Elastohydrodynamic penetration length
= Elastohydrodynamic penetration length
{title2=$\ell_\omega=(B/(c_\perp\omega))^{1/4}$}
The elastohydrodynamic penetration length balances the transverse viscous force $c_\perp\omega y$ against the bending force $By/\ell_\omega^4$ in a periodically driven filament. The <sperm number> $L/\ell_\omega$ distinguishes nearly rigid, weakly deformed filaments from long filaments whose response is localized near the actuator.