Traditional fluid mechanics edifies the indifference between liquid and gas flows as long as certain similarity parameters—most prominently the Reynolds number—are matched. This may or may not be the case for flows in nano- or microdevices. The customary continuum, Navier–Stokes modeling is ordinarily applicable for both air and water flowing in macrodevices. Even for common fluids such as air or water, such modeling is bound to fail at sufficiently small scales, but the onset for such failure is different for the two forms of matter. Moreover, when the no-slip, quasi-equilibrium Navier–Stokes system is no longer applicable, the alternative modeling schemes are different for gases and liquids. For dilute gases, statistical methods are applied and the Boltzmann equation is the cornerstone of such approaches. For liquid flows, the dense nature of the matter precludes the use of the kinetic theory of gases, and numerically intensive molecular dynamics simulations are the only alternative. The present article discusses the above issues as well as outline physical phenomena unique to liquid flows in minute devices.

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