Giant fluctuations in a free diffusion process

Nonequilibrium fluctuations during diffusion in liquid layers

“…The parameters that are used in the calculations are reported in Table I; on the represented length scales, a liquid crystal monolayer suspended in medium vacuum should be a good approximation of a 2D fluid. It can be clearly appreciated that the fluctuations have a negligible amplitude in three-dimensional fluids, when the macroscopic concentration gradient extends over a thickness of the order of millimeters, such as that usually employed in experiments [8,9,11,12,16]. Under the same conditions, the fluctuations are quite intense in the two-dimensional fluids.…”

“…In the past, the results of simulations [42,[67][68][69] and experiments [9,12] suggested that the fronts of diffusion could have a fractal structure [43]. In the 3D case, further theoretical analysis showed that the fronts of diffusion do not have a scale-invariant structure [19] but exhibit instead a self-affine structure.…”

“…Until now, experiments based on the dissolution of the phases generated through a second-order phase transition have been carried out in a critical binary mixture in 3D [9,66], but so far no evidence of nonequilibrium fluctuations has been reported during the investigation of the dissolution of lipid domains in bilayers mimicking cell membranes. Indeed, the detection of such fluctuations could represent a significant step forward in understanding whether the interactions between the membrane proteins can be affected by Casimir-like forces generated by the nonequilibrium fluctuations.…”

“…Theory and experiments have shown that in the absence of gravity the fluctuations do not exhibit any characteristic length scale, beyond the molecular and macroscopic ones that provide intrinsic lower and upper bounds for the size of fluctuations [12,13]. On Earth, a characteristic length scale is determined by the presence of the gravity force, which quenches long-wavelength fluctuations [5,6,8,9,[14][15][16].…”

“…The presence of a concentration gradient puts the fluid in a nonequilibrium condition, characterized by the presence of a macroscopic diffusive mass flux [1]. Nonequilibrium fluctuations orders of magnitude larger than the equilibrium ones have been predicted theoretically [2][3][4][5][6][7] and reported experimentally [8][9][10][11] during diffusion between couples of ordinary miscible fluids. These nonequilibrium fluctuations are originated by thermally excited velocity fluctuations, which give rise to vortices at mesoscopic length scales displacing volumes of fluid in layers with different concentration.…”