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Partial slip in liquids studied with high-frequency shear oscillations
 Vlachová, Jana ; doc.Ing.Marian Lehocky, Ph.D. (oponent) ; Johannsmann, Diethelm (vedoucí práce)
The sphere-plate contact under normal and tangential load in water was studied. As a source of shear oscillation the acoustic resonator (a quartz crystal microbalance - QCM) was used. The contact of a sphere with the surface of resonator induces a shift of resonance frequency and bandwidth. The dependence of the frequency shift f and bandwidth shift on the amplitude of oscillation were measured. With increasing amplitude, a decrease in f and an increase in were observed. This behaviour is characteristic for partial slip. Applying the Cattaneo-Mindlin model, the contact radius and the friction coefficient were calculated. The contact radius in the limit of small amplitudes increases with increasing normal load. For this dependency, the data fit well to the JKR model. The friction coefficient is of the order of unity, as it should be. It slightly decreases with increasing external normal force, which can be explained with an adhesive force contributing to the total normal force. The formalism yields two separate values for friction coefficient, the first is derived from the frequency shift and the second is derived from the shift in bandwidth. These two values agree with each other within ± 20 % for experiments in liquids, while they differ by about a factor of two for experiments with hydrophilic surfaces in air. This is tentatively explained with capillary forces. The discrepancy points to a shortcoming of the Cattaneo Mindlin theory.
Úplný záznam
Partial slip in liquids studied with high-frequency shear oscillations
 Vlachová, Jana ; doc.Ing.Marian Lehocky, Ph.D. (oponent) ; Johannsmann, Diethelm (vedoucí práce)
The sphere-plate contact under normal and tangential load in water was studied. As a source of shear oscillation the acoustic resonator (a quartz crystal microbalance - QCM) was used. The contact of a sphere with the surface of resonator induces a shift of resonance frequency and bandwidth. The dependence of the frequency shift f and bandwidth shift on the amplitude of oscillation were measured. With increasing amplitude, a decrease in f and an increase in were observed. This behaviour is characteristic for partial slip. Applying the Cattaneo-Mindlin model, the contact radius and the friction coefficient were calculated. The contact radius in the limit of small amplitudes increases with increasing normal load. For this dependency, the data fit well to the JKR model. The friction coefficient is of the order of unity, as it should be. It slightly decreases with increasing external normal force, which can be explained with an adhesive force contributing to the total normal force. The formalism yields two separate values for friction coefficient, the first is derived from the frequency shift and the second is derived from the shift in bandwidth. These two values agree with each other within ± 20 % for experiments in liquids, while they differ by about a factor of two for experiments with hydrophilic surfaces in air. This is tentatively explained with capillary forces. The discrepancy points to a shortcoming of the Cattaneo Mindlin theory.
Úplný záznam

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