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

Abstract

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.

Positive and negative birefringence in packed films of binary spherical colloidal particles

“…Therefore, drying colloidal suspensions with a binary size distribution has attracted much attention for a decade. 9 One of the most attractive issues is the segregation or stratication phenomenon of small or large particles that occurs during drying. [10][11][12][13] Sun et al 14 investigated drying of a sessile droplet of a binary suspension.…”

On the effect of particle surface chemistry in film stratification and morphology regulation

“…Pé helps to determine the dominant process (evaporation or diffusion) during the film formation process. 17,32,33 The diffusion coefficient (D) of a particle is given by D = k B T/(6ZpR), where k B is the Boltzmann constant, T is the absolute temperature, R is the radius of the particle and Z is the viscosity of the solvent. Hence the Péclet number can be depicted as expressed in eqn (1).…”

“…16 Furthermore, the experimental results were correlated with simulation and modeling of stratified films and it was concluded that higher particle size ratio favors stratification in dilute colloidal dispersions. 17 Similarly, Zhou et al has showed that stratification was favored by high particle size ratio and high Péclet number, 18 i.e. the ratio between mass transport by convection flow and diffusion of the particles.…”

Self-stratification of amphiphilic Janus particles at coating surfaces

“…The difference in diffusion rate for particles of different sizes will lead to a different amount of accumulation at the drying front. [25][26][27][28][29] The Peclet number, the ratio of the evaporation rate against the diffusion rate, is often used to quantify and predict the stratification behaviour. However, the Peclet number does not consider the surface adsorption of particles at all.…”