Elastohydrodynamic lubricant flow with nanoparticle tracking

Efficient photobleaching of rhodamine 6G by a single UV pulse

While photobleaching can be detrimental in applications focusing on fluorescence as it lowers the signal strength, it can advantageously provide non-fluorescent tracers in a fluorescent flow and hence be implemented in tracking techniques such as molecular tagging velocimetry (MTV). The photobleaching of rhodamine 6G under a single UV pulse is described with a simple three-level model of fluorescence. It offers a convenient estimate of the order of magnitude of irradiance required to observe significant photobleaching for a uniform beam. In an effort to improve the application to MTV, analytical formulas enable to determine the photobleached signal strength from a Gaussian UV laser beam overlapping with a green laser sheet and imaged from the side, as well as the apparent width of the imaged photobleached line.

Shear heating, flow, and friction of confined molecular fluids at high pressure

Understanding the molecular-scale behavior of fluids confined and sheared between solid surfaces is important for many applications, particularly tribology where this often governs the macroscopic frictional response. In this study, nonequilibrium molecular dynamics simulations are performed to investigate the effects of fluid and surface properties on the spatially resolved temperature and flow profiles, as well as friction. The severe pressure and shear rate conditions studied are representative of the elastohydrodynamic lubrication regime. In agreement with tribology experiments, flexible lubricant molecules give low friction, which increases linearly with logarithmic shear rate, while bulky traction fluids show higher friction, but a weaker shear rate dependence. Compared to lubricants, traction fluids show more significant shear heating and stronger shear localization. Models developed for macroscopic systems can be used to describe both the spatially resolved temperature profile shape and the mean film temperature rise. The thermal conductivity of the fluids increases with pressure and is significantly higher for lubricants compared to traction fluids, in agreement with experimental results. In a subset of simulations, the efficiency of the thermostat in one of the surfaces is reduced to represent surfaces with lower thermal conductivity. For these unsymmetrical systems, the flow and the temperature profiles become strongly asymmetric and some thermal slip can occur at the solid-fluid interface, despite the absence of velocity slip. The larger temperature rises and steeper velocity gradients in these cases lead to large reductions in friction, particularly at high pressure and shear rate.