Stratification in Drying Films Containing Bidisperse Mixtures of Nanoparticles

Abstract

Large scale molecular dynamics simulations for bidisperse nanoparticle suspensions with an explicit solvent are used to investigate the effects of evaporation rates and volume fractions on the nanoparticle distribution during drying. Our results show that "small-on-top" stratification can occur when Pe ϕ ≳ c with c ∼ 1, where Pe is the Péclet number and ϕ is the volume fraction of the smaller particles. This threshold of Pe ϕ for "small-on-top" is larger by a factor of ∼α than the prediction of the model treating solvent as an implicit viscous background, where α is the size ratio between the large and small particles. Our simulations further show that when the evaporation rate of the solvent is reduced, the "small-on-top" stratification can be enhanced, which is not predicted by existing theories. This unexpected behavior is explained with thermophoresis associated with a positive gradient of solvent density caused by evaporative cooling at the liquid/vapor interface. For ultrafast evaporation the gradient is large and drives the nanoparticles toward the liquid/vapor interface. This phoretic effect is stronger for larger nanoparticles, and consequently the "small-on-top" stratification becomes more distinct when the evaporation rate is slower (but not too slow such that a uniform distribution of nanoparticles in the drying film is produced), as thermophoresis that favors larger particles on the top is mitigated. A similar effect can lead to "large-on-top" stratification for Pe ϕ above the threshold when Pe is large but ϕ is small. Our results reveal the importance of including the solvent explicitly when modeling evaporation-induced particle separation and organization and point to the important role of density gradients brought about by ultrafast evaporation.

Control of Stratification in Drying Particle Suspensions via Temperature Gradients

“…We have previously shown that particles of different sizes have different thermophoretic responses to a thermal gradient. [30] In our previous work, only a thin layer of the liquid solvent adjacent to the bottom wall is thermalized at T l during evaporation, as shown in Fig. 1(a).…”

“…[30] In certain cases the "small-on-top" stratification expected by the existing theory [25] was even converted to "largeon-top" in the presence of strong evaporative cooling. [30] Based on the physical picture depicted above, it is natural to investigate the effects of a controlled thermal gradient on stratification in a drying suspension. In this paper, we explore this idea by comparing three types of thermalization schemes as sketched in Fig.…”

“…1(a)]. [30] Evaporative cooling leads to a negative temperature gradient in the suspension toward the interface. In Scheme B, all solvent beads in the simulation cell are thermalized at T l [ Fig.…”

“…The competition between thermophoresis generated by evaporative cooling and diffusiophoresis can thus suppress "small-on-top" stratification at ultrafast drying rates or even turn the stratification into "large-on-top". [30] This discovery further indicates that thermophoresis, with a controlled thermal gradient other than the naturally occurring evaporative cooling, may be used to control stratification. In this paper, we employ MD modeling to test this idea in detail and demonstrate that stratification in a drying suspension can be controlled on demand with a temperature gradient imposed on the system, i.e., via controlled thermophoresis.…”

Directional freezing of binary colloidal suspensions: a model for size fractionation of graphene oxide

“…One can incorporate the hydrodynamic interactions by including the concentration-dependence in the diffusion constant. There are many recent studies on the effect of hydrodynamics on the non-equilibrium colloidal dynamics, mostly by evaporation [30][31][32][33][34][35] . One study on polymer solutions by Statt et al 32 showed that stratification happens in implicit solvent simulations, but not in explicit solvent simulations when hydrodynamic effects are included.…”