An experimental study of the Rayleigh-Bénard convection in a yield stress fluid (Carbopol 980) is presented. By combined integral measurements of the temperature difference between two parallel plates and local flow velocity in a wide range of heating powers P two distinct regimes are observed. For heating powers smaller then a critical value Pc a purely conductive regime is observed. A gradual increase of the heating power beyond this onset reveals a convective regime manifested through a nonlinear dependence of the temperature difference between plates on the heating power. Simultaneously with this, local measurements of the flow fields simultaneously reveal a nonlinear increase of the roll pattern amplitude. Regardless the concentration of Carbopol, the Rayleigh-Bénard convection in the Carbopol gel is found to emerge as an imperfect bifurcation that can be correctly modelled by the Landau-Ginsburg theory of phase transitions. A critical slowing down phenomenon is observed corresponding to the onset of convection. The scaling laws of the convective onset Pc and of the Landau-Ginsburg model parameters with the relevant material properties are discussed. The paper closes with a comparison of our findings with existing previous works.
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