Quantification/mechanism of interfacial interaction modulated by electric potential in aqueous salt solution

Abstract

With the development of surface and interface science and technology, methods for the online modulation of interfacial performance by external stimuli are in high demand. Switching between ultra-low and high friction states is a particular goal owing to its applicability to the development of precision machines and nano/micro-electromechanical systems. In this study, reversible switching between superlubricity and high friction is realized by controlling the electric potential of a gold surface in aqueous salt solution sliding against a SiO 2 microsphere. Applying positive potential results creates an ice-like water layer with high hydrogen bonding and adhesion at the interface, leading to nonlinear high friction. However, applying negative potential results in free water on the gold surface and negligible adhesion at the interface, causing linear ultra-low friction (friction coefficient of about 0.004, superlubricity state). A quantitative description of how the external load and interfacial adhesion affected friction force was developed, which agrees well with the experimental results. Thus, this work quantitatively reveals the mechanism of potential-controlled switching between superlubricity and high-friction states. Controlling the interfacial behavior via the electric potential could inspire novel design strategies for nano/micro-electromechanical and nano/micro-fluidic systems.

Potential-Dependent Interfacial Frictional Behavior between Charged Microspheres and Gold in Aqueous Solutions

“…Our own experiments on adhesion confirmed the hydrogen-bonding effect revealing that while the adhesion between a GE and hydrophilic silica tip changed with the gold potential, it did not change when the tip was replaced with a hydrophobic polystyrene tip that is incapable of hydrogen bonding. Furthermore, our group recently demonstrated a reversible switch between superlubricity and high friction using the potential of a GE . In this study, we determined that under a positive gold potential, the frictional force, adhesion force, and load obeyed the binomial law of friction, confirming the role of electrostatically induced hydrogen bonding with the ice-like water layer …”

“…Furthermore, our group recently demonstrated a reversible switch between superlubricity and high friction using the potential of a GE . In this study, we determined that under a positive gold potential, the frictional force, adhesion force, and load obeyed the binomial law of friction, confirming the role of electrostatically induced hydrogen bonding with the ice-like water layer …”

“…For the GE surfaces with different electric charges, the ions and water molecules between the interfaces would have different structures. According to previous studies, when a gold surface has the appropriate positive potential, water molecules adsorb on the GE to form an ordered ice-like water layer with viscosities 10 5 –10 7 times that of bulk water (Figure a,b). , The ordered water layer exposes a large number of dangling hydroxyl groups, which are capable of hydrogen bonding with polar hydrophilic probes ,,, (panel 1 of Figure c). However, at positive potentials, the Cl – adsorption is inevitable, which would inhibit the formation of ice-like water. , In this experiment, the maximum potential used, V app = +0.6 V, and the amount of adsorbed Cl – was insufficient to disrupt the ice-like water structure, similar to the findings of previous studies. ,, In contrast, when the GE potential was negative, the ordered water structure was disrupted and ice-like water did not form. ,, Due to electrostatic attraction, the hydrated cations were adsorbed on the interface water layer at the GE (Figure ).…”

“…As shown in the abovementioned examples, most electrochemical AFM studies of the frictional behavior in GE interfaces use the negatively charged silica colloidal probe as a counter-frictional material. ,, Currently, studies that directly compare colloidal probes with different surface electric properties are lacking; such comparative studies could have a wide-range impact. For example, while GEs are widely used as test substrates in the biomedical field, such as cancer cell recognition and targeted drug transport, − the surfaces of most drugs or cells of interest are either positively or negatively charged …”

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

“…Natural water-lubricated systems can exhibit exceptionally low coefficients of friction (COF) of 𝜇 < 0.002 [10]. Experiments show that the COF for aqueous solutions on gold electrodes can also be electrotuned [11][12][13][14]. Prior aqueous friction studies have used different experimental techniques, including the surface-force apparatus [11,15,16], atomicforce [12] and colloidal-probe [13,14] microscopy.…”