  Diffusion processes are widespread in biological and chemical systems, where
they play a fundamental role in the exchange of substances at the cellular
level and in determining the rate of chemical reactions. Recently, the
classical picture that portrays diffusion as random uncorrelated motion of
molecules has been revised, when it was shown that giant non-equilibrium
fluctuations develop during diffusion processes. Under microgravity conditions
and at steady-state, non-equilibrium fluctuations exhibit scale invariance and
their size is only limited by the boundaries of the system. In this work, we
investigate the onset of non-equilibrium concentration fluctuations induced by
thermophoretic diffusion in microgravity, a regime not accessible to analytical
calculations but of great relevance for the understanding of several natural
and technological processes. A combination of state of the art simulations and
experiments allows us to attain a fully quantitative description of the
development of fluctuations during transient diffusion in microgravity. Both
experiments and simulations show that during the onset the fluctuations exhibit
scale invariance at large wave vectors. In a broader range of wave vectors
simulations predict a spinodal-like growth of fluctuations, where the amplitude
and length-scale of the dominant mode are determined by the thickness of the
diffuse layer.
