We are grateful to Sergio Ciliberto and David Stephens for useful discussions. We also wish to thank Sergio Ciliberto for making available to us the raw data of the experiment reported in  which allowed one of us (T.B.) to analyze the data and plot the Fig. 2 in Sect. 4.3. The Research of T.B. was partly supported by ANR-11-LABX-0040-CIMI within the Program ANR-11-IDEX-0002-02 and by ANR Contract ANR-14-CE25-0003-0. The Research of V.J. was partly supported by NSERC. The work of C.-A.P. was partly funded by Excellence Initiative of Aix-Marseille University, A*MIDEX, a French “Investissements d’Avenir” Programme.