177 research outputs found
Frictional Duality Observed during Nanoparticle Sliding
One of the most fundamental questions in tribology concerns the area
dependence of friction at the nanoscale. Here, experiments are presented where
the frictional resistance of nanoparticles is measured by pushing them with the
tip of an atomic force microscope. We find two coexisting frictional states:
While some particles show finite friction increasing linearly with the
interface areas of up to 310,000nm^2, other particles assume a state of
frictionless sliding. The results further suggest a link between the degree of
surface contamination and the occurrence of this duality.Comment: revised versio
Nanoscopic Study of the Ion Dynamics in a LiAlSiO Glass Ceramic by means of Electrostatic Force Spectroscopy
We use time-domain electrostatic force spectroscopy (TD-EFS) for
characterising the dynamics of mobile ions in a partially crystallised
LiAlSiO glass ceramic, and we compare the results of the TD-EFS
measurements to macroscopic electrical conductivity measurements. While the
macroscopic conductivity spectra are determined by a single dynamic process
with an activation energy of 0.72 eV, the TD-EFS measurements provide
information about two distinct relaxation processes with different activation
energies. Our results indicate that the faster process is due to ionic
movements in the glassy phase and at the glass-crystal interfaces, while the
slower process is caused by ionic movements in the crystallites. The spatially
varying electrical relaxation strengths of the fast and of the slow process
provide information about the nano- and mesoscale structure of the glass
ceramic.Comment: 5 pages, 4 figure
Nanotribological studies using nanoparticle manipulation: Principles and application to structural lubricity
Evaluation der klinisch-funktionellen Ergebnisse und der Lebensqualität von Patienten mit Kniegelenksarthrose nach Hyaluronsäurebehandlung
Diese prospektive, randomisierte, doppelblinde und placebokontrollierte Studie befasst sich mit einer intraartikulaeren Injektion mit nicht-tierischer, stabilisierter Hyaluronsaeure bei Gonarthrosepatienten. Ziel war die Wirksamkeitsbewertung unter Beruecksichtigung der Lebensqualitaet. An 30 Patienten im Kellgrenstadium II - III und deutlichen Schmerzen wurde der therapeutische Nutzen mittels visueller Analogskala (VAS), Lequesne-Score, Knee Society Score, dem Western Ontario and McMaster Universities Osteoarthritis Index und der Medical Outcomes Study Short Form-36 ueber 12 Wochen ermittelt. Je eine Ganganalyse mit Elektromyographie vor Injektion und am Studienende dienten der zusaetzlichen Beurteilung. Im statistischen Vergleich zeigte sich keine Ueberlegenheit der nicht-animalischen, stabilisierten Hyaluronsaeure
Sublattice identification in noncontact atomic force microscopy of the NaCl(001) surface
We compare the three-dimensional force field obtained from frequency-distance measurements above the NaCl(001) surface to atomistic calculations using various tip models. In the experiments, long-range forces cause a total attractive force even on the similarly charged site. Taking force differences between two sites minimizes the influence of such long-range forces. The magnitude of the measured force differences are by a factor of 6.5–10 smaller than the calculated forces. This is an indication that for the particular tip used in this experiment several atoms of the tip interact with the surface atoms at close tip-sample distances. The interaction of these additional atoms with the surface is small at the imaging distance, because symmetric images are obtained. The force distance characteristics resemble those of a negative tip apex ion which could be explained, e.g., by a neutral Si tip.Peer reviewe
Ageing of a Microscopic Sliding Gold Contact at Low Temperatures
Nanometer-scale friction measurements on a Au(111) surface have been performed at temperatures between 30 and 300 K by means of atomic force microscopy. Stable stick slip with atomic periodicity is observed at all temperatures, showing only weak dependence on temperature between 300 and 170 K. Below 170 K, friction increases with time and a distortion of the stick-slip characteristic is observed. Low friction and periodic stick slip can be reestablished by pulling the tip out of contact and subsequently restoring the contact. A comparison with molecular dynamics simulations indicates that plastic deformation within a growing gold junction leads to the observed frictional behavior at low temperatures. The regular stick slip with atomic periodicity observed at room temperature is the result of a dynamic equilibrium shape of the contact, as microscopic wear damage is observed to heal in the sliding contact
Temperature dependence of the energy dissipation in dynamic force microscopy
The dissipation of energy in dynamic force microscopy is usually described in
terms of an adhesion hysteresis mechanism. This mechanism should become less
efficient with increasing temperature. To verify this prediction we have
measured topography and dissipation data with dynamic force microscopy in the
temperature range from 100 K up to 300 K. We used
3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) grown on KBr(001), both
materials exhibiting a strong dissipation signal at large frequency shifts. At
room temperature, the energy dissipated into the sample (or tip) is 1.9
eV/cycle for PTCDA and 2.7 eV/cycle for KBr, respectively, and is in good
agreement with an adhesion hysteresis mechanism. The energy dissipation over
the PTCDA surface decreases with increasing temperature yielding a negative
temperature coefficient. For the KBr substrate, we find the opposite behaviour:
an increase of dissipated energy with increasing temperature. While the
negative temperature coefficient in case of PTCDA agrees rather well with the
adhesion hysteresis model, the positive slope found for KBr points to a
hitherto unknown dissipation mechanism
Friction anomalies at first-order transition spinodals: 1T-TaS2
Revealing phase transitions of solids through mechanical anomalies in the friction of nanotips sliding on their surfaces, a successful approach for continuous transitions, is still an unexplored tool for first-order ones. Owing to slow nucleation, first-order structural transformations occur with hysteresis, comprised between two spinodal temperatures where, on both sides of the thermodynamic transition, one or the other metastable free energy branches terminates. The spinodal transformation, a collective one-shot event without heat capacity anomaly, is easy to trigger by a weak external perturbation. Here we show that even the gossamer mechanical action of an AFM-tip can locally act as a trigger, narrowly preempting the spontaneous spinodal transformation, and making it observable as a nanofrictional anomaly. Confirming this expectation, the CCDW-NCCDW first-order transition of the important layer compound 1T-TaS2 is shown to provide a demonstration of this effect
- …