Comparison of friction measurements using the atomic force microscope and the surface forces apparatus: the issue of scale

Results are presented of lateral force measurements using the atomic force microscope (AFM) and the surface forces apparatus (SFA). Two different probes are used in the AFM measurements; a sharp silicon nitride tip (radius R20 nm) and a glass ball (R15 m). The lateral force is measured between the (silicon nitride or glass) probe and a mica surface which has been coated by a thin lubricant film. In the SFA, a thin lubricant film separates two molecularly smooth mica surfaces (R1 cm) which are slid relative to each other. Perfluoropolyether (PFPE) and polydimethylsiloxane (PDMS) were used as the lubricant films. In the SFA where the contact diameter is largest, the PFPE film shows much lower friction than PDMS. As the size of the probe decreases, the difference in the measured friction decreases. For sharp AFM tips, no clear distinction between the tribological properties of the films can be made. Hence, the measured coefficient of friction varies according to the length scale probed, at least for small dimensions.     

The Boundary Lubrication Properties of Model Esters

The friction of three chemically distinct esters was measured in order to determine how molecular architecture influences friction. The friction coefficients of mica surfaces separated by a thin film (<2 nm) of -chlorodecyl benzoate, -chlorodecyl pentafluoro benzoate, and -chlorodecyl perfluoro hexanoate were measured to be 0.15±0.015, 0.13±0.012, and 0.12±0.02, respectively. The friction coefficients for the esters are lower than the previously measured friction coefficients of simple hydrocarbon liquids such as n-tetradecane (=0.8), but are comparable to the friction coefficients of surfactant monolayer coated surfaces (=0.001–0.2). The results suggest that the ester molecules adsorb onto the mica surface with the (phenyl or hexyl) carbonyl next to the surface and the hydrocarbon tail pointing away from the surface. Hence, the friction is controlled by the packing density and properties of the hydrocarbon tail. Changes in the chemistry and structure of the carboxylic acid portion of the ester only give rise to small changes in the friction coefficient.     

FRICTION AND ADHESION MEASUREMENTS BETWEEN A FLUOROCARBON SURFACE AND A HYDROCARBON SURFACE IN AIR

The friction and adhesion between a fluorocarbon monolayer-coated surface against a hydrocarbon monolayer-coated surface has been directly measured. The friction was found to be lower than the friction between a hydrocarbon monolayer against a hydrocarbon monolayer and a fluorocarbon monolayer against a fluorocarbon monolayer. No stick-slip sliding was observed for speeds from 0.8?µm/s to 2.6?µm/s. The fluorocarbon–hydrocarbon interface was adhesive, with the energy of interaction measured to be 14.9?mJ/m²?±?1.0?mJ/m². As predicted from theory, the magnitude of the adhesion of a fluorocarbon monolayer against a hydrocarbon monolayer is between that measured for a fluorocarbon monolayer against a fluorocarbon monolayer and a hydrocarbon monolayer against a hydrocarbon monolayer. One may note that the interfacial energy, γ, follows the general trend γ_FC/FC?<?γ_HC/FC?<?γ_HC/HC, whereas the shear stress, τ, varies according to τ_FC/HC?<?τ_HC/HC?<?τ_FC/FC.     

Maximum Force Technique for the Measurement of the Surface Tension of a Small Droplet by AFM

An atomic force microscope (AFM) is used to measure the meniscus force on a vertical quartz rod as the rod is pulled through an air/liquid interface. A fluid bridge forms between the liquid and the base of the rod as the rod is withdrawn from the liquid. The force reaches a maximum as the bridge necks down and finally detaches from the rod. The maximum force on the rod is independent of the material of the rod and can be used to calculate the surface tension of the liquid. Alternately, if the surface tension of the liquid is known, the maximum force of the meniscus can be used to calibrate the spring constant of the AFM cantilever. The contact angle of the liquid on the rod was calculated as the rod was inserted into the liquid droplet. Contact angle hysteresis was observed. Results are presented of the measurement of the meniscus force of water, 10^-3 M cetyl trimethyl ammonium bromide (CTAB) and tetradecane as the rod is withdrawn from the liquid.     

Aerobic spore-forming bacteria in bulk raw milk: factors influencing the numbers of psychrotrophic, mesophilic and thermophilic Bacillus spores

Psychrotrophic, mesophilic and thermophilic spore concentrations of Bacillus spp. were determined on a weekly basis in bulk raw milk samples obtained from a central processing facility, over one calendar year. These data were correlated with concentrations of metal ions, free amino acids and somatic cell counts obtained from the same samples, as well as local meteorological mean temperature and relative humidity measurements relating to the same test period. A heat treatment of 80°C for 20 min followed by the addition of L-alanine to the milk at 0.1% w/v and incubation at 55°C for 7 days gave optimal recovery conditions for thermophilic spores. Free amino acids, metal ions, somatic cell counts, temperature, and relative humidity measurements were each significantly correlated with the recovery of spores of Bacillus spp. from bulk raw milk, although no single factor was shown to demonstrate a consistent effect with the psychrotrophic, mesophilic and thermophilic spore groups studied.     

Maximum Force Technique for the Measurement of the Surface Tension of a Small Droplet by AFM

An atomic force microscope (AFM) is used to measure the meniscus force on a vertical quartz rod as the rod is pulled through an air/liquid interface. A fluid bridge forms between the liquid and the base of the rod as the rod is withdrawn from the liquid. The force reaches a maximum as the bridge necks down and finally detaches from the rod. The maximum force on the rod is independent of the material of the rod and can be used to calculate the surface tension of the liquid. Alternately, if the surface tension of the liquid is known, the maximum force of the meniscus can be used to calibrate the spring constant of the AFM cantilever. The contact angle of the liquid on the rod was calculated as the rod was inserted into the liquid droplet. Contact angle hysteresis was observed. Results are presented of the measurement of the meniscus force of water, 10^?3 M cetyl trimethyl ammonium bromide (CTAB) and tetradecane as the rod is withdrawn from the liquid.