Physics of ultra-thin surface films on molecularly smooth surfaces

Abstract

This paper investigates the physics of ultra-thin surface films in nano-scale counterformal conjunctions. A novel approach is proposed in which the transient behaviour of the fluid film is integrated with the contact mechanics of the approaching bodies. The method predicts film thickness and pressure distribution, as well as local elastic deflection and resulting sub-surface stress tensor. It is found that inertial dynamics of bodies, as determined by local squeeze film effect, prevents diminution of film below certain thickness and reduces the solvation effect. This approach has direct applications for micro-scale mechanisms, where prediction of thin surface adhered film thickness is required for tribological purposes, as well as structural in-service integrity of contacts of vanishing dimensions.

Frictional characteristics of molecular length ultra-thin boundary adsorbed films

“…This effect depends on the packing density of the molecules within the conjunction, which is a complex function of the sliding velocity, and seems to be only effective over a range of speeds (molecular layers), before a bulk lubricant film of hydrodynamic characteristics is formed. This finding is in line with the experimental works in [7,8,9,34] and complex numerical analysis, such as that for a progressively diminishing gap by Teodorescu et al [35]. The main contribution of this paper is in demonstrating that such characteristics can be predicted using a relatively simple analytical approach, based on the assumption of a Langmuir-Blodgett layer subjected to Eyring thermal activation energies.…”

Frictional characteristics of molecular length ultra-thin boundary adsorbed films

“…This effect depends on the packing density of the molecules within the conjunction, which is a complex function of the sliding velocity, and seems to be only effective over a range of speeds (molecular layers), before a bulk lubricant film of hydrodynamic characteristics is formed. This finding is in line with the experimental works in [7,8,9,34] and complex numerical analysis, such as that for a progressively diminishing gap by Teodorescu et al [35]. The main contribution of this paper is in demonstrating that such characteristics can be predicted using a relatively simple analytical approach, based on the assumption of a Langmuir-Blodgett layer subjected to Eyring thermal activation energies.…”

Formation of ultra-thin bi-molecular boundary adsorbed films

“…They showed that mutual approach of smooth surfaces inhibits solvation by increasing the load capacity of the thin surface films. This was also confirmed by Teodorescu et al [23] in a subsequent study. Al-Samieh and Rahnejat [14] also included the effect of meniscus force in the vanishing conjunction of a pair of very smooth bounding solids.…”

Dry and wet nano-scale impact dynamics of rough surfaces with or without a self-assembled monolayer

“…However, the presence of moisture and relative motion of surfaces can also bring about formation of very thin polar films that can generate electrostatic and hydrodynamic pressures. At very close range, with molecularly smooth surfaces reordering of water molecules near the surfaces can also cause solvation pressure [14][15][16]. The current model should be extended to include such effects, which may be prevalent in practice.…”

“…no asperity interactions/adhesion). In nanoscale, such conditions correspond to molecularly smooth surfaces in mutual approach, where the presence of any moisture can lead to a host of other phenomena, such as electrostatic repulsion, solvation, van der Waal's interactions, and even formation of a hydrodynamic film, depending on velocity of approach and applied load [15][16][17]. With higher average asperity heights, the rebound velocity decreases with increasing impact time, until the latter becomes long enough for menicus action around the interacting asperity tips, and when both u 1 and u 2 remain acute.…”

Frictional characteristics of molecular length ultra-thin boundary adsorbed films

“…This effect depends on the packing density of the molecules within the conjunction, which is a complex function of the sliding velocity, and seems to be only effective over a range of speeds (molecular layers), before a bulk lubricant film of hydrodynamic characteristics is formed. This finding is in line with the experimental works in [7,8,9,34] and complex numerical analysis, such as that for a progressively diminishing gap by Teodorescu et al [35]. The main contribution of this paper is in demonstrating that such characteristics can be predicted using a relatively simple analytical approach, based on the assumption of a Langmuir-Blodgett layer subjected to Eyring thermal activation energies.…”

Formation of ultra-thin bi-molecular boundary adsorbed films

“…They showed that mutual approach of smooth surfaces inhibits solvation by increasing the load capacity of the thin surface films. This was also confirmed by Teodorescu et al [23] in a subsequent study. Al-Samieh and Rahnejat [14] also included the effect of meniscus force in the vanishing conjunction of a pair of very smooth bounding solids.…”

Dry and wet nano-scale impact dynamics of rough surfaces with or without a self-assembled monolayer

“…However, the presence of moisture and relative motion of surfaces can also bring about formation of very thin polar films that can generate electrostatic and hydrodynamic pressures. At very close range, with molecularly smooth surfaces reordering of water molecules near the surfaces can also cause solvation pressure [14][15][16]. The current model should be extended to include such effects, which may be prevalent in practice.…”

“…no asperity interactions/adhesion). In nanoscale, such conditions correspond to molecularly smooth surfaces in mutual approach, where the presence of any moisture can lead to a host of other phenomena, such as electrostatic repulsion, solvation, van der Waal's interactions, and even formation of a hydrodynamic film, depending on velocity of approach and applied load [15][16][17]. With higher average asperity heights, the rebound velocity decreases with increasing impact time, until the latter becomes long enough for menicus action around the interacting asperity tips, and when both u 1 and u 2 remain acute.…”

Frictional characteristics of molecular length ultra-thin boundary adsorbed films

“…This effect depends on the packing density of the molecules within the conjunction, which is a complex function of the sliding velocity, and seems to be only effective over a range of speeds (molecular layers), before a bulk lubricant film of hydrodynamic characteristics is formed. This finding is in line with the experimental works in [7,8,9,34] and complex numerical analysis, such as that for a progressively diminishing gap by Teodorescu et al [35]. The main contribution of this paper is in demonstrating that such characteristics can be predicted using a relatively simple analytical approach, based on the assumption of a Langmuir-Blodgett layer subjected to Eyring thermal activation energies.…”

Formation of ultra-thin bi-molecular boundary adsorbed films

“…They showed that mutual approach of smooth surfaces inhibits solvation by increasing the load capacity of the thin surface films. This was also confirmed by Teodorescu et al [23] in a subsequent study. Al-Samieh and Rahnejat [14] also included the effect of meniscus force in the vanishing conjunction of a pair of very smooth bounding solids.…”

Dry and wet nano-scale impact dynamics of rough surfaces with or without a self-assembled monolayer

“…However, the presence of moisture and relative motion of surfaces can also bring about formation of very thin polar films that can generate electrostatic and hydrodynamic pressures. At very close range, with molecularly smooth surfaces reordering of water molecules near the surfaces can also cause solvation pressure [14][15][16]. The current model should be extended to include such effects, which may be prevalent in practice.…”

“…no asperity interactions/adhesion). In nanoscale, such conditions correspond to molecularly smooth surfaces in mutual approach, where the presence of any moisture can lead to a host of other phenomena, such as electrostatic repulsion, solvation, van der Waal's interactions, and even formation of a hydrodynamic film, depending on velocity of approach and applied load [15][16][17]. With higher average asperity heights, the rebound velocity decreases with increasing impact time, until the latter becomes long enough for menicus action around the interacting asperity tips, and when both u 1 and u 2 remain acute.…”