Nanoscale Friction Switches: Friction Modulation of Monomolecular Assemblies Using External Electric Fields

Abstract

This paper presents experimental investigations to actively modulate the nanoscale friction properties of a selfassembled monolayer (SAM) assembly using an external electric field that drives conformational changes in the SAM. Such "friction switches" have widespread implications in interfacial energy control in micro/ nanoscale devices. Friction response of a low-density mercaptocarboxylic acid SAM is evaluated using an atomic force microscope (AFM) in the presence of a DC bias applied between the sample and the AFM probe under a nitrogen (dry) environment. The low density allows reorientation of individual SAM molecules to accommodate the attractive force between the −COOH terminal group and a positively biased surface. This enables the surface to present a hydrophilic group or a hydrophobic backbone to the contacting AFM probe depending upon the direction of the field (bias). Synthesis and deposition of the low-density SAM (LD-SAM) is reported. Results from AFM experiments show an increased friction response (up to 300%) of the LD-SAM system in the presence of a positive bias compared to the friction response in the presence of a negative bias. The difference in the friction response is attributed to the change in the structural and crystalline order of the film in addition to the interfacial surface chemistry and composition presented upon application of the bias. Keywords Chemistry DisciplinesChemistry | Mechanical Engineering | Nanoscience and Nanotechnology CommentsReprinted with permission from Langmuir 25 (2009) This paper presents experimental investigations to actively modulate the nanoscale friction properties of a selfassembled monolayer (SAM) assembly using an external electric field that drives conformational changes in the SAM. Such "friction switches" have widespread implications in interfacial energy control in micro/nanoscale devices. Friction response of a low-density mercaptocarboxylic acid SAM is evaluated using an atomic force microscope (AFM) in the presence of a DC bias applied between the sample and the AFM probe under a nitrogen (dry) environment. The low density allows reorientation of individual SAM molecules to accommodate the attractive force between the -COOH terminal group and a positively biased surface. This enables the surface to present a hydrophilic group or a hydrophobic backbone to the contacting AFM probe depending upon the direction of the field (bias). Synthesis and deposition of the low-density SAM (LD-SAM) is reported. Results from AFM experiments show an increased friction response (up to 300%) of the LD-SAM system in the presence of a positive bias compared to the friction response in the presence of a negative bias. The difference in the friction response is attributed to the change in the structural and crystalline order of the film in addition to the interfacial surface chemistry and composition presented upon application of the bias.

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

“…Water-based lubricants have been widely studied owing to their low viscosity and environmentally friendly properties [19][20][21]. The application of external fields is also an effective strategy for controlling and regulating interfacial friction [22][23][24]; the electric potential of a surface can be especially useful for controlling its interfacial friction behavior in solution [4,25,26]. Thus, studying the effects of electric potential on friction can aid our understanding of friction mechanisms.…”

Role of Interfacial Water and Applied Potential on Friction at Au(111) Surfaces

“…Controlling small-scale friction during sliding has potential application in a variety of areas including nano/micro-electro-mechanical systems (NEMS/MEMS) (Kim et al, 2007;Achanta and Celis, 2015). The application of external stimuli, which enables reversible modification of interfacial properties, is an effective strategy to control and tune friction (Binggeli et al, 1993;Park et al, 2006;Karuppiah et al, 2009). Particularly, application of the electrical potential has been shown to influence the tribological behavior at interfaces in various solutions (Labuda et al, 2011;Sweeney et al, 2012;Strelcov et al, 2015).…”

Molecular Dynamics Simulation of Conformational Transition and Frictional Performance Modulation of Densely Packed Self-Assembled Monolayers Based on Electrostatic Stimulation

“…In the case of sparsely distributed SAMs, the greater interchain spacing allows conformation change in the backbone chain under application of electrical fields, which could lead to significant alteration of surface properties such as hydrophobicity and work of adhesion as well as mechanical performance, such as friction. 22 Karuppiah et al 23 utilized atomic force microscopy (AFM) to evaluate the adhesion and friction response of a low-density mercaptocarboxylic acid (MHA) SAM based on electrical field modulation with different polarities. They discovered higher adhesion forces when negative electrical fields were applied upon the SAMs and attributed this performance alteration to surface chemistry transition.…”