Title: 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_sΦ_s ≳ c with c ~1, where Pe_s is the Péclet number and Φ_s is the volume fraction of the smaller particles. This threshold of Pe_sΦ_s for “small-on-top” is larger by a factor of ~α² 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 themore » 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_sΦ_s above the threshold when Pe_s is large but Φ_s is small. Lastly, 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.« less

Authors:

Tang, Yanfei ^[1]; Grest, Gary S.  ^[2];

• Search DOE PAGES for author "Grest, Gary S."
• Search DOE PAGES for ORCID "0000-0002-5260-9788"

Cheng, Shengfeng  ^[1]

• Search DOE PAGES for author "Cheng, Shengfeng"
• Search DOE PAGES for ORCID "0000-0002-6066-2968"

---

+ Show Author Affiliations

1. Virginia Polytechnic Inst. and State Univ. (Virginia Tech), Blacksburg, VA (United States)
2. Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)

Publication Date:

Wed May 23 00:00:00 EDT 2018

Research Org.:

Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)

Sponsoring Org.:

USDOE National Nuclear Security Administration (NNSA)

OSTI Identifier:

1457520

Report Number(s):

SAND-2018-6692J
Journal ID: ISSN 0743-7463; 664619

Grant/Contract Number:  

AC04-94AL85000

Resource Type:

Accepted Manuscript

Journal Name:

Langmuir

Additional Journal Information:

Journal Volume: 34; Journal Issue: 24; Journal ID: ISSN 0743-7463

Publisher:

American Chemical Society

Country of Publication:

United States

Language:

English

Subject:

77 NANOSCIENCE AND NANOTECHNOLOGY; 36 MATERIALS SCIENCE; 71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; Evaporation; Drying; Nanoparticle; Suspension; Stratification; Péclet Number; Molecular Dynamics