Most important hydrodynamic regime

Fig. 2 shows a map of the most important hydrodynamic regimes for downward not-foaming systems. Without discussing these regimes, the conditions mostly used in industry are a., b., and occasionally c. for more inviscid systems. 

In hydrodynamic lubrication, the load is supported by the pressure developed due to relative motion and the geometry of the system. In the regime of hydrodynamic or fluid film lubrication, there is no contact between the solids. The film thickness is governed by the bulk physical properties of the lubricants, the most important being viscosity friction arises purely from shearing of viscous lubricant. 

Although prevention of contact is probably the most important function of a lubricant, there is stiU much to be learned about the transition from hydrodynamic and elastohydrodynamic lubrication to boundary lubrication. This is the region in which lubrication goes from the desirable hydrodynamic condition of no contact to the less acceptable boundary condition, where increased contact usually leads to higher friction and wear. This regime is sometimes referred to as a condition of mixed lubrication. 

The size, shape and charge of the solute, the size and shape of the organism, the position of the organism with respect to other cells (plankton, floes, biofilms), and the nature of the flow regime, are all important factors when describing solute fluxes in the presence of fluid motion. Unfortunately, the resolution of most hydrodynamics problems is extremely involved, and typically bioavailability problems under environmental conditions are in the range of problems for which analytical solutions are not available. For this reason, the mass transfer equation in the presence of fluid motion (equation (17), cf. equation (14)) is often simplified as [48] ... 

Several examples of experimental approaches to thin-film lubrication have been reported [3]. It is important in examining these techniques to make the distinction between methods that are used to determine lubricant film thickness under hydrodynamic or elastohydrodynamic conditions (e.g., optical interference, electrical capacitance, or x-ray transmission), and methods that are used to determine the occurrence or frequency of contact. As we will see later, most experimental studies of synovial joint lubrication have focused on friction measurements, using the information to determine the lubrication regime involved this approach can be misleading. 

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