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.
Highlights
The evaporation of a sessile droplet, which is by itself an interesting encounter in everyday life, causes a complex, non-equilibrium fluid dynamic phenomenon[1]
Since the polyethylene glycol (PEG) concentration profile varies with time as shown in Supplementary Fig. 1(e), it would take a time to build up a potential to generate the Marangoni flow
It is known that the deposition pattern can be altered by change of surface activity through pH control[12], temperature-dependent surface tension gradient[2, 13,14,15], surfactant-induced surface tension gradient[16,17,18], surface-mediated repulsion of different shaped particles[10], and etc
Summary
The PS particles in the DIw droplet are strongly and quickly driven and collected near the contact line by the outward flow (Fig. 1g). Both SDS and PEG solutions formed multiple-ring patterns as shown in Fig. 3(a) and (b). These multiple rings were a result of the repeated pinning and receding process. Since the PEG concentration profile varies with time as shown in Supplementary Fig. 1(e), it would take a time to build up a potential (surface tension gradient) to generate the Marangoni flow. When the Marangoni flow begins, it may be called as an onset (or critical) condition of Marangoni phenomena, which will be discussed later
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