2025MNRAS.541.3101S

We present new observations of the PDS 70 disc obtained with the Atacama Large Millimeter/submillimetre Array (ALMA) in Band 9 (671 GHz) at 0.242 arcsec resolution, which provide valuable insights into the spatial distribution of submillimetre grains in the disc. The data reveal a ring-like morphology, with a radial peak located between those previously observed at infrared wavelengths and longer millimetre observations. Additionally, we detect a tentative outer shoulder in Band 9 that is not observed at longer wavelengths. These findings suggest that small grains ($\sim 100\ \mu$m) traced by Band 9 may be escaping from the pressure bump both radially inwards and outwards, or may be tracing different disc layers than those probed at longer wavelengths. A multiwavelength analysis of the disc at millimetre wavelengths and the best fit to the spectral energy distribution shows the presence of centimetre grains around the ring location, where the dust surface density also peaks, compatible with dust trap models. The grain size in the disc cavity is not well constrained but is consistent with grains as small as 10 $\mu$m, supporting the hypothesis that small dust grain filters through the cavity. We use dust evolution models to demonstrate that a turbulent viscosity of $\alpha \, \gtrsim \, 10^{-3}$ allows small grains to filter through the disc gap, while $\alpha \, \lesssim \, 5\, \times \, 10^{-3}$ is required to retain large grains in the pressure bump. The Band 9 observations of PDS 70 validate theoretical models and confirm the presence of pebble flux through the disc gap.