title: Discrete Element Model for Suppression of Coffee-Ring Effect.
abstract: When a sessile droplet evaporates, coffee-ring effect drives the suspended particulate matters to the droplet edge, eventually forming a ring-shaped deposition. Because it causes a non-uniform distribution of solid contents, which is undesired in many applications, attempts have been made to eliminate the coffee-ring effect. Recent reports indicated that the coffee-ring effect can be suppressed by a mixture of spherical and non-spherical particles with enhanced particle-particle interaction at air-water interface. However, a model to comprehend the inter-particulate activities has been lacking. Here, we report a discrete element model (particle system) to investigate the phenomenon. The modeled dynamics included particle traveling following the capillary flow with Brownian motion, and its resultant 3D hexagonal close packing of particles along the contact line. For particles being adsorbed by air-water interface, we modeled cluster growth, cluster deformation, and cluster combination. We found that the suppression of coffee-ring effect does not require a circulatory flow driven by an inward Marangoni flow at air-water interface. Instead, the number of new cluster formation, which can be enhanced by increasing the ratio of non-spherical particles and the overall number of microspheres, is more dominant in the suppression process. Together, this model provides a useful platform elucidating insights for suppressing coffee-ring effect for practical applications in the future.
authors.affiliation: Department of Mechanical and Biomedical Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Hong Kong Special Administrative Region, China.
authors.forename: Ting
authors.initials: T
authors.lastname: Xu
authors.affiliation: School of Creative Media, City University of Hong Kong, 83 Tat Chee Avenue, Hong Kong Special Administrative Region, China.|Centre for Robotics and Automation, City University of Hong Kong, 83 Tat Chee Avenue, Hong Kong Special Administrative Region, China.|CityU Shenzhen Research Institute, 8 Yuexing 1st Road, Shenzhen Hi-Tech Industrial Park, Nanshan District, Shenzhen 518057, China.
authors.forename: Miu Ling
authors.initials: ML
authors.lastname: Lam
authors.affiliation: Department of Mechanical and Biomedical Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Hong Kong Special Administrative Region, China.|School of Creative Media, City University of Hong Kong, 83 Tat Chee Avenue, Hong Kong Special Administrative Region, China.|Centre for Robotics and Automation, City University of Hong Kong, 83 Tat Chee Avenue, Hong Kong Special Administrative Region, China.|CityU Shenzhen Research Institute, 8 Yuexing 1st Road, Shenzhen Hi-Tech Industrial Park, Nanshan District, Shenzhen 518057, China.
authors.forename: Ting-Hsuan
authors.initials: TH
authors.lastname: Chen