Abstract : Using a fluid dynamical scenario for pp scattering at 7 TeV, we compute correlation functions for pi+pi+ paires. Femtoscopic radii are extracted based on threedimensional parameterizations 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 an 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 threedimensional hydrodynamical expansion (high multiplicity).