Thin film flow in the kitchen

Fluid dynamics is ubiquitous in the kitchen. Here, we discuss examples of cultural kitchen practices that involve the hydrodynamics of a thin-film, namely, practices related to decanting residual fluids with varying viscosity from a carton and draining leftover water after washing a cooking pot to minimize rusting. By performing calculations on the thin-film equation and experiments in the kitchen, we show why it is hard to empty a jar or dry a wok by flipping them for just a brief instant of time. The duration of decantation to recover 90% of the residual fluid was estimated for a variety of kitchen fluids through experiments and theoretical calculations. By comparing the time scales of viscous flow and evaporation in the case of an iron wok, we discuss how a two-dump method reduces rusting of its inner surface by one order of magnitude. The results of our calculation based on thin-film fluid flow offer a quantitative justification for these traditional kitchen practices.