Nanofluids: Physical phenomena, applications in thermal systems and the environment effects- a critical review

When a colloidal suspension droplet evaporates from a solid surface, it leaves a characteristic deposit in the contact region. These deposits are common and important for many applications in printing, coating, or washing. By the use of superamphiphobic surfaces as a substrate, the contact area can be reduced so that evaporation is almost radially symmetric. While drying, the droplets maintain a nearly perfect spherical shape. Here, we exploit this phenomenon to fabricate supraparticles from bidisperse colloidal aqueous suspensions. The supraparticles have a core-shell morphology. The outer region is predominantly occupied by small colloids, forming a close-packed crystalline structure. Toward the center, the number of large colloids increases and they are packed amorphously. The extent of this stratification decreases with decreasing the evaporation rate. Complementary simulations indicate that evaporation leads to a local increase in density, which, in turn, exerts stronger inward forces on the larger colloids. A comparison between experiments and simulations suggest that hydrodynamic interactions between the suspended colloids reduce the extent of stratification. Our findings are relevant for the fabrication of supraparticles for applications in the fields of chromatography, catalysis, drug delivery, photonics, and a better understanding of spray-drying.

Segregation in Drying Binary Colloidal Droplets.

On the colloidal and chemical stability of solar nanofluids: From nanoscale interactions to recent advances

For a wide range of applications, films are deposited from colloidal particles suspended in a volatile liquid. There is burgeoning interest in stratifying colloidal particles into separate layers within the final dry film to impart properties at the surface different to the interior. Here, we outline the mechanisms by which colloidal mixtures can stratify during the drying process. The problem is considered here as a three-way competition between evaporation of the continuous liquid, sedimentation of particles, and their Brownian diffusion. In particle mixtures, the sedimentation of larger or denser particles offers one means of stratification. When the rate of evaporation is fast relative to diffusion, binary mixtures of large and small particles can stratify with small particles on the top, according to physical models and computer simulations. We compare experimental results found in the scientific literature to the predictions of several recent models in a quantitative way. Although there is not perfect agreement between them, some general trends emerge in the experiments, simulations and models. The stratification of small particles on the top of a film is favoured when the colloidal suspension is dilute but when both the concentration of the small particles and the solvent evaporation rate are sufficiently high. A higher particle size ratio also favours stratification by size. This review points to ways that microstructures can be designed and controlled in colloidal materials to achieve desired properties.

A critical and quantitative review of the stratification of particles during the drying of colloidal films

Development of high-performance roll-to-roll-coated gas-diffusion-electrode-based fuel cells

When suspensions containing colloidal particles of two different sizes are coated on substrates to form films by evaporating the host fluids, the smaller particles can segregate to the top surface of the films. We investigate the effects of the evaporation rate on the segregation by use of Langevin dynamics simulations. The evaporation rate is scaled by the Brownian diffusion rate of the particles, yielding a dimensionless number which we define as the particle drying Peclet number. We show that there is a Peclet number at which the segregation is the most enhanced. Our result indicates the need for the regulation of the evaporation rate to control the segregation.

Rei Tatsumi

Papers

Effects of the evaporation rate on the segregation in drying bimodal colloidal suspensions

Numerical simulations of dispersion and aggregation behavior of surface-modified nanoparticles under shear flow

Numerical Simulation of Dispersion and Aggregation Behavior of Surface-modified Nanoparticles in Organic Solvents

Numerical Simulation of Structure Formation of Surface-Modified Nanoparticles during Solvent Evaporation

Numerical simulations of lift force and drag force on a particle in cross-flow microfiltration of colloidal suspensions to understand limiting flux