On the Friction Behavior of SiO2 Tip Sliding on the Au(111) Surface: How Does an Amorphous SiO2 Tip Produce Regular Stick-Slip Friction and Friction Duality?

Friction behaviors of an amorphous SiO2 tip sliding on the Au(111) surface in atomic force microscopy (AFM) are investigated through molecular dynamics (MD) simulations. We observed a regime of extremely low, close-to-zero friction at low normal loads with clear stick-slip friction signals. The friction is almost independent of the applied normal load below a threshold value. However, above this loading threshold, friction can remain low or increase sharply. Such an unexpected friction duality is attributed to the high probability of defect formation at the sliding interface that can induce plowing friction in a high-friction state. The energy difference between the low-friction state and the high-friction state is surprisingly low, which is comparable to kT (∼25 meV) at room temperature. These findings are consistent with previous AFM friction measurements using silicon AFM tips. Further MD simulations show that one can always use an amorphous SiO2 tip to image the crystalline surface with regular stick-slip friction signals. This is largely due to the fact that there is always a small fraction of contacting Si and O atoms at the sliding interface that are sitting on the relatively stable, close-to-hollow sites of the crystalline Au(111) surface during the stick stage; thus, they are capable of sampling local energy minima. We anticipate that regular stick-slip friction can be achieved even in the intermediate loading range, so long as the low-friction state is maintained when friction duality occurs.

Will Polycrystalline Platinum Tip Sliding on a Gold(111) Surface Produce Regular Stick-Slip Friction?

Friction measurements by an atomic force microscope (AFM) frequently showed regular stick-slip friction signals with atomic-scale resolutions. Typically, for an AFM metal tip sliding on a metal crystal surface, the microstructure of the tip made from the thermally evaporated metal coating on a silicon cantilever was polycrystalline. Our detailed molecular dynamics(MD) simulations of a polycrystalline Pt tip (R = 10 nm in radius) sliding on an Au(111) surface revealed how the geometry of the polycrystalline tip took effect on the friction behavior at the contact interface. We found that the apex of the Pt tip with multiple grains near the edge of contact could induce severe plastic deformations of the gold substrate, leading to irregular stick-slip frictions upon sliding. Simulation results showed that in order to achieve a clear stick-slip friction signal with single atomic slips, the apex of the Pt tip must adopt a single crystalline protrusion without any neighboring grains involved in the metal contact. We showed that such a single crystalline protrusion, which presumably could be achieved during initial run-in or wear-out of high-energy Pt atoms in the neighboring grains, was passivated by a large number of gold atoms due to metal adhesion in the contact periphery. Using such a crystalline protrusion tip, we demonstrated that the stick-slip friction produced was very "tolerant" to the adhesion of a large number of gold atoms on the tip apex. We further showed that AFM tip mass used in MD simulations also played an important role in determining the transition between friction regimes, which could be well explained by the Prandtl-Tomlinson thermal activation model.

Nucleation of Frank Dislocation during the Squeeze-Out Process in Boundary Lubrication: A Molecular Dynamics Study.

Liquid-vapor molecular dynamics (LVMD) simulations are performed to reinvestigate the phase transition and solvation force oscillation behavior of a simple argon liquid film confined between two solid surfaces. Our simulations present a novel scenario in which the n → n - 1 layering transitions are accompanied by the formation, climb, and annihilation of Frank partial dislocations during the squeeze-out process under compression. This is indicated by the splitting of the repulsive peaks in the solvation force profile. The detailed analysis reveals that the formation-climb-annihilation mechanism of Frank dislocation occurs during approach and disappears during receding, which would result in force hysteresis. In combination with our recent works, this study provides new insights into the physical property of nanoconfined lubricant films in boundary lubrication.

[Quantification of type I and III collagen content in normal human skin in different age groups].

OBJECTIVE: To quantify the content of type I, III collagen and their ratio in normal human skin of different age, and to explore the regulation of changes. METHODS: The normal human skin specimens were obtained from 6 spontaneously aborted fetus and 56 burn patients of different ages, including infants (newborn -3 years), pre-school group ( > 3, < or =7 years), adolescent group ( >7, < or = 18 years), youth and middle age group ( > 18, < or = 50 years), and elderly group ( > 50 years), were studied. The total collagen content were determined by hydroxyproline method. The contents of type I, Ill collagen and their ratio were examined by immunohistochemistry. RESULTS: The total collagen content decreased along with increase in age, and it was highest in fetus [(543 +/- 13) microg/g]. The ratio between type I and Ill collagen increased along with increase in age. The content of type III collagen was highest in fetus [(278 +/- 7) microg/g], and it decreased along with increase in age. The content of type I collagen content was [(265 +/- 7) microg/g] in fetus, and it was increased slightly in infant and pre-school groups, then decreased along with advance in age. CONCLUSION: Decomposition of type III collagen in normal human skin may exceed its synthesis after birth immediately, leading to its reduction. Synthesis of type I collagen in normal human skin is dominant before 8 years old, and it shows an opposite tendency afterwards.