Diffusion in liquid mixtures

Abstract

The understanding of transport and mixing in fluids in the presence and in the absence of external fields and reactions represents a challenging topic of strategic relevance for space exploration. Indeed, mixing and transport of components in a fluid are especially important during long-term space missions where fuels, food and other materials, needed for the sustainability of long space travels, must be processed under microgravity conditions. So far, the processes of transport and mixing have been investigated mainly at the macroscopic and microscopic scale. Their investigation at the mesoscopic scale is becoming increasingly important for the understanding of mass transfer in confined systems, such as porous media, biological systems and microfluidic systems. Microgravity conditions will provide the opportunity to analyze the effect of external fields and reactions on optimizing mixing and transport in the absence of the convective flows induced by buoyancy on Earth. This would be of great practical applicative relevance to handle complex fluids under microgravity conditions for the processing of materials in space.

Fig. 1. Non-equilibrium concentration fluctuations.

False-colour shadowgraph images of non-equilibrium concentration fluctuations induced by the Soret effect in a polystyrene polymer solution on Earth (left) and in space (right) during the GRADFLEX experiment on FOTON M3. Scale bar: 2.0 mm.

Fig. 2. Effect of gravity on colloidal particles.

Schematic representation of some key effects of gravity on small (colloidal) or large (granular) particles in a fluid. a In the absence of gravity, thermal fluctuations lead to a uniform distribution of colloidal particles. b For granular particles this process can be prohibitively slow, and hence injection of mechanical energy is required for the system to remain in a fluid state. c Gravity promotes a net flux of particles (sedimentation) along the direction of gravity. d If the mass distribution within each particle is not spherically symmetric, gravity results in a net torque on the particles, which can promote their orientational ordering. e For a diluted colloidal suspension, the steady-state distribution of particles in the presence of gravity is not homogeneous along the direction of gravity but corresponds to an exponential concentration profile. f In a granular medium, the particles densely accumulate at the bottom of the container, where they minimize their gravitational potential energy and form load-bearing frictional contacts.

Fig. 3. Interfacial dynamics of fluids in microgravity.

Complex interfacial dynamics with FC-40 and 20 cSt silicone oil vibrated at 50 Hz in microgravity.

Fig. 4. Carbon capture and sequestration.

Schematic representation of carbon capture and sequestration in reservoirs. The inherent temperature gradient induces thermal diffusion of impurities from the bottom to the cap-rock of the reservoir. During injection, sequestration and storage, CO₂ goes through different thermodynamic states as depicted in the right panel. The temperature and pressure dependence of the Widom line for pure CO₂ was determined by the maxima of the isobaric heat capacity. The area between the blue and red lines represents the transition region between the liquid-like and gas-like subdomains of the supercritical fluid. Image of thermal-power plant by vectorpocket on Freepik.com (https://www.freepik.com/free-vector/heavy-industry-factory-working-thermal-power-plant-station-with-electricity_4758475.htm).

Fig. 5. Marangoni-flow-driven chemical fronts.

The front propagates from left to right in the presence of buoyancy (top) and in the absence of gravity (bottom). Scale bar: 5.0 mm.

Fig. 6. Reaction-diffusion-advection front in a chemical system.

Radial reaction-diffusion-advection front generated by a simple bimolecular reaction A + B → C when a reactant A is injected at a constant flow rate in a quasi-2D reactor prefilled with the reactant B. In the absence of buoyancy-driven currents under microgravity, the dark product C spreads as an expanding circle. Scale bar: 10 mm.