Using a fluid dynamical scenario for pp scattering at 7 TeV, we compute
correlation functions for π+π+ pairs. Femtoscopic radii are extracted
based on three-dimensional parametrizations of the correlation functions. We
study the radii as a function of the transverse momenta of the pairs, for
different multiplicity classes, corresponding to recent experimental results
from ALICE. We find the same decrease of the radii with kT, more and more
pronounced with increasing multiplicity, but absent for the lowest
multiplicities. In the model we understand this as transition from string
expansion (low multiplicity) towards a three-dimensional hydrodynamical
expansion (high multiplicity)