Transport phenomena involves the transfer of momentum, energy, and mass through fluids. The three processes - momentum (viscosity), energy (thermal conductivity), and mass (diffusivity) transfer - occur simultaneously in nature and are described by closely related equations. For momentum transfer, Newton's law of viscosity states that the shear stress between parallel fluid layers is proportional to the velocity gradient between the layers. For Newtonian fluids, viscosity is a property of the fluid that does not depend on pressure but does generally decrease with increasing temperature.