Experimental study of nonequilibrium fluctuations during free diffusion in nanocolloids using microscopic techniques

Abstract

We report quantitative experimental results regarding concentration fluctuations based on a small-angle light-scattering setup. A shadowgraph technique was used to record concentration fluctuations in a free-diffusion cell filled with colloids. Our experimental setup includes an objective attached to the CCD camera to increase the field of view. We performed two separate experiments, one with 20 nm gold and the other with 200 nm silica colloids, and extracted both the structure factors and the correlation time during the early stages of concentration fluctuations. The temporal evolution of fluctuations was also qualitatively investigated using recursive plots and spatial-temporal sections of fluctuating images. We found that the correlation time versus wavenumber for gold nanocolloids is concave shaped, whereas, for silica colloids, it is convex shaped. The difference in correlation time behavior is not only due to the size of the particle, but also to possible plasmonic interactions in gold colloids.

Mass transport properties of the tetrahydronaphthalene/n-dodecane mixture measured by investigating non-equilibrium fluctuations

“…Under non-equilibrium conditions, the gradient of one variable couples the spontaneous fluctuations of the velocity of the fluid molecules with the fluctuations of the relevant variable, thus providing a huge enhancement of the intensity of the fluctuations, increasing with their size. This phenomenon has been widely investigated in the latest decades and a sound theoretical description (see for example [59][60][61][62][63]) as well as many experimental studies [46,47,49,[64][65][66][67] can be found in the literature. Here, we would like to recall the essential equations that describe the intensity of the light scattered by concentration fluctuations generated in a fluid out of equilibrium and in the presence of the terrestrial gravitational field.…”

The NEUF-DIX space project - Non-EquilibriUm Fluctuations during DIffusion in compleX liquids

“…So far, theoretical models able to describe the properties of non-equilibrium fluctuations have been developed only for ideal conditions like steady states, small gradients, and high dilutions [4]. Several experiments have shown the desirability of a deeper understanding of non-equilibrium fluctuations under non-ideal conditions, such as large concentrations, large gradients and time dependent processes [34,37,21,31]. The available theoretical models rely mostly on linearized hydrodynamics, but several recent theoretical works have shown the importance of second order terms, which give rise to nonlinear effects such as the non-equilibrium Casimir Forces [23,24,26,27,6].…”

“…The available theoretical models rely mostly on linearized hydrodynamics, but several recent theoretical works have shown the importance of second order terms, which give rise to nonlinear effects such as the non-equilibrium Casimir Forces [23,24,26,27,6]. Beyond their relevance in several natural and technological processes, complex liquids represent the ideal system to investigate non-equilibrium fluctuations, due to the opportunity of tuning the microscopic size of the system and the range of the interaction, together with the typical diffusive timescales [17,18,36,37,20,21,31]. One of best tools to investigate complex liquids is represented by light scattering.…”

“…Examples of these complex fluids are crude oil reservoirs, solutions of electrolytes and polyelectrolytes, solutions of polydisperse polymers, stabilized colloidal dispersions, and biological systems -to mention only a few. The experimental investigation of NEF in complex fluids has been rather limited and focused mostly onto dilute polymer solutions [17,[20][21][22] and dilute colloidal suspensions [36,37]. Only very recently experimentalists became interested in dense colloidal suspensions [31], a promising experimental field, almost unexplored also from the theoretical point of view.…”

“…The goal of the project is to deal with several challenging problems that emerged during the latest years, such as: i) the theoretical prediction of Casimir-like forces induced by non-equilibrium fluctuations [23,24,26,27]; ii) the understanding of the non-equilibrium fluctuations in multi-component mixtures, in relation to the transport coefficients [32,33]; iii) the understanding of the non-equilibrium fluctuations in polymer solutions, in relation to their behavior close to a glass transition [34,35]; iv) the understanding of the non-equilibrium fluctuations in concentrated colloidal suspensions [36,37,31], a problem closely related with the detection of Casimir forces; v) the investigation of the onset of fluctuations during transient diffusion [21].…”