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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.

In this study the friction and wear behavior of medical grade ultra-high molecular weight polyethylene (UHMWPE) (GUR 1050 resin) were evaluated as a function of polymer crystallinity. Crystallinity was controlled by heating UHMWPE samples to a temperature above its melting point and varying the hold time and cooling rates. Degree of crystallinity of the samples was evaluated using differential scanning calorimetry (DSC). Quantitative friction experiments were conducted at two different scales. A custom-made microtribometer with commercially available spherical Si3N4 probes in dry conditions was used to test friction at the microscale. An atomic force microscope with commercially available Si3N4 probes under dry conditions was used for nanoscale experiments. A higher degree of crystallinity in the UHMWPE resulted in lower friction force and an increase in scratch resistance at both scales. Reciprocating wear tests preformed using the tribometer show that higher crystallinity also results in lower friction, as well as lower wear depth and width.

Medical-grade UHMWPE samples with two different surface finishing treatments, milling and melting/reforming were exposed to 10% bovine serum albumin solution and their friction responses were quantified using atomic force microscopy. The observed friction increase upon exposure to proteins was attributed to the formation of a layer of denatured proteins on the surface. Changing the crystallinity and surface energy of UHMWPE affected the protein adsorption mechanism and the resulting increase in friction behavior.

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