Unpinning of trapped oil droplets via non-resonant acoustic streaming in capillary tubes

Abstract

We establish self-consistent analytical model demonstrating that trapped non-wetting liquid phases in narrow capillary channels can be successfully unpinned via non-resonant, second-order acoustic streaming coupled with background static drive gradients. Moving away from boundary-guided or resonant mechanisms, our approach exploits the bulk acoustic-wind force density generated by the steady-state momentum flux of attenuated first-order linear wave interactions. By expanding the hydrodynamic equations up to second order, we determine the critical assisted acoustic wave amplitude required to break capillary pinning thresholds and derive an explicit formulation for steady transport velocity under viscous wall constraints. Furthermore, incorporating both boundary-layer wall effects and bulk core thermo-viscous dissipation reveals a natural mathematical optimum condition where the spatial absorption coefficient matches half the inverse distance to the target droplet ($α= 1/(2x_0)$). This condition is then numerically validated and cross-correlated against legacy industrial frequency baselines, providing a fundamental theoretical framework for minimizing transducer power requirements while maximizing localized mobilization velocities in geological pore networks. Finally, we demonstrate that this optimal operational frequency scales inversely with the transmission distance, providing an analytical framework to optimize downhole acoustic tools according to spatial damping constraints of the specific formation rather than relying on rigid hardware parameters.