Viscosity vs oscillation amplitude reveals a critical lengthscale of ~4 μm controlling mechanics.

<p>Viscosity, <i>η</i> (Pa-s), as a function of oscillation amplitude for 2 (green), 4.5 (cyan), 6 (blue) and 10 μm (magenta) probes. Note that the viscosities for 4.5 and 6 μm probes are nearly identical. Smaller <i>η</i> values for 2 μm probes indicate a water-like regime (<i>l</i>_<i>1</i>) which only transitions to the continuum regime for amplitudes >4 μm (<i>l</i>_<i>2</i>, dashed line). The reduced <i>η</i> for 10 μm probes is an artifact of the probe being forced out of the trap, likely by larger stiffer structures in the mucus (<i>l</i>_<i>3</i>).