Stick to slip transition and adhesion of lubricated surfaces in moving contact

Abstract

The friction of dry self-assembled monolayers, chemically attached to a solid surface and comprising a well-defined interface for sliding, is compared to the case of two solids separated by an ultrathin confined liquid. The monolayers were condensed octadecyltriethoxysilane (OTE). The liquid was squalane (C,,H,,), a film 2.0 nm thick confined between parallel plates of mica. The method of measurement was a surface forces apparatus, modified for oscillatory shear. The principal observations were the same in both cases: (1) Predominantly elastic behavior in the linear response state was followed by a discontinuous transition to a mostly dissipative state at larger deformations. The elastic energy stored at the transition was low, of the order of 0.1 kT per molecule. This transition was exactly repeatable in repetitive cycles of oscillation and reversible with pronounced hysteresis. (2) The dissipative stress in the sliding state was almost independent of peak sliding velocity when this was changed over several decades. Significant (although smaller) elastic stress also persisted, which decreased with increasing deflection amplitude but was almost independent of oscillation frequency. (3) The adhesive energy in the sliding state was significantly reduced from that measured at rest. This similarity of friction in the two systems, dry and wet sliding, leads us to speculate that, similar to plastic deformation of solids, sliding in the confined liquid films is the result of slippage along an interface.

Structure and Shear Response in Nanometer‐Thick Films

“…However, the equilibration time may be extremely long due to the small ratio between h (∼ 1nm) and the contact diameter (∼ 100µm) and the immense viscosities of confined films [41]. Equilibration is more likely in steady sliding experiments where the contact area changes completely [4,6,11], than in the experiments with small (∼ 10nm) sinusoidal motions that we compare to here [7,10,37].…”

Nanorheology of liquid alkanes

“…Reiter et al [23] in their SFA experiments with squalane speculate that the molecules have a rather globular shape with a diameter of about 1.0 nm. These results suggest that confined flow SFA experiments may yield a useful, perhaps even quantitative, model of practical lubrication applications because the SFA experiments impose a similar structure on the fluid yielding similar power law rheological behavior as in lubrication but at experimentally accessible strain rates.…”

Nonlinear Rheology of Confined Polymer Melts under Oscillatory Flow

“…A number of papers are devoted to this problem, in particular to the nanorheology of confined polymer melts. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] Confined polymer liquids show some features that are different from those of polymer liquids in the bulk. They show extremely large relaxation time even in the regime of rather short polymer chains which cannot create entanglements.…”