  Using a fluid dynamical scenario for $pp$ scattering at 7 TeV, we compute
correlation functions for $\pi^+\pi^+$ 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 $k_T$, 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).
