Friction friction and wear in so-called self-lubricating materials. Fluid friction occurs between fluid layers that are moving relative to each other. This internal resistance to flow is named "viscosity". In everyday terms, the viscosity of a fluid is described as its "thickness". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity. The less viscous the fluid, the greater its ease of deformation or movement. All real fluids (except superfluids) offer some resistance to shearing and therefore are viscous. For teaching and explanatory purposes it is helpful to use the concept of an inviscid fluid
Mass flow action within their cells . Solute flow is driven by a difference in hydraulic pressure created from the unloading of solutes in the sink tissues. That is, as solutes are off-loaded into sink cells (by active or passive transport), the density of the phloem liquid decreases locally, creating a pressure gradient. Mass flow "This article is about the flow of fluids in biological systems. For use of "mass flow" in physics see Fluid Dynamics. For "bulk flow" in physics see Advection. For "protein transport" in cell biology see Bulk movement." Mass flow, also known as “mass transfer” and “bulk flow”,