  In optically thin parts of protoplanetary disks photophoresis is a
significant force not just for dust grains, but also for macroscopic bodies.
The absolute strength on the supposedly highly porous objects is not known in
detail as yet. We set up a low pressure torsion balance and studied
photophoretic forces. We investigated the dependence on plate dimensions and on
ambient pressure and considered the influence of channels through the plates.
As samples for full (no channel) plates we used tissue with 2mm thickness and
circular shape with diameters of 10mm, 30mm and 50mm. The influence of channels
was probed on rectangular-shaped circuit boards of 35mm x 35mm area and 1.5mm
thickness. The number of channels was 169 and 352. At low pressure, the
absolute photophoretic force is proportional to the cross section of the
plates. At high pressure, gas flow through the channels enhances the
photophoretic force. The pressure dependence of the radiative force can
(formally) be calculated by photophoresis on particles with a characteristic
length. We derived two characteristic length scales l depending on the plate
radius r_1, the channel radius r_2, and the thickness of the plate which equals
the length of the channel d as l=r^{0.35} x d^{0.65}. The highest force is
found at a pressure p_max = 15 x l^{-1}Pa mm. In total, the photophoretic force
on a plate with channels can be well described by a superposition of the two
components: photophoresis due to the overall size and cross section of the
plate and photophoresis due to the channels, both with their characteristic
pressure dependencies. We applied these results to the transport of large
solids in protoplanetary disks and found that the influence of porosity on the
photophoretic force can reverse the inward drift of large solids, for instance
meter-sized bodies, and push them outward within the optically thin parts of
the disk.
