text: Hele-Shaw flow is defined as flow taking place between two parallel flat plates separated by a narrow gap satisfying certain conditions, named after Henry Selby Hele-Shaw, who studied the problem in 1898. Various problems in fluid mechanics can be approximated to Hele-Shaw flows and thus the research of these flows is of importance. Approximation to Hele-Shaw flow is specifically important to micro-flows. This is due to manufacturing techniques, which creates shallow planar configurations, and the typically low Reynolds numbers of micro-flows.
  The conditions that needs to be satisfied are
  where  is the gap width between the plates,  is the characteristic velocity scale,  is the characteristic length scale in directions parallel to the plate and  is the kinematic viscosity. Specifically, the Reynolds number  need not always be small, but can be order unity or greater as long as it satisfies the condition  In terms of the Reynolds number  based on , the condition becomes 
  The governing equation of Hele-Shaw flows is identical to that of the inviscid potential flow and to the flow of fluid through a porous medium (Darcy's law). It thus permits visualization of this kind of flow in two dimensions.