Hydrodynamic Brachistochrone: Conflicting Paths of Time and Energy Minima within Viscous Media

Abstract

We experimentally and theoretically study the hydrodynamic analog of the classical brachistochrone problem: the time- and energy-minimizing paths for a spherical particle rolling down an incline within a viscous fluid. We show that in the presence of viscous dissipation, the paths of minima diverge from the classical cycloid, into curves of opposing curvature for time and energy, and are characterized by an effective dimensionless parameter, $St_p$, representing the ratio of the particle's viscous response time scale to its gravitational time scale. Using a generalized variational framework, we show that the fastest path reduces to nearly straight ramps, however, beginning and terminating in localized cycloids of curvature, $κ_c \sim St_p^{-2}$