Added mass effect in coupled Brownian particles

Abstract

The added mass effect is the contribution to a Brownian particle's effective mass arising from the hydrodynamic flow its motion induces. For a spherical particle in an incompressible fluid, the added mass is half the fluid's displaced mass, but in a compressible fluid its value depends on a competition between timescales. Here we illustrate this behavior with a solvable model of two harmonically coupled Brownian particles of mass $m$, one representing the sphere, the other the immediately surrounding fluid. The measured distribution of the Brownian particle's velocity, $P(\bar{v})$, follows a Maxwell-Boltzmann distribution with an effective mass $m^*$. Solving analytically for $m^*$, we find that its value is determined by three relevant timescales: the momentum relaxation time, $t_p$, the harmonic oscillation period, $\tau$, and the velocity measurement time resolution, $\Delta t$. In limiting cases $\Delta t \ll \tau,t_p$ and $\tau\ll\Delta t\ll t_p$, our expression for $m^*$ reduces to $m$ and $2m$, respectively. We find similar behavior upon generalizing the model to the case of unequal masses.