Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session S68: Non-equilibrium Dynamics of Film Formation During DryingInvited
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Sponsoring Units: DPOLY DSOFT Chair: Gary Grest, Sandia National Laboratories Room: Four Seasons 4 |
Thursday, March 5, 2020 11:15AM - 11:51AM |
S68.00001: Sandwich Layering in Binary Colloidal Films During Evpaorative 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. In this talk, I will present our recent results from microbeam 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. Additionally, I will present recent modeling results with different forms of the interaction potential, some of which qualitatively capture formation of "sandwich" structures. |
Thursday, March 5, 2020 11:51AM - 12:27PM |
S68.00002: Illuminating the dynamics of drying suspensions in droplets and films Invited Speaker: Joris Sprakel The drying of a suspension, e.g. in an inkjet printed droplet or a coating film, driven by evaporation or capillary imbibition into the substrate, leads to complex changes in the internal dynamics and mechanics of the system. As the particle concentration increases, diffusivities decrease, collective phenomena emerge, particle begin to interact, aggregate and coalesce. Often, these phenomena occur simultaneously without clear temporal separation, at a wide range of characteristic time scales, and in a spatially-inhomogeneous manner. Here we give an overview of recent advances in the development of an experimental tool, Laser Speckle Imaging, that can unravel many of these phenomena across a wide range of time scales, and in a spatially-resolved manner, even in complex and realistic coating formulations. We show how LSI can be used to pinpoint the coating open time and moment of film formation, give insight into subsurface transport and the emergence and propagation of cracks. |
Thursday, March 5, 2020 12:27PM - 1:03PM |
S68.00003: Drying Process in Films of Nanoparticle Suspensions and Polymer Solutions Invited Speaker: Shengfeng Cheng The drying process of polymer solutions or nanoparticle suspensions provides an excellent playground to explore nonequilibrium physics. For example, novel stratification phenomena have recently been discovered in polydisperse particle suspension films that undergo rapid drying. In these systems, the complex interplay of solvent evaporation, fluid dynamics, diffusion, phoresis, and capillarity leads to rich, far-from-equilibrium phenomena. I will describe our efforts using large scale molecular dynamics (MD) simulations to study the drying process of colloid suspensions, polymer solutions, and their mixtures. For bidisperse particle suspensions, a state diagram of stratification is determined and the counterintuitive "small-on-top" stratification, with an enrichment of the smaller particles at the receding liquid-vapor interface during fast drying, is observed. Results are compared to the predictions of several theoretical models based on diffusiophoresis. We show that the temperature gradient induced by evaporative cooling can lead to size-dependent thermophoretic responses of nanoparticles, which compete with diffusiophoresis and suppress the “small-on-top'” stratification. On the basis of this observation, an approach to control stratification using various externally imposed thermal gradients is suggested and validated with MD simulations. We further show that a mixture of two liquids stratifies when rapidly dried with the less volatile component concentrated near the evaporating interface. This phenomenon can be used to separate and stratify particles which have different couplings to the components of a liquid mixture. Finally, we demonstrate a new strategy of unfirmly dispersing nanoparticles into a polymer matrix using fast solvent evaporation, based on the stratification behavior of polymer-particle mixtures in a far-from-equilibrium situation induced by rapid drying. |
Thursday, March 5, 2020 1:03PM - 1:39PM |
S68.00004: 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 5, 2020 1:39PM - 2:15PM |
S68.00005: The role of hydrodynamic interactions in models and simulations of drying Invited Speaker: Michael P Howard Nonequilibrium molecular modeling is valuable for predicting the microstructure of drying films. For example, simulations were recently key to uncovering the “small-on-top” stratification of binary colloidal mixtures. In the absence of solvent effects, drying can be well-described using implicit models without hydrodynamic interactions; however, these models tend to overestimate the solute migration speeds and stratification observed in experiments. Here, I will highlight two examples from our recent work that clearly demonstrate the important role of hydrodynamic interactions in nonequilibrium models of drying. I will first discuss the evaporation-induced crystallization of a colloidal dispersion, where we found that the crystal nucleation and growth depended sensitively on the hydrodynamic interactions because of the colloid distribution in the drying film. I will then discuss the stratification of a mixture of two differently sized polymers; the mixture stratified without hydrodynamic interactions, as predicted, but did not stratify when they were included. Our work shows that although implicit-solvent models neglecting hydrodynamic interactions are computationally efficient, the corresponding sacrifice in accuracy may not be acceptable for faithfully predicting structure in drying films. |
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