title: Altering the coffee-ring effect by adding a surfactant-like viscous polymer solution.
abstract: A uniform deposition of the suspended particles in an evaporating droplet is necessary in many research fields. Such deposition is difficult to achieve, because the coffee-ring effect dominates the internal flow in a droplet. The present study adopts a biocompatible, surfactant-like polymer (Polyethylene glycol, PEG) to break the coffee-ring effect and obtain a relatively uniform deposition of the microparticles with yielding multi-ring pattern over a droplet area. Movements of the suspended particles in evaporating droplets and deposition patterns of them on a glass substrate were analyzed with microscopic images and video files. The PEG in the droplets successfully altered the coffee-ring effect because of the surface tension variation, which induced a centripetal Marangoni flow. Balancing these two phenomena apparently generated the Marangoni vortex. For PEG solution droplets, the pinning-depinning process during evaporation was periodically repeated and multiple rings were regularly formed. In conclusion, adding a surfactant-like viscous polymer in a droplet could provide a uniform coating of suspended particles, such as cells and various biomaterials, which would be essentially required for droplet assays of biomedical applications.
authors.affiliation: School of Mechanical Engineering, Korea University, Seoul, 136-701, Republic of Korea.
authors.forename: Changdeok
authors.initials: C
authors.lastname: Seo
authors.affiliation: School of Mechanical Engineering, Korea University, Seoul, 136-701, Republic of Korea.
authors.forename: Daeho
authors.initials: D
authors.lastname: Jang
authors.affiliation: School of Mechanical Engineering, Korea University, Seoul, 136-701, Republic of Korea.
authors.forename: Jongjin
authors.initials: J
authors.lastname: Chae
authors.affiliation: School of Mechanical Engineering, Korea University, Seoul, 136-701, Republic of Korea. lexerdshin@korea.ac.kr.
authors.forename: Sehyun
authors.initials: S
authors.lastname: Shin