Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session S01: Non-equilibrium Dynamics of Film Formation During DryingInvited Live
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Sponsoring Units: DPOLY Chair: Gary Grest, Sandia National Laboratories |
Thursday, March 18, 2021 11:30AM - 12:06PM Live |
S01.00001: Stratification in Drying Colloidal and Polymer Films Invited Speaker: Shengfeng Cheng Drying of colloidal and polymer films is a ubiquitous nonequilibrium process where evaporation, fluid dynamics, diffusion, phoretic motion, and capillarity are coupled. Recently, various auto-stratification phenomena have been discovered when polydisperse colloidal suspensions are dried rapidly. We employ molecular dynamics simulations to model drying solutions of colloids, polymers, and their mixtures. Our purpose is twofold: first, to elucidate the physical mechanism underlying the stratification phenomena; and secondly, to explore various means of controlling the resulting stratified structures. For suspensions of bidisperse particles, we observe the counterintuitive “small-on-top” stratification when the suspensions are dried quickly. Our results generally support the diffusiophoretic mechanism of stratification but the drying process can be complicated by the emergence of temperature gradients during rapid solvent evaporation. The corresponding thermophoretic response of colloidal particles is size-dependent and can be utilized to control the structure of the resultant stratified film by imposing externally controlled temperature gradients. Stratification can also be induced and controlled using a binary solvent with the two components having different volatilities. During fast evaporation, the two components develop opposite concentration gradients with the less volatile one enriched at the evaporating front. These concentration gradients can be used to stratify colloidal particles that have contrasting preference to the solvent components, even when the particles are identical otherwise. The stratification behavior also occurs in drying suspensions of polymer-particle mixtures, which leads to useful strategies of fabricating polymer nanocomposites with controlled dispersion of filler particles. |
Thursday, March 18, 2021 12:06PM - 12:42PM Live |
S01.00002: Stratification in Drying Colloidal Films: A competition between diffusion, evaporation and diffusiophoresis Invited Speaker: Alexander Routh Waterborne dispersions are an environmentally friendly formulation for coatings. There are a number of issues associated with drying such coatings such as reduced open time and crack formation. Blending dispersions with hard and soft components has been shown to allow better film formation; however, stratification of components can lead to regions of cracking. Alternatively stratification can be beneficial and used to place different components at desired regions within the film. This could be used for expensive anti-fungal agents in bathroom coatings or catalysts in the walls of tubular reactors. |
Thursday, March 18, 2021 12:42PM - 1:18PM Live |
S01.00003: The role of hydrodynamic interactions in models and simulations of drying Invited Speaker: Michael Howard Nonequilibrium molecular modeling has proven valuable for predicting morphology evolution during drying; for example, simulations were recently key to uncovering the “small-on-top” stratification of colloidal mixtures. In the absence of solvent effects, such morphologies can be rationalized and predicted using implicit-solvent models; however, these models tend to overestimate the extent of stratification observed in experiments. Using a mixture of two differently sized polymers as a case study, I will systematically demonstrate that this overestimation is due to neglect of hydrodynamic interactions in prior models and simulations. I will compare hybrid simulations accounting for hydrodynamic interactions between polymers through the multiparticle collision dynamics technique (and in which the dried morphology is homogeneous) with free-draining Langevin dynamics simulations neglecting the same (and in which the dried morphology is stratified). The simulation methodology unambiguously attributes the drastically different morphologies to the treatment of the solvent. The role of the hydrodynamic interactions is further quantified and rationalized using multicomponent diffusion (Onsager) coefficients. This work indicates that it is important to include hydrodynamic interactions in faithful models of evaporation-induced stratification. |
Thursday, March 18, 2021 1:18PM - 1:54PM Live |
S01.00004: Sandwich Layering in Binary Colloidal Films During Evaporative Drying Invited Speaker: Surita Bhatia Multicomponent films based on colloidal dispersions have a wide range of applications, including antimicrobial coatings for medical instruments, conductive textiles for flexible electronics, anti-reflective coatings for optical devices, paints for humid environments that are resistant to mold growth, and drug-loaded coatings for medical implants. Often, there is a need to spatially control location of certain components in the film. For example, silver nanoparticles can be used to impart antimicrobial activity to paints, but this component is expensive and may only be needed in the top few layers of the coating, not throughout the entire film. In principle, evaporative drying of multicomponent dispersions can be used to create films with a prescribed vertical concentration profile in a one-step process. In this talk, we present our recent results from atomic force microscopy (AFM) and small-angle X-ray scattering (SAXS) on films prepared from binary colloidal dispersions containing large and small particles of varying size and initial volume fraction. Our results show evidence of different types of stratification behavior, including large-on-top (e.g., large particles migrating to the top surface of the film), small-on-top, and “sandwich”-like layering. We discuss these results in terms of recent theories for stratification during evaporative drying. |
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