning fork oscillate along a shear (red) and a normal (blue) mode simultaneously. The two curves at the top show the corresponding resonance curves. The mechanical excitation $F^{*}=F{e}^{i\omega t}$ [$N$] is induced by the actuation of an external piezodither. An accelerometer glued on one prong records the dynamic oscillation amplitude $a^{*}=a{e}^{(i\omega t+\varphi )}$ [$m$]. A lock-in amplifier extracts the amplitude and phase of the accelerometer signal. The phase difference is continuously kept at zero by use of a phase-locked loop (PLL) which varies the excitation frequency ($\omega$) accordingly. A feedback loop (PID) controls the excitation intensity $F$ in order to keep the oscillation amplitude a chosen value. A spherical probe, glued at the end of one prong, is immersed in an ionic liquid drop deposited on a gold substrate. The setup is placed in a reduced pressure ($P<{10}^{-1}\mathrm{mbar}$) environment. (b) The ionic liquid ${\mathrm{BmimBF}}_4$ is confined between the substrate and the gold-coated sphere of radius $R\approx 1.5\mathrm{mm}$. A piezoscanner varies the confinement distance $D$ over tens of micrometers with subnanometric precision. A potential difference $\mathrm{\Delta }V\approx 25\mathrm{mV}$ is applied between the polarizable surfaces. (c) Electric current flowing through the contact upon decreasing the separation distance $D$.

Figure 2

Nanorheological and nanotribological measurements. (a) Inverse of the normal dissipative impedance $1/Z_N^{\prime \prime }$ as a function of separation between the surfaces. The dashed line results from a linear fit characteristic of viscous damping. The normal oscillation amplitude is $a_N\sim 2\mathrm{nm}$. (b) Evolution of the conservative $Z_S^{\prime }$ (black) and dissipative $Z_S^{\prime \prime }$ (red) shear impedance as the separation distance is varied. A solidlike mechanical behavior of the nanoconfined RTILs is observed from $\sim 40$-nm confinement (blue zone) down to the direct metallic contact (red zone and white data points). The shear oscillation amplitude is $a_S\sim 3\mathrm{nm}$. (c) Corresponding shear dissipative impedance $Z_S^{\prime \prime }$ as a function of shear elastic impedance $Z_S^{\prime }$ highlighting an abrupt change of frictional behavior.

Figure 3

Shear mechanical properties of the solid interfacial phase. (a) Shear mechanical impedance $Z_S^{*}$ as a function of applied shear strain for a fixed confinement distance $D=10\mathrm{nm}$. (b),(c) Stress-strain curves at constant shear amplitude $a_S=6\mathrm{nm}$ and $a_S=18\mathrm{nm}$, respectively. (d) Dissipative shear modulus $G^{\prime \prime }$ as a function of shear amplitude obtained from fitting the linear part of the different stress-strain curves. Error bars represent the standard error.

Figure 4

Shear frictional properties of the solid interfacial phase. (a) Logarithmic dependence of the dissipative friction force with respect to the shear amplitude for a fixed confinement distance $D=25\mathrm{nm}$. (b) Transition from a power-law to solid friction for fixed separation distance $D=3\mathrm{nm}$. Error bars represent the standard error and correspond to 10% for single points.

Figure 5

AFM image of (a) the gold substrate (rms roughness $\approx 0.9\mathrm{nm}$) and (b) the gold-coated sphere (rms roughness $\approx 1.5\mathrm{nm}$). For the sphere, the spherical background of the raw AFM image is substracted.

Figure 6

Shear impedance measurement for ionic liquid (IL) and silicon oil. The shear oscillation amplitude is 6 nm for both measurements.

Figure 7

Comparison of the hydrodynamic zero $D_0$ and the position of the frictional response $D_f$ as a function of the applied shear oscillation amplitude $a_S$ for insulating confinement surfaces.

Figure 8

Shear impedance measurement with different applied voltage difference. Shear amplitude is 5 nm.