Possible Liquid Slip Mechanism

The slip flow phenomena can be explained by different mechanism. The fluid slip can be described as true or apparent slip. The true slip occurs at a molecular level, where liquid molecules are effectively sliding on the solid surface. The apparent slip occurs not at the solid/fluid interface but at the fluid/fluid interface where a thin layer of liquid/gas molecules is tightly bound to the solid surface. For apparent slip, the velocity gradient close to the solid is so high that the molecules beyond the layer of liquid/gas molecules appear to slide on the surface. 

The true slip phenomena can be attributed to the liquid-liquid and liquid-solid interactions. If the viscous friction between liquid molecules at the interface is stronger than between molecules of the liquid and molecules of the solid, then the molecules can slide on the surface. This is true for hydrophobic surfaces but might also hold for hydrophilic surfaces. If the dimensions of the liquid molecules are of comparable size as the corrugation on the solid surface, the molecules are trapped in the pits on the surface resulting in no-slip BCs. But if their size is much smaller or much larger, they can slide on the surface. 

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The liquid slip phenomenon has been presented in the following sections. The experimental investigation quantifying the liquid slip has been discussed to begin with followed by the results from molecular simulation. Subsequently, the factors responsible for liquid slip and possible mechanism responsible for liquid slip are discussed. 

Liquid slip can be promoted by the adsorption of long chain surfactants from solution on to smooth solid surfaces. This suggests a possible new mechanism of boundary lubrication by organic friction modifiers - encouragement of local slip of the liquid lubricant against the bounding solids to produce a local reduction in shear stress. Such a mechanism would imply that friction modifiers might be able reduce friction in full film lubrication conditions. 

The simplest mechanical design basis possible is a separator with a homogeneous inflow in which gas and liquid are separated, a weir plate to provide suitable safety criteria for the oil phase, and three outlets for the respective (clean) water, oil, and gas phases. The traditional simplified view is to assume plug flow in the liquid and gas phase, and slip between the liquid and gas, thus neglecting inlet effects and the possible slip between oil and water. Furthermore, Stokes law is used for mass transport between the bulk phases (assuming rapid coalescence). This view is depicted in Fig. 6. 

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