What is it about?
Quartz crystal microbalance with dissipation (QCM-D) response data was used to determine the friction coefficient (μ) between soft solid surface and ethanol. A self-assembly of Dodecanethiol (DCT) film on gold was carried out. The number of DCT grafted (Γ) onto gold surface was investigated at concentrated solutions (10–33% v/v). Such large concentration leads to an increase on grafting density, viscosity, roughness and water contact angle favoring the reduction of friction coefficient (∼0.0922). These results allow proposing a soft selfassembled hydrophobic film to reduce friction between fluids into conduit, tube, or some other device.
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Why is it important?
The aim was to characterize the interactions between a hydrophobic soft solid surface and a fluid (ethanol) to identify those terms that make the soft solid surface a good boundary lubricant. The response data of quartz crystal microbalance with dissipation provides information to calculate the number of grafted molecules, normal forces, friction forces and friction coefficient between soft solid film and a fluid, dodecanethiol film adsorbed on gold was used as a model. The effect of highly DCT concentrated solutions on friction coefficient was investigated; concentration affected the number of grafted molecules and thus the friction coefficient. In particular at 10% DCT concentration, a low friction coefficient 0.094 was obtained, while at 33% concentration only achieved a 3% reduction and friction coefficient of 0.092. Topography images demonstrate that high DCT concentration produces an increase in grafting density. Based on ellipsometry results, which indicate that the films have a thickness greater than DCT molecules, probably due to the high concentration of the bulk liquid, molecules of DCT get trapped in the adsorbed film, thus increases roughness, water contact angle and viscosity. Although the DCT films formed are very thin, low friction coefficients values were obtained.
Perspectives
Friction coefficient between soft solid surface and a fluid has been reported for first time by QCM-D data successfully. As a model a selfassembly dodecanethiol (DCT) film onto gold surface was carried out. The high concentrations effect of bulk DCT solution in mass adsorption, grafting density, viscosity, parallel and perpendicular forces that affect the interface between soft solid surface and a fluid were calculate by tracking the changes in frequency and energy dissipation. The motion equations that govern the laminar flow into the QCM-D device were studied. Increasing the concentration of the bulk solution of DCT rises grafting density, viscosity, roughness and water contact angle favoring the friction coefficient reduction. Ellipsometry measures and AFM images indicate that a monolayer is not formed, even though the end group (CH3) is not reactive, such large concentration cause the possible formation of DCT dimers by sulfur bridges and DCT molecules permanently trapped in the film, which increases the roughness, viscosity and therefore reduces the friction coefficient. Friction coefficient of the film from bulk liquid concentration of DCT from 10 to 33% was 0.0941 to 0.0922. This is a low friction coefficient, even at the lower bulk liquid DCT concentration (10%), and decreases slightly (∼3%) with increasing bulk liquid concentration. The gold plate water contact angle (67 ± 1.9°) augmented to 111.4 ± 1.5° when growing the DCT concentration to 33%. These results allow to propose self-assembled hydrophobic films to reduce friction between fluids into conduit, tube, or some other device.
Dr Rosalba Patiño Herrera
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This page is a summary of: Friction coefficient between a hydrophobic soft solid surface and a fluid: Determined by QCM-D, Chemical Physics, September 2018, Elsevier,
DOI: 10.1016/j.chemphys.2018.06.023.
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